Drainage Strategy - Sidlesham (Manhood Peninsula) - Southern Water
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Drainage Strategy
– Sidlesham
(Manhood Peninsula)
We’ll update and republish this Drainage
Strategy as we continue to engage with our
customers and stakeholders.
You can find further information on how to share
your feedback via our website:
southernwater.co.uk/drainagestrategies
Southern Water
Southern House
Yeoman Road
Worthing
West Sussex
BN13 3NX
4593_04.18
Page 2 Drainage Strategy – Sidlesham Drainage Strategy – Sidlesham Page 3
Executive summary
The Drainage Strategy for Sidlesham Long-term outcomes Current issues and future challenges
Environment Agency and other organisations
(Manhood Peninsula) focuses on to find and fix the causes of pollution which
Our Five-year Business Plan 2015 to 2020 for Sidlesham (Manhood Peninsula) are preventing the bathing water from being
the current pressures and future sets out details of the improved water and consistently rated as ‘excellent’. Some
Over the next 25 years, population growth is
challenges that will impact on the wastewater services we will provide in expected to result in a 12% increase in domestic rivers, groundwater and other water bodies
sewerage network and wastewater the future. It was developed as a result of properties connecting to our sewers in the in the catchment were classified as ‘bad’
treatment works. the company’s biggest ever consultation Sidlesham wastewater catchment. Climate to ‘moderate’ status in 2015 according to
programme which included feedback from change and urban creep are expected to add the Water Framework Directive (WFD) and
more than 34,000 customers and stakeholders. to surface water flows into our sewers which, in require improvement to ‘good’ status by 2027.
It will enable us to take a more strategic Partnership working will be key to achieving the
approach to drainage planning across the area, The business plan is built around the delivery turn, will increase the number of properties at
risk from surface water flooding. higher environmental standards required under
providing a long-term (25-year) strategy to of six outcomes which our customers and the WFD.
ensure we deliver a reliable and sustainable stakeholders told us were their priorities. For Maintaining and improving bathing water
wastewater service for the region while each outcome, we have developed a clear set quality is essential for encouraging tourism and We have assessed the risks to the delivery of
accommodating population growth, new of promises that detail what we will achieve for supporting economic prosperity in the region. our key outcomes if we were to do nothing,
development, climate change and stricter our customers. Three of these outcomes are The Manhood Peninsula has a ‘Blue Flag’ summarised in table 1. These risks will be
environmental quality standards. relevant to the Drainage Strategy for Sidlesham award for its West Wittering beach. At Selsey, identified, assessed and reduced through our
and are shown in figure 1 below alongside the we are working with the local authorities, the ‘business as usual’ activities.
The Sidlesham wastewater catchment
relevant promises associated with them.
includes the villages of Almodington, Birdham,
Bracklesham, Selsea, Sidlesham, East/West
Table 1. Key outcomes at risk in the Sidlesham catchment (Manhood Peninsula) if we do nothing
Wittering and West Itchenor in the Manhood
Peninsula, south of Chichester. Risk of not delivering the outcome by:
Key outcome Comments
2020 to 2025 2040
Figure 1. Outcomes relevant to the Drainage Strategy for Sidlesham (Manhood Peninsula)
Maintain compliance at Medium to high High Sidlesham Wastewater Treatment Works is currently operating at
wastewater treatment its treatment capacity and is likely to require an uplift in treatment
Removing wastewater Looking after Better information works capacity and dry weather flow consent in line with the anticipated
growth in new connections. Additional capacity could be delivered in
effectively the environment and advice parallel with development.
Minimise flooding due to Medium Medium to high The Sidlesham network can accommodate approximately 53% of the
• M
inimise flooding due to • Minimise
wastewater pollution • Improve
advice on how to wastewater forecast new wastewater connections over the next 25 years without
wastewater and surface water so that by 2020 there will be prevent blocked drains to help an unacceptable deterioration in our wastewater service. Additional
with a 25% reduction in internal no serious (Category 1 & 2) reduce blockages capacity could be delivered in parallel with development.
sewer flooding incidents by 2020 pollution incidents caused by our Minimise flooding due Medium Medium to high Groundwater infiltration is considered to be significant across the
operations. We are also aiming to surface water and catchment and has been an issue in the village of Sidlesham where
to halve the number of minor groundwater repairs have been carried out. Groundwater levels may get higher as
a consequence of rising sea levels due to climate change.
