Jersey Strategic Flood Risk Assessment

Government of Jersey

Project number: 60627145

April 2021

Jersey Strategic Flood Risk Assessment AECOM Project Number: 60627145

AECOM

Quality information

Prepared by Checked by Verified by Approved by

Hannah Booth

Graduate Water

Consultant

Bernadine Maguire

Principal Flood Risk &

Coastal Consultant

Sarah Littlewood

Principal Flood Risk

Consultant

Emily Craven

Associate

Bernadine Maguire

Principal Flood Risk &

Coastal Consultant

Revision History

Revision Revision date Details Authorized Name Position

1 April 2020 Draft for comment BM Bernadine Maguire Principal

2 December 2020 Final draft BM Bernadine Maguire Principal

3 January 2021 Final BM Bernadine Maguire Principal

4 March 2021 Final BM Bernadine Maguire Principal

5 April 2021 Final BM Bernadine Maguire Principal

Prepared for:

Government of Jersey

Prepared by:

AECOM Infrastructure & Environment UK Limited

Midpoint, Alencon Link

Basingstoke

Hampshire RG21 7PP

United Kingdom

T: +44(0)1256 310200

aecom.com

This document has been prepared by AECOM Infrastructure & Environment UK Limited (“AECOM”) for sole use

of our client (the “Client”) in accordance with generally accepted consultancy principles, the budget for fees and

the terms of reference agreed between AECOM and the Client. Any information provided by third parties and

referred to herein has not been checked or verified by AECOM, unless otherwise expressly stated in the

document. No third party may rely upon this document without the prior and express written agreement of

AECOM.

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Table of Contents

User Guide ............................................................................................................................. i

1. Introduction................................................................................................................ 1

1.1 Study area overview .............................................................................................................. 1

2. Island Plan Review .................................................................................................... 3

2.1 Island Plan ............................................................................................................................ 3

3. Methodology .............................................................................................................. 6

3.1 Overview ............................................................................................................................... 6

3.2 Understanding flood risk ........................................................................................................ 6

3.3 Actual and residual risk .......................................................................................................... 8

3.4 Data collection ....................................................................................................................... 8

3.5 Assessing each source of flooding ......................................................................................... 9

4. Flood risk overview .................................................................................................. 12

4.1 Coastal flooding................................................................................................................... 12

4.2 Inland flooding ..................................................................................................................... 16

4.3 Reservoir flooding................................................................................................................ 24

4.4 Sewer Flooding ................................................................................................................... 27

4.5 Summary ............................................................................................................................ 28

5. Flood risk mitigation measures in Jersey ................................................................. 29

5.1 Current flood risk management measures ............................................................................ 29

5.2 Future flood risk management measures .............................................................................. 33

6. Strategic flood risk management ............................................................................. 38

6.1 Flood storage areas ............................................................................................................. 38

6.2 Catchment and floodplain restoration ................................................................................... 39

6.3 Upstream natural catchment management methods ............................................................. 40

6.4 Opportunities for strategic flood risk management in Jersey .................................................. 41

7. Flood risk framework ............................................................................................... 42

7.1 Overview ............................................................................................................................. 42

7.2 Flood risk categories............................................................................................................ 42

7.3 Vulnerability classifications................................................................................................... 43

7.4 Development suitability and planning approach .................................................................... 45

8. Site specific flood risk management measures ........................................................ 47

8.1 Site measures ..................................................................................................................... 47

8.2 Building measures ............................................................................................................... 51

9. Cumulative development and land use change ....................................................... 55

9.1 Cumulative impacts ............................................................................................................. 55

9.2 Land-use change ................................................................................................................. 55

10. Recommendations for policy and guidance ............................................................. 56

10.1 Overview ............................................................................................................................. 56

10.2 Guidance ............................................................................................................................ 56

Appendix A Data Register ................................................................................................... 57

Appendix B Maps ................................................................................................................ 60

Appendix C Pumping Station Catchment Areas .................................................................. 61

Appendix D Site Specific Flood Risk Assessments ............................................................. 62

D.1 What is a Flood Risk Assessment?....................................................................................... 62

D.2 When is a FRA required? ..................................................................................................... 62

D.3 What should a FRA contain? ................................................................................................ 62

D.4 How detailed should a FRA be? ........................................................................................... 64

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Figures

Figure 3-1 Summary of coastal flooding mechanisms ......................................................................................... 6

Figure 3-2 Source Pathway Receptor Model ...................................................................................................... 7

Figure 4-1 Grands Vaux catchment area

....................................................................................................... 25

Figure 5-1 Pumping station locations ............................................................................................................... 31

Figure 5-2 Jersey Water Supply Schematic (Jersey Water, 2019) ..................................................................... 33

Figure 5-3 Coastal Management Unit locations, Jersey Shoreline Management Plan ........................................ 35

Figure 6-1 Online and Offline Flood Storage Areas........................................................................................... 38

Tables

Table 2-1 Strategic Spatial Options considered for the Island Plan (2021-2030) .................................................. 4

Table 3-1 Expressing the probability of flooding .................................................................................................. 7

Table 4-1 Areas of development modelled to be at high risk of flooding from the tide (present day, not including

the presence of defences) ............................................................................................................................... 12

Table 4-2 Areas modelled to be at risk of flooding from the tide as a result of climate change ............................ 13

Table 4-3 Records of historic coastal flooding .................................................................................................. 15

Table 4-4 Areas at high risk of flooding from inland flooding .............................................................................. 17

Table 4-5 Peak rainfall intensity allowance in small and urban catchments (use 1961 to 1990 baseline) ............. 19

Table 4-6 Records of historic inland flooding .................................................................................................... 19

Table 4-7 Flooding Hotspots ............................................................................................................................ 27

Table 5-1 Coastal Flood Defences ................................................................................................................... 29

Table 5-2 Water Impounding Areas .................................................................................................................. 31

Table 5-3 Policy Recommendations from the SMP ........................................................................................... 36

Table 6-1 Natural Flood Management guidance documents .............................................................................. 40

Table 7-1 Flood Risk Categories ...................................................................................................................... 43

Table 7-2 Development Vulnerability Classifications ......................................................................................... 44

Table 7-3 Development Suitability and Planning Approach – Built up areas ....................................................... 45

Table 7-4 Development Suitability and Planning Approach – Countryside areas ................................................ 45

Table 8-1 Typical SuDS Components (Y: primary process, * some opportunities subject to design) .................... 50

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User Guide

This Level 1 Strategic Flood Risk Assessment (SFRA) report has been produced with a range of end users in mind –

Government of Jersey, developers, flood risk consultants and emergency planners. The table below describes the

sections of the SFRA and helps each user find key information in the SFRA for their use.

Section and Content Summary

Flood Risk

Information

Government of

Jersey

Developers

Emergency

Planners

1 Introduction

An overview of the purpose and objectives of the SFRA

ü ü ü

2 Island Plan Review

An overview of the Island Plan Review.

ü ü

3 Methodology

An overview of the supplied data for use in the SFRA.

4 Flood Risk Overview

An overview of the flood risk from all sources across the study area, including the

impact of climate change where this information is available. Based on the data

recorded in Appendix A and the mapping in Appendix B.

ü ü ü ü

5 Flood Risk Mitigation Measures in Jersey

An overview of current flood mitigation and management measures.

ü ü ü ü

6 Strategic Flood Risk Management

An overview of potential options for improved strategic flood risk management.

ü ü ü

7 Flood Risk Framework

Recommendations for a Flood Risk Framework based on the findings of the SFRA.

ü ü

8 Site-specific Flood Risk Management Measures

A summary of potential site-specific flood risk management measures that developers

can use to manage the impacts of flooding to and from the site.

ü ü

9 Cumulative Development and Land Use Change

An overview of the potential flood risk impacts from cumulative development and land

use change.

ü ü

10 Recommendations for Policy and Guidance

A Data Register List of all data used in the SFRA, including source, format and

limitations.

ü ü ü

B Maps providing information on the different sources of flooding which affect Jersey.

ü ü ü ü

C Pumping Station Catchment Areas Maps providing information on the catchment

areas for the main pumping stations in Jersey.

ü ü

D Site Specific Flood Risk Assessments Guidance for developers undertaking a Flood

Risk Assessment (FRA) for proposed development sites.

ü ü

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1. Introduction

The Jersey Strategic Flood Risk Assessment (SFRA) provides an island-wide assessment of the risks associated

with flooding from all sources, including the effects of climate change. The SFRA also provides guidance on the

mitigation of flooding and identifies potential opportunities to reduce flooding.

The SFRA identifies methods to deliver strategic policies, which will support long-term flood risk management in

Jersey. Through this, the SFRA will support current policy in Jersey including the Common Strategic Policy

(2018-22), the Government Plan (2020-23) and Future Jersey (2017-2037), the Island’s 20-year community

vision. The SFRA will also feed into the Island Plan review process, forming part of the baseline evidence for

flood risk and climate change. This will support the next iteration of the Island Plan which is due to be adopted

and published in 2022 as the Island Plan (2022-24).

The development of the SFRA has been carried out broadly in line with the guidance on how to prepare a

strategic flood risk assessment

for England, developed by the Environment Agency. However, as an

independent island state, the SFRA does not need to strictly follow this guidance, and the SFRA has been

tailored specific to Jersey and the requirements of the island.

The aims of the SFRA for Jersey are as follows:

· Identify the risk of flooding from all sources;

· Outline the potential cumulative impact that development or changing land use would have on the risk of

flooding;

· Identify the potential effect of climate change on flood risk;

· Identify potential opportunities to reduce the causes and impacts of flooding;

· Develop a flood risk framework and supporting policy recommendations for consideration in the Island

Plan review;

· Provide information on methods to manage and mitigate flood risk;

· Provide guidance on producing site specific flood risk assessments.

1.1 Study area overview

1.1.1 Island Parishes

The Channel Island of Jersey is divided into twelve administrative parishes. All have a coastal boundary and

share a name with their ancient parish churches. The parish boundaries are shown on the figures within

Appendix B. The risk of flooding from all sources of flooding to each of these parishes is discussed in Section 3.

1.1.2 Topography

Light Detection and Ranging (LiDAR) topographic survey data is presented in Appendix B Figure 1. The highest

points on the island are approximately 110 to 140 metres Ordnance Datum (m OD) and are located in the north of

the island, in the parishes of Trinity and St John. The lowest points approximately 10 m OD, are located along the

coastline to the south, south-east and west. A number of valleys are also present across the island, sloping from

the north of the island to the lower lying areas along the southern coastline.

How to prepare a strategic flood risk assessment (Environment Agency, 2019). Available online at:

https://www.gov.uk/guidance/local-planning-authorities-strategic-flood-risk-assessment

Appendix B, Figure 1 Topography

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1.1.3 Geology

The Jersey coastline is diverse in geology, and creates a landscape which is highly valuable, and draws in

visitors. The Government of Jersey Integrated Landscape and Seascape Character Appraisal

defines the

underlying solid and drift geology across the island. Cliffs are most prominent along the north and southwest

coastlines, with elevated cliffs on the north coast made from hard volcanic rocks and granite. Those in the

southwest are characterised by metamorphic granite and deports of porphyritic granite. Other headlands on the

coast are less extensive in extent. The southeast and west areas of the coastline are composed of softer

geology.

Jersey Integrated Landscape and Seascape Character Appraisal (Fiona Fyfe Associates, 2020) available online at:

https://www.gov.je/government/pages/statesreports.aspx?reportid=5271

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2. Island Plan Review

2.1 Island Plan

The Government of Jersey is currently preparing the Island Plan review, which will guide development in Jersey.

In 2019 the Minister for the Environment launched the Island Plan Review Programme, with a view of developing

a new 10-year Island Plan for the 2021 to 2030 period. However, the impact of the Covid-19 pandemic has meant

that it is no longer possible to deliver an Island Plan as originally envisaged. To best respond to the current

context, the Minister is developing a shorter-term ‘bridging’ plan that will exist between two longer-term plans (the

current Island Plan 2011 to 2021; and a future Island Plan 2025 to 2034). The new bridging Island Plan will set

the means to facilitate the island’s positive future growth over a period of significant uncertainty and provide a

new framework against which planning decisions will be made. The plan is key to delivering sustainable

development; balancing the future economic, environmental and social needs of the island in a way that is best

for Jersey and which reflects the vision and aspirations of islanders.

The Government of Jersey adopted the current Island Plan in 2011, however, much has changed since then and

there is a need to respond to current economic, environmental and social challenges faced by the island. The

review of the Island Plan will need to consider how best to address:

· the need for homes;

· safeguarding the environment;

· supporting the economy;

· mitigating the impacts of climate change;

· responding to an ageing population; and

· securing good design and creating better places.

The Government of Jersey is aiming for the Island Plan (2022-2024) to be adopted in 2022.

2.1.1 Managing growth

The objective of the Island Plan is to deliver sustainable development where a balance is struck between the

need to protect the sensitive coast, countryside, biodiversity, and heritage assets, and the need to develop new

housing, land for employment opportunities, major public infrastructure and community facilities, including open

space and an improved public realm.

Previous Island Plans have focused on allocating new development in and around established centres.

Historically, the town of St Helier has absorbed much of the island’s growth, spilling beyond the boundaries of the

parish of St Helier to embrace parts of the parishes of St Saviour and St Clement. This leaves 85% of the island

as open countryside.

The Government of Jersey has proposed six broad spatial options for development within their Stage 1 Strategic

Issues and Options consultation undertaken as part of the Island Plan Review

. These options have been

summarised in Table 2-1.

Island Plan Review 2021 to 2030, Stage 1: strategic issues and options consultation, (n.d), Available online at:

https://www.gov.je/Government/Consultations/Pages/IslandPlanReviewStage1.aspx

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Table 2-1 Strategic Spatial Options considered for the Island Plan (2021-2030)

Strategic Spatial Option Description

Option 1: Increasing density within the town of St Helier. The town of St Helier stretches from First Tower in the west, to La

Pouquelaye in the north, and Grève D’Azette in the east. Increasing

the density of new development here would maintain and give

greater emphasis to the existing policy direction of the current

Island Plan.

Option 2(a): Outward expansion of town to the north,

east and west

The urban fringes of the town comprise open countryside that could

be developed to help meet development needs. Building on the

edge of town would represent a change from the current Island

Plan.

Option 2(b): Outward expansion of town to the south There is also potential to change the existing use of parts of the

harbour to enable the expansion of the town to the south. Such

schemes would need to be commercially viable and address

current safety issues. The current Island Plan includes the

development of the St Helier Waterfront, which is effectively an

expansion of the town to the south

Option 3: Increasing density in other built up areas Other built-up areas include the coastal strip from St Aubin to

Gorey; Red Houses and Les Quennevais; and a range of smaller

built-up areas including parish centres. Increasing the density of

new development here would maintain and give greater emphasis

to the existing policy direction of the current Island Plan.

Option 4: Outward expansion of other built up areas The edges of the other built-up areas (described under Option 3)

generally comprise open countryside. Most is currently defined as

Green Zone, but some is within the Coastal National Park. Some of

the land on the edges of existing built areas could be used to help

meet development needs. Expanding these built-up areas, by

releasing land on the edge of them for new development, would

represent a change from the policy direction of the current Island

Plan.

Option 5: A new settlement or the significant expansion

of an existing settlement

The boldest option would be to create a new settlement, or

significantly expand an existing one. The development of a

significant amount of new housing in Les Quennevais and Maufant

are examples of this approach, where it has been undertaken in the

past. This would likely require an extensive process of land

acquisition and the development of open countryside. This would

represent a significant change from the policy direction of the

current Island Plan.

Option 6: Development in the countryside This option would involve relaxing rules about converting,

demolishing and rebuilding, and extending existing buildings in the

countryside. It would mean increasing the density of development

in the countryside and developing more areas of under-used or

open land around the edge of existing clusters of buildings in the

countryside. The redevelopment and consolidation of buildings or

building clusters in the countryside would represent a significant

change from the policy direction of the current Island Plan.

Island Plan, Strategic issues and options. (n.d) Available online at:

https://www.gov.je/SiteCollectionDocuments/Environment%20and%20greener%20living/ID%20Island%20Plan%20Review%20

Stage%201%20AM.pdf

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Following the public consultation on these options, held in Summer 2019, the responses received showed most

support for focussing development within St Helier (Option 1), including expansion of the town to the south

(Option 2b), with a strong resistance to development within the countryside

. The preferred spatial strategy, which

was assessed relative to the current challenges that the island faces, has been published by the Government of

Jersey

and is summarised below:

· focus development in the Town of St Helier:

─ facilitate the development of key urban opportunity sites; and

─ use public land to meet immediate needs.

· generally maintain the existing definition of built-up areas:

─ encourage the re-use and redevelopment of already developed land at higher densities,

appropriate to the context.

· enable the sustainable and proportionate growth of some built-up areas – involving:

─ the planned release of greenfield land including;

─ extending some built-up area edges; and

─ around some parish centres, where this contributes to the overall community wellbeing and

sustainability of an existing settlement.

· limit development around the undeveloped coast and in the countryside to those uses which require a

specific location.

· positive consideration of future land-reclamation proposals in St Helier.

This SFRA provides background information to ensure that the materiality of flood risk is considered as part of the

Island Plan Review and for the assessment of individual sites.

Findings Report: Island Plan Strategic issues and options consultation (Government of Jersey). Available online at

https://www.gov.je/SiteCollectionDocuments/Planning%20and%20building/IP-findings%20report-digital%20111219.pdf

Preferred Strategy Report: Island Plan Review – Technical Evidence Base (Government of Jersey). Available online at

https://www.gov.je/SiteCollectionDocuments/Planning%20and%20building/201022%20R%20Island%20Plan%20Review%20Pr

eferred%20Strategy%20Report%20FINAL.pdf

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3. Methodology

3.1 Overview

The methodology for the assessment of flood risk from all sources is outlined below; Section 3.4 provides a

description of the datasets used to assess the risk of flooding from each source, further details of which are

included within the data register in Appendix A.

3.2 Understanding flood risk

3.2.1 What is flood risk

Flooding is a natural process and can happen at any time in a wide variety of locations. It constitutes a temporary

covering of land not normally covered by water and presents a risk when people, human, and environmental

assets are present in the area that floods. Assets at risk from flooding can include housing, transport and public

service infrastructure, commercial and industrial enterprises, agricultural land and environmental and cultural

heritage. Major sources of flooding include:

· Coastal – inundation of floodplains by the sea; overtopping of defences; breaching of defences; wave

action, as summarised in Figure 3-1.

· Inland – inland flooding covers two main sources including overland run-off from adjacent land (also

referred to as pluvial or surface water) and out of bank flow from watercourses (this can result from

natural water levels exceeding the bank levels, blockage of culverts or bridges etc.).

