strategic flood risk assessment update and sequential test of … · 2020-01-03 · strategic flood...

West Sussex Joint Minerals Local Plan: Soft Sand Review SFRA Update and Sequential Test of Soft Sand Sites

West Sussex Single Issue Soft Sand Review Strategic Flood Risk Assessment Update and Sequential Test of Soft Sand Sites November 2019


2

Contents Page Contents Page ............................................................................................................. 2 Executive Summary ..................................................................................................... 3 1.0. Introduction ......................................................................................................... 4

1.1. What is a Strategic Flood Risk Assessment? ......................................................... 4 1.2. What is the Sequential Test? .............................................................................. 4 1.3. Soft Sand Review (SSR) .................................................................................... 6 1.4. Sources of Flooding in West Sussex ..................................................................... 7 1.5. Flood Zones in West Sussex ............................................................................... 8 1.6. Climate Change ................................................................................................ 9

2. Flood Risk Assessment of Potential Mineral Sites ......................................................12 2.1. What minerals need to be planned for? .............................................................. 12 2.2. What are the Reasonably Available Sites? .......................................................... 12 2.3. Flood Risk Assessments at Planning Application Stage ......................................... 13 2.4. Summary of Flood Risk at Potential Soft Sand Sites............................................. 13 2.5. Consideration of the Principle of the Exception Test for high risk groundwater sites . 14

Appendix A: Extracts from Planning Practice Guidance .....................................................18 Appendix B: Ground Water and Surface Water Flood Risk Mapping Methodology .................21 Appendix C: Flood Risk Assessments: Climate Change Allowances.....................................23 Appendix D: Flood Risk Maps of Soft Sand Sites ..............................................................25 Appendix E: References ...............................................................................................35


3

Executive Summary West Sussex County Council and the South Downs National Park Authority (‘the Authorities’) adopted the Joint Minerals Local Plan (JMLP) in July 2018. The Authorities are required to undertake a Single Issue Soft Sand Review of the JMLP, which will review Policy M2 (Soft Sand) to address the shortfall in soft sand to the end of the plan period (2033) and, as required, allocate sites to meet the need, through Policy M11. In allocating sites to meet a steady and adequate supply of minerals, local planning authorities should apply the ‘sequential test’ to demonstrate that there are no reasonably available sites in areas with a lower probability of flooding. This should be informed by a Strategic Flood Risk Assessment (SFRA). The West Sussex SFRA (2010) still forms part of the evidence base for the Soft Sand Review (SSR) but this document presents the most up to date flood risk information and the ‘sequential test’ for the sites considered at the detailed technical assessment stage. Sand and gravel sites are classed as ‘water compatible’ and therefore appropriate for development in flood risk terms, however, national policy states that the allocation of sites should still follow a ‘sequential approach’ (including exceptions testing) and should consider the need to avoid flood risk from sources other than rivers and the sea. Other sources of flooding are also included for each site to inform the ‘sequential approach’ to site allocation. Updated climate change allowances were published in November 2018 (UKCP18) and are the official source of information on how the climate of the UK may change over the rest of this century. The allowances published in February 2016 (UKCP16) are still the best national representation of how climate change is likely to affect flood risk for peak river flow and peak rainfall intensity. Research that is due to be published in 2019 may result in changes to these allowances and will be taken into account, where necessary, in an updated version of this document.


4

1.0. Introduction 1.0.1. Flooding is an important issue in West Sussex with over 109,000 properties at risk

from flooding (Local Flood Risk Management Strategy, 2014). The risk of flooding is likely to increase as a result of climate change as sea levels rise, rainfall increases and extreme storm events become more frequent. The planning system can help to reduce the risk of flooding by locating development in areas that avoid flood risk, ensuring it does not exacerbate flooding elsewhere and ensuring new development is resilient to the effects of flooding.

Paragraphs 155 to 165 of the National Planning Policy Framework - NPPF (2019) sets out the requirement for Local Planning Authorities to take account of flood risk. The NPPF requires strategic policies to be informed by an SFRA and to develop policies to manage flood risk; a sequential, risk-based approach to the location of development taking into account the current and future impacts of climate change. Development should not be allocated if there are reasonably available sites in areas with a lower risk of flooding. The SFRA provides the basis for applying this test.

1.1. What is a Strategic Flood Risk Assessment?

1.1.1. A SFRA is a study carried out by one or more local planning authorities to assess the risk to an area from flooding from all sources, now and in the future, taking into account the impacts of climate change, and to assess the impact that land use changes and development in the areas will have on flood risk. In 2010, West Sussex County Council (WSCC) commissioned consultants, Capita Symonds, to prepare a SFRA1.