(Category 3) incidents from 2017
onwards Surface water flooding has been a significant issue due to the limited
capacity of the land drainage system. Flooding is likely to increase
• M
aintain wastewater treatment due to more intense storms caused by climate change and more
works compliance by ensuring run-off due to urban creep.
that any releases to the Minimise pollution Low Low to medium The number of pollution incidents in 2015 and 2016 were relatively
environment comply with permit low. The likelihood and severity of pollution incidents may increase
conditions due to additional wastewater, surface water and groundwater in the
sewerage system.
• Maintain
‘excellent’ bathing Improve water bodies Medium Medium Nine out of 11 water bodies on the Manhood Peninsula are predicted
water quality and improving (river, lake, coastal to achieve ‘good’ status by 2027 with the remaining two at
other bathing waters groundwater) ‘moderate’ status. Partnership working will be key to achieving the
higher environmental standards required under the WFD.
Maintaining ‘excellent’ Low to medium Low to medium Three out of four bathing waters in the region have generally
bathing water quality ‘excellent’ bathing water quality. At Selsey, we are working with
partners to improve bathing water quality to ‘excellent’ by 2019–20.
Page 4 Drainage Strategy – Sidlesham Drainage Strategy – Sidlesham Page 5
Table 3. Action plan for the Drainage Strategy for Sidlesham (Manhood Peninsula)
No. Action Lead Supporting Due date Action delivery status
action action
owner owners
1 Through the SW WSCC, EA, End of 2016 A draft drainage strategy was issued to external stakeholders in
drainage strategy, CDC November 2016 for comment and discussion. Feedback has been
We are assessing a range of traditional identify current received from WSCC, CDC and the EA during the winter of 2016-17.
engineering and alternative strategies to Next steps drainage issues,
key risks, shared
tackle the current drainage issues and future We will update and revise this drainage outcomes and
risks that have been identified for the area. strategy to reflect consultations with our potential actions
We are also investing in the development customers and stakeholders in the region. 2 Publish the drainage SW All Following The drainage strategy will be updated with feedback from external
of innovative techniques that will bring a We will continue to work with the local strategy on the stakeholders receipt of stakeholders before publication.
significant improvement to our capabilities and community to gain important feedback from Southern Water comments
performance. Potential strategies are being our customers and local stakeholders which we website from
assessed against whole life costs, constraints external Customers and other stakeholders will have the opportunity to
will use to shape our solutions. We have already provide feedback on the final drainage strategy.
stake-
and benefits to both the environment and begun discussions with our regional drainage holders
stakeholders. Table 2 lists the key outcomes at partners and we will work collaboratively to
risk and offers potential strategies which could agree and implement a strategy that ensures a 3 Improve Selsey’s SW WSCC, EA, 2019–20 In May 2017, Selsey was named as one of seven bathing waters
be implemented to overcome the issues and bathing water CDC to be improved to ‘excellent’ as detailed in the action plan for the
reliable and effective wastewater service
quality to ‘excellent’ bathing water.
challenges that threaten them. for the region in the long term.
4 Review the needs SW WSCC, EA, End of 2017 In March 2015, a meeting was held with external stakeholders to
Table 3 provides an action plan for this strategy and options CDC identify risks in the Sidlesham catchment to inform the DAP.
which we will implement in partnership with identified in the
Sidlesham DAP Structural, operational, growth, flooding and environmental needs
other stakeholders in the region. and potential options have been identified and are being reviewed.
5 Produce an action SW WSCC, EA, Summer A Sidlesham DAP action plan is provided in appendix B of this
Table 2. Key outcomes at risk and potential strategies for Sidlesham (Manhood Peninsula) plan following CDC 2017 drainage strategy.
completion of the
Key outcomes at risk Potential strategies Sidlesham DAP
Maintain compliance at • Collaboration with the local authority and developers to understand the timing, size and location of 6 Complete the SW 2018 The Pagham DAP will be produced as part of Southern Water’s
wastewater treatment works developments with planning certainty Pagham DAP which ongoing 2015 to 2020 DAP programme prior to updating the
• Reduce groundwater infiltration of foul sewers covers part of the drainage strategy for the Manhood Peninsula.