· Sewer – surcharging of piped drainage systems (public sewers, highway drains etc.).

· Groundwater – caused by the water table rising after prolonged rainfall to emerge above ground level

remote from a watercourse; most likely to occur in low-lying areas underlain by permeable rock

(aquifers); groundwater recovery after pumping for industry has ceased.

· Infrastructure failure – reservoirs; industrial processes; burst water mains; blocked sewers or failed

pumping stations.

Flooding can also occur from a combination of many different sources.

Figure 3-1 Summary of coastal flooding mechanisms

Jersey Shoreline Management Plan (2020). Available online at

https://www.gov.je/government/pages/statesreports.aspx?reportid=5173

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Flood risk is the product of the likelihood (or probability) of flooding and the consequence of flooding. The

consequences of flooding can vary based on the vulnerability to flooding (taking into account any flood defences

and the potential for those defences to breach, fail or be overtopped) and the total value of the assets at risk of

flooding (population and development). With investment in flood protection or the changing/managing of the

location of assets in flood risk areas, the risk of flooding will be reduced.

The risk of flooding is assessed using the source – pathway – receptor model (as shown in Figure 3-2). This

method is a standard environmental risk model, common to the identification of environmental hazards and is the

starting point of identifying receptors at risk of flooding. For a flood risk to be present there has to be a source (for

example, sea or rainfall), pathway (for example, overtopping, asset failure, exceedance or overland flow route)

and receptor (for example, people, property and the environment). The consequences are varied and can include

loss of life, stress, material damage, loss of land, environmental degradation, cultural loss and economic impact.

Figure 3-2 Source Pathway Receptor Model

Different types and forms of flooding present a range of different risks. The speed of inundation, depth and

duration of flooding can vary greatly between different flood events. With climate change, the frequency, pattern

and severity of flooding are expected to change and become more damaging.

3.2.2 Defining the likelihood of flooding

The likelihood of flooding is expressed as the percentage probability and/or return period, as outlined in Table

3-1. The percentages are based on the average frequency measured (or extrapolated) from records over a large

number of years. The lower the percentage, the less chance there is of that flood event happening in any given

year; the higher the percentage then the more chance there is of that flood event happening in any given year.

Although rare, floods with a low probability are likely to have greater impacts that are often far more severe

compared to the high probability, more frequent, events.

Table 3-1 Expressing the probability of flooding

Probability Percentage range Return period (years)

High

Low

Greater than 1.3% 1 in 75 or greater

Between 0.5% - 1.3% Between 1 in 200 to 1 in 75

Less than 0.5% Less than 1 in 200

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3.2.3 Consequences of flooding

The consequences of flooding vary between different flood events, and can result in property damage, disruption

to lives and businesses and personal implications for people (e.g. financial loss, emotional distress and health

problems). Consequences of flooding depends on the severity of the flooding, and is influenced by the following

factors:

· Depth of flood water

· Velocity of flood water

· Rate of onset

· Duration

· Wave action effects

· Water quality

The vulnerability of receptors also has a significant impact on possible damage and additional consequences

caused by flooding. The potential for damage can be influenced by the following factors:

· type of development

· age-structure of the population

· presence and reliability of mitigation measures

The combined influence of these factors will determine the potential flood risk in any area.

3.3 Actual and residual risk

Flood risk is not static, as it cannot be described simply as a fixed water level or an area at risk if flooding was to

occur. Risk varies depending on the severity of event, the source, the pathways of flooding (such as the condition

of flood defences) and the vulnerability of receptors.

3.3.1 Actual risk

Actual risk is the risk taking into account any flood management measures that are in place. Flood management

measures typically provide a minimum Standard of Protection (SoP) which can vary between different defence

types and different areas.

It is important to recognise that risk comes from many different sources and that the SoP provided will vary. The

actual risk of flooding from the tide may be low to a settlement behind a coastal defence but moderate from

inland sources, which may pond behind the defence in low spots and is unable to discharge into the sea during

high water levels.

3.3.2 Residual risk

Even when flood management measures are in place there is always a possibility that these could be overtopped

or exceeded or that they could fail or breach. In areas where this could occur, there is still a risk of flooding. This

is known as a residual risk. Flood management infrastructure failure can lead to rapid inundation of fast flowing

and potentially deep floodwaters, which may pose significant risk to the people, property and environment.

Because of this, it is never appropriate to class an area as ‘not at risk’ due to flood management measures being

in place.

3.4 Data collection

A large quantity of information and datasets have been made available by the stakeholder organisations and

used to inform the assessment of flood risk. Descriptions of the datasets that have been used, along with details

of their appropriate use or limitations, are included in Appendix A Data Register. These datasets have been used

to develop the maps included in Appendix B.

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3.5 Assessing each source of flooding

The identification of data sets and areas at risk of flooding has been carried out broadly in line with the guidance

on how to prepare a strategic flood risk assessment

for England, developed by the Environment Agency.

However, as an independent island state, the SFRA does not need to strictly follow this guidance, and the SFRA

has been tailored specific to Jersey and the requirements of the island.

3.5.1 Coastal flood risk

Coastal modelling from the Jersey Shoreline Management Plan (SMP)

has been used to identify the areas at

risk of flooding from the sea. The modelling included Still Water Level Flooding Analysis (excluding the presence

of flood defences) as well as including the analysis of the effects of climate change to 2120. This modelling

produced flood frequency outlines as return periods. The flood frequency has been expressed as annual

exceedance probability (AEP)

, which is the probability of that flood event occurring in any given year. The

modelled flood events have been identified below ranked from the lowest risk to the highest risk.

· 1 in 200 year Return Period (0.5% AEP)

· 1 in 75 year Return Period (1.33% AEP)

· 1 in 20 year Return Period (5% AEP)

· 1 in 1 year Return Period (100% AEP)

The following return periods were modelled to account for climate change:

· 1 in 200 year Return Period (0.5% AEP) including short (2040), medium (2070) and long-term epochs

(2120) to account for climate change.

· 1 in 75 year Return Period (1.33% AEP) including short (2040), medium (2070) and long-term epochs

(2120) to account for climate change.

· 1 in 20 year Return Period (5% AEP) including short (2040), medium (2070) and long-term epochs

(2120) to account for climate change.

· 1 in 1 year Return Period (100% AEP) including short (2040), medium (2070) and long-term epochs

(2120) to account for climate change.

For the assessment of coastal risk, high risk has been identified using the 1 in 200 year outline (0.5% AEP) for

the present day and medium risk has been identified using the 1 in 200 year outline (0.5% AEP) for 2120. This

return period was chosen due to the size of the flood extents creating the worst-case scenario for coastal

flooding; and for consistency with the established approach used in parts of the United Kingdom.

The coastal modelling assessed the risk of the overtopping of some of the existing defences. This data has been

used to identify areas potentially at risk of flooding as a result of overtopping and areas in which improvements

will be needed. Wave overtopping modelling was undertaken for the following high priority areas:

· St Ouen’s Bay

· St Brelade’s Bay

· St Aubin’s Bay

· Havre des Pas

· La Grève D’Azette

How to prepare a strategic flood risk assessment (Environment Agency, 2019). Available online at:

https://www.gov.uk/guidance/local-planning-authorities-strategic-flood-risk-assessment

AECOM (2019), Jersey Shoreline Management Plan, Available online at:

https://www.gov.je/Government/Pages/StatesReports.aspx?ReportID=5173

Annual exceedance probability is the chance or probability of a natural hazard event (usually a rainfall or flooding event)

occurring annually and is usually expressed as a percentage. Bigger rainfall events occur (are exceeded) less often and will

therefore have a lesser annual probability.

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· La Mare

· Le Nez Point to Le Hocq Point

· La Rocque

· Royal Bay of Grouville, and,

· Archirondel (north).

Information on coastal flood defences as well as current and future management interventions have been taken

from the Jersey Shoreline Management Plan and information provided by the Government of Jersey. A list of the

data sources provided for this assessment has been included in Appendix A.

3.5.2 Inland flood risk

Surface water hydraulic modelling was undertaken for the whole island as part of the SMP using a 2D TUFLOW

model. The topographical data used was a composite Digital Terrain Model (DTM) with a 1m grid resolution

sourced from the 1m LiDAR supplied by Government of Jersey (2017). The 2D TUFLOW model was set up with

a grid resolution of 5m. This was chosen as 5m resolution represented the finest resolution that could be

achieved whilst retaining practical model run times. Due to the scale and resolution of the island-wide hydraulic

model, structures were included through filtering of LiDAR DTM data and provide a worst-case assessment of

flood risk. Buildings were represented in the model by raising threshold levels by 0.3m above LiDAR DTM within

the 2D domain.

In surface water hydraulic models, rainfall boundaries are usually applied across the entire model domain. In this

case, the model domain was determined as being the whole island of Jersey. Therefore, a large proportion of the

grid will present very shallow depths prior to runoff, and as such, the model results are filtered to be presented as

mapped results. Areas of surface water flood depth of less than 0.1m were removed to present a clearer

representation of flood risk. Further details on the modelling methodology are available within Section 8 of the

Hydraulic Modelling Report technical appendix of the SMP

The modelling was completed in line with UK Environment Agency surface water flood maps, and the results

were presented as areas of low, medium and high risk of flooding.

· High risk of flooding means the area has a chance of flooding corresponding to a 1 in 30 year return

period event (3.33% AEP);

· Medium risk of flooding means the area has a chance of flooding corresponding to a 1 in 100 year

return period event (1% AEP); and,

· Low risk of flooding means the area has a chance of flooding corresponding to a 1 in 1000 year return

period event (0.1% AEP).

Sensitivity analysis was undertaken on the three return periods for two storm durations (2 hour and 6 hour) and

three continuing loss (infiltration) rates (6.5mm/hr, 12.5m/hr, 18.5mm/hr).

The assessment of inland flood risk has been carried out using the 1 in 30 year, 1 in 100 year, 1 in 1000 year

outlines for the 2 hour storm duration with 6.5mm/hr continuing loss rate. This scenario was chosen as it displays

the worst-case extent of inland flooding.

Modelling was also undertaken for two return periods (1 in 30 year and 1 in 100 year) for two climate change

uplifts (+20%, and +40%). These scenarios have been mapped as part of the SFRA to account for the potential

impact of climate change.

Jersey SMP Appendix B – Hydraulic Modelling Report available at

https://www.gov.je/Government/Pages/StatesReports.aspx?ReportID=5173#:~:text=The%20Jersey%20Shoreline%20Manage

ment%20Plan,years%20(up%20to%202120).

Jersey Strategic Flood Risk Assessment AECOM Project Number: 60627145

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Modelling was also undertaken for four surface water pumping stations (Baudrette Brook, Bel Royal, Samarês

Marsh and West of Albert), to determine the effect they will have on flood risk to Jersey. Their locations can be

viewed in Figure 5-1 in Section 5.1.2 of this report. The model and results represent a best case scenario in

which all the pumps at the four pumping stations are in good condition, well-maintained and are in operation for

the whole simulation at the maximum pump rate.

Information on mitigation measures as well as current and future management interventions has been taken from

the Jersey Shoreline Management Plan and information provided by the Government of Jersey. A list of the data

sources provided for this assessment has been included in Appendix A.

3.5.3 Groundwater flood risk

It has been identified by the Government of Jersey and Jersey Water that due to the geology of the island,

groundwater flooding is not a significant risk. There are locations in the island in which groundwater is accessible,

however these are not located near development and are used as a source of potable water. the risk of flooding

from groundwater to the island has, therefore, been excluded from the rest of this assessment.

3.5.4 Sewer flood risk

The risk of sewer flooding has been assessed using the Government of Jersey flooding hotspots data. This data

has been used to identify areas which may have previously experienced sewer flooding. It should be noted that

this data is currently being updated by the Government of Jersey and is subject to revision.

3.5.5 Reservoir flood risk

The assessment of flood risk from reservoirs has been completed using information provided by the Government

of Jersey and Jersey Water. Modelling of the possible flood routeing and predicted depth and velocity of water

from the Grands Vaux Reservoir has been taken from the Grands Vaux Flood Plan. A list of additional data

sources provided for this assessment has been listed in Appendix A.

Jersey Strategic Flood Risk Assessment AECOM Project Number: 60627145

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4. Flood risk overview

This Section provides the strategic assessment of flood risk from each of the sources of flooding across the

Island of Jersey. For each source of flooding, details of any historical incidents are provided, and where

appropriate, the impact of climate change on the source of flooding is described. This Section should be read

with reference to the figures in Appendix B.

4.1 Coastal flooding

4.1.1 Physical structure of the coast

The Jersey coastline has evolved over time, influenced by geology, coastal processes and human interaction.

The island encompasses a diverse range of natural features, the interactions between which have combined to

create the unique character of the coastline. These physical features of the coastline can influence the way the

urban environment inland is developed and can determine the types of coastal defences which are appropriate

for defending against coastal erosion and flooding. The Jersey coastline is dynamic, and change is predicted to

influence the boundary between land and sea over the next 100 years. Climate change is predicted to cause

rising still water levels, which will cause increased wave heights and increased severity and occurrence of

storms. This will increase the risk of coastal flooding on the island in the future.

4.1.2 Risk of flooding from the tide

The risk of coastal flooding is very varied across the island with some areas at limited risk, and others at

significant risk. Appendix B Figures 2 and 2A-2H identify the baseline coastal flood risk: this is where the risk of

flooding is modelled without the presence of any flood defences. The approach to future coastal flood risk

management measures for the island is set out in the Jersey Shoreline Management Plan (refer to Section 5.2.1).

Areas of existing development that are located in an area at high risk of coastal flooding in each parish have

been discussed in Table 4-1. It should be noted that the modelled outputs represent the present day flood risk

from the still water level. There are areas that are known to flood (such as Beaumont and St Aubin) which are not

identified at high risk of flooding from the still water level, but it is acknowledged that these areas can flood as a

result of a number of contributing factors including wave overtopping.

Table 4-1 Areas of development modelled to be at high risk of flooding from the tide (present day, not

including the presence of defences)

Parish Summary of present day coastal flood risk Corresponding map

St Helier The majority of the Parish of St Helier is not in an area at risk of coastal

flooding. Flood water is restricted to the coastline along the parish edge

within St Helier and Havre des Pas

. Parts of the Port of St Helier are also

identified as being at risk if coastal flooding was to occur.

Figure 2F and 2G

St Saviour The Parish of St Saviour has a small coastline limited to the area of Le

Dicq slipway which is at risk of flooding. Hotel de Normandie is directly

opposite Le Dicq slipway and in very close proximity to the seawall that

overtops.

Figure 2H

St Clement There is considerable urban development along the majority of coastline in

the Parish of St Clement. Flood water is restricted to the coastline along the

parish edge, including La Mare slipway (opposite Rue de Maupertuis).

Figure 2G and 2H

Grouville There is considerable urban development along sections of the coastline in

the Parish of Grouville. Le Hurel slipway (adj. Grande Route des Sablons)

is identified as being at risk from coastal flooding

Figure 2H

St Martin Gorey Pier and harbour are identified as being at risk from coastal flooding. Figure 2D and 2H

Trinity There is limited urban development along the coastline in these parishes

and the majority of development is on land above or set back from the

coastline. No residential or commercial development is present in areas at

Figure 2C and 2D

St John Figure 2B and 2C

St Mary Figure 2A and 2B

Appendix B Figure

2 and 2A

Jersey Strategic Flood Risk Assessment AECOM Project Number: 60627145

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Parish Summary of present day coastal flood risk Corresponding map

St Ouen risk of flooding. Flood water is predominantly restricted to the coastline

along the parish edge.

Figure 2A and 2E

St Peter Figure 2E and 2F

St Brelade The majority of development is set back from the coastline. No residential

or commercial development is present in areas at risk of flooding. Flood

water is predominantly r

estricted to the coastline along the parish edge.

Figure 2E and 2F

St Lawrence Development in this area is predominantly located along the waterfront

however flood water is predominantly restricted to the coastline along the

parish edge.

Figure 2F and 2G

4.1.3 Climate change

Climate change poses a significant risk to the Jersey coastline. The effects of climate change on coastal flooding

include:

· Rising still water levels

· increased wave heights

· increased severity and occurrence of storms.

These changes will increase the risk of coastal flooding in the island in the future. The impact of climate change

on areas at risk of coastal flooding has been assessed using three epoch scenarios. These scenarios are:

· Short term (2040)

· Medium term (2070), and

· Long term (2120).

The future impact of climate change on flood risk was assessed using the UK National Oceanography Centre

guidance. The 50th percentile results for the Intergovernmental Panel on Climate Change (IPCC) ‘RCP8.5’

climate change emission scenario (“business as usual”) have been used. This gives a resulting sea level rise

prediction of 0.83 metres by 2120.

The areas of existing development at risk of coastal flooding (as a result of climate change) in each parish have

been discussed in Table 4-2.

Table 4-2 Areas modelled to be at risk of flooding from the tide as a result of climate change

Parish Areas identified at risk of coastal

flooding in the future

Climate Change scenarios that

would affect the area

Corresponding

map

St Helier Parts of the Port of St Helier Short term (2040) Figure 3F and 3G

Parts of St Helier town centre Short term (2070)

Wider Port of St Helier Long term (2120)

Wider area within Havre des Pas Long term (2120)

Parts of Victoria Avenue Short term (2040)

St Saviour Parts of Georgetown, Grève d’Azette and

Rue des Pres

Medium (2070) to Long term

(2120)

Figure 3G

St Clement

Grève D’Azette, St Clement Golf Course,

Plat Douet, Samarès and Rue des Prés

Medium (2070) to Long term

(2120)

Figure 3G and 3H

Le Hocq Long term (2120)

Grouville Le Hurel Medium (2070) to Long term

(2120)

Figure 3H

Appendix B Figure

s 3 and 3 A

Jersey Strategic Flood Risk Assessment AECOM Project Number: 60627145

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Parish Areas identified at risk of coastal

flooding in the future

Climate Change scenarios that

would affect the area

Corresponding

map

Rue de la Forge, Royal Jersey Golf Club

Grouville Common, Grouville Marsh

Long term (2120)

Grouville Marsh Short (2040) to Long term (2120)

St Martin Gorey Promenade and Pier

Gorey Harbour; Gorey Village

Anne Port and Archirondel

Medium (2070) to Long term

(2120)

Figure 3D and 3H

RNLI St Catherine’s Bay (Lifeboat Station),

Rozel

Short (2040) to Long term (2120)

Trinity Rozel Short (2040) to Long term (2120) Figure 3C and 3D

Bouley Bay Long term (2120)

St John Bonne Nuit Pier Long term (2120) Figure 3B and 3C

St Mary Grève de Lecq Medium term (2070) Figure 3A

St Ouen Grève de Lecq Medium term (2070) Figure 3A

L’Etacq

Medium (2070) to Long term

(2120)

St Peter Beaumont, Goose Green

St Aubin’s Bay promenade

Le Marais

Long term (2120) Figure 3F

St Brelade St Brelade’s Bay Short (2040) to Medium term

(2070)

Figure 3E and 3F

St Aubin

St Aubin’s Bay promenade

Short (2040) to Long term (2120)

St Lawrence Bel Royal

St Aubin’s Bay promenade

Long term (2120) Figure 3F

4.1.4 Areas benefiting from defences

There are extensive man-made coastal defences present around parts of the island’s coastline, which are further

described in Section 5.1.1. Some of these defences are higher than the still water level tide which reduces the

risk of flooding. Appendix B Figure 9 identifies the areas benefiting from coastal defences where the defence is

higher than the still water level tide during a high risk coastal flood event in 2120.