1.1.2. Although the 2010 SFRA still forms part of the evidence base for the Soft Sand

Review (SSR), this document presents the most up to date flood risk information for the soft sand sites to inform the sequential test, the sustainability appraisal and the selection of sites. The SFRA has been prepared with updated flood risk data provided by the Environment Agency and the West Sussex County Council Flood Risk Management Team and in its statutory function as Lead Local Flood Authority (LLFA) under the Flood and Water Management Act 2010.

1.1.3. This is a ‘live’ document that will be updated when new guidance and/or flood risk

information becomes available.

1.2. What is the Sequential Test? 1.2.1. The SFRA is used to apply the sequential test that aims to steer new development

to areas with the lowest probability of flooding. If there are alternative sites in

1 Strategic Flood Risk Assessment of West Sussex, January 2010.


5

areas of lower flood risk, development should not be allocated or permitted. If it is not possible for the development to be located in zones with a lower probability of flooding, it must be demonstrated that the development provides wider sustainability benefits that outweigh flood risk and a site-specific flood risk assessment must demonstrate that the development will be safe for its lifetime without increasing flood risk. This is called the ‘exception test’.

1.2.2. Table A2 (Appendix A) categorises different types of uses and development

according to their vulnerability to flood risk. Sand and gravel workings, which includes soft sand extraction, are classed as ‘water compatible development’ and other mineral workings are classed as ‘less vulnerable’. The NPPF acknowledges that minerals can only be worked where they occur and although sand and gravel extraction are classed as ‘water compatible’ the sequential approach to allocating sites for mineral extraction and processing should still be taken. The NPPF also acknowledges that minerals sites may provide opportunities to reduce flood risk by providing flood storage and attenuation. Due to the scale of minerals development, there may also be opportunities for applying the sequential approach at the site level, such as, locating offices and processing plants in areas at lowest risk of flood risk.

*Other sources of flooding also need to be considered. Figure 1: Application of the Sequential Test for Local Plan Preparation (Source: NPPF, 2019)


6

1.3. Soft Sand Review (SSR) 1.3.1. The Authorities are preparing a SSR that will consider the strategy for how the

shortfall in soft sand will be met to ensure that there is a steady and adequate supply of minerals.

1.3.2. An Issues and Options document was published for consultation in January 2019. This set out the ‘need’ for soft sand, the strategy options for meeting the need and the potential sites. Detailed information about the nine soft sand sites is presented in the Soft Sand Site Selection Report (January, 2019).

1.3.2. The potential soft sand sites have also been subject to Sustainability Appraisal (SA)

that considered the likely environmental, social and economic implications of the sites being assessed. This SFRA update will inform the next iteration of the SA process.


7

1.4. Sources of Flooding in West Sussex 1.4.1. West Sussex is affected by six sources of flooding. Fluvial (river), tidal (the sea)

and groundwater affect the largest areas. Other sources of flooding include flooding from surface water, sewers and from artificial sources (e.g. canals and reservoirs).

Flooding from Rivers (Fluvial)

1.4.2. The largest area affected by flooding from rivers is along the River Adur and River

Arun and their tributaries. The floodplain from these rivers is also expected to have the deepest floodwaters during large flood events. The West Sussex Rifes are a group of smaller watercourses, including the River Lavant, River Ems, the Bosham Stream, the Aldingbourne Rife and other coastal streams along the Manhood Peninsula. The latest data for flooding from rivers was obtained from the Environment Agency flooding datasets. Since the last SFRA update for the JMLP (January, 2017) there has been an update to the River Lavant flood model but these changes do not affect any of the sites included in this document. Flooding from the Sea (Tidal)

1.4.3. The low lying parts of the West Sussex Coastline are at risk of open flooding from

high tides and storm surges on the English Channel. In most instances, sea defences are constructed to offer protection from the 0.5% AEP storm surge but there is still a residual risk of flooding from behind these defences as a result of extreme high tides and tidal surges and wave action. The River Arun and Adur are also tidally influenced and flooding from sea can affect land a significant distance away from the coast. The latest data for tidal flooding was obtained from the Environment Agency flooding datasets. Flooding from Groundwater

1.4.4. The large chalk bands across West Sussex means a significant proportion of the

county is affected by groundwater flooding. Since the 2010 SFRA, the data for groundwater flooding data has been updated due to improved modelling. A summary of the methodology used to map flooding from groundwater is given in Appendix B.

Flooding from Surface Water

1.4.5. Surface water flooding is highly variable across the county due to the varied

geology, soil types and rainfall patterns. Surface water flooding data has been updated since the 2010 SFRA due to improved modelling. A summary of the methodology used to map flooding from surface water is given in Appendix B.