Manhood Peninsula
• Provide additional treatment capacity at the works as required
7a Identify sewerage SW Not stated An investigation of the sewer flooding issues in the eastern part
• Transfer wastewater to other treatment works with spare capacity flooding issues in SWMP of Selsey has been carried out as part of the Sidlesham DAP using
and remedial hydraulic modelling of the sewerage system. A potential cause of
Minimise flooding and • Collaboration with the local authority and developers to understand the timing, size and location of
measures in Selsey sewer flooding has been identified as a lack of capacity in parts of
pollution due to wastewater developments with planning certainty
(SWMP Ref. the sewerage system. Possible options to reduce sewer flooding
• Upsize foul/combined sewers, pumps and rising mains as required SELS_004) have been identified and will be considered during the assessment
of all options identified in the DAPs in the Southern Water region.
• Reduce groundwater infiltration of foul sewers
7b Identify sewerage SW Not stated As part of the Sidlesham DAP, sewer flooding issues have been
• Transfer wastewater to other treatment works with spare capacity flooding issues and in SWMP investigated in property flooding clusters to the south and west
• Construct offline storage tanks to attenuate high flows remedial measures of Sidlesham village. The flooding mechanism has been identified
in Sidlesham. Local as possibly being due to groundwater infiltration or surface water
• Reduce surface water flows in combined sewers residents have inundation of the foul sewer network.
reported concerns
Minimise flooding and • Remove misconnections of surface water to foul sewers Southern Water carried out surveys and repairs for groundwater
about foul flows in
pollution due to surface infiltration in the village of Sidlesham in 2013. Ongoing monitoring of
• Maintain public sewers, highway drains and land drainage system Jury Lane. (SWMP
water and groundwater flow levels are to be carried out to check whether the repairs have
Ref. SIDL_009)
• Upsize surface water/combined sewers and pumps as required been successful or whether further infiltration reduction is required.
Southern Water does not manage sewers in Jury Lane, Sidlesham.
• Construct offline storage tank/s to manage the impact of high flows
7c The foul pumping SW Not stated An investigation of sewer flooding issues in West Wittering was
• Installation of sustainable drainage systems (SuDS) station on Pound in SWMP included in the Sidlesham DAP. Hydraulic modelling has predicted
• Separation of surface water from foul water in combined sewers Road in West potential flooding in Pound Road. Groundwater infiltration of the foul
Wittering is thought sewer network may be a cause of the flooding and requires further
• Reduce groundwater infiltration of foul sewers to be the cause investigation using long term flow monitoring and possible CCTV
Improve water bodies to • Implementation of Integrated Water Cycle Management and working with external stakeholders to reduce of sewer flooding surveys of the sewers.
‘good’ status by 2021–27 the pollution of water bodies (SWMP Ref.
WWIT_004)
Maintain ‘excellent’ bathing • Minimise pollution due to additional wastewater, surface water and groundwater flows using the potential
waters strategies described above 8 Investigate the use SW WSCC, EA, 2018 Consultants have been appointed to carry out a study on the
of SuDS to manage CDC benefits of SuDS for Southern Water and its customers. Drainage
surface water issues partners are to be consulted on SuDS policy and implementation in
each region.
9 Provide support WSCC Community Ongoing Communities are being encouraged to prepare for and reduce the
and funding groups, town risk and impacts of flooding in West Sussex through the provision of
for Operation and parish funding of £1.25m in 2013, £1.1m in 2014–15 and £0.5m in 2016–17.
Watershed Active councils
Communities Fund
(SW=Southern Water, EA =Environment Agency, WSCC=West Sussex County Council, CDC=Chichester District Council, DAP= Drainage Area
Plan, SuDs=sustainable drainage systems, SWMP=surface water management plan)
Page 6 Drainage Strategy – Sidlesham Drainage Strategy – Sidlesham Page 7
Contents 1. Introduction
2 Executive Summary The Manhood Peninsula is located connected to the public sewerage system and
south of Chichester and falls within also treats the wastewater produced by visitors
7 1. Introduction to the region and trade effluent where these
the administrative boundary of businesses connect to public sewers. Some
8 2. What is a Drainage Strategy?
Chichester District Council and West rural areas are not connected to the public
9 3. Working in partnership Sussex County Council. sewerage system and properties in these areas
10 4. Current drainage and flooding issues in the Manhood Peninsula may discharge their wastewater into septic
The region extends from Chichester Harbour tanks or private sewage treatment systems
10 4.1 Environmental quality in the west to Pagham Rife in the east and to which are not operated by Southern Water.