4.1.5 Wave overtopping

Coastal flooding has the potential to have significant implications on urban areas in Jersey if flood defences are

overtopped or are breached. Wave overtopping modelling was undertaken for the following high priority areas:

· St Ouen’s Bay

· St Brelade’s Bay

· St Aubin’s Bay

· Havre des Pas

· La Grève D’Azette

· La Mare

· Le Nez Point to Le Hocq Point

· La Rocque

Appendix B Figure

Jersey Strategic Flood Risk Assessment AECOM Project Number: 60627145

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· Royal Bay of Grouville, and,

· Archirondel (north).

Appendix B Figure 10 and Figures 10 A-H identifies the areas at risk from overtopping during a high risk coastal

flood event in 2040. The potential depth of flooding (in metres above the existing ground level) is also indicated

on the maps.

4.1.6 Historic coastal flooding

The Government of Jersey has provided information on historic coastal flooding in the island. Records of historic

coastal flooding are listed in Table 4-3.

Table 4-3 Records of historic coastal flooding

Date Parish Details

3 January 2018 St Helier

St Ouen

St Peter

Storm Eleanor causes coastal flooding which closes roads and increases water

level within watercourses. A section of sea wall collapsed at St Helier. Victoria

Avenue was closed while Gloucester Street/Esplanade area and Five Mile Road

also flooded

3 March 2014 St Ouen

St Helier

High tide (12m) and heavy winds combine to cause flooding. Rue Verte at

L’Etacq severely damaged by the high tides. Victoria Avenue closed

2 February 2014 Unknown Coastal flooding associated with storms. Coastal roads flooded

17 October 2012 Unknown High tides cause flooding to various areas, including Beaumont

8 March 2008 St Brelade

St Helier

St Lawrence

Storm Johanna causes flooding. Water overtopped flood defences which were

breached in four locations. Victoria Avenue was closed at First Tower. Roads

flooded in St Aubin, La Haule, Beaumont and The Gunsite. The sea wall was

damaged at West Park with flooding onto Victoria Avenue, West Park,

Esplanade, Gloucester Street and Seaton Place. Houses and businesses in this

area were also flooded

16 17 18 19

23 November 1984 St Helier Severe storm noted for comparison with March 2008 event. Flooding in St

Helier

. Seawall between First Tower and West Park damaged. Properties

flooded First Tower to Castle Street.

27 February 1967 St Helier Severe storm noted for comparison with March 2008 event. Flooding affected St

Helier and was exacerbated by heavy rain

October 1965 Unknown

Severe storm noted for comparison with March 2008 event, but no further

information given

October 1964

It should be noted that a decrease in coastal flooding at St Helier was observed following the land reclamation

which was undertaken during the 1970s.

The Government of Jersey Department for Infrastructure, Housing and Environment has noted a number of

observations from historic coastal flood events within different parishes, as detailed below:

· St Helier - historic flooding of properties and roads along the First Tower to Bay View section of Victoria

Avenue. Coastal flooding has reached as far as Mont Cochon and Tower Road. Properties flooded

between the slip road and Paris Lane.

Jersey Evening Post (2018) Jersey suffers coastal flooding. Available online at:

https://jerseyeveningpost.com/news/2018/01/03/jersey-suffers-coastal-flooding/

(2014). Channel Islands flooded after 'highest tide of the year’. Available online at: https://www.bbc.co.uk/news/world-

europe-jersey-26390204

https://www.youtube.com/watch?v=tdO18kuP870

https://www.youtube.com/watch?v=vPlYf8u5jMs

Jersey Evening Post (2018). Available online at: https://jerseyeveningpost.com/news/2018/01/02/jersey-facing-biggest-

flooding-threat-since-the-storm-of-march-2008/

ARUP. (June 2017) Jersey Future Hospital Flood Risk Assessment.

Surge Watch (n.d) Storm Event 10th March 2008. Available online at: https://www.surgewatch.org/events/12/

The exceptional tide, storm survey and damage on 10 March 2008, as of 1 May 2008. Frank Le Blancq and John Searson,

Jersey Meteorological Department, May 2008.

Appendix B Figure

s 10 and 10 A

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· St Martin - historic flooding of properties at Gorey Pier and harbour has occurred.

· St Mary - Café Romany, at Grève de Lecq has flooded.

· St Peter - major flooding of the Beaumont area and properties on numerous occasions.

· St Brelade - major flooding of St Aubin’s village, harbour and north to La Haule and properties on

numerous occasions

4.2 Inland flooding

4.2.1 What is inland flooding

Inland flooding is defined as areas of overland flow and watercourses that cause flooding when water is unable to

soak into the ground or enter drainage systems. It can run quickly off land and result in localised flooding. Water

will naturally flow to the lowest point, so it is often possible tell where surface water will collect in a flood by

looking at the topography of the ground and using that to identify flow paths and watercourses.

Areas of overland flow can be defined as individual catchments. A catchment is the area of land, including the

hills, woodlands, and buildings which water drains from, before flowing into watercourses and into the sea. The

outside edge of a catchment is always the highest point. Gravity causes all rain and run-off in the catchment to

run downhill where it naturally collects in a watercourse. Rain falling outside the edge of one catchment is falling

on a different catchment and will flow into other streams and watercourses.

Intense rainstorms and poorly managed overland flow paths and watercourses can mean the potentially

destructive power of the water can cause damage to land, property and possibly lives. If overland flow is

obstructed, it can act as a dam and cause a build-up of water that, if released, can result in significant

consequences.

4.2.2 The role of topography in inland flooding

The topography of Jersey (as shown in Appendix B Figure 1) highlights the areas at the highest elevation are

located in the north of the island (approximately 110 to 140 mAOD), There are a noticeable number of narrow

valleys which slope from the north, gradually descending to approximately 10 mAOD on the south side of the

island adjacent to the sea. Overland flow collects in these valleys and topographic depressions and forms

watercourses that flow into the parishes of St Helier, St Lawrence and St Clement, along the coastline.

Flow paths in the east and west of the island are less defined. The highest areas of elevation are located to the

north east of the western part of the coastline and the north west of the eastern part of the island. The topography

of the western part of the island slopes west with the lower points located along the western coastline. In the east

of the island, the land gradually slopes south east, towards Grouville. A number of small watercourses are

present in these areas.

4.2.3 Inland flooding in Jersey

The risk of inland flooding has been defined into the following categories:

· High risk of flooding means the area has a chance of flooding corresponding to a 1 in 30 year return

period event (3.33% AEP);

· Medium risk of flooding means the area has a chance of flooding corresponding to a 1 in 100 year return

period event (1% AEP); and,

· Low risk of flooding means the area has a chance of flooding corresponding to a 1 in 1000 year return

period event (0.1% AEP).

Appendix B Figures 4A-4H identify the baseline inland flood risk. The results presented indicate a worst-case

scenario and do not include allowance for surface water pumping. There are four surface water pumps located

on the south coast which have the potential to reduce flooding. The effects of these pumps have been discussed

in Section 4.2.5.

Jersey Strategic Flood Risk Assessment AECOM Project Number: 60627145

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The primary flow paths can be seen conveying the water along natural valley contours toward the south coast.

Parts of the coastal plain is at risk of flooding from overspill from St Ouen’s Pond and other watercourses in St

Ouen’s Bay. There is a flow path running north to south crossing the eastern end of Jersey Airport which

indicating a low to medium risk of inland flooding. La Rue de L’Eglise/Mont du Jubilé and L’Avenue de Reine

Elziabeth II, surrounding the airport, are identified at high risk of flooding. Beaumont in St Aubin’s Bay is identified

at low risk from inland flooding, however there is also a high risk of ponding on the fields behind La Route de la

Haule on Goose Green Marsh and at Sandybrook.

A large area of St Helier is at low to medium risk from inland flooding, where the primary flow paths can be seen

converging in the valleys of Grands Vaux and Vallée des Vaux through the lower parts of the town toward the

harbour. There are a few isolated areas of existing wetland on the edge of town being at high risk including Rue

des Près Marsh adjacent the industrial estate; and at Fountain Lane north of Bagot in St Saviour. There is a small

area of ponding identified to the east of Pier Road potentially impacting on buildings on Pier Road; and also to

the north-east of the Millennium Town Park.

There is a large flow path of high risk from inland flooding to both fields and houses from Grouville Marsh to

Gorey Village, which is held back from the coast by La Rue à Don.

Areas of development that are at risk of flooding in each parish have been discussed in Table 4-4.

Table 4-4 Areas at high risk of flooding from inland flooding

Parish Areas with existing development which are identified at risk of inland flooding Corresponding map

St Helier St Helier town centre Figure 4G

Fern Valley/ Bellozanne Valley and First Tower

Millbrook

Grands Vaux/ Town Mills/ Springfield/ Town Park

Vallée des Vaux

Queen’s Road to Great Union Road

Clos du Fort

Westmount Quarry

Havre des Pas

St Saviour Grands Vaux

Rue des,Prés/ Plat Douet/ Georgetown

Bagot

Figure 4F and 4H

Havre des Pas

St Clement Le Marais Figure 4G and 4H

Samarès Le Squez & Le Marais

Le Rocquier/ Le Hocq

Pontac

Le Bourg

Grouville Fauvic

Le Bourg/ Les Près/ Gorey Village

Figure 4H

La Ville-ès-Renauds

St Martin Gorey Village North Figure 4D and 4H

Mont des Landes

St Catherine’s Woods

Rozel Valley

Trinity Rozel Figure 4C and 4D

Bouley Bay

St John Le Mourier valley Figure 4B and 4C

Appendix B Figure

s 4A

Jersey Strategic Flood Risk Assessment AECOM Project Number: 60627145

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Parish Areas with existing development which are identified at risk of inland flooding Corresponding map

St Mary Le Mourier Valley

Grève de Lec

Figure 4A and 4B

St Ouen Grève de Lecq Figure 4A and 4E

St Peter Beaumont

Sandbrook

Figure 4E and 4F

St Brelade St Brelade’s Bay Figure 4E and 4F

St Aubin

Some parts of Les Quennevais

Lawrence

Millbrook Figure 4F and 4G

Bel Royal

4.2.4 Reservoirs

Reservoirs in Jersey have a significant influence in the flow of water in the surface water catchments in the

island. The island has six raw water reservoirs which fall under the Reservoir (Jersey) Law 1996. Each reservoir

has a catchment area, the size of which depends on the geography of the surrounding countryside. Rainfall in a

catchment area runs off the land into streams which feed the reservoir. The reservoirs restrict the amount of

water flowing down into the lower part of the catchment and, therefore, have a significant impact on the risk of

inland flooding downstream of the reservoir location. Water yield for the reservoirs is also supported by other

indirect stream water catchments which are pumped to storage in the reservoirs.

Further information on the risk of flooding from reservoirs has been provided in Section 4.3.

4.2.5 Pumping stations

Appendix B Figures 5A-5E identifies the area at risk of inland flooding including the operation of the West of

Albert, Samarès Marsh, Baudrette Brook, Bel Royal pumping stations. Further information on these pumping

stations is supplied in Section 5.1.2.

The general topography of Jersey promotes overland flow paths towards the south coast, where the four

pumping stations assessed are located. A large area in Beaumont and St Aubin’s, is identified at low risk of

inland flooding, with some areas at high and medium risk. The area to the east of Goose Green Marsh, a

designed flood plain, where Bel Royal pumping station extracts water from, has an area at medium risk of inland

flooding, whereas to the west of Goose Green Marsh there is a large area at high risk of inland flooding on the

marsh, fields and Goose Green car park behind La Route de la Haule. A large area of St Helier is at low risk from

inland flooding, with a few isolated areas (particularly highways) identified at medium risk.

Goose Green Marsh experiences a reduced risk of inland flooding when the pumps from Bel Royal are operating,

although it should be noted that the Goose Green Marsh outfall to the foreshore is separate to that of the pumped

Le Perquage/St Peter’s Valley catchment. A large area of St Helier benefits from the pumps at West of Albert

operating to extract surface water from the roads and residential areas upstream of the pumping station. For the

1% AEP, the pumps at Baudrette Brook and Samarès Marsh removes surface water from roads, St Clement’s

Golf Course, Rue des Prés marsh area, and fields behind Clos de la Mare.

4.2.6 Climate change

Climate change is projected to result in more extreme rainfall events which will generate more surface runoff

resulting in an increased likelihood of inland flooding. More extreme rainfall can cause the water levels within

watercourses to rise rapidly and also increase the pressure on drains and sewers.

As no climate change predictions are specifically available for Jersey, the representation of climate change over

time was extracted from values for UK Government peak rainfall intensity allowance in small and urban

catchments provided for England

and included in Table 4-5. From the below table, the total potential change

Flood risk assessments: climate change allowances, Environment Agency, 2017, available at

https://www.gov.uk/guidance/flood-risk-assessments-climate-change-allowances, accessed November 2018

Appendix B Figure

Jersey Strategic Flood Risk Assessment AECOM Project Number: 60627145

AECOM

anticipated for the ‘2080s’ for climate change Central uplift percentage of +20% and Upper end uplift of +40%

have been used.

Table 4-5 Peak rainfall intensity allowance in small and urban catchments (use 1961 to 1990 baseline)

Climate Change Scenario Total potential change

anticipated for the ‘2020s’

(2015 to 2039)

Total potential change

anticipated for the ‘2050s’

(2040 to 2069)

Total potential change

anticipated for the ‘2080s’

(2070 to 2115)

Upper end +10% +20% +40%

Central +5% +10% +20%

The impact of climate change on inland flooding has been defined for the following categories:

· High risk of flooding means the area has a chance of flooding corresponding to a 1 in 30 year return

period event (3.33% AEP); including a 20% and 40% uplift to account for climate change;

· Medium risk of flooding means the area has a chance of flooding corresponding to a 1 in 100 year return

period event (1% AEP); including a 20% and 40% uplift to account for climate change

Appendix B Figures 6A- 6E identifies the area at risk of inland flooding with the uplift to account for climate

change.

The high risk of flooding scenario shows an increased risk of inland flooding in Beaumont. The Goose Green car

park and adjacent field are already at high risk of inland flooding and the extent increases into the surrounding

fields behind La Route de la Haule for the climate change uplift of 20% and 40%. Inland flooding at Gorey also

extends significantly, with the extent being similar for the increased climate change uplift of 20% and 40%.

The medium flood risk scenario indicates that properties in Beaumont are at an increased risk of inland flooding

with climate change uplift of 20% and +40%. In addition, a larger area at St Helier is at risk of inland flooding

compared to the baseline present day scenario. The increased extent of area at risk from inland flooding is

towards the coast affecting the area between the Esplanade and Broad Street. At Gorey, the flow path remains

west of La Rue à Don and mostly results in inundation of local fields, however some properties along La Rue à

Don and Gouray Village Main Road are at an increased risk of inland flooding.

4.2.7 Historic inland flooding

The Government of Jersey has provided information on historic inland flooding in the island. Records of historic

inland flooding are listed in Table 4-6.

Table 4-6 Records of historic inland flooding

Date Parish Location Details

21 May 1983

St Brelade, St Peter and

St Mary

St Aubin, Grève de

Lecq and St Peter’s

Valley

Two severe storms, both bringing over 50mm of rain

in one hour in some places. Records show 43mm in

one hour at Jersey Airport with a possible 25mm in

six minutes. Flooding affected St Aubin, Grève de

Lecq and St Peter’s Valley

21.

5 June 1983

28 February 2010

St Saviour, St

Lawrence, St Peter, St

Clement, St Helier, St

Ouen

Various locations

across the Island

Numerous calls to the Fire Service to report flooding

to roads, residential properties and gardens and

commercial premises.

2014 St Helier & St Saviour Grands Vaux, Nicholson

Close and Pillar

Gardens

Road, properties and land flooded. Insufficient

capacity of main culvert in valley.

2014 St Peter Tesson Mill Vallée de St

Pierre, Rue du Moulin

de Tesson

Road, properties and land flooded. Upstream leat

stream overflowed to lower stream and through

property.

Severe Storms in Jersey, 31 May and 5 June 1983, David V Randon, Journal of Meteorology, Vol 8, No 84, 1983.

Appendix B Figure

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Date Parish Location Details

2014 St Lawrence

Tesson Mews Rue du

Moulin de Tesson

Road and properties flooded. Two private pipes

restrict flow which then floods properties and road.

2014 St Lawrence Sandybrook Road and properties flooded. Water flooded road

from above.

2014 St Lawrence Millbrook House Road and properties flooded. Flood of water from

Parish road.

2014 St Helier Beresford Street Road and properties flooded. Failure of penstock

gate to open at Weighbridge.

12 June 2015 Unknown Unknown

Roads flooding in response to heavy rainfall, some

areas having over 28mm

22.

8 February 2016 St Helier Victoria Avenue Storm Imogen floods roads including Victoria

Avenue

23.

16 September 2017 St Ouen and St Peter Various Flash flooding in Jersey. Roads left underwater after

torrential rain. St Ouen and St Peter badly affected,

particularly St Peter’s Valley. The road between St

Ouen and St Peter was closed and roads below

Grève de Lecq hill flooded. The area around St

Ouen’s Manor also flooded

24.

27 November 2017 St Peter and St

Lawrence

Beaumont, St Peter, St

Lawrence and Grands

Vaux.

Torrential downpours cause flooding. Roads and

properties flooded at Beaumont; several inches

deep at the bottom of Beaumont Hill. Also flooding

in St Peter, St Lawrence and Grands Vaux.

August 2019 St. Peter La Vallée de St. Pierre Land flooded caused by a collapsed pipe in a field.

August 2019 St. Peter Le Vieux Beaumont Properties flooded. Road gullies blocked causing

overflow of surface water into private property.

August 2019 Trinity

La Grande Route de St.

Jean

Road and properties flooded. Ponding on water on

road surface affecting property.

August 2019 St. John

La Grande Route de St.