8

Flooding from Sewers 1.3.3. The SFRA (2010) uses historic incidences of sewer flooding to identify areas more

prone to sewer flooding. The West Sussex Flood Risk Management Team also provided any updated information for historic incidences of sewer flooding for the specific sites.

Flooding from Artificial Sources

1.4.6. This includes the potential of flooding from canals and reservoirs. Although the

2010 SFRA did not include a detailed assessment of flooding from these sources it does show the locations of these sources that should be taken into account. Chapter 9 of Volume II of the SFRA (2010) explains how the data for flooding from artificial sources was derived.

1.5. Flood Zones in West Sussex 1.5.1. For the purposes of applying the Sequential Test, areas at risk from flooding

include land within Flood Zones 2 and 3 (a and b). It can also include an area in Flood Zone 1 that the Environment Agency has notified the local planning authority as having critical drainage problems. The definitions of the Flood Zones are given in table 1. Figure 1 (taken from paragraph 21 of the Planning Practice Guidance) sets out the process that has been used in undertaking the sequential test.

Table 1: Definitions of Flood Zone Flood Zone Definition Zone 1 Low Probability

Land having a less than 1 in 1,000 annual probability of river or sea flooding. (Shown as ‘clear’ on the Flood Map – all land outside zones 2 and 3).

Zone 2 Medium Probability

Land having between a 1 in 100 and 1 in 1,000 annual probability of river flooding; or Land having between a 1 in 200 and 1 in 1,000 annual probability of sea flooding. (Land shown in light blue on the Flood Map).

Zone 3a High Probability

Land having a 1 in 100 or greater annual probability of river flooding; or Land having a 1 in 200 or greater annual probability of sea flooding. (Land shown as dark blue on the Flood Map)

Zone 3b The Functional Floodplain

This zone comprises land where water has to flow or be stored in times of flood. Local planning authorities should identify in their Strategic Flood Risk Assessments area of functional floodplain and its boundaries accordingly, in agreement with the Environment Agency. (Not separately distinguished from Zone 3a on the Flood Map)


9

Climate Change 1.5.2. All plans should apply a sequential, risk-based approach to the location of

development – taking into account the current and future impacts of climate change.

1.5.3. The climate change allowances are based on projections and different scenarios of carbon dioxide emissions to the atmosphere. They include predictions of anticipated change for:

• Peak river flow by river basin district; • Peak rainfall intensity; • Sea level rise; • Offshore wind speed and extreme wave height.

1.5.4. Updated climate change allowances were published in November 2018 (UKCP18)

and are the official source of information on how the climate of the UK may change over the rest of this century. The allowances published in February 2016 (UKCP16) are still the best national representation of how climate change is likely to affect flood risk for peak river flow and peak rainfall intensity (Appendix C).

1.5.5. The climate change allowances for sea level rise will also be updated again in 2019

and until then it is reasonable to continue to use the ‘Flood risk assessments: climate change allowances’ published in February 2016 because the allowance that have been used represent the high end of the range of sea level rise projected by UK Climate Projections 18. Research that is due to be published in 2019 may result in changes to these allowances and will be taken into account, where necessary, in an updated version of this document.

1.6.4. Climate change is predicted to have an impact on flooding in West Sussex. Rainfall is likely to become more frequent, more intense and covering longer periods. Information on the latest climate change allowances has been reviewed in terms of the impacts on the sites in Table 2. For mineral sites in West Sussex, only peak river flow and peak rainfall intensity are considered to have an impact. Sea level rise and offshore wind speed and extreme height are not relevant to the mineral sites as none of the sites are affected by tidal flooding or situated close to the coast.

1.6.5. The climate change allowances have been applied to three of the sites in the report that are at risk of fluvial flooding and the results are presented in Table 2. The climate change allowances are not relevant to the other sites because they are not at risk of tidal or fluvial flooding.

West Sussex Joint Minerals Local Plan: Soft Sand Review November 2019 SFRA Update and Sequential Test of Soft Sand Sites Table 2: Peak River Flow and Peak Rainfall Allowances for Brick Kiln Farm and West Heath Common Mineral Site

Mineral type Current Fluvial Flood Risk

Peak River Flow Allowances (Percentage of site in flood risk zone in brackets)

Peak Rainfall Allowances (Percentage of site in flood risk zone once climate change allowance applied in brackets)

East of West Heath Common

Soft Sand (water compatible)

Moderate risk (22% of site in Fluvial FZ2/3)

Central Allowance There is a 50% chance that peak river flows could increase by: 2020s (2015 to 2039) = 10% 2050s (2040 to 2069) = 20% 2080s (2070 to 2115) = 35%

Upper End Allowance River levels could rise by the following: 2020s (2015 to 2039) = 10% 2050s (2040 to 2069) = 20% 2080s (2070 to 2115) = 40% Central Allowance River levels could rise by the following: 2020s (2015 to 2039) = 5% 2050s (2040 to 2069) = 10% 2080s (2070 to 2115) = 20%

Severals West

Soft Sand (Water Compatible)

Low Risk (FZ1) but the site borders FZ2/3.