11 4.2 Protection of groundwater sources Selsey at the southernmost tip of the peninsula. The wastewater from the Manhood Peninsula
11 4.3 Bathing waters Villages in the region include Almodington, villages of North Mundham and Hunston flows
Birdham, Bracklesham, East/West Wittering, southwards to the Pagham WTW which serves
13 4.4 Shellfish waters Sidlesham and West Itchenor. a catchment area of approximately 4km² and
13 4.5 Flooding Sidlesham Wastewater Treatment Works (WTW) approximately 4,000 properties connected to
treats wastewater that Southern Water collects the public sewers. This drainage strategy will
18 5. Wastewater services in the Sidlesham catchment be reviewed and updated when information is
from a catchment which covers an area of
18 5.1 Wastewater collection and treatment process approximately 16km² as shown in figure 2. The available from the Pagham drainage area plan
works serves approximately 10,500 properties as discussed in section 2.
19 5.2 Wastewater catchment maps and schematic
20 5.3 Wastewater service performance
Figure 2: Location of wastewater catchments serving the Manhood Peninsula
22 6. Risks to Wastewater services
22 6.1 Growth
22 6.1.1 Network capacity for growth
24 6.1.2 Treatment capacity for growth
25 6.2 Climate change KENT
26 6.3 Urban creep
26 6.4 Asset deterioration EAST SUSSEX
27 6.5 Infiltration HAMPSHIRE WEST SUSSEX
28 6.6 Clean water consumption
29 6.7 Environmental legislation
30 7. Strategic assessment
30 7.1 Outcomes for the Sidlesham catchment (Manhood Peninsula) ISLE OF WIGHT
32 7.2 Key outcomes at risk
33 7.3 Strategies: Business as usual activities
34 7.4 Strategies: Least whole life cost approach
35 7.5 Strategies: Innovation Key:
35 7.6 Strategies: Sustainable drainage systems Southern Water water supply
and wastewater treatment area
36 7.7 Strategies: Surface water separation
Designated bathing waters
36 7.8 Strategies: Removal of misconnections
37 7.9 Strategies: Integrated Water Cycle Management
37 7.10 Strategic assessments for the delivery of key catchment outcomes
39 8. Next Steps
40 9. Action Plan
41 10. Glossary of Terms
43 11. References
44 Appendix A – Strategic assessments
56 Appendix B – Sidlesham DAP Action PlanPage 8 Drainage Strategy – Sidlesham Drainage Strategy – Sidlesham Page 9
2. What is a Drainage Strategy? 3. Working in partnership
The Drainage Strategy for Sidlesham What is a Drainage Area Plan? To ensure we develop and implement We will work with our key stakeholders to
(Manhood Peninsula) will enable us the most effective and sustainable improve our knowledge and understanding
This drainage strategy incorporates information of issues such as population growth, climate
to take a more strategic approach to from the Sidlesham Drainage Area Plan
strategies, we will need to work in change and urbanisation of green spaces.
drainage planning across the area. (Southern Water/MWH, 2015). partnership with other organisations
A high level representation of the main
who also have important drainage stakeholders who contribute to the
Southern Water maintains a rolling programme
It will provide a long-term (25-year) strategy to
of drainage area plans (DAPs). The Pagham responsibilities. development of an effective drainage strategy
ensure a reliable and sustainable wastewater is shown in figure 4.
DAP is included in our DAP programme for
service for the region while accommodating
2015 to 2020 and when it is completed we will For the Sidlesham catchment (Manhood Since the South-West Sussex Internal Drainage
population growth, new development, climate
update this drainage strategy. Peninsula) this will include: Board was dissolved recently, there is no
change and stricter environmental quality
standards. We prioritise the production of DAPs with • Chichester District Council internal drainage board for the Manhood
the highest priority being those catchments Peninsula.
To help develop the strategy, we have adopted
with existing issues such as sewer flooding,
•
West Sussex County Council
the Drainage Strategy Framework (Environment (Lead Local Flood Authority)
and where there are proposals for future
Agency et al., 2013), outlined in figure 3. This
framework recommends a four-stage planning
development which may affect the performance • Environment Agency
of the sewerage system. Sidlesham was
process when preparing drainage strategies.
selected as a priority DAP for these reasons.