Jean

Road and properties flooded. Water coming off field

to the east flooding road.

August 2019 Trinity La Route de la Trinité Road flooded. Blocked private culvert.

August 2019 Trinity La Route de la Trinité Blocked private culvert.

August 2019 St. Helier La Route de la

Liberation

No details.

August 2019 St. Peter Le Mont des Routeurs /

Route des Hetres

No details.

August 2019 St. Ouen La Rue des Cosnets Road and properties flooded. Road drainage to

soakaway. Limited capacity.

August 2019 St. Martin La Rue d'Aval Road and properties flooded. Insufficient capacity of

road drainage.

August 2019 St. Ouen La Grande Route de St.

Ouen

No details.

September 2019 St. Saviour Plat Douet Road Road partially flooded; properties flooded.

October 2019 St. Helier The Tunnel

(Westbound)

Road partially flooded. Gravity sewer under capacity

due to build-up of grease. New Pumps and Control

Panel at Le Doicq Pumping Station. Heavy and

persistent rainfall.

November 2019 St. Lawrence Mont Félard Properties flooded. Blocked outlet on private land.

Jersey Evening Post (2015). St Helier Struck by lightning as thunderstorm and heavy rain batter the island. Available online

at: https://jerseyeveningpost.com/news/2015/06/12/st-helier-home-struck-by-lightening-as-thunderstorm-and-heavy-rain-

batterthe-island/

BBC News (2016). Storm Imogen winds close Channel Island roads and cause flooding. Available online at:

https://www.bbc.co.uk/news/world-europe-jersey-35526934

Jersey Evening Post (2017). Jersey hit by flash flooding. Available online at:

https://jerseyeveningpost.com/news/2017/09/16/jersey-hit-by-flash-flooding/

Jersey Evening Post (2017). Jersey hit by flooding. Available online at:

https://jerseyeveningpost.com/news/2017/11/27/jersey-hit-by-flooding/

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Date Parish Location Details

November 2019 Grouville Rue à Don Road flooded. Outlets to Marsh blocked.

November 2019 Grouville Rue à Don Road partially flooded; properties flooded. Gulley at

Grouville Church blocked (upstream) leading to

downstream gullies becoming surcharged and

blocked.

November 2019 St. Clement La Grande Route de St.

Clement

Road partially flooded. Gullies on La Grande Route

de St. Clement at junction of Rectory Close blocked

and surcharged due to amount of silt coming

downhill from Rue du Hocq / La Verte Rue / La Rue

Pignon.

November 2019 St. Martin La Route de Ste.

Catherine

Blocked road drainage.

November 2019 Trinity La Rue de la Monnaie Property flooded. Insufficient drainage infrastructure

of culvert under road.

November 2019 St. John La Route du Mont Mado Road flooded; properties flooded. Poor maintenance

of culvert and grating of culvert under road.

November 2019 St. Lawrence La Route de St. Aubin Capacity of pipe through park, tree roots.

November 2019 St. Brelade La Rue de la Sergente Insufficient road drainage.

November 2019 St Peter Of Route de Beaumont Blocked culvert flooded properties, field and road

junction.

November 2019 St. Peter Route de Beaumont Properties flooded. Low lying property pumps out

onto road.

December 2019 St. Peter Rue des Landes Road flooded. Single Road Gulley blocked with

leaves

December 2019 St. Saviour Rue du Trot Road flooded. Insufficient capacity of road drainage

pipe across fields to the east.

January 2020 St. Helier La Grande Route de St.

Jean

Properties flooded. Single road gulley cannot cope

upstream.

February 2020 St. John Les Chenolles Properties flooded. Culvert blocked under property.

Grating required upstream in field J744.

February 2020 St Peter Route de l'Aleval Road flooded. Culvert keeps blocking.

February 2020 St Peter Vallée de St Pierre Road flooded. Culvert capacity needs to be

reviewed.

March 2020 St Brelade Route du Sud Road and property flooded. Gullies becoming

blocked; lack of gullies upstream (on Rue des

Champs).

March 2020 St Brelade La Route du Petit Port

Properties and land flooded. Single gulley on

downstream side of entrance. Upstream fields have

been planted and covered in polythene.

March 2020 St Martin Le Mont de Rozel / La

Vallee de Rozel

Properties flooded. Culvert in parish road and under

Le Mont de Rozel full of rubble.

March 2020 St Helier Vallee des Vaux Road, land and properties flooded. Culvert getting

beaten, overflowing into road and bypassing

impounding pond and leat, re-enters downstream of

Rossmore into lower stream. Lower stream has also

blocked, leading to flooding.

March 2020 St Lawrence Rue de Haut Road flooded. Insufficient capacity of private culvert.

March 2020 St Helier New Road Properties flooded. Poor maintenance of private

culvert.

March 2020 St Clement Rue de Maupertuis Properties flooded.

March 2020 St Brelade Charing Cross

Road and properties flooded. Culvert unable to

drain at high tide.

March 2020 St Brelade Chemin des Pietons Properties flooded. Culvert becomes blocked.

March 2020 Grouville Rue Horman

Land and properties flooded. Grating becomes

blocked.

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Date Parish Location Details

March 2020 St Clement Vieux Chemin

Road flooded. Parish and private drainage does not

have capacity.

March 2020 St Clement

Grande Route de St

Clement

Road and property flooded. Water from Rue

Laurens enters into property on south side of road.

March 2020 St Clement Rue du Prebytere

Road flooded. Public drainage becomes blocked

with debris

March 2020 St Saviour New York Lane Road and properties flooded. Private culvert to north

does not have capacity.

March 2020 St Helier Wellington Road Road and properties flooded. Surface water sewer

does not have capacity and flows onto St Saviours

Road.

March 2020 St Helier Mont Cochon Road and properties flooded. Road drainage system

does not have capacity.

March 2020 St Ouen Mont Huelin Private culvert becomes blocked.

March 2020 St Ouen Route de Ste Marie Property flooded. Private culvert becomes blocked.

Blocked grating to downstream culvert.

March 2020 St Peter Avenue de la Reine

Elizabeth II

Road flooded. Gullies become blocked flooding

primary route.

March 2020 St Peter Grande Route de St

Pierre

Property flooded. Problems with road drainage

system.

March 2020 St Peter La Dimerie Property flooded. Historic drainage due to blocked

culvert.

March 2020 Trinity Rue de la Fontaine de

Colard

Road and properties flooded. Flooding of properties

from water off Route d'Ebenezer.

March 2020 Trinity Rue de la Monnaie

Properties flooded. Insufficient size of downstream

culvert.

March 2020 St Brelade Route de la Moye

Property flooded. Road drainage system does not

have capacity.

March 2020 St Saviour Route de la Hougue Bie Road flooded. Road drainage becomes blocked.

March 2020 St Martin Grouville Rue d'Aval Road flooded. Road drainage becomes choked with

mud debris from fields to the east.

March 2020 St Brelade Route des Genets Road drainage system through property to small.

March 2020 St Martin Grande Route de St

Martin

Road and property flooded. Private culvert on Rue

du Hucquet and road drainage becomes blocked.

March 2020 St Saviour Route de Maufant Road and property flooded. Road drainage blocked

outlet culvert choked.

March 2020 St Saviour Route de Maufant Road flooded. Road drainage blocked.

March 2020 St Lawrence Waterworks Valley Road flooded. Road drainage blocked.

March 2020 St Mary Rue de la Frontiere Road flooded. Potential issues with capacity of

culvert under road.

March 2020 St Mary Mont de Ste Marie Road flooded. Potential issues with capacity of

culvert under road.

March 2020 St Saviour St Saviour's Hill Properties flooded. Capacity of culvert through

properties

March 2020 Grouvllle Route de Grouville Road and property flooded. Blocked road gullies

and downstream culvert.

March 2020 Grouvllle Route de Grouville Road and properties flooded. Blocked road gullies

and downstream culvert.

March 2020 St Saviour /Grouville Longueville Road Road flooded. Blocked road drainage.

March 2020 Grouville Grande Route des

Sablons

Road and property flooded. Insufficient road

drainage system through property.

March 2020 St Clement Rue du Pontietaut Road and property flooded. Insufficient road

drainage floods property lower than road.

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Date Parish Location Details

March 2020 St Clement

Grande Route de St

Clement

Road, land and property flooded. Main road

drainage choked and flooded through school sports

hall.

March 2020 St Saviour Route de la Hougue Bie Road and properties flooded. Insufficient drainage

for water off fields to the north.

March 2020 St Martin Rue St Julian Road and properties flooded. Blocked culvert.

March 2020 St Helier & St Saviour Grands Vaux Road and properties flooded. Pond requires to be

de-silted to increase capacity.

March 2020 St Mary La Verte Rue Road and land flooded. Road gully becomes

blocked and threatens property.

March 2020 St John Route du Mont Mado Road flooded. Gullies and downstream private

culvert needs clearing.

March 2020 Trinity Route d'Ebenezer Road flooded. Capacity of culvert under road and

blocked gullies.

March 2020 Grouville Route des Cotils Road flooded. Insufficient road drainage with run-off

from Bel Horizon.

March 2020 St Clement Rue au Long Road and land flooded.

March 2020 St Brelade Route de la Baie Road and properties flooded. Gullies become

choked, insufficient road drainage.

March 2020 St Helier Commercial Street Basements flooded due to capacities in public

sewer.

March 2020 St Helier Castle Street Basements flooded due to capacities in public

sewer.

March 2020 St Ouen Grande Route de St

Ouen

Road flooded. Water off fields inundates road

drainage system.

March 2020 St Lawrence Route de St Aubin Property flooded. Insufficient capacity of private

culvert under garage.

The Government of Jersey Department for Infrastructure, Housing and Environment has noted a number of

observations from historic inland flood events within different areas, as detailed below:

Beaumont – Flooding occurs at the filter-in-turn junction with Route de Beaumont and Route de la Haule.

Currently the road drainage system drains to the foul water system, which has limited capacity, and water can

pond at the foot of the hill, at the junction. This affects commercial properties as well as private residences.

The removal of road drainage to a separate surface water disposal system will require a pumping station to be

built in the area of the Gunsite slipway so that, on high tide levels, storm water can be over-pumped, similar to

the pump station on Le Perquage, 200 metres to the east. This would also drain the Goose Green Marsh area

and overflow from Le Perquage main stream.

The most recent flooding of this area was in February 2020 when the ‘usual’ properties were flooded, however,

the situation was compounded by the blocking of the Beaumont valley culvert that discharges to the foreshore,

alongside the Sugar Basin slipway. As a consequence, this event also flooded four properties, opposite the Co-

Op Locale store, just to the north of the junction.

Tesson Mill – The main flow of water down St Peter’s Valley, one of the largest catchments in the island, flows

along the old leat stream together with a lower natural stream in the bottom of the valley. Flooding occurs at

Tesson Mill when the leat stream overtops into the natural stream and flows through a private culvert beneath the

mill which surcharges and floods through the floors. Lack of maintenance of the private culvert, possibly a loose

granite stone structure, could be a contributing factor. Downstream of the mill, and running alongside the parish

road Rue du Mouiln de Tesson, the pen stream in private ownership has been bridged with significantly smaller

pipes and this causes both the road, and Tesson Mews, to flood.

Vallée des Vaux – The pen stream that drains this valley has been culverted, just upstream of the Rossmore

impounding pond, and flooding occurs in high flows. Debris build-up on the grating for the culvert can also

exacerbate flooding, which affects the parish road. When floodwater is diverted from the Rossmore impounding

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area, it follows the road to the south until it reaches a higher point on the road surface where it discharges into a

field north of The Farm and supermarket. The private culvert that drains the natural stream under the

supermarket has become blocked in the past which can result in flooding of the supermarket and car park, which

is at a lower level.

Home Farm, Rue de Grouville – This sits in a natural bowl of land that drains water to Grouville Marsh further to

the east. In the past the lack of maintenance and capacity of the stream culvert under the road has seen water

flooding in an east to west direction across the road and into the Home Farm complex, flooding properties. This

last happened earlier in 2020. The adjacent Meadowvale Cottage has also been affected in the past.

St Clement – Clos Lempriere and adjacent fields C41 & C42. Water collects in these fields and is unable to drain

away other than as ground water and evaporation. Historic flooding of Clos Lempriere estate has occurred on

many occasions.

4.3 Reservoir flooding

4.3.1 Overview

The island has six raw water storage reservoirs which are located at:

· Grands Vaux

· Val de la Mare

· Queen’s Valley

· Millbrook

· Dannemarche, and

· Handois.

The reservoirs store untreated water collected from streams and pumped from raw water abstraction points and

the desalination plant. Each reservoir has a catchment area, the size of which depends on the geography of the

surrounding countryside. Further description of each of the reservoirs is provided in the following sections.

4.3.2 Reservoir safety

The inspection and maintenance of Jersey Reservoirs falls under the Reservoir (Jersey) Law 1996. In

accordance with the law and best practice they are subject to a Section 12 inspection biannually by an appointed

Supervising Engineer. Also, under the law, inspection by an independent All Reservoirs Panel Engineer (ARPE)

is undertaken every 5 years. Measures to manage the risk of reservoir flooding (infrastructure failure) from the

reservoirs in Jersey are implemented by Jersey Water, therefore the risk of flooding is residual.

Each of the Jersey Water reservoirs are classed as either a category ‘A’ or category ‘B’ reservoir as described in

Table 1 of the Floods and Reservoir Safety 4

Edition (ICE, 2015). Jersey Water has undertaken a flood study for

each of the reservoirs to assess compliance with the standard safety check recommended by the guide in

respect of sufficient freeboard to accommodate surcharges from floods and wind waves during specific flood

events. The results show no significant risk to overtopping the crest; and the capacity of the spillways appears to

be adequate at Grands Vaux, Val de la Mare, Queen’s Valley, Handois, Dannemarche and Millbrook Reservoirs.

Details on each of the reservoirs are discussed below. Further information on the management measures

currently in place and mitigation measures that can be implemented is detailed in Sections 5 and 6.

4.3.3 Grands Vaux Reservoir

Grands Vaux Reservoir is situated approximately 2 km northeast of the town of St Helier. It is impounded by a 15

m high concrete gravity dam, which was constructed in 1952. The full supply level of the reservoir is 36.58 m

above datum and the capacity is 229,600 m

. The catchment which feeds the reservoir is mainly open farmland

Appendix B Figure

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and woodland and has an area of approximately 9.09 km

(see Figure 4-1) The reservoir drains to a small bypass

culvert downstream, situated between the dam and the primary school.

The overflow passes over the crest of the central monoliths (i.e. Block Nos. 5–14). The weir is set at 36.58 m OD

and is slightly lower over a 30.48 m wide section across the central five blocks. On either side there are 24.38 m

sections which are 75 mm higher. The combined width of the weir is 79.25 m. Water passing the weir flows over

the downstream face and falls into the concrete channel that runs along the dam toe. The channel drains out into

a short stilling basin below Block No.9 and out over a weir into an unlined earth channel. The stream passes

down the valley before disappearing into a series of culverts which have largely replaced the natural watercourse

on its route through St Helier to the sea.

Figure 4-1 Grands Vaux catchment area

Having the largest water catchment in Jersey the Grands Vaux Reservoir is critical to the Jersey Water Supply

Infrastructure. It is periodically used to capture and transfer water to Queen’s Valley Reservoir and to allow

general supply schemes to operate. This means that there is limited capacity in the reservoir to attenuate flood

flows generated from significant rainfall within the catchment. ,

The Grands Vaux Reservoir basin is small and can fill very rapidly during a storm. The flood volume, in severe

weather, can be greater than the capacity of the basin and, in combination with the steep topography of the land

around the reservoir, the reservoir frequently overflows as it is designed to do. The topography of the area

immediately downstream of the dam consists of a steep sided, narrow valley which channels any overtopped

water down the valley into St Helier. As a control measure, the downstream (school) bypass culvert is

continuously monitored for level and blockages by Jersey Water’s 24 hour control room. The existing drainage

network in St Helier can accommodate low return period events like the 1 in 1 year (100% AEP) but would be

unable to handle events of greater magnitude (lower AEP)

Modelling of the Grands Vaux Reservoir indicates that a 1 in 100 year (0.1% AEP) return period rainfall event,

with more than 53mm of rain within a 6 hour duration, could have significant effects on the area downstream. The

latest States of Jersey Grands Vaux Flood Plan

was updated in July 2018 and contains details of the multi-

agency response to flooding due to an extreme storm event resulting in an increase of water in the catchment

and subsequent reservoir overflow. The flood plan details the triggers and activation process of the plan if a

Report on Grands Vaux Catchment and Flood Protection Measures (States of Jersey, June 2000)

Grand Vaux Flood Plan (2018). Available online from:

https://www.gov.je/SiteCollectionDocuments/Staying%20safe/SoJ%20Grand%20Vaux%20Flood%20Plan%20v2.1.pdf

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forecasted or un-forecasted flood event was to occur. The flood plan also shows the area at risk of flooding if the

reservoir was to overflow during an extreme storm event.

It should be noted that a review of the treatment of reservoirs within a planning policy context has been

undertaken to understand how the risks associated with the overflow of the Grands Vaux Reservoir during more

extreme storm events might be considered within the Island Plan Review. This is of particular interest for

Government of Jersey as St Helier is the primary urban centre in the island and may be the focus for growth

within the Plan Review period.

4.3.4 Val de la Mare Reservoir

Val de la Mare Reservoir

is situated in the Parishes of St Peter and St Ouen with a direct catchment area of

3.28 km

. The reservoir was formed by the construction of a 29 m high concrete gravity dam across the mouth of

the Val de la Mare valley. It has a maximum depth of about 20 m and a capacity of 0.94 Mm3. The surface area

at the full supply level of 46.02 m OD is about 105,000 m

. The dam comprises 27 No. mass concrete

monoliths, each of which is 6.7 m wide. The upstream face of each monolith is vertical while the downstream

face is sloped.

The overflow passes over the crest of the central monoliths (i.e. Blocks 12-14) with the weir set at 46.02 m OD.

The weir is set slightly higher at 46.1 m OD over the adjacent monoliths (i.e. Blocks 8-11 and 15-18).

Overflowing water passes down the downstream face of the dam and into a collection channel along the toe from

which it passes into a concrete channel that drains into a 36-inch diameter pipe that continues below the valley

floor and eventually emerges into the natural watercourse downstream of the road, La Route du Moulin.