Central Allowance Under climate change scenarios, there is a 50% chance that the western edge of the site could fall within FZ2/3 as the river levels in the adjacent river could rise by the following amounts: 2020s (2015 to 2039) = 10% 2050s (2040 to 2069) = 20% 2080s (2070 to 2115) = 35%

Upper End Allowance Under peak rainfall allowances, levels of the adjacent river could rise by the following: 2020s (2015 to 2039) = 10% 2050s (2040 to 2069) = 20% 2080s (2070 to 2115) = 40% Central Allowance 2020s (2015 to 2039) = 5% 2050s (2040 to 2069) = 10% 2080s (2070 to 2115) = 20%

Duncton Common


Moderate (10% of site in FZ2/3)

Central Allowance Under climate change scenarios, there is a 50% chance that peak river flows could increase by the following amounts: 2020s (2015 to 2039) = 10% 2050s (2040 to 2069) = 20% 2080s (2070 to 2115) = 35%

Upper End Allowance Under peak rainfall allowances, levels of the adjacent river could rise by the following: 2020s (2015 to 2039) = 10% 2050s (2040 to 2069) = 20% 2080s (2070 to 2115) = 40% Central Allowance 2020s (2015 to 2039) = 5% 2050s (2040 to 2069) = 10% 2080s (2070 to 2115) = 20%

Coopers Moor


High risk (25% of site in FZ2/3)

Central Allowance Under climate change scenarios, there is a 50% chance that peak river flows could increase by the following amounts:

Upper End Allowance Under peak rainfall allowances, levels of the adjacent river could rise by the following: 2020s (2015 to 2039) = 10% 2050s (2040 to 2069) = 20%

West Sussex Joint Minerals Local Plan: Soft Sand Review November 2019 SFRA Update and Sequential Test of Soft Sand Sites

11

2020s (2015 to 2039) = 10% 2050s (2040 to 2069) = 20% 2080s (2070 to 2115) = 35%

2080s (2070 to 2115) = 40% Central Allowance 2020s (2015 to 2039) = 5% 2050s (2040 to 2069) = 10% 2080s (2070 to 2115) = 20%

West Sussex Joint Minerals Local Plan: Soft Sand Review SFRA Update and Sequential Test of Soft Sand Sites 2. Flood Risk Assessment of Potential Mineral Sites 2.1. What minerals need to be planned for?

Sand and Gravel

2.1.1. The Local Aggregate Assessment (LAA) provides the evidence base for the supply and demand for aggregates. The 2018 LAA (2019) can be found on the West Sussex County Council website (www.westsussex.gov.uk/mwdf) and concludes that over the plan period there is likely to be a shortfall of soft sand that will need to be planned for taking account of the previous 10 years sales (2008–2017), and the following assumptions; o Assumption 1: the construction of new residential dwellings in West

Sussex is projected to grow by 26.8% o Assumption 2: Up to 91% of sand and gravel may be used in the

construction of residential dwellings

2.1.2. The LAA presents three scenarios for the shortfall in soft sand taking into account the different assumptions and gives a shortfall of between 1,652,062 tonnes and 2,832,887 tonnes.

2.1.3. The LAA is updated on an annual basis and further evidence gathered as part of the

Plan preparation may change shortfall.

2.2. What are the Reasonably Available Sites? 2.2.2. A methodology was devised to identify potential soft sand sites. This methodology

is explained in the Soft Sand Site Selection Report, January, 2019 (www.westsussex.gov.uk/mwdf). A ‘short list’ of nine sites, which are considered viable and deliverable, was identified from a ‘long list’ of 21 sites. For the purposes of applying the Sequential Test, the authorities have considered these to be ‘reasonably available’ sites to direct mineral extraction to lower flood risk areas first. Sites that have been ruled out are listed in Appendix 3 of the Soft Sand Site Selection Report, January 2019.

2.2.3. The nine sites will be subject to further desk-based assessment (including this SFRA) to provide the evidence upon which decisions will be made about which sites whether each site is ‘acceptable in principle’. ‘Acceptable in principle’ means that a site is considered to be suitable for development, available, and considered to be viable. Those sites that are ‘acceptable in principle’ will be considered for allocation in the Proposed Submission Plan.

http://www.westsussex.gov.uk/mwdf



13

2.3. Flood Risk Assessments at Planning Application Stage 2.3.2. Any proposals for mineral development will need to address Policy M19 (Flood Risk

Management) of the JMLP which can be found online at www.westsussex.gov.uk/mwdf.