• Parish Councils
This is similar to that used in the development
of surface water management plans. The schemes identified from all DAPs are
• Manhood Peninsula Partnership
The Drainage Strategy for Sidlesham (Manhood
reviewed and prioritised against competing •
Local Flood Action Groups (eg West
needs across the region and are critical in Manhood, Birdham & Earnley, Sidlesham)
Peninsula) covers the first (initialise/prepare)
identifying investment requirement in the
stage and the second (risk assessment) stage.
production of five- yearly business plans. Figure 4. Roles and responsibilities for drainage
Our activities as part of stage three include
DAPs are internal Southern Water documents
an options appraisal of both traditional
and are not intended to be public-facing. DAPs
and alternative strategies which we will be
are a major source of information for drainage
undertaking in collaboration with our key local
strategies which have a wider remit and are
drainage partners.
designed for public consumption.
Figure 3. Four stage process for the development of Drainage Strategies
Partnership Uncertainty Risk based
Whole life and Innovative and
Live process
costs benefits sustainable
Initialise/prepare Risk assessment Options appraisal Intervention
Establish
Aligned delivery
partnership and Consultation Consult
and collaborative
consultation on risk issues on options
solutions
process
Define Quantify Plan for Innovative and
uncertainties uncertainties uncertainties sustainable
Demonstrate
Prepare Quantify Live
whole life cost
risk data risks and visible
and benefitPage 10 Drainage Strategy – Sidlesham Drainage Strategy – Sidlesham Page 11
4. Current Drainage and Flooding
Issues in the Manhood Peninsula The Broad Rife had an overall ‘bad’ ecological
status in 2015. The Environment Agency 4.2 Protection of
(EA) has identified saline intrusion affecting groundwater sources
4.1 Environmental quality invertebrates as a main contributor to the West Sussex has a high number of groundwater
‘bad’ status. A ‘poor’ to ‘moderate’ ecological sources which provide clean, healthy drinking
The Manhood Peninsula benefits from a high-quality environment which includes a number of status for the Broad Rife has been identified
designated sites where wildlife, habitats, landscapes and heritage are protected as detailed in water that Southern Water and Portsmouth
as being due to dissolved oxygen and Water supplies to customers in the region.
table 4. phosphates in continuous sewage discharge This groundwater is vulnerable to
Table 4. Designated sites on the Manhood Peninsula
from wastewater treatment sources. However, contamination from natural and man-made
phosphates in rivers can also be due to run-off pollutants on the ground surface which pass
Designated Site Description Sites on the Manhood Peninsula from agricultural land and leaking septic tanks. down through thin soils and fissures into the
The Sidlesham Wastewater Treatment Works chalk aquifers.
Ramsar and Special Protection Areas
Pagham Harbour, Chichester & Langstone Harbours discharges into the Broad Rife and further
(wetlands, rare and migratory birds)
work is required to understand the source of Groundwater source protection zones show
Special Areas of Conservation
(habitats and non-bird species)
Solent Maritime phosphates and dissolved oxygen and how the risk of contamination from any activities
Southern Water can assist in improvements to that might cause pollution in the area and can
Marine Conservation Zones Selsey Bill and the Hounds, Pagham Harbour the Broad Rife. be seen on maps at the EA’s ‘What’s in your
backyard’ website. There are no protection
Sites of Special Scientific Interest Selsey East Beach, Pagham Harbour, Bracklesham Bay, Chichester Harbour The Bremere Rife had a ‘bad’ ecological status zones in the Manhood Peninsula apart from
in 2015 which the EA has identified as being a small zone 1 (inner) which is just south of
Local Nature Reserves Pagham Harbour probably due to pollution in rural areas from Fishbourne and is outside the Sidlesham
agricultural sources. wastewater catchment. The main protection
Under the Water Framework Directive (WFD), the South-East River Basin Management Plan aims to
achieve at least a ‘good’ status for all water bodies by 2027 with some expected to reach this status The Pagham Rife had a ‘failed’ chemical status zones are in the chalk downlands to the north
by 2021 (Environment Agency, 2016). Table 5 shows the current and predicted status of the different in 2015 which is probably due to diffuse sources of Chichester.
water bodies on the Manhood Peninsula. of pollution from rural areas, towns, cities and Under the WFD, the South East Hants
transport (EA, 2016). Partnership working is Bracklesham groundwater body had a ‘poor’
Table 5. Water body status on the Manhood Peninsula in December 2015 (EA, 2016) key to achieving the higher environmental chemical status in 2015 which the EA identified
Water body name Category Status type Classification Predicted Predicted
standards required under the Water Framework as probably due to pollution from landfill
(2015) outcome outcome Directive (WFD). leaching and industry.