4.3.5 Queen’s Valley Reservoir

Queen’s Valley Reservoir

is situated in the parishes of Grouville and St Martin with a direct catchment area of 5

. There are three dams in cascade: the main dam, the intermediate dam and the silt pond dam. The reservoir

was formed by construction of a 35 m high rockfill dam with a central dense bituminous concrete core, it has a

maximum water depth of 21 m and a capacity of 1,190 Mm

. The surface area at the full supply level of 36 m OD

is about 77,000 m

. The crest is approximately 5.5 m wide with a crest height of 38.6 m OD at the centre of the

dam dropping to 38.1 m OD at the abutments. The crest has a wave wall that extends 900 mm above the crest,

and at its lowest point is 39 m OD. The top of the bituminous core is 500 mm below crest level, 37.6 m OD at the

abutments. The upstream slope is protected against wave erosion by rip-rap down to 25 m OD, with rockfill

exposed below this level.

The overflow structure is a circular ogee weir on a bellmouth, at a level of 36 m OD and 11.8 m in diameter. The

weir encircles the shaft and discharges down a segment of the shaft on its downstream side. The shaft enters

the overflow and access tunnel through a throttled section of tunnel, 1.6 m high at the upstream end and 0.8 m

high at the downstream end where it discharges into a 2.1 m high tunnel. The tunnel has an overall diameter of

3.0 m, the bottom segment of which is filled in to accommodate the draw-off and scour pipes. The invert of the

tunnel at the upstream end is 12.8 m OD and it is approximately 140 m long. At the downstream end it opens

into a covered stilling basin where a jump forms to reduce velocity to sub-critical. Downstream of the stilling

basin, there is an outlet pond leading to an outlet stream that eventually discharges into the sea near Gorey.

4.3.6 Handois, Dannemarche and Millbrook Reservoirs

Handois, Dannemarche and Millbrook Reservoirs

are all located in the Parish of St Lawrence within

Waterworks Valley. The three reservoirs form a cascade starting with Handois at the upstream end, then

Dannemarche and finally Millbrook Reservoir. Handois is located to the north of the valley and Millbrook at the

south.

Handois Reservoir was formed by construction of a 7 m high concrete gravity dam. The reservoir has a capacity

of 187,000 m3. The surface area at the full supply level of 88.69 m OD is 31,700 m

. The dam has a crest level

of 90.88 m OD and stands approximately 7 m above existing ground level. There are two overflow structures, an

original spillway that was built into the dam near the left abutment and an auxiliary spillway that was cut into the

left flank and passes around the west side of the dam. The modified original spillway has a 6.1 m long weir set at

the full supply level of 88.69 m OD. This discharges down a stepped masonry channel to the toe before passing

Val de la Mare reservoir Flood Study (2018)

Queens Valley Reservoir Flood Study (2018)

Waterworks Valley Cascade Flood Study (2018)

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around the Treatment Works in a series of channels and culverts on its way back to the natural watercourse

downstream. The total catchment area is 2.71 km2. The catchment is predominantly rural and the land is

mainly given over to mixed farmland and woodland.

Dannemarche Reservoir was formed by construction of an 8 m high, masonry gravity dam. The impounding

reservoir has a capacity of 93,000 m

. The surface area at the full supply level of 46.02 m OD is 20,200 m

Reservoir construction was completed in 1909. Dannemarche dam is a masonry faced concrete gravity dam with

a nominal crest level of 47.199 m OD. There are three overflows, two of which are located at the left abutment

and the third is at the right abutment. The one at the left abutment is the original spillway and is now referred to

as the emergency spillway. The total catchment area is 4.71 km2, of which 2.00 km2 drains directly to the

reservoir, while the remaining 2.71 km2 is routed through Handois Reservoir upstream. The additional catchment

area between Handois and Dannemarche reservoirs has similar characteristics to the Handois catchment.

Millbrook Reservoir was formed by construction of a 6 m high dam. The impounding reservoir has a capacity of

36,000 m

. The surface area at the full supply level of 19.84 m OD is 15,700 m

. Millbrook Dam has a masonry

mass concrete wall that is backed by puddle clay and supported by an earthfill embankment on the downstream

side. It has a nominal crest level of 20.71 m OD and a maximum height of 6 m. There are three overflow

spillways. The original spillway is located at the left abutment and passes around the east of the dam. It is 9.4 m

long and is an ‘L’ shaped weir set at 19.84 m OD. The weir discharges into a narrow tumble bay where it is

joined by the bywash channel, which flows around the left side of the reservoir. The channel cascades down the

left abutment to the valley bottom. Two auxiliary spillways are located on either side of the valve tower. Each has

a 10 m long weir set at 20.55 m OD. The weirs discharge down stepped chutes and over the car park. The total

catchment area is 5.98 km2, of which 1.27 km2 drains directly to the reservoir. The remaining area is routed at

varying degrees through Handois and Dannemarche upstream. The additional catchment area has similar

characteristics to that of Handois and Dannemarche upstream.

The flood study undertaken for the Waterworks Valley reservoirs found that as the auxiliary spillways at Millbrook

do not discharge into a channel, any flood event in which these spillways come into use could result in flooding

downstream of the reservoir. The study found that the original spillway can pass the flows from up to a 1 in 500

year event before reservoir water levels rise to auxiliary spillway weir level. Flood inundation mapping for

Millbrook Reservoir

identify the areas that could become inundated downstream during significant flood events

which result in the auxiliary spillway weir level being reached. Whilst it was found that the risk is within the range

of tolerability it could be beneficial to develop a suitable emergency plan to provide warning to residents/owners

in the event of severe flooding.

4.3.7 Historic Flooding

The Government of Jersey Department for Infrastructure, Housing and Environment has identified that the

Grands Vaux area, in particular around Nicholson Close and Pillar Gardens, is a known area for flooding. The

estates have been flooded on several occasions over the last ten years, including 28

February 2010. The cause

is linked to the intensity of the storm, overflow from the reservoir, and capacity with the downstream drainage

infrastructure The flooding is caused by a designed overflow coming into operation when the upstream flow in the

1500mm pipe that bypasses Grand Vaux Primary School reaches an ‘open’ chamber with a restricted discharge

to the downstream 1140mm sewer. The level of water rises in the ‘open’ manhole and discharges onto Grands

Vaux road, the designed overflow. The flood travels down the road entering properties on the lower east side

before reaching the low point in the road at the entrance to Nicholson Close and Pillar Gardens. Parish road

drainage along the route is unable to drain the flow of water. The water then floods into the low-lying area of the

estates and enters properties.

4.4 Sewer Flooding

The Government of Jersey flooding hotspots data has been used to identify areas which may have previously

experienced sewer flooding. Table 4-7Table 4-7 identifies the areas which may have experienced sewer flooding.

It should be noted that this data is currently being updated by the Government of Jersey and is subject to

revision.

Table 4-7 Flooding Hotspots

Parish Location of flooding hotspot Number of flooding hotspots

Inundation Mapping and Quantitative Risk Assessment: Millbrook Reservoir (2019)

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St Helier St Helier 8

St Clement La Grève D’Azette 1

Pontac 1

St Lawrence Millbrook 1

First Tower 1

St Peter Beaumont 3

St Brelade St Aubin 1

St Ouen L'Étacq 1

Trinity La Ville à L'Évêque 1

St Martin Rozel Bay 1

4.5 Summary

The SFRA provides background information to ensure that the materiality of flood risk is considered as part of the

assessment of spatial options for the Island Plan Review and for the assessment of individual sites. The

assessment of flood risk in this chapter leads to the conclusion that careful consideration of local flood risk is

required in some of the island’s urban areas, including those in the parishes of St Helier, St Lawrence, St Peter,

St Brelade, Grouville and St Martin, where most existing development is located, and where new development

could be located in in the future.

To help mitigate the risk of flooding in Jersey, a number of mitigation and management measures are currently in

place which reduce the risk of flooding from different sources. Information on these mitigation measures is

provided in Section 5. The standard of protection and residual risk of flooding from these mitigation measures

should be considered when assessing the potential risk of flooding to any new developments.

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5. Flood risk mitigation measures in

Jersey

5.1 Current flood risk management measures

5.1.1 Coastal flooding

There are extensive man-made coastal defences present around parts of the island’s coastline. The defences are

generally in a good condition; however, some are not to a high specification and, therefore, there is a risk they

may not withstand increased water depths as a result of climate change. Further information on the main flood

defences assets located in each parish is provided below in Table 5-1 and mapped in Appendix B Figure 8.

Table 5-1 Coastal Flood Defences

Parish Location Flood Defence Type Condition

Potential risk of flooding

from the 1 in 200 year flood

levels

Associated

Map

St Helier Bellozanne Sea Wall 2- Good Defence higher than still

water level tide

Figure 8F

and 8G

St Helier Structure 1- Very good Still water level tide exceeds

existing defence height but is

lower than surrounding

ground level

Stonework structure 3- Fair

Still water level tide exceeds

existing defence height

Sea Wall 2- Good

Still water level tide exceeds

existing defence height

Structure 2- Good Defence higher than still

water level tide

Saviour

Structure 2- Good Defence higher than still

water level tide

Figure 8H

Clement

Le Squez Wall 2- Good Still water level tide exceeds

existing defence height

Figure 8G

and 8H

Structure 1- Very good Defence higher than still

water level tide

Samarès Wall and Revetment 2- Good Defence higher than still

water level tide

Revetment 2- Good Defence higher than still

water level tide

Le Haguais Wall 2- Good Still water level tide exceeds

existing defence height but is

lower than surrounding

ground level

Le Hocq Revetment 1- Very good Defence higher than still

water level tide

Wall 2- Good Still water level tide exceeds

existing defence height but is

lower than surrounding

ground level

Pontac Revetment 2- Good Still water level tide exceeds

existing defence height

Le Bourg Wall and Revetment 2- Good Defence higher than still

water level tide

Wall 2- Good Defence higher than still

water level tide

See Appendix B Figure 9

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Parish Location Flood Defence Type Condition Potential risk of flooding

from the 1 in 200 year flood

levels

Associated

Map

Grouville La Rocque Wall 2- Good Defence higher than still

water level tide

Figure 8H

Slope 2- Good Defence higher than still

water level tide

Le Hurel Revetment 2- Good Still water level tide exceeds

existing defence height but is

lower than surrounding

ground level

Rock/Shingle 2- Good Defence higher than still

water level tide

Fauvic, La

Ville-ès-

Renauds,

Gorey,

Wall 2- Good Defence higher than still

water level tide

St Martin Faldouët Wall 2- Good Defence higher than still

water level tide

Figure 8D

and 8H

Archirondel Wall 2- Good Defence higher than still

water level tide

Fliquet Revetment and Wall 2- Good Defence higher than still

water level tide

Fliquet Wall 2- Good Defence higher than still

water level tide

Trinity Bouley Bay Wall 2- Good Defence higher than still

water level tide

Figure 8C

and 8D

St John Mont Mado Wall 2- Good Defence higher than still

water level tide

Figure 8B

and 8C

St Ouen La Grève de

Lecq

Wall 2- Good Defence higher than still

water level tide

Figure 8A

and 8E

L'Étacq Revetment 2- Good

Defence higher than still

water level tide

Les Mielles

Nature

Reserve

Wall

1- Very good to 3-

Fair

Defence higher than still

water level tide

St Peter Les Mielles

Nature

Reserve

Wall and Rock Armour 2- Good Defence higher than still

water level

tide

Figure 8E

and 8F

Brelade

Le Port Wall 2- Good Defence higher than still

water level tide

Figure 8E

and 8F

Le Braye Revetment and Wall 2- Good

Defence higher than still

water level tide

Wall 2- Good

Defence higher than still

water level

tide

Ouaisné Bay Wall

1- Very good to 3-

Fair

Defence higher than still

water level tide

Lawrence

La Haule Wall and Revetment 2- Good

Defence higher than still

water level tide

Figure 8F

and 8G

Beaumont Wall

1- Very good to 2-

Good

Defence higher than still

water level tide

The preferred policies to manage the risk of coastal flooding and an associated action plan for implementation

are detailed in the Jersey Shoreline Management Plan (as discussed in Section 5.2.1).

Appendix B Figure

8 and 8A

Appendix B Figure

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5.1.2 Inland flooding

Six pumping stations are present in the island, as illustrated in Figure 5-1, and they pump surface water from the

drainage network into the sea. Each pumping station drains a different catchment area and varies in the rate of

pumping. The catchment areas for the Baudrette, Bel Royal, Samarès Marsh and West of Albert pumping

stations are illustrated in Appendix C.

Bel Royal pumping station pumps Le Perquage stream and does not protect Beaumont. West of Albert pumping

station drains the central and eastern areas of St Helier, but not the western areas – West Park, Rouge Bouillon,

Queen’s Road areas. The Underpass pumping station only drains a low lying road.

As discussed in Section 4.2.5, the pumping stations reduce the severity of inland flooding in a number of

locations (as mapped in Appendix B Figures 5A-5H).

Figure 5-1 Pumping station locations

There are also a number of water impounding areas which provide storage of water across the island. Further

information on the water impounding areas located in each parish is provided below in Table 5-2 and mapped in

Appendix B Figure 8 and 8A-8H.

Table 5-2 Water Impounding Areas

Parish Location Type of structure

St Mary Fields MY957A & MY973 off Rue de la Vallee Restricted outlet chamber

St Peter Fields P786, P787 & P788 north of The Mermaid Earth bund with restricted discharge and

overspill

St Brelade Field P303 to the north of Clos Saut Falluet Earth bund with restricted discharge and

overspill

St Brelade Creepy Valley (flood plain) Discharge to valley floor

St Brelade Field B224 Pont Marquet Country Park Concrete dam with overspill

St Brelade Field B771 Railway Walk off Mont les Vaux Open outlet no restriction

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Parish Location Type of structure

St Lawrence Fields L861, L862, L862A and L863 (flood plain) Stream overflow to fields

St Helier Rossmore Farm, Vallee des Vaux

Granite wall/earth bund with

St Helier Grands Vaux Pond, Grands Vaux Concrete dam with limited discharge and

overspill

St Saviour

Field S620 Government House, St Saviour’s

Hill

Earth bund with restricted discharge and

overspill

St Saviour

Fields S846 & S847 Fountain Lane

Earth bund with restricted discharge and

overspill

St Saviour

Field S765 Longueville Manor – Swiss Valley

Earth bund with restricted discharge and

overspill

St Saviour

Fields S27, S28 AND S29 Rue des Pres Trading

Estate

Earth bund with restricted discharge and

overspill & control gate

St Saviour

La Becquetterie (Scout Hall) Rue des Pres

Concrete channel with overspill

St Clement

Field C3, La Blinerie

Earth bund with raised outlet

St Clement

Le Petit Marais, Le Marais

Earth bund with raised outlet

St Clement

Le Hocq Marsh, South of Le Rocquier School

Earth bund with restricted discharge

Grouville

Field G146 Grouville Marsh, off Rue Horman

Concrete control gate structure

Grouville

The Willows (Jersey Potteries) off Rue Horman

Earth bund with raised outlet

5.1.3 Reservoir flooding

In Jersey a number of the catchment areas generate more water than can be stored in the respective reservoir.

To overcome this problem and fully utilise the reservoir and catchment area capacity, a partial system of raw

water transfer mains has been developed which, in some cases, allows water to be moved between the

reservoirs. The pumped stream abstraction points can be controlled and are not used when the reservoirs are

full. A summary of the Jersey Water supply network is illustrated in Figure 5-2.

When full, the reservoirs hold enough useable water to provide approximately 120 days of supply to the Island.

Reservoirs are equipped with monitoring equipment allowing water levels and certain quality parameters to be

continuously monitored. When required for treatment, the stored water is pumped from the reservoirs to either

Handois or Augrès Water Treatment Plants.

Appendix B Figure

8 and 8A

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Figure 5-2 Jersey Water Supply Schematic (Jersey Water, 2019)

5.2 Future flood risk management measures

5.2.1 Coastal flooding

The Jersey SMP provides an island-wide assessment of the risks associated with flooding and erosion from

coastal sources. It sets out a framework to manage these risks to the community, environment and economy of

Jersey in a sustainable manner over the next 100 years (up to 2120).

The SMP details the management intent for the island's coastline over the next 100 years (up to 2120). The aim

is to prevent and manage the effects of coastal erosion and flooding. The impact of climate change on rising sea

levels over time has been assessed. The plan considers risks to the community, environment and economy of

Jersey. It takes into account the coastal defences that are in place around the island.

A summary of the Shoreline Management Plan (SMP) and its policies is provided below.

5.2.1.1 Jersey Shoreline Management Plan

The Jersey SMP considers risks to the community, environment and economy of Jersey. The plan considers the

coastal defences that are around the island and identified where improvements may be needed. Three time

periods were considered as part of the assessment (Present day (2020-2040), Medium Term (2040-2070) and

Long Term (2070-2120)) and mitigation options for each of these scenarios was proposed.

To assess the best option for the different parts of the coastline, the coastline was divided into smaller units. The

coastline was divided into six Coastal Management Areas (CMAs). Each CMA has similar risks of flooding,

coastal erosion and levels of development. These CMAs were further subdivided into 36 Coastal Management

Units (CMUs). The policy options were set at the CMU level so that the management intent is appropriate at a

local scale. Figure 5-3 shows the location of each CMU.

Taken from Jersey Water’s website- https://www.jerseywater.je/

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Four management policies were considered for implementation within the SMP, which include:

· No Active Intervention – a policy decision to not invest in coastal defences or maintenance work. The

shoreline is left to naturally evolve without intervention. This policy generally applies to natural areas of

the coastline which are currently undefended;

· Maintain the Defence Line – existing coastal defences are maintained. The level of flood protection may

decrease in some locations over time due to climate change. This policy generally applies where the

existing defences provide a reasonable standard of flood protection or prevent erosion of the shoreline;

· Adaptive Management – a policy to proactively manage and mitigate coastal flood or erosion risk. The

policy will be delivered through various management schemes / initiatives depending on the level of risk

and the circumstances. This could include improving the standard of flood protection for an existing sea

defence, constructing new defences, raising awareness of local flood risk or recommending property

level flood protection; and,

· Advance the Line – new sea defences are built seaward of existing defences. This policy will only be

implemented in areas where there is a significant risk of coastal flooding or erosion, or where it will

deliver additional benefits for the community, environment and economy, such as creating a new

amenity space.

A summary of the policy options for each policy area is shown in Table 5-3.