2.3.3. A Flood Risk Assessments (FRAs) will also be required at the planning application stage to demonstrate how flood risk from all sources of flooding to the development itself and flood risk to other will be managed now and taking climate change into account. A drainage strategy will also be required at planning application stage and early liaison with the Lead Local Flood Authority is strongly recommended.

2.4. Summary of Flood Risk at Potential Soft Sand Sites 2.4.2. Table 3 summaries the flood risk information for each site and the conclusion from

the sequential test exception test processes carried out in accordance with the NPPF. Maps of each site showing the extent of the flood zones are provided in Appendix D. Although soft sand sites are classed as ‘water compatible’, the allocation of sites should follow the sequential approach and should consider the need to avoid flood risk from sources other than rivers and the sea in accordance with Planning Practice Guidance. The sequential approach should also be applied at the site level to ensure that more vulnerable uses are located in areas at lowest flood risk. A ‘RAG’ rating has been devised to differentiate between sites according to the percentage of the site that falls within an area of flood risk and to help identify which sites may be more ‘at risk’ from flooding from a particular source to assist with the ‘sequential test’ process (figure 1) which will inform the site selection process. Identifying which parts of the site are at higher or lower risk of flooding will help with the ‘sequential approach’ at site level, for example, it may be possible to locate ancillary facilities such as processing plant and offices in areas at the lowest flood risk. ‘RAG’ Rating and Key to Table 3:

A large part of site (More than 30%) falls in an area at risk of flooding from this

source Part of the site (11 – 30%) falls in an area at risk of flooding from this source

A small part of the site (10% or less) falls within an area at risk of flooding from this source

Site is not affected by flooding from this source



14

2.5. Consideration of the Principle of the Exception Test for high risk

groundwater sites 2.5.2. Four of the nine sites are considered to have a high risk to groundwater flooding;

therefore there is a requirement to consider the principle of the exception test for each, which is set out below. Buncton Manor Farm

2.5.3. Buncton Manor Farm is considered to pass the principle of the exception test due to the characteristics of sand extraction during operational phases of such development. Suitable mitigation, in the form of pumps, could be deployed to ensure that there is no flood risk to any potential receptors. There is low risk to any direct receptors with respect to vulnerability. The sustainability benefit of this site, providing a natural resource, of which a steady and adequate supply is required, would outweigh the immediate and consequential flood risk, which can be effectively managed during any operation phase under a permit and/or a suitable Site Management Plan. A site specific Flood Risk Assessment would be required at planning application stage, as required by Policy M19 of the JMLP, and validated prior to any commencement of operations. Coopers Moor

2.5.4. Coopers Moor is considered to pass the principle of the exception test due to the

characteristics of sand extraction during operational phases of such development. Suitable mitigation, in the form of pumps, could be deployed to ensure that there is no flood risk to any potential receptors. There is low risk to any direct receptors with respect to vulnerability. The sustainability benefit of this site providing a natural resource, of which a steady and adequate supply is required, would outweigh the immediate and consequential flood risk, which can be effectively managed during any operation phase under a permit and/or a suitable Site Management Plan. A site specific Flood Risk Assessment would be required at planning application stage, as required by Policy M19 of the JMLP, and validated prior to any commencement of operations. East of West Heath Common

2.5.5. East of West Heath Common is considered to pass the principle of the exception

test due to the characteristics of sand extraction during operational phases of such development. Suitable mitigation, in the form of pumps, could be deployed to ensure that there is no flood risk to any potential receptors. There is low risk to any direct receptors with respect to vulnerability. The sustainability benefit of this site providing a natural resource, of which a steady and adequate supply is required, would outweigh the immediate and consequential flood risk, which can be


15

effectively managed during any operation phase under a permit and/or a suitable Site Management Plan. A site specific Flood Risk Assessment would be required at planning application stage, as required by Policy M19 of the JMLP, and validated prior to any commencement of operations.

Severals West

2.5.6. Severals West is considered to pass the principle of the exception test due to the

characteristics of sand extraction during operational phases of such development. Suitable mitigation, in the form of pumps, could be deployed to ensure that there is no flood risk to any potential receptors. There is low risk to any direct receptors with respect to vulnerability. The sustainability benefit of this site providing a natural resource, of which a steady and adequate supply is required, would outweigh the immediate and consequential flood risk, which can be effectively managed during any operation phase under a permit and/or a suitable Site Management Plan. A site specific Flood Risk Assessment would be required at planning application stage, as required by Policy M19 of the JMLP, and validated prior to any commencement of operations.