(2021) (2027)
Ecological Bad Poor Good
Broad Rife River
Chemical Good Good Good
Ecological Bad Bad Good
4.3 Bathing waters
Bremere Rife River
Chemical Good Good Good The Manhood Peninsula has four designated
bathing water sites which have weekly
Ecological Moderate Moderate Good
Pagham Rife River assessments of bathing water quality during
Chemical Fail Fail Fail the bathing season from May to September.
Ecological Moderate Moderate Good Table 6 shows the historical and current annual Blue Flag Seaside
Chichester Harbour Transitional water assessment for each of these bathing waters. award award
Chemical Good Good Good
Ecological Moderate Moderate Moderate Table 6. Bathing waters classification for the Manhood Peninsula (Based on EU Bathing Waters Directive 2006/7/EC)
Pagham Harbour Transitional water
Chemical Good Good Good
Defra’s four-year rolling assessment
Ecological Good Good Good
Pagham Lagoon Transitional water Bathing Water 2009–12 2010–13 2011–14 2012–15 2013–16 2014–17
Chemical Good Good Good
Ecological Moderate Moderate Good Bracklesham Bay Excellent Excellent Excellent Excellent Excellent Excellent
Sussex Coastal water
Chemical Good Good Good
Ecological Moderate Moderate Good Pagham Excellent Excellent Excellent Good Good Good
Chichester Canal Canal
Chemical Good Good Good
Quantitative Good Good Good Selsey Good Good Sufficient Good Good Good
South East Hants Bracklesham Group Groundwater
Chemical Poor Poor Good
Quantitative Poor Good Good West Wittering Excellent Excellent Excellent Excellent Excellent Excellent
Sussex Lambeth Group Groundwater
Chemical Good Good Good
Quantitative Poor Good Good
Littlehampton Anticline West Groundwater
Chemical Good Good GoodPage 12 Drainage Strategy – Sidlesham Drainage Strategy – Sidlesham Page 13
West Wittering beach has been assessed as We are working with local authorities, the •
West Sussex Local Flood Risk Management
having ‘excellent’ bathing water quality over EA and other organisations over the next 4.4 Shellfish waters
Strategy 2013 to 2018 (WSCC, 2014)
the last five years and was awarded a Blue Flag five years to find and fix sources of pollution There is a shellfish water protected area in
for its beach in 2017. In Bracklesham Bay and preventing seven bathing waters in our region Chichester Harbour (Thornham Channel) to
•
M anhood Peninsula Surface Water
Pagham, bathing water quality has also been from achieving ‘excellent’ standard – this is one Management Plan (ch2m for WSCC, 2015)
the west of the Manhood Peninsula (Defra,
consistently ‘excellent’ or ‘good’ over the last measure required for bathing waters to gain 2016) which is harvested for oysters, clams Groundwater flooding
five years. ‘Blue Flag’ status. This added benefit is over and cockles. Further information is available in
and above our statutory duties, which water the Local Action Plan for Shellfish Harvesting The geology of the Manhood Peninsula
Over the last 20 years, Southern Water has
companies are normally only funded to meet. Beds in Chichester Harbour (Chichester District determines whether the region is vulnerable to
invested significantly to meet the requirements
Council and Havant Borough Council, 2015). groundwater flooding. The chalk South Downs
of the EU Urban Wastewater Treatment We have undertaken a rigorous selection
are immediately north of Chichester from which
Directive (91/271/EEC) to improve the quality of process to shortlist 21 bathing waters from Sidlesham WTW discharges into the Broad groundwater emerges at Fishbourne Springs
treated wastewater that we discharge into the the 83 in our region for further investigation. Rife which flows into Bracklesham Bay in the northern extremity of the Manhood
sea. This investment has included the provision Based on the Department for Environment, approximately 9km east of Chichester Harbour. Peninsula. It is believed that the groundwater
of additional treatment at Sidlesham WTW Food & Rural Affairs’ (Defra’s) four-year rolling
flow in the South Downs may contribute to the
and at two large wastewater treatment works assessment, table 6 shows that the Selsey
baseflow (the level of ground water that seeps
(Ford near Arundel and Budds Farm, Havant) bathing water has generally been of ‘good’ 4.5 Flooding into the banks of a river or riverbed) in the
which discharge into the sea in the vicinity of quality over the last five years, although it was The Manhood Peninsula is at risk of flooding upper reaches of the Pagham Rife.