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Figure 5-3 Coastal Management Unit locations, Jersey Shoreline Management Plan

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Table 5-3 Policy Recommendations from the SMP

KEY:

NAI: No Active Intervention

MTDL: Maintain the Defence Line

AM: Adaptive Management

ATL: Advance the Line

Coastal Management Unit (CMU)

Preferred Policy Option

Subject to

flooding

Subject

erosion

Present day

2020-2040

Medium term

2040-2070

Long term

2070-2120

1.1 Noirmont Common NAI NAI NAI

✗ ✔

1.2 Belcroute Bay MTDL MTDL MTDL

✗ ✔

1.3 La Housse NAI NAI NAI

✗ ✔

1.4 St Aubin’s Harbour AM AM ATL MTDL

✔

✗

1.5 St Aubin’s Bay AM AM ATL MTDL

✔

✗

1.6 St Helier MTDL AM MTDL

✔ ✗

1.7 La Collette MTDL MTDL MTDL

✗

1.8 Havre des Pas AM ATL MTDL MTDL

✔ ✗

1.9 La Grève d’Azette AM AM MTDL

✔ ✔

1.10 Le Hocq / Pontac AM AM MTDL

✔ ✔

2.1 Royal Bay of Grouville AM AM MTDL

✔

✗

2.2 Gorey Harbour MTDL AM MTDL

✔ ✔

3.1 La Route de la Cote MTDL MTDL MTDL

✔ ✔

3.2 Archirondel Tower AM MTDL MTDL

✔

✗

3.3 St Catherine’s Bay MTDL MTDL MTDL

✔

✗

3.4 La Coupe MTDL MTDL MTDL

✗ ✔

4.1 La Coupe to Rozel Bay NAI NAI NAI

✗

4.2 Rozel Bay MTDL MTDL MTDL

✔ ✗

4.3 Le Catel NAI NAI NAI

✗

4.4 Bouley Bay MTDL MTDL MTDL

✔

✗

4.5 Egypt NAI NAI NAI

✗

✔

4.6 Bonne Nuit MTDL MTDL MTDL

✔ ✔

4.7 La Perruque NAI NAI NAI

✗ ✗

4.8 Ronez Quarry NAI NAI NAI

✗

4.9 Crabbé NAI NAI NAI

✗ ✗

4.10 Grève de Lecq MTDL MTDL MTDL

✔

✗

4.11 Plemont NAI NAI NAI

✗

5.1 St Ouen’s Bay MTDL MTDL MTDL

✔

✗

5.2 Petit Port MTDL MTDL MTDL

✗

6.1 Gorselands NAI NAI NAI

✗ ✔

6.2 Les Creux NAI NAI NAI

✗

✔

6.3 St Brelade’s Bay AM AM MTDL

✔ ✔

6.4 Ouaisne Bay MTDL MTDL MTDL

✔

✗

6.5 La Cotte de St Brelade NAI NAI NAI

✗

✔

6.6 Portelet Common NAI NAI NAI

✗ ✔

6.7 Portelet Beach NAI NAI NAI

✗

✔

5.2.2 Inland flooding

The Government of Jersey Department for Infrastructure, Housing and Environment has identified that there are

no immediate plans for future inland attenuation schemes as an overarching inland flooding strategy for the

island has yet to be developed to help identify the priority areas requiring intervention.

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5.2.3 Sewer flooding

5.2.3.1 Waste Water Strategy

In March 2014, the Government of Jersey published its Waste Water Strategy

outlining plans for the next 20

years. The strategy focuses on the need to replace the sewage treatment works (STW) at Bellozanne and the

deterioration of the sewers and drains that collect and transport sewage to Bellozanne STW and present a risk of

flooding.

To address these issues, the Waste Water Strategy provides a prioritised and sustainable plan for improvement

works over the 20 year period of the strategy.

The Strategy also looks at the sewerage network (the sewers and drains that collect and transport sewage to

Bellozanne STW for treatment and surface water to the sea). In order, the priorities are:

1. to repair and refurbish the sewerage network

2. to continue to undertake projects to install new drains so that the surface water is separated from the

sewage. This reduces the amount of water that does not require costly treatment, going to Bellozanne

STW, and allowing it to go straight to sea

3. to connect appropriate properties to the network that currently have no connection

5.2.3.2 Jersey Sewerage Assessment

In 2014, Grontmij were commissioned by the Government of Jersey to update a sewer network model for the

island to identify any areas that required maintenance and upgrade works. The following issues were identified in

the study:

· Pollution and eutrophication in St. Aubin’s Bay from waste water treatment works discharges.

· Issues with various combined sewer overflow discharges.

· Concern over the potential impact on Ramsar sites and oyster beds.

Flooding has been reported in the town centre of St Helier, surface water flooding is known to occur intermittently

in the St Aubin’s / Charing Cross area and flooding also occurs at Gunsite Tower (flooding in the marsh area)

near Beaumont Sewage Pumping Station (SPS).

Government of Jersey (2014), Waste Water Strategy. Available online at:

https://www.gov.je/News/2014/Pages/WasteWaterStrategy.aspx

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6. Strategic flood risk management

This Section identifies strategic flood management options which could be used to help reduce the risk of inland

flooding in Jersey.

6.1 Flood storage areas

Flood storage areas (FSA) aim to reduce the flows passed downstream in watercourses to mitigate flooding

further along in the catchment. Flood storage areas are designed to detain runoff or flow within a watercourse,

releasing it downstream at a slower rate. There are two types of water storage:

Offline storage

Offline storage is where water is diverted from the watercourse, stored in a separate area which may still be part

of the floodplain, then later released back to the watercourse (Figure 6-1). An inflow structure, such as a weir,

diverts water to the storage area when the watercourse level exceeds a predetermined value, and the outlet

structure returns the water to the watercourse after the flood peak has passed, either via gravity, pump or both.

Online storage

Online storage is where water is temporarily stored within a watercourse channel and floodplain, usually behind a

dam or impoundment structure (Figure 6-1). The flow control structures such as pipes, flumes or sometimes

gates, are normally located inside the impoundment structure and control the outflow of water from the storage

area back into the channel.

Figure 6-1 Online and Offline Flood Storage Areas

FSAs along watercourses work by removing a volume of water from the watercourse at high flows thereby

reducing the flow of water downstream. To work efficiently, the FSAs need to be designed such that they remove

the high flood flows. If an FSA is at capacity before the peak flow arrives, it will have a negligible impact on

reducing flood level rise downstream and therefore be ineffective.

Impounding FSAs, also known as online FSAs, are constructed across watercourses and restrict the peak size of

downstream flows along the watercourse. Impoundment is usually achieved through construction of a dam and

flows are restricted by means of a culvert (or pipe). The culvert may be fitted with a flow control device to better

control the magnitude of the flows passing through.

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On impounding reservoirs, the size of the culvert passing through the dam is critical. If the culvert in the dam is

too large it will allow too much water to pass through and thereby not achieve the required standard of protection

downstream. If the culvert through the dam is too small, it will start restricting the flow of water too early and the

FSA will be full before the peak flows have occurred. The design of culvert size is further complicated because

the amount of water passing through the culvert will vary with varying depths of water retained in the FSA. This

problem may be overcome through the use of a vortex flow control device.

Non-impounding FSAs, also known as offline FSAs, are not constructed across a watercourse but are located

next to them. Typically, non-impounding FSAs comprise an inlet structure, a dam or retaining structure and an

outlet structure. Peak water flows are removed from the watercourse, are stored temporarily, and then are

returned once the peak flows have passed.

The inlet structures to non-impounding FSAs can take many different forms including weirs or culverts and

channels, both of which may have control gates. In the case of inlet weirs or inlet channels, the inlet level needs

to be set such that the storage area starts to fill thus removing the peak of the flood flow from the river system.

Where inlets are gated, the gate needs to open once the river has reached a pre-defined level.

Outlets from non-impounding FSAs can either be via gravity through an open or gated culvert. The gravity outlet

commences operation once the peak river flow has passed and requires no manual intervention. The gravity

outlet ensures that the FSA commences emptying as soon as possible after a flood event and so is ready for re-

use should there be another flood in the watercourse. In contrast, gated outlets require manual intervention and

the FSAs will remain full until the gate is opened. FSAs vary considerably in size, shape and nature. They can be

relatively simple having only one inlet and outlet or they can be more complex with multiple inlet and outlets.

The type of FSA suitable for a particular location depends on a number of factors. In general, however, where

high watercourse levels occur over a long period, it is more efficient to use a non-impounding FSA. Where

watercourse levels rise and fall quickly, it is better to use an impounding FSA.

6.2 Catchment and floodplain restoration

Compared to flood defences and flood storage, floodplain restoration represents the most sustainable form of

strategic flood risk solution, by allowing watercourses to return to a more naturalised state, and by creating space

for naturally functioning floodplains working with natural processes. Although the restoration of floodplain is

difficult in previously developed areas where development cannot be rolled back, the following measures are

methods that could be implemented to help catchment and floodplain restoration:

· Promoting existing and future brownfield sites, that are adjacent to watercourses, to naturalise banks as

much as possible.

· Buffer areas around watercourses provide an opportunity to restore parts of the floodplain.

· Removal of redundant structures to reconnect the watercourse and the floodplain.

· Avoid placing new development within the floodplain. For those sites considered within the Island Plan

and / or put forward by developers, that are at risk of inland flooding associated with a watercourse

flowing through or past them, development (where possible) should be located away from the

watercourses. This will ensure the watercourses retain their connectivity to the floodplain. Loss of

floodplain connectivity could potentially increase flooding. Further guidance is provided in Section 8.

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6.3 Upstream natural catchment management

methods

Opportunities to work with natural processes to reduce flood risk should be sought where available. Working with

natural flood prevention processes has many benefits including a positive impact on the natural environment and

a reduction in the costs of flood mitigation schemes. Methods of working with natural processes to mitigate

flooding are discussed below.

Natural Flood Management

Natural Flood Management (NFM) involves techniques that aim to work with natural hydrological and

morphological processes, features and characteristics to manage the sources and pathways of flood waters.

NFM techniques include the following:

· watercourse restoration and enhancement

· catchment and floodplain woodlands

· instream large woody structure

· land and soil management practices

· agricultural and upland drainage practices

· non floodplain wetlands

· overland sediment traps

· flood storage areas

Details of further guidance on NFM techniques, their benefits, and methods of implementation are provided in

Table 6-1.

Table 6-1 Natural Flood Management guidance documents

Document title Web Link

SEPA Natural Flood Management Handbook https://www.sepa.org.uk/media/163560/sepa-natural-flood-management-

handbook1.pdf

Natural Flood Management Measures: A

Practical Guide for Farmers (North West)

https://catchmentbasedapproach.org/learn/natural-flood-management-

measures-a-practical-guide-for-farmers-north-west/

Working with Natural Processes to reduce

flood risk- The evidence behind Natural Flood

Management

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/

attachment_data/file/654440/Working_with_natural_processes_one_page_su

mmaries.pdf

Working with Natural Processes –Evidence

Directory

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/

attachment_data/file/681411/Working_with_natural_processes_evidence_dire

ctory.pdf

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6.4 Opportunities for strategic flood risk

management in Jersey

There is considerable risk of inland flooding identified for the catchment which drains towards the Grands Vaux

Reservoir, which is at risk of overtopping from significant rainfall events. The areas upstream of the reservoir are

comprised of steep areas immediately adjacent to the watercourses, however, the nature of the land uses

(agricultural etc) within the wider catchment present potential opportunities for Natural Flood Management (NFM).

Implementation of catchment woodlands and instream large woody structures in the two streams that flow into

the Grands Vaux Reservoir would help to mitigate flood risk.

A natural flood storage area could also be implemented to the north of Beaumont which could mitigate the rate of

flow in the watercourse which flows through the east of Beaumont. Instream mitigation and flood woodland would

be suitable for the other watercourses along the south of the island which flow into Millbrook and Bellozanne.

Recommendation

The Government of Jersey could further explore opportunities for strategic management of inland flooding

throughout the island through development of a Catchment Flood Management Plan (CFMP). This can cover

the whole island and/or selected key catchments identified to contribute to existing and predicted inland

flooding.

A CFMP should consider all types of inland flooding including surface water, watercourse and reservoir and

include:

· the likely impacts of climate change

· the effects of how we use and manage the land

· how areas could be developed to meet our present day needs without compromising the ability of

future generations to meet their own needs

A CFMP can help the Government of Jersey and their partners to plan and agree the most effective way to

manage inland flood risk in the future.

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7. Flood risk framework

7.1 Overview

Considering the present day and future flooding risks that face the island, the Government of Jersey is seeking to

adopt an approach within the Island Plan Review to ensure that development will achieve suitable resilience to

the challenges of flood risk. A Flood Risk Framework has been developed for Jersey and is set out in this

Section. The purpose of the Flood Risk Framework is to:

· Define Flood Risk Categories, based on the probability of flooding from coastal and inland flooding;

· Define Vulnerability Classifications for different types of development based on their sensitivity to

flooding;

· Use the Flood Risk Categories and Vulnerability Classifications to (1) specify the types of development

that may or may not be acceptable in different Flood Risk Categories, and (2) define the appropriate

approach to planning decisions for each scenario.

Where possible, development which is vulnerable to flooding; or which could increase the probability of flooding

elsewhere should be located away from areas at risk of flooding. The avoidance of flood risk, by not locating

development in areas at risk of flooding, is recognised as a key part of delivering sustainable flood risk

management.

7.2 Flood risk categories

Flood risk categories have been established, based on the probability of coastal flooding and inland flooding.

The risk of coastal flooding has been defined in the following categories:

· High risk has been identified using the 1 in 200 year outline (0.5% AEP) for the present day; and

· Medium risk has been identified using the 1 in 200 year outline (0.5% AEP) for 2120.

These return periods were chosen due to the size of the flood extents creating the worst-case modelled scenario

for coastal flooding; and for consistency with the established approach used in parts of the United Kingdom.

The risk of inland flooding has been defined into the following categories:

· High risk of flooding means the area has a chance of flooding corresponding to a 1 in 30 year return

period event (3.33% AEP);

· Medium risk of flooding means the area has a chance of flooding corresponding to a 1 in 100 year

return period event (1% AEP); and,

· Low risk of flooding means the area has a chance of flooding corresponding to a 1 in 1000 year return

period event (0.1% AEP).

These return periods were chosen for consistency with the established approach used in parts of the United

Kingdom. These flood risk categories are also summarised in Table 7-1.

Appendix B Figure

2 and 2A

Appendix B Figure

3 and 3A

Appendix B Figure

4 and 4A

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Table 7-1 Flood Risk Categories

Risk Category Inland Flooding Coastal Flooding

Little or No Risk

Annual probability of inland flooding is less than 0.1%

AEP (1 in 1000-year probability).

Low Risk Annual probability of 0.1% AEP (1 in 1000-year

probability) inland flooding risk.

Medium Risk Annual probability of 1% AEP (1 in 100-year probability)

inland flooding risk.

Annual probability of 0.5% AEP (1 in 200-year

probability plus a 2120 epoch for climate

change) flood event.

High Risk Annual probability of 3.3% AEP (1 in 30-year probability)

inland flooding risk.

Annual probability of 0.5% AEP (1 in 200-year

probability for the present day) flood event.

The calculated probability of a flood occurring should be regarded as a best estimate and not a precise forecast.

The annual probabilities referred to in Table 7-1 relate to the land at the time a planning application is made, or

a development plan is prepared.

7.3 Vulnerability classifications

When making decisions about the suitability of development in relation to the risk of flooding, it is also necessary

to consider the sensitivity of the proposed development or land use to flooding. This is referred to as the

vulnerability of the development. Development types have been assigned a vulnerability classification based of

the significance of the impacts that would occur if the development were to flood. The vulnerability classifications

are defined in Table 7-2.

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Table 7-2 Development Vulnerability Classifications

Vulnerability Classification Development Definitions

Essential Civil Infrastructure

· Essential transport infrastructure (including mass evacuation routes) which has to cross

the area at risk.

· Essential utility infrastructure which has to be located in a flood risk area for operational

reasons, including electricity generating power stations and grid and primary

substations; and water treatment works that need to remain operational in times of

flood.

· Wind turbines.

· Police and ambulance stations; fire stations and command centres; telecommunications

installations required to be operational during flooding.

· Hospitals.

Highly Vulnerable

· Emergency dispersal points.

· Basement dwellings.

· Caravans, mobile homes and park homes intended for permanent residential use.

· Installations requiring hazardous substances consent. (Where there is a demonstrable

need to locate such installations for bulk storage of materials with port or other similar

facilities, or such installations with energy infrastructure or carbon capture and storage

installations, that require coastal or water-side locations, or need to be located in other

high flood risk areas, in these instances the facilities should be classified as ‘Essential

Infrastructure’).

· Residential institutions such as residential care homes, children’s homes, social

services homes, prisons and hostels.

· Buildings used for dwelling houses, student halls of residence, drinking establishments,

nightclubs and hotels.

· Non–residential uses such as health services, nurseries and educational

establishments.

· Buildings used for shops; financial, professional and other services; restaurants, cafes

and hot food takeaways; offices; general industry, storage and distribution; and

assembly and leisure.

· Landfill and sites used for waste management facilities for hazardous waste.

· Sites used for holiday or short-let caravans and camping, subject to a specific warning

and evacuation plan.

Less Vulnerable

· Land and buildings used for agriculture and forestry.

· Waste treatment (except landfill and hazardous waste facilities).

· Minerals working and processing (except for sand and gravel working).

· Water treatment works which do not need to remain operational during times of flood.

· Sewage treatment works, if adequate measures to control pollution and manage

sewage during flooding events are in place.

Water Compatible

· Flood control infrastructure.

· Water transmission infrastructure and pumping stations.

· Sewage transmission infrastructure and pumping stations.

· Sand and gravel working.

· Docks, marinas and wharves.

· Navigation facilities.

· Defence installations.

· Ship building, repairing and dismantling, dockside fish processing and refrigeration and

compatible activities requiring a waterside location.

· Water-based recreation (excluding sleeping accommodation).

· Lifeguard and coastguard stations.

· Amenity open space, nature conservation and biodiversity, outdoor sports and

recreation and essential facilities such as changing rooms.

· Essential ancillary sleeping or residential accommodation for staff required by uses in

this category, subject to a specific warning and evacuation plan.

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7.4 Development suitability and planning approach

Using the Flood Risk Categories (Table 7-1) and the Vulnerability Classifications (Table 7-2), this Section sets out

the approach the Government of Jersey is recommended to take in relation to planning decisions across the

island.

The approach differs for built up areas and countryside areas to avoid locating more vulnerable developments

within areas which may not have supporting flood management measures. Table 7-3 (for built up areas) and

Table 7-4 (for countryside areas) specify the types of development that may or may not be acceptable in different

Flood Risk Categories and define the appropriate approach to planning decisions for each scenario.

Where a development site is identified at risk of flooding, even if it is only a low risk, it is necessary for the

development proposals to acknowledge this risk and identify suitable mitigation so the impacts of flooding can be

managed, enabling the development and its occupants to be more resilient to flooding and climate change. It is

recommended that a Flood Risk Assessment (FRA) is prepared for any development within an area identified at

low, medium or high risk of flooding in order to assess the risk of flooding; potential mitigation; and its

acceptability in relation to flood risk. The level of detail required within the FRA should be proportionate to the

level of risk and vulnerability category of the proposed development. Further information regarding FRAs is

included in Appendix D.