Table 3: Summary of Flood Risk at Potential Mineral Sites Fluvial Flood

Zone Tidal Flood Zone

Climate Change Scenario (February 2016 Allowances)

Surface Water Groundwater Sewers and Artificial Sources**

Flood History Existing Flood Defences

Outcome of Sequential Test

Outcome of Exception Test

Buncton Manor Farm

Low Risk (100% in FZ1)

Low Risk (100% in FZ 1)

No change Low risk, although watercourse running across and along boundary which will need to be managed at all times.

High risk (15%); Moderate risk (17%). Remainder of the site is negligible susceptibility.

No foul sewers. Existing highway drainage along Washington Road (A283) and Water Lane.

Minor highway flooding recorded in the area but no incidents at the site itself.

None known Exception Test Required due to Groundwater Risk

Development is appropriate (with mitigation)

Chantry Lane Extension

Low Risk (100% FZ1)

Low Risk (100% FZ1)

No change Low risk susceptibility

Negligible risk susceptibility

Medium Risk None known None known Development is appropriate

n/a

Coopers Moor 25% of site in FZ2/3

Low risk (100% FZ1)

See table 2 25% of site at high risk, associated primarily with the watercourse that runs NW through the site.

High risk (38%); Moderate risk (60%). Remainder of the site is negligible susceptibility.

Low Risk None known Not protected by flood defences. Part of site falls in a flood warning area.

Exception Test Required due to Groundwater Risk


Duncton Common

Moderate (10% of site in FZ2/3)

Low risk (100% in FZ 1)

See table 2 Low risk (10% of site at higher risk) associated with watercourse near south eastern boundary.

High Risk (7%); Moderate Risk (11%); remainder of site seasonal high ground water Levels are 5m or more below ground.

Low Risk None known Part of site falls in a flood warning area.

Development is appropriate*

n/a


Moderate risk (22% of site in Fluvial FZ2/3)


See table 2

Low risk (outside of ordinary water courses)

High2 Low Risk None known Most of the area outside the river plain is on higher ground and the site is in a flood warning area.

Exception Test Required due to Groundwater Risk


Ham Farm Low Risk (100% FZ1)

Low Risk (100% FZ1)

No change Low risk susceptibility

Moderate (17.5%). Remainder of the site is low risk.

Low Risk None known Development is appropriate

n/a

Minsted West Low Risk (100% in FZ 1)


No change Low risk (<5% at higher risk)

Moderate3 Medium Risk (Close to pond at Stedham Common Sand Pit)

None known None known Development is appropriate

n/a

2 High Risk: Seasonal groundwater levels are either at the surface or within 0.025m of the surface. 3 Moderate Risk: Seasonal GW levels reach between 0.025m and 0.5m of the surface.


17

Severals East Low Risk (100% in FZ)


No change Low risk Moderate Risk (30%)

No data Localised flooding experienced in 2013/14 at Woolbeding Estate and Gardens

None known Development is appropriate

n/a

Severals West

Low Risk (100% in FZ 1 but borders Fluvial FZ2/3)


See table 2 Low risk (5% at higher risk)

High risk (16%); Moderate Risk (70%)

No data None known None known Exception Test Required due to Groundwater Risk


1 High Risk: Seasonal groundwater levels are either at the surface or within 0.025m of the surface. 1 Moderate Risk: Seasonal groundwater levels reach between 0.025m and 0.5m of the surface. *In Flood Zone 3, water compatible uses should be designed and constructed to:

remain operational and safe for users in times of flood; result in no net loss of floodplain storage; not impede water flows and not increase flood risk elsewhere.

**For sites that are classed as ‘Medium risk’ to flooding from artificial sources, an amber colour has been used as no information is available about the proportion of the site at risk of flooding from this source. Data has been taken from the 2010 SFRA, where available.


Appendix A: Extracts from Planning Practice Guidance Table A1: Flood Risk Vulnerability Classification (Source: Planning Practice Guide) Classification Uses Essential 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. Highly Vulnerable • Police and ambulance stations; fire stations and

command centres; telecommunications installations required to be operational during flooding.

• 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’).

More Vulnerable • Hospitals • 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 for health services, nurseries and educational establishments.

• 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 • Police, ambulance and fire stations which are not

http://planningguidance.planningportal.gov.uk/blog/guidance/hazardous-substances/planning-for-hazardous-substances/

http://planningguidance.planningportal.gov.uk/blog/guidance/hazardous-substances/planning-for-hazardous-substances/


19

required to be operational during flooding. • Buildings used for shops; financial, professional

and other services; restaurants, cafes and hot food takeaways; offices; general industry, storage and distribution; non-residential institutions not included in the ‘More Vulnerable’ class; and assembly and leisure.