the Manhood Peninsula. assessed as ‘sufficient’ in 2014. As Selsey’s from groundwater, surface water, river and tidal
bathing water has failed to reach ‘excellent’ In the central region of the Manhood Peninsula,
Bathing water is affected by various sources of sources. The EA’s ‘What’s in your backyard’
quality in recent years, it was included in the the chalk layer is much deeper and is confined
pollution such as: website provides maps which show the flood
21 shortlisted bathing waters. below thick layers of impermeable ‘London
risk in the region.
•
t he release of surface water after heavy
We spent a year carrying out a range of
Clay’ and ‘Reading Beds’ so that groundwater
rain which contains contaminated rainwater It is Southern Water’s responsibility to ensure in the chalk is generally unable to flow to the
detailed investigations including watercourse our customers can continue to use their
run-off from roads surface. At the southern end of the peninsula,
sampling, DNA analysis and CCTV surveys of wastewater services in the event of a flooding around Selsey, Earnley and Wittering, the
•
diluted, partially-treated wastewater from sewers to understand the causes of pollution incident. We will work alongside local councils, underlying geology is the ‘Bracklesham Group’.
combined sewer overflows or pumping at each of the shortlisted bathing waters. We the EA and others to help alleviate the effects This formation comprises a mixture of silt, sand
stations that has been released to prevent also held a series of customer focus groups of flooding on the community. and clay and may contain some groundwater in
heavy rainfall overwhelming the sewerage and have surveyed more than 3,600 customers
the more permeable areas.
system and flooding properties and 300 businesses across the Southern Water As a flood risk management authority, Southern
region to get a better understanding of the Water has the following responsibilities: The underlying strata are generally not
•
misconnections of foul sewage into
views and needs of the communities at each exposed at the surface and are covered by
surface water sewers which discharge into
location.
• adopting new-build sewers thin superficial drift deposits. These deposits
watercourses without treatment
In May 2017, seven bathing waters including
• managing public sewer flooding may contain coarse sands and gravels which
•
wastewater from private treatment works
Selsey were selected for improvements to •
b eing scrutinised by the democratic
can contain groundwater which is recharged
and leaking septic tanks which are not relatively quickly following rainfall. This
enable them to reach ‘excellent’ quality by processes of the lead local flood authority
owned by Southern Water ‘shallow’ groundwater will then generally
2019–20. An action plan has been developed
•
acting in accordance with the national and discharge into drainage ditches that flow into
•
decaying seaweed and algae reaching which provides an overview of the issues our
local strategies for flooding performing as the rifes (rivers) on the peninsula.
nuisance levels investigations uncovered at Selsey and the
a Category 2 responder to flood incidents
steps required to improve bathing water quality. Consequently, groundwater may increase
•
heavy rainfall run-off from agricultural land under the Civil Contingencies Act.
flows in ditches and rifes which can contribute
into rivers This project is part of our long-term plan to
Southern Water’s management of public sewer to the risk of river flooding when surface water
improve bathing water quality in our region.
• animals and seabirds on or near beaches As part of this, we have committed to work
flooding is discussed later in this section. from heavy rainfall is added to the flow. Direct
groundwater flooding from emerging springs is
• waste from boats. with other agencies, local communities Further information on groundwater, surface unlikely to occur in the Manhood Peninsula.
and landowners to bring all coastal waters water and river flooding risk in the Manhood
Our business plan for 2015–20 was developed at bathing beaches in the region up to the Peninsula and West Sussex can be found in the High groundwater levels can prevent surface
through the biggest programme of customer standard required to achieve Blue Flag status following documents: water dispersing from sewers into the ground
research in our history. During this process, our by 2040, provided there is continued support in soakaways which can result in surface water
customers indicated that clean bathing waters from our customers and regulator to do so..
•
Arun and Western Streams Catchment Flood
flooding. There have been reports of flooding
and beaches are a priority and they are willing Management Plan (EA, 2009)
around soakaways in Stockbridge which is
to contribute more towards improvements
•
Strategic Flood Risk Assessment of West just south of the A27 (CH2M for West Sussex
where these are required. We are committed to County Council, 2015).