Table 7-3 Development Suitability and Planning Approach – Built up areas

Built up areas

Flood Risk Category

(See Table 7-1)

Essential Civil

Infrastructure

Highly vulnerable Less vulnerable Water compatible

High X

— —

Medium

— —

ü ü

Low

—

ü ü ü

Little or No risk

ü ü ü ü

ü – Development is appropriate ü – Development is appropriate subject to mitigation

— - Development will need to identify wider justification for its location X – Development should not be permitted

Table 7-4 Development Suitability and Planning Approach – Countryside areas

Countryside Areas

Flood Risk Category

(See Table 7-1)

Essential Civil

Infrastructure

Highly vulnerable Less vulnerable Water compatible

High X X

—

Medium

— —

ü ü

Low

—

ü ü ü

No risk

ü ü ü ü

ü – Development is appropriate ü – Development is appropriate subject to mitigation

— – Development will need to identify wider justification for its location X – Development should not be permitted

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The allocation of land as part of the Island Plan review process (i.e. where ‘countryside’ is effectively being

proposed to be incorporated into the ‘built-up area’) should be undertaken on a risk based approach where

development is steered towards those sites at least risk, prior to those with a higher risk. A site assessment

spreadsheet has been developed to support this assessment process. The aim is to steer new development to

areas with the lowest risk of flooding. If it is not possible for development to be located in zones with a lower risk

of flooding, further assessments may be required to provide justification for the location of the development

and/or identification of suitable mitigation measures in accordance with the Flood Risk Framework outlined

above.

Policy Recommendation 1

Where possible, development which is vulnerable to flooding or could increase the probability of flooding

elsewhere should be located away from areas at risk of flooding.

Where it is not possible to allocate development away from areas at risk of flooding, the Government of Jersey

require development to be assessed based on its spatial location and subsequent exposure to inland and/or

coastal flooding within a risk category (as set out in Table 7-1) and its vulnerability to flooding within a risk

category (as set out in Table 7-2). Based on this categorisation, Table 7-3 provides a strategic flood risk

framework against which the suitability of development in built-up areas for each of these risk categories can

be assessed. Table 7-4 provides the same for the strategic assessment of flood risk for development in the

remainder of the island’s coast and countryside.

A Flood Risk Assessment (FRA) is required for any development within an area identified at low, medium or

high risk of flooding.

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8. Site specific flood risk

management measures

The majority of the island of Jersey is protected by the presence of coastal flood defences that mitigate the

current flood levels. The likelihood of flooding from the tide is a residual risk, and flooding would only occur if the

defences are overtopped. The risk of flooding from climate change poses a significant risk to Jersey so it is

important to ensure that new development will not be at risk from flooding as a result of climate change. The

following mitigation measures should be used to manage the risk of tidal and inland flooding in new

developments. Mitigation measures should be implemented at a site level to help avoid and prevent flooding in a

development site, before implementing building design measures to manage the impacts of flooding to a

development.

8.1 Site measures

8.1.1 Development along watercourses, coastline and coastal

flood defences

The Government of Jersey should seek a buffer strip alongside areas of coastline or coastal defences to ensure

access for maintenance purposes and the implementation of future risk management schemes. An undeveloped

buffer strip alongside watercourses for maintenance purposes should also be required. Developers are asked to

explore opportunities for watercourse restoration or additional natural methods to help reduce inland flooding

(refer to Sections 6.2 and 6.3 for further information).

In England, the Environment Agency have flood risk permitting distances of 8m from a main river fluvial

watercourse and 16m from any tidal main river or flood defence structure (whether the structure is tidal or fluvial).

These distances are not exclusion zones, but distances where they will scrutinise proposals with regard to:

a) potential impacts on defence integrity e.g. loading on ground close to the defence, potential damage to

ground anchors and tie rods etc.

b) impacts on the ability to access defences for inspection, repair and maintenance.

c) the future ability to reconstruct or improve the defence (need to safeguard land).

The Government of Jersey should explore an appropriate width of buffer strip alongside areas of coastline,

coastal defences and watercourses. A minimum distance of five meters from existing designated defences is

currently required by the Drainage (Jersey) Law 2005.

8.1.2 Vulnerability of proposed uses

Flood risk should be considered at an early stage in deciding the layout and design of a site to provide an

opportunity to avoid and reduce the risk of flooding within the development. Most large development proposals

include a variety of land uses of varying vulnerability to flooding. Where possible, built development should be

located in the lowest risk areas (considering all sources of flooding) e.g. residential elements should be restricted

to areas at lower probability of flooding whereas parking, open space or proposed landscaped areas can be

placed on lower ground where there may be a higher probability of flooding.

Policy Recommendation 2

Retain an undeveloped buffer strip of predetermined width alongside areas of coastline or tidal flood

defences. Retain an undeveloped buffer strip of predetermined width alongside any watercourses.

Developments should seek to implement opportunities for watercourse restoration or additional natural

attenuation on sites which are bound by a watercourse (natural or culverted) or it is contained within the

boundary.

Policy Recommendation 3

Where possible, development should be located in the areas at lowest risk of flooding within the site.

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8.1.3 Land raising

In areas at risk of flooding of low depths (<0.3m), land raising can be used as a way of mitigating the risk of

flooding. Land raising can be used to prevent flooding on a short term basis to provide time to move valuables

out of an area at risk of flooding. It is important to ensure that land raising does not impact development nearby to

the site.

8.1.4 Landscaping

In order to demonstrate that flood risk is not increased elsewhere, development in the floodplain will need to

prove that flood routeing is not adversely affected by the development. Careful consideration should be given to

the use of fences and landscaping walls so as to prevent causing obstruction to flow routes and increasing the

risk of flooding to the site or neighbouring areas.

Potential overland flow paths should be determined, and appropriate solutions proposed to minimise the impact

of the development, for example by configuring road and building layouts to preserve existing flow paths or using

earth bunds to manage flood water and improve flood routeing. It is important to ensure any mitigation measures

do not divert water towards other properties elsewhere.

8.1.5 Sustainable drainage systems

Sustainable drainage systems (SuDS) can be used in all types of development to provide a natural approach to

managing drainage. SuDS are used to prevent water pollution and flooding in urban areas and can also create

green spaces and habitat for wildlife.

SuDS are typically softer engineering solutions inspired by natural drainage processes such as ponds and swales

which manage water as close to its source as possible. Wherever possible, a SuDS technique should seek to

contribute to each of the three goals identified below:

· Reduce flood risk (to the site and neighbouring areas);

· Reduce pollution; and,

· Provide landscape and wildlife benefits.

Generally, the aim should be to discharge surface water run-off as high up the following hierarchy of drainage

options as reasonably practicable:

1. Store rainwater for later use

2. Use infiltration techniques, such as porous surfaces;

Policy Recommendation 4

All new development in areas at risk of inland flooding should not adversely affect flood routeing and thereby

increase flood risk elsewhere. Opportunities should be sought within the site design to make space for water,

such as:

· Removing boundary walls or replacing with other boundary treatments such as hedges, fences (with

gaps).

· Considering alternatives to solid wooden gates or ensuring that there is a gap beneath the gates to

allow the passage of floodwater.

· On uneven or sloping sites, consider lowering ground levels to extend the floodplain without creating

ponds. The area of lowered ground must remain connected to the floodplain to allow water to flow

back to the watercourse when levels recede.

· Create under-croft car parks or consider reducing the ground floor footprint and creating an open

area under the building to allow flood water storage.

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3. Attenuate run-off in open water features for gradual release to a watercourse;

4. Attenuate run-off by storing in tanks or sealed water features for gradual release to a watercourse;

5. Discharge run-off direct to a watercourse;

6. Attenuate rainwater by storing in tanks or sealed water features for gradual release to a surface water drain;

and

7. Discharge rainwater to the public surface water sewer

SuDS should be used to reduce and manage surface water run-off to and from proposed developments as near

to source as possible. SuDS must be implemented for all development sites unless it is demonstrated that SuDS

are not suitable, such as:

· They would be likely to cause significant land or water pollution;

· The site’s ground conditions would preclude their use;

· The size of the site would prevent their use;

· They would cause damage to adjacent buildings or sites.

Discharges of surface water to groundwater, or to local watercourses and waterbodies will be required to meet

quality standards and conditions set by the Government of Jersey and will not be permitted where this would lead

to pollution.

All developments should not result in an increase in overland flow, and where possible, should demonstrate

betterment in terms of rate and volumes of overland runoff.

SuDS techniques can be used to reduce the rate and volume and improve the water quality of surface water

discharges from sites to the receiving environment (i.e. natural watercourse or public sewer etc.). The SuDS

Manual

identified several processes that can be used to manage and control runoff from developed areas.

Each option can provide opportunities for storm water control, flood risk management, water conservation and

groundwater recharge.

· Infiltration: the soaking of water into the ground. This is the most desirable solution as it mimics the

natural hydrological process. The rate of infiltration will vary with soil type and condition, the antecedent

conditions and with time. However, due to the geology of the island, this is unlikely to be possible in the

majority of locations.

· Detention/Attenuation: the slowing down of surface flows before their transfer downstream, usually

achieved by creating a storage volume and a constrained outlet. In general, though the storage will

enable a reduction in the peak rate of runoff, the total volume will remain the same, just occurring over a

longer duration.

· Conveyance: the transfer of surface runoff from one place to another, e.g. through open channels,

pipes and trenches.

· Water Harvesting: the direct capture and use of runoff on site, e.g. for domestic use (flushing toilets) or

irrigation of urban landscapes. The ability of these systems to perform a flood risk management function

will be dependent on their scale, and whether there will be a suitable amount of storage always available

in the event of a flood.

As part of any SuDS scheme, consideration should be given to the whole life management and maintenance of

the SuDS to ensure that it remains functional for the lifetime of the development. Table 8-1 outlines typical SuDS

techniques.

The application of SuDS is not limited to a single technique per site. Often a successful SuDS solution will utilise

a combination of techniques, providing flood risk, pollution and landscape/wildlife benefits. In addition, SuDS can

be employed on a strategic scale, for example with a number of sites contributing to large scale jointly funded

and managed SuDS. It should be noted; each development site must offset its own increase in runoff and

attenuation cannot be “traded” between developments.

CIRIA C697 SuDS Manual. Available online from: http://www.ciria.org/Resources/Free_publications/the_suds_manual.aspx

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Table 8-1 Typical SuDS Components (Y: primary process, * some opportunities subject to design)

Technique Description

Conveyance

Detention

Infiltration

Harvesting

Pervious Surfaces Pervious surfaces allow rainwater to infiltrate through the surface into an

underlying storage layer, where water is stored before infiltration to the

ground, reuse, or release to surface water.

Y Y *

Filter Drains

Linear drains/trenches filled with a permeable material, often with

perforated pipe in the base of the trench. Surface water from the edge of

paved areas flows into the trenches, is filtered and conveyed to other parts

of the site.

Y Y

Filter Strips

Vegetated strips of gently sloping ground designed to drain water evenly

from impermeable areas and filter out silt and particulates.

* * *

Swales

Shallow vegetated channels that conduct and/or retain water and can

permit infiltration when unlined.

Y Y *

Ponds Depressions used for storing and treating water. Y * Y

Wetlands As ponds, but the runoff flows slowly but continuously through aquatic

vegetation that attenuates and filters the flow. Shallower than ponds.

Based on geology these measures can also incorporate some degree of

infiltration.

* Y * Y

Detention Basin Dry depressions designed to store water for a specified retention time. Y

Soakaways Sub-surface structures that store and dispose of water via infiltration. Y

Infiltration Trenches As filter drains but allowing infiltration through trench base and sides. * Y Y

Infiltration Basins Depressions that store and dispose of water via infiltration. Y Y

Green Roofs Green roofs are systems which cover a building’s roof with vegetation.

They are laid over a drainage layer, with other layers providing protection,

waterproofing and insulation. It is noted that the use of brown/green roofs

should be for betterment purposes and not to be counted towards the

provision of on-site storage for surface water. This is because the hydraulic

performance during extreme events is similar to a standard roof (CIRIA

C697).

Rainwater

Harvesting

Storage and use of rainwater for non-potable uses within a building, e.g.

toilet flushing. It is noted that storage in these types of systems is not

usually considered to count towards the provision of on-site storage for

surface water balancing because, given the sporadic nature of the use of

harvested water, it cannot be guaranteed that the tanks are available to

provide sufficient attenuation for the storm event.

* * * Y

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8.1.6 Reducing flood risk from sewers

Developers of any sites (regardless of their size) that could result in an increase in discharge to the public sewer

network should check the capacity of the local public sewer network with the Government of Jersey at the earliest

possible stage in the planning process to ensure that there is sufficient capacity. All new developments should

avoid illegal connection of surface water to the foul water system, seek to improve the drainage infrastructure and

reduce the risk of flooding, without increasing the risk of flooding elsewhere.

Simple measures including non-return valves fitted to foul chambers and wastewater outlets can help protect

individual properties from both inland flooding and sewer flooding by preventing water entering the property via

the drains and sewers. These can be easily incorporated into both new and existing developments.

8.2 Building measures

8.2.1 Flood resilient design

While flood defences protect the majority of the island from coastal flooding, the risk of flooding from climate

change poses a significant risk. It is important to ensure that new development will be resilient to flooding from

climate change.

Policy Recommendation 5

New development and redevelopment should incorporate Sustainable Drainage Systems (SuDS) into the

overall design. Areas of impermeable surfaces should be kept to a minimum in all new developments.

Surface water run-off should be discharged as high up the following hierarchy of drainage options as

practicable:

1. Store rainwater for later use

2. Use infiltration techniques, such as porous surfaces;

3. Attenuate run-off in open water features for gradual release to a watercourse;

4. Attenuate run-off by storing in tanks or sealed water features for gradual release to a watercourse;

5. Discharge run-off direct to a watercourse;

6. Attenuate rainwater by storing in tanks or sealed water features for gradual release to a surface water

drain; and

7. Discharge rainwater to the public surface water sewer

SuDS should be used to reduce and manage surface water run-off to and from proposed developments as

near to source as possible. SuDS must be implemented for all development sites unless it is demonstrated

that SuDS are not suitable, such as:

· They would be likely to cause significant land or water pollution

· The site’s ground conditions would preclude their use

· The size of the site would prevent their use

· They would cause damage to adjacent buildings or sites.

Discharges of surface water to groundwater, or to local watercourses and waterbodies will be required to

meet quality standards and conditions set by the Government of Jersey and will not be permitted where this

would lead to pollution.

All developments should not result in an increase in overland flow, and where possible, should demonstrate

betterment in terms of rate and volumes of overland runoff.

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Internal flooding of a development could occur if the defences are overtopped and flood depths are high enough

that water would enter a development. If flooding of depths of greater than 0.6m was to occur as a result of

defence overtopping, it is likely that structural damage could occur in traditional masonry construction due to

excessive water pressures. In these circumstances, the strategy should be to allow water into the building, but to

implement careful design in order to minimise damage and allow rapid re-occupancy. This is referred to as the

Water Entry Strategy. These measures are appropriate for uses where temporary disruption is acceptable.

Materials should be used which allow the passage of water whilst retaining their structural integrity and they

should also have good drying and cleaning properties. Alternatively, sacrificial materials can be included for

internal and external finishes; for example, the use of gypsum plasterboard which can be removed and replaced

following a flood event. Flood resilient fittings should be used to at least 0.1m above the design flood level.

Resilience measures are either an integral part of the building fabric or are features inside a building that will limit

the damage caused by floodwaters.

In areas at risk of frequent or prolonged flooding, implement flood resilience measures such as:

· Use materials with either, good drying and cleaning properties, or, sacrificial materials that can easily be

replaced post-flood.

· Design for water to drain away after flooding.

· Design access to all spaces to permit drying and cleaning.

· Raise the level of electrical wiring, appliances and utility metres.

· Coat walls with internal cement-based renders; apply tanking on the inside of all internal walls.

· Ground supported floors with concrete slabs coated with impermeable membrane.

· Tank basements, cellars or ground floors with water resistant membranes.

· Use plastic water resistant internal doors.

Further specific advice regarding suitable materials and construction techniques for floors, walls, doors and

windows and fittings can be found in ‘Improving the Flood Performance of New Buildings, Flood Resilient

Construction’

Structures such as bus, bike shelters, park benches and refuse bins (and associated storage areas) located in

areas with a high flood risk should be flood resilient and be firmly attached to the ground and designed in such a

way as to prevent entrainment of debris which in turn could increase flood risk and/or breakaway posing a danger

to life during high flows.

8.2.2 Finished floor levels

Where developing in an area at medium or high risk of coastal flooding is unavoidable, the recommended method

of mitigating flood risk to people, is to ensure internal floor levels are raised a certain amount above the 1 in 200

year (0.5% AEP) flood level (also known as a freeboard level). Highly vulnerable development should also aim to

raise floor levels 300mm above the 1 in 200 year (0.5% AEP) plus the 2120 epoch for climate change predicted

flood level. Where this is not achievable, further mitigation measures (shown in the following sections) should be

incorporated to manage the risk. These measures should be detailed within the FRA.

In certain situations (e.g. for proposed extensions to buildings with a lower floor level or conversion of existing

historical structures with limited existing ceiling levels), it could prove impractical to raise the internal ground floor

levels to sufficiently meet the general requirements. In these cases, the Government of Jersey should be

CLG (2007) Improving the Flood Performance of New Buildings, Flood Resilient Construction. Available online from:

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/7730/flood_performance.pdf

Policy Recommendation 6

Developments located in areas at high, medium or low risk of flooding should be designed to be resilient to

flooding, where it is deemed appropriate that they will be subject to internal flooding. This includes

structures which form part of the public realm, such as bins and benches.

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approached to discuss options for a reduction in the minimum internal ground floor levels provided flood

resistance measures are implemented up to an agreed level.

A summary of the still water levels around the island is presented in detail within Appendix B(b) of the Jersey

Shoreline management Plan (Still Water Level Report)

. The still water levels used in the coastal modelling are

summarised for the 0.5% AEP below:

· 0.5% AEP still water level projected for 2020 – 6.95 m OD

· 0.5% AEP still water level projected for 2120 – 7.77 m OD

Appendix B Figures 10 and 10 A - 10H identifies potential depth of flooding (in metres above the existing ground

level) during a high risk coastal flood event in 2040 in the priority areas at risk from overtopping (St Ouen’s Bay,

St Brelade’s Bay, St Aubin’s Bay, Havre des Pas, La Grève D’Azette, La Mare, Le Nez Point to Le Hocq Point, La

Rocque, Royal Bay of Grouville and Archirondel (north)).