• 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 development • 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. • Ministry of Defence 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.


20

Table A2: Flood Risk Vulnerability and Flood Zone Compatibility (Source: Planning Practice Guidance) Flood Risk Vulnerability Classification

Essential Infrastructure

Highly Vulnerable

More Vulnerable

Less Vulnerable

Water Compatible

Zone 1 Zone 2 Exception

Test Required

Zone 3a† Exception Test

Required†

Exception Test

Required

Zone 3b* Exception Test

Required*

*

= Development is appropriate = Development should not be permitted. † In Flood Zone 3a essential infrastructure should be designed and constructed to remain operational and safe in times of flood. * In Flood Zone 3b (functional floodplain) essential infrastructure that has to be there and has passed the Exception Test, and water-compatible uses, should be designed and constructed to:

• remain operational and safe for users in times of flood; • result in no net loss of floodplain storage; • not impede water flows and not increase flood risk elsewhere.

West Sussex Joint Minerals Local Plan: Soft Sand Review SFRA Update and Sequential Test of Soft Sand Sites Appendix B: Ground Water and Surface Water Flood Risk Mapping Methodology Surface Water Flooding General notes on national models

• Surface water flood risk maps were created nationally by the Environment Agency using a model of the ground levels to see where rain water flows and ponds in different storm events.

• Assumptions are made about whether drainage infrastructure exists in certain areas, how much water it can take before reaching capacity and the effects that the infiltration of rain into the soil can have.

• For each storm event a depth of rain water falling in a certain period is calculated and added to the map across the whole country

• The modelling does not take into account any non-surface water influences such as rivers, the sea or groundwater.

• Because the mapping is done on a national scale and, generally, highly detailed local information is not taken into account, the maps should only be used as an indication of where surface water flooding is likely to occur.

• The Flood Maps for Surface Water identify drainage routes and low areas in floodplains, and flooding caused by local rainfall. It does not indicate flooding that occurs from overflowing watercourses, surcharging drainage systems or public sewers caused by catchment-wide rainfall events or river flow. As such, it only shows areas where surface water would be expected to flow or pond.

• The flood risk information is not usually suitable for identifying individual properties at risk of flooding. Although it is possible to estimate whether the local area is at risk of flooding, some assumptions have to be made in any flood modelling and so the maps can never be perfect. It is not possible to say whether an individual property is at risk of internal flooding, as this is influenced by factors such as the height of door steps, air bricks or the height of surrounding walls.

Notes on Updated Flood Map for Surface Water (uFMfSW 2014) • This model uses a particular set of assumptions to give an indication about how surface

water would flow along roads, around buildings and into low spots in the ground. The ground height was raised to represent buildings (typically by 0.3m), flow paths were better represented through structures such as bridges and rail embankments, and roads were lowered (by 0.125m) so flood flow paths are better represented. Ground roughness was also varied to take into account different land use.

• Rainfall depths were adjusted to take into account infiltration (to represent the differences between urban and rural areas) and drainage (assuming a constant rate of flow is removed in all urban areas).

• Very shallow flooding and very small areas of flooding were removed from the model. • In certain areas, such as semi-rural locations or other places where positive drainage

infrastructure has been assumed to be in place in the model but in fact does not exist, the depth or extent of flooding may be greater than indicated.


22

Notes on Flood Map for Surface Water (FMfSW 2010) • Two rainfall events: the 1 in 30 year and the 1 in 200 year were used. The former was

mapped showing depths greater than 300mm and the latter was mapped showing depths greater than 100mm.

• Two adjustments are made to calculate effective rainfall: o In rural areas, runoff equates to 39% of rainfall to represent infiltration; o In urban areas, runoff equates to 70% of rainfall to represent infiltration then

runoff is reduced by 12mm/hr to represent the effects of sewers. • No allowance was made for differing infiltration of different soil types and so a single

infiltration figure was used in urban or rural areas. • The threshold of 300mm represents the depth that significant property damages begins

to occur and it is the depth that moving through floodwater (driving or walking) becomes difficult. The Flood Map for Surface Water shows predictions of flooded areas but does not show whether individual properties will be affected by surface water flooding or have been affected in the past.