8.2.3 Property flood protection devices

There are a range of property flood protection devices available on the market, designed specifically to resist the

passage of floodwater. These include removable flood barriers and gates designed to fit openings, vent covers

and stoppers designed to fit WCs. These measures can be appropriate for preventing water entry associated with

inland flooding and sewer flooding. The efficacy of such devices relies on their being deployed before a flood

event occurs. It should also be borne in mind that devices such as air vent covers, if left in place by occupants as

a precautionary measure, may compromise safe ventilation of the building in accordance with Building

Regulations.

Such measures are not encouraged within new developments or the refurbishment of existing development as

they require active intervention to achieve a reduction in the impact of flooding. These measures should only be

considered as a last resort if all other mitigation options have been considered and robustly justified that they are

not achievable on the individual development site.

8.2.4 Flood Evacuation Plans for areas at risk of flooding from

overtopping of a reservoir

As discussed in Section 4.2.4, the town of St Helier is potentially at risk of flooding from the Grands Vaux

Reservoir if it was to overflow as a result of a significant rainstorm affecting the catchment. Jersey Water manage

the levels of water in the reservoirs for the purpose of water supply.

If the levels of water are unable to be mitigated due to the volume of rainfall within the catchment, evacuation

may be required where residents of an area need to be evacuated due to the risk of flooding in the valley. Flood

warnings may be able to be issued if the storm is predicted and levels are expected to cause reservoir

overtopping. However, overtopping may still occur due to flash flooding (which was unpredictable) upstream of

the reservoir.

To enable timely action by residents or occupants to allow evacuation to take place unaided, i.e. without the

deployment of trained personnel to help people from their homes, businesses and other premises, a Flood

Government of Jersey (2020) Jersey Shoreline Management Plan Appendix B(b) Still Water Level Report. Available online

at: https://www.gov.je/SiteCollectionDocuments/Environment%20and%20greener%20living/R-

Shoreline%20Management%20Plan%20Appendix%20B(b)%20Still%20Water%20Level%20Report%2020200205%20HL.pdf

Policy Recommendation 7

All development in areas at medium or high risk of coastal flooding (as defined in Table 7-1) should have

Finished Floor Levels a minimum of 300mm above the 1 in 200 year (0.5% AEP) flood level. Highly

Vulnerable development in areas at medium or high risk of coastal flooding should have Finished Floor

Levels a minimum of 300mm above the 1 in 200 year (0.5% AEP) flood level for the year 2120 to account for

the future impact of climate change.

Appendix B Figure

s 10 and 10A

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Evacuation Plan should be prepared. Rescue by the emergency services is likely to be required where flooding

has occurred, and prior evacuation has not been possible.

For all developments at risk of flooding from reservoir overflow a Flood Evacuation Plan should be prepared to

identify the evacuation route from the site, and to demonstrate their development will not impact on the ability of

the local authority and the emergency services to safeguard people within existing properties. Flood warnings

and guidance on large-scale evacuations will be provided by the Government of Jersey as identified in its Grands

Vaux Flood Plan

Government of Jersey (2018) Grand Vaux Flood Plan. Available online at:

https://www.gov.je/SiteCollectionDocuments/Staying%20safe/SoJ%20Grand%20Vaux%20Flood%20Plan%20v2.1.pdf

Policy Recommendation 8

A Flood Evacuation Plan should be prepared for any new developments which are located with an area

identified at risk of inundation from reservoir overtopping associated with either the Grands Vaux or

Millbrook Reservoirs. The Flood Evacuation Plan should demonstrate the route which would be used to

evacuate a site during a reservoir overtopping flood event.

The development proposals should also be supported with a Flood Risk Assessment in order to assess the

risk of flooding and detail the potential site specific mitigation measures required to support the Flood

Evacuation Plan.

To support implementation of this recommendation it will be necessary for the Government of Jersey to

clearly identify the areas at risk of inundation from reservoir overtopping associated with both the Grands

Vaux and Millbrook reservoirs. The spatial extent for Grands Vaux Reservoir is indicated in the Grands Vaux

Flood Plan but the information is incomplete in the published version. The spatial extent for Millbrook

Reservoir is not currently in the public domain.

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9. Cumulative development and land

use change

9.1 Cumulative impacts

In addition to consideration of flood risk within the development site, it is important to consider the effect on the

surrounding area. Where development takes place on an area subject to flooding, it may change the pattern of

flood risk. In particular, an additional built-up area may reduce the space for flood water causing it to be diverted

elsewhere. This is an important consideration for areas at risk of tidal and inland flooding as the diversion or

displacement of flood water could put properties that are not modelled to be at risk of flooding at risk. To mitigate

the displacement and diversion of flood water, mitigation measures and appropriate building placement should be

used to avoid this. Techniques such as providing alternative flood plain storage, diverting inland flood flows into

areas of planting or the use of sustainable drainage systems (SuDS) can be used to mitigate the risk of flooding.

Further guidance on methods to mitigate flooding are provided in Section 8.

Furthermore, it is not sufficient to assume that locating development away from areas of tidal risk and inland

flooding areas and the use of SuDS will automatically render flood risk to third parties adequately low irrespective

of location. A situation may arise in which there is no spare capacity in a drainage system to take additional flows

from a new development. It is important to confirm with the Government of Jersey that sufficient capacity is

available in the drainage system to ensure that flooding does not occur in the development or further down the

drainage system.

9.2 Land-use change

9.2.1 Paving

The paving over of front and back gardens can increase the risk of flooding in urbanised catchments. An increase

in impermeable areas causes larger amounts of runoff (during periods of heavy rain) than a traditional lawn or

permeable surface. This runoff can cause a build-up of water on roads and in areas vulnerable to inland flooding.

Increased runoff can also contribute to increased pressure on drainage and sewer systems. Where possible,

permeable methods should be used to ensure runoff is not increased. Further guidance on permeable surfaces is

provided in Section 8.

9.2.2 Rural land management

The increasing frequency and magnitude of flooding can also be partially attributed to changes in rural land use

management, for example tree removal, land drainage and intensification of crop cycles and animal stocking. The

effects on overland flows and inland flooding are usually subtle and difficult to predict especially at larger scales.

There is strong evidence that small scale catchment management approaches can deliver flood risk management

benefits. Farmland can store floodwater to reduce the risk of flooding downstream within the catchment. Large

washland (storage) areas can be designed to store excess water and slow down flood peaks. Additional guidance

on using Natural Flood Management (NFM) methods is provided in Section 6.3.

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10. Recommendations for policy and

guidance

10.1 Overview

This SFRA has identified the areas which are at risk of flooding from a number of different sources. To ensure

that future development in Jersey addresses these risks and does not contribute towards increasing them, site

specific flood mitigation measures should be implemented to manage flooding. Development should also not

hinder the preferred long term management approaches outlined within the SMP and other flood mitigation

strategies in Jersey.

It is recommended that the policy recommendations identified throughout the SFRA are implemented into future

updates in flood risk planning policy.

10.2 Guidance

The Flood Risk Framework and supporting policy recommendations make reference to the need to prepare Flood

Risk Assessments (FRAs) for developments within areas identified at risk of flooding. Further guidance has been

provided on the preparation of site specific FRAs in Appendix D.

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Appendix A Data Register

Dataset Source Format Description Limitations Map

Topography

LiDAR data

(DTM, ASCII)

Government

of Jersey

GIS Layer

The Government of

Jersey LiDAR data

contains digital elevation

data derived from surveys

carried out in 2017. The

data has a spatial

resolution of 2m.

None

Appendix B

Figure 1

Coastal

Risk of tidal

flooding

present day

Government

of Jersey

GIS Layer This dataset covers the

modelled outline of the

0.5% annual exceedance

probability (AEP) of tidal

flooding (1 in 200 chance

each year).

The information provided is

largely based on modelled

data and is therefore

indicative rather than specific.

Locations may also be at risk

from other sources of

flooding, such as inland

flooding from heavy rain, or

failure of infrastructure such

as sewers and culverts.

The information indicates the

flood risk to areas of land and

is not sufficiently detailed to

show whether an individual

property is at risk of flooding,

therefore properties may not

always face the same chance

of flooding as the areas that

surround them. This is

because we do not hold

details about properties and

their floor levels.

Appendix B

Figure 2 and

Figures 2A –

Risk of tidal

flooding with

future climate

change

allowances

Government

of Jersey

GIS Layer

This dataset covers the

modelled outline of the

0.5% annual exceedance

probability (AEP) of tidal

flooding (1 in 200 chance

each year) in the future

epochs:

Short term – 2040

Medium term – 2070

Long term – 2120

Appendix B

Figure 3 and

Figures 3A –

Areas at risk

of coastal

defence

overtopping

Government

of Jersey

GIS Layer This dataset covers the

defences which are

modelled to be

overtopped during a 0.5%

AEP plus long term

(2120) climate change

flood risk scenario.

Appendix B

Figure 9

Inland

Risk of inland

flooding

present day

Government

of Jersey

GIS Layer Provides an indication of

the broad areas likely to

be at risk of inland

flooding, i.e. areas where

surface water would be

expected to flow or pond.

This dataset covers the

modelled outline of the

following rainfall events:

· 3.3% AEP of inland

flooding (1 in 30

chance each year)

· 1% AEP of inland

flooding (1 in 100

chance each year)

· 0.1% AEP of inland

flooding (1 in 1000

chance each year)

This dataset does not show

the susceptibility of individual

properties to inland flooding.

Appendix B

Figure 4 and

Figures 4A –

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Dataset Source Format Description Limitations Map

Risk of inland

flooding

including the

effect of

pumping

stations

Government

of Jersey

GIS Layer

Provides an indication of

the broad areas likely to

be at risk of inland

flooding taking into

account the main

pumping stations. This

dataset covers the

modelled outline of the

following rainfall events:

· 3.3% AEP of inland

flooding (1 in 30

chance each year)

· 1% AEP of inland

flooding (1 in 100

chance each year)

· 0.1% AEP of inland

flooding (1 in 1000

chance each year)

This dataset does not show

the susceptibility of individual

properties to inland flooding.

Appendix B

Figure 5 and

Figures 5A –

Risk of inland

flooding with

climate

change

allowances

Government

of Jersey

GIS Layer

Provides an indication of

the broad areas likely to

be at risk of surface water

flooding, i.e. areas where

surface water would be

expected to flow or pond.

This dataset covers the

modelled outline of the

following rainfall events:

· 3.3% AEP of inland

flooding (1 in 30

chance each year)

· 1% AEP of inland

flooding (1 in 100

chance each year)

Two percentage uplifts

were considered for both

of these return periods

(+20% and +40%).

This dataset does not show

the susceptibility of individual

properties to surface water

flooding.

Appendix B

Figure 6 and

Figures 6A –

Historic

Flooding

incidents and

records

Government

of Jersey

Email &

spreadsheet

Anecdotal details of flood

risk areas and a

spreadsheet with records

of historic flood events.

Some of the data is based on

circumstantial and subjective

evidence. There is not always

available metadata, e.g. exact

date of flood event or type of

flooding etc.

Not mapped

Reservoirs

Flood Study

Reports

Jersey

Water

Reports A flood study for each of

the reservoirs was

undertaken to assess

compliance with the

standard safety check

recommended by the

guide in respect of

sufficient freeboard to

accommodate surcharges

from floods and wind

waves during specific

flood events.

None

Not mapped

Grands Vaux

Reservoir

Flood Plan

Government

of Jersey

Report

Emergency flood plan to

manage the potential

inundation resulting from

overflow of the Grands

Vaux Reservoir.

The flood plan only considers

the inundation resulting from

overtopping during a

significant rainfall event.

Not mapped

Inundation

mapping and

Quantitative

Risk

Assessment

– Millbrook

Reservoir

Jersey

Water

Report

Inundation mapping and

quantitative risk

assessment for the

potential overflow of

Millbrook Reservoir.

None

Not mapped

Flood

Risk

Mana

Coastal flood

assets

Government

of Jersey

GIS Layer Provides an overview of

the defence condition

Snapshot of the condition of

the defences in 2019.

Appendix B

Figure 8 and

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Dataset Source Format Description Limitations Map

assessment undertaken

in 2019 as part of the

Jersey SMP.

Figures 8A –

Water

impounding

areas

Government

of Jersey

Drawings

Identifies the type and

location of the different

water impounding areas

across the island.

Locations approximated from

the drawings provided.

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Appendix B Maps

Figure 1 – Topography

Figure 2 and 2A-2H – Risk of flooding from the tide

Figure 3 and 3A-3H – Risk of flooding from the tide including allowances for climate change

Figure 4 and 4A-4H – Risk of inland flooding present day

Figure 5 and 5A-5H – Inland flooding including the effect of pumping stations

Figure 6 and 6A-6H – Risk of inland flooding including an allowance for climate change

Figure 7 – Raw water storage reservoirs

Figure 8 and 8A-8H – Flood risk management features

Figure 9 – Areas benefiting from defences

Figure 10 and 10A-10H – Flood depth map for tidal flooding (overtopping)

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Appendix C Pumping Station

Catchment Areas

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Appendix D Site Specific Flood Risk

Assessments

D.1 What is a Flood Risk Assessment?

A Flood Risk Assessment (FRA) is a report prepared for a specific site suitable for submission with a planning

application, which provides an assessment of flood risk to and from a proposed development. The FRA should

demonstrate how flood risk will be managed now and over the lifetime of the development, taking climate change

into account, and with regard to the vulnerability of its users.

The objectives of a site-specific FRA are to establish:

· whether a proposed development is likely to be affected by current or future flooding from any source;

· whether it will increase flood risk elsewhere;

· whether the measures proposed to deal with these effects and risks are appropriate.

D.2 When is a FRA required?

A Flood Risk Assessment (FRA) is required for any development within an area identified at low, medium or high

risk of flooding (Table 7-1).

D.3 What should a FRA contain?

Table D-1 sets out the type of information that a FRA should typically contain.

Table D-1 Site specific FRA Checklist (developed from guidance in PPG

Error! Bookmark not defined.

)

What to include in the FRA Source(s) of Information

1. Site Description

Site address - -

Site description - -

Location plan Including geographical features and street

names

Government of Jersey Mapping

Site plan Plan of site showing development proposals and

any structures which may influence local flow

paths e.g. bridges, pipes/ducts crossing

watercourses, culverts, screens, embankments,

walls, outfalls and condition of channel

Government of Jersey Mapping

Site Survey

Topography Include general description of the topography

local to the site.

Plans showing existing and proposed levels.

SFRA Appendix B Figure 1

Geology General description of geology local to the site. -

2. Assessing Flood Risk

The level of assessment will depend on the degree of flood risk and the scale, nature and location of the proposed

development. Not all of the prompts listed below will be relevant for every application.

Flooding from the tide Identify any parts of the site at risk of coastal

flooding.

Identify any historic flooding that has affected the

site.

Discuss how the site is likely to be affected by

climate change?

SFRA Appendix B Figure 2 and 2a-2h

SFRA Appendix B Figure 3 and 3a-3h

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Flooding from inland

sources

Identify any parts of the site at risk of inland

flooding.

Identify any historic flooding that has affected the

site.

Discuss how the site is likely to be affected by

climate change?

SFRA Appendix B Figure 4 and 4a-4h

SFRA Appendix B Figure 5 and 5a-5h

SFRA Appendix B Figure 6 and 6a-6h

Flooding from Sewers

Identify any historic flooding that has affected the

site.

Flooding from Reservoirs Review the Risk of Flooding from Reservoirs

mapping.

SFRA Appendix B Figure 7

3. Proposed Development

Current use Identify the current use of the site. -

Proposed use Will the proposals increase the number of

occupants / site users on the site such that it

may affect the degree of flood risk to these

people?

Vulnerability Classification

Determine the vulnerability classification of the

development. Is the vulnerability classification

appropriate within the Flood Zone?

SFRA Table 7-2

4. Managing and Mitigating Flood Risk

Section 8 of the SFRA presents measures to manage and mitigate flood risk and when they should be implemented. Where

appropriate, the following should be demonstrated within the FRA to address the following questions:

How will the site/building be protected from flooding, including the potential impacts of climate change, over the

development’s lifetime?

How will you ensure that the proposed development and the measures to protect your site from flooding will not increase

flood risk elsewhere?

Are there any opportunities offered by the development to reduce flood risk elsewhere?

What flood-related risks will remain after you have implemented the measures to protect the site from flooding (i.e. residual

risk) and how and by whom will these be managed over the lifetime of the development (e.g. flood warning and evacuation

procedures)?

Development Layout Plan showing how sensitive land uses have

been placed in areas within the site that are at

least risk of flooding.

SFRA Section 9

Finished Floor Levels Plans showing finished floor levels in the

proposed development taking account of

indicated flood depths.

Flow Routeing

Provide evidence that proposed development

will not impact flood flows to the extent that the

risk to surrounding areas is increased.

Watercourse or Coastline

Development Buffer Zone

Provide plans showing how a buffer zone of

relevant width will be retained adjacent to any

watercourse, coastline or flood defence in

accordance with requirements of the

Government of Jersey.

Drainage and Surface

Water Management

Provide evidence that SuDS have been used to

manage overland flow (or sufficient justification if

these cannot be used) and evidence that these

will sufficiently manage the risk of surface water

flooding. Demonstrate compliance with the

existing legal requirements under the Drainage

(Jersey) Law, 2005.

Flood Evacuation Plan (for

areas at risk of flooding

from the Grands Vaux

Reservoir)

Where appropriate reference the Evacuation Plan

or Personal Flood Plan that has been prepared for

the proposed development (or will be prepared by

site owners).

In applying the Flood Risk Framework (Section 1), developers and the Government of Jersey should also

consider:

· the characteristics of the site,

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· the use and design of the proposed development,

· the size of the area likely to flood,

· existing flood prevention measures – extent, standard and maintenance regime,

· cumulative effects of development, especially the loss of flood storage capacity,

· effects of a flood on access including by emergency services,

· effects of a flood on proposed open spaces including gardens, and

· the extent to which the development, its materials and construction are designed to be water resistant.

D.4 How detailed should a FRA be?

Site-specific FRAs should be proportionate to the degree of flood risk, the scale and nature of the development,

its vulnerability classification (Table 7-2). Site-specific FRAs should also make optimum use of readily available

information, for example the mapping presented within this SFRA.

For example, where the development is an extension to an existing house which would not significantly increase

the number of people present in an area at risk of flooding, the Government of Jersey would generally need a

less detailed assessment to be able to reach an informed decision on the planning application. For a new

development comprising a greater number of houses in a similar location, or one where the flood risk is greater,

the Government of Jersey may require a more detailed assessment. As a result, the scope of each site-specific

FRA may vary.

A FRA should comply with the Government of Jersey Island Plan policies and laws. Failure to provide sufficient

information will result in an application being refused.

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