Groundwater Flooding Notes on ESI Groundwater Flooding Susceptibility The map is a general purpose indicative screening tool, and is intended to provide a useful initial view for a wide variety of applications. However, it does not provide an alternative to a proper site-specific assessment, and a detailed risk assessment should be used for any site where the impact of groundwater flooding would have significant adverse consequences. Mapped classes combine an understanding of likelihood, model and data uncertainty and possible severity. Likelihood is ranked according to whether we expect flooding to occur at a site due to extreme elevated groundwater levels, estimated to have a return period of 1 in 200 years. Severity relates to the expectations of the amount of property damage or other harm that groundwater flooding at that location might cause. Uncertainty relates to the confidence that the map accurately represents locations where groundwater may emerge and cause flooding. Overall risk is presented on the indicative 1 in 200 year return period map. Groundwater susceptibility is based on modelling and whilst indicting that there is a potential risk it does not mean that the site has or will be flooded from this source. Neither does the site be indicated NOT at risk mean that it will not be at risk – purely that the available mapping does not show it.


23

Appendix C: Flood Risk Assessments: Climate Change Allowances Updated climate change allowances were published in November 2018 (UKCP18) and are the official source of information on how the climate of the UK may change over the rest of this century. The allowances published in February 2016 (UKCP16) are still the best national representation of how climate change is likely to affect flood risk for peak river flow and peak rainfall intensity. The peak river flow allowances are based on basin district. For the South East Basin District the allowances are: C1: Peak River Flow Allowances for the South East River Basin District Allowance category

Total potential change anticipated for the 2020s (2015 to 2039)



Upper end 25% 50% 105% Higher central 15% 30% 45% Central 10% 20% 35%

For ‘water compatible development,’ such as sand and gravel sites, the central allowance should be used for sites in flood zone 3a and 3b. No allowance is required for sites in flood zone 2. For less vulnerable uses, such as clay sites, the central allowance should be used for sites in flood zone 2 and the central and higher central allowances should be used to asses a range in flood zone 3a. Development should not be permitted in flood zone 3b.

Peak Rainfall Intensity Allowance

Changes have been made to the anticipated changes in extreme rainfall intensity in small and urban catchments. For flood risk assessments and strategic flood risk assessments, the central and upper end allowances should be assessed to understand the range of impact. Mineral sites are greenfield sites outside urban areas and are not expected to be affected by these changes.

C2: Peak rainfall intensity allowance in small and urban catchments Applies across all of England

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%


24

Sea Level Allowances There is a single regional allowance for each time frame for sea level. These allowances account for slow land movement. This is due to ‘glacial isostatic adjustment’ resulting from the release of pressure after ice that covered large parts of northern Britain melted at the end of the last ice age. The northern part of the country is slowly rising and the southern part is slowly sinking. This is why net sea level rise is less for the north west and north east than the rest of the country.

All the mineral sites considered in this report are not affected by sea level flooding, therefore these allowances have not been considered in this report.

C3: Sea level allowance for the south east of England for each epoch in millimetres (mm) per year with cumulative sea level rise for each epoch in brackets (use 1990 baseline)

1990 to 2025 2026 to 2055 2056 to 2085 2086 to 2115 Cumulative rise to 2115/metres (m)

4 (140mm) 8.5 (255mm) 12 (360mm) 15 (450mm) 1.21m Offshore wind speed and extreme wave height allowance Wave heights may change because of increased water depths resulting from climate change. The frequency, duration and severity of storms could also change. All the mineral sites considered in this report are located in-land and would not be affected by extreme wave height.

C4: Offshore wind speed and extreme wave height allowance (use 1990 baseline)

Applies around all the English coast 1990 to 2055 2056 to 2115

Offshore wind speed allowance +5% +10%

Offshore wind speed sensitivity test +10% +10%

Extreme wave height allowance +5% +10%

Extreme wave height sensitivity test +10% +10%


25

Appendix D: Flood Risk Maps of Soft Sand Sites

0 110 22055 METRES © Crown copyright and database rights 2016 Ordnance Survey. 100023447

Buncton Manor Farm

±

Flood DefencesAreas Benefiting From DefencesFlood Zone 3Flood Zone 2Surface Water Flooding risk 1:30Surface Water Flooding risk 1:100

Historic Flood LocationsHistoric Flooding EventsGroundwater Flooding High RiskGroundwater Flooding Moderate RiskGroundwater Flooding Low RiskGroundwater Flooding Negligible Risk


Chantry Lane Extension

±




Coopers Moor

±




Duncton Common

±




Ham Farm

±




Minsted West

±




Severals East

±




Severals West

±




35

Appendix E: References Capita Symonds (2010). Strategic Flood Risk Assessment of West Sussex. MHC&LG (2019). National Planning Policy Framework. MHC&LG (2014). Planning Practice Guidance. West Sussex Joint Minerals Local Plan (January, 2019). Soft Sand Site Selection Report. West Sussex Joint Minerals Local Plan (January, 2019). Assessment of Need for Aggregates: Local Aggregates Assessment.

strategic flood risk assessment update and sequential test of … · 2020-01-03 · strategic flood...

Documents