Sussex (Capita Symonds for WSCC, 2010)
maintaining the number of our bathing waters
with ‘excellent’ water quality at 54 beaches •
West Sussex Preliminary Flood Risk High groundwater levels can also cause
across Hampshire, Isle of Wight, Kent and Assessment (WSCC, 2011) infiltration of sewers which is discussed further
Sussex. in section 6.5.
•
West Sussex County Council Report on June
2012 Flood Event (WSCC, 2012)Page 14 Drainage Strategy – Sidlesham Drainage Strategy – Sidlesham Page 15
River and tidal flooding We also have a stock of temporary flood Surface Water Management Plans (SWMP) are •
run-off into the ditch network needs to
barriers and mobile generators which can be used to identify and assess flood risks from be controlled including run-off from new
The Manhood Peninsula is a low lying coastal used to ensure that our sites can continue to surface water and also local flooding due to developments and glass houses
area which is artificially drained by a series of operate at normal output if the site should be groundwater and ordinary watercourses as
main rivers known as ‘the rifes’ which are fed flooded. defined in the Flood and Water Management •
t he continuity of the ditch network is critical.
by the ditch network. The rifes discharge into Act 2010. A SWMP will also include actions to Land drainage consents are important
Chichester Harbour, Pagham Harbour, the area Sidlesham WTW is in a high risk EA flood zone in preventing culverting or infilling of
reduce local flooding that have been agreed by
protected by the Medmerry scheme or directly 3 for river flooding (greater than 1% annual watercourses where it will increase the
the drainage partners. Progress on the actions
into the sea. probability) or sea flooding (greater than 0.5% flood risk
is reviewed periodically. These partners
annual probability). However, the Medmerry
In 2005, tidal flooding occurred to the west include the lead local flood authority, highways •
a consequence-based approach should be
coastal flood defence scheme provides some
of Selsey when coastal defences were authorities, EA, sewerage undertaker and adopted which focuses on the most critical
protection to the works from tidal flooding.
overtopped. Since then, the Medmerry coastal others. parts of the drainage network.
flood defence scheme has been completed Surface water flooding In 2015, the Manhood Peninsula Surface Water
between Selsey and Bracklesham and protects Southern Water is fully participating in
Significant flooding has occurred on several Management Plan was completed (CH2M for the development and implementation of
over 300 properties from tidal flooding. The low West Sussex County Council, 2015) based on
lying nature of the area means it is at particular occasions in the Manhood Peninsula due to the the Manhood Peninsula Surface Water
incapacity of the drainage ditches and rifes to the following objectives: Management Plan with our drainage partners.
risk from tidal flooding due to potential sea
level rises as a result of climate change. drain surface water flows during prolonged wet •
to understand the surface water drainage Actions include:
winters – such as those experienced in 2012–13 in the area and gather additional data to
High tides can prevent the rifes from draining and 2013–14.
•
ongoing maintenance of the public
reduce gaps in knowledge sewerage network, pumping stations
into the sea which can create back-up in the
rifes and inland flooding. The EA is currently In June 2012, extreme rainfall resulted in the •
to identify pinch points in the network and and combined sewer overflows to ensure
reviewing the operation of its Ferry Road flooding of approximately 110 properties on connectivity issues surface water flows can flow freely to
pumping station which pumps flows from the the Manhood Peninsula in the villages of East treatment works and/or controlled releases
Wittering, Bracklesham, Almodington, Earnley, •
to identify potential improvement works
Broad Rife when it is tide-locked. Southern
Birdham and Somerley (WSCC, 2012). The to reduce flood risk to communities in •
t he removal of properties which have been
Water is currently considering options to the peninsula including capital and listed on a register called the DG5 (see
ensure that treated effluent can continue to likelihood of this volume of rainfall is around
maintenance measures, building on the glossary) as having experienced sewer
be discharged from Sidlesham WTW when the once every 200 years and it overwhelmed
work already undertaken by communities, flooding from water-pressure overloads
Broad Rife is tide-locked. the Manhood Peninsula land drainage system
partnerships and other organisations caused by excessive surface water entering
which is typically designed to withstand rainfall
During extreme rainfall events or following the sewerage system
levels that occur between once in 25 to 100 •
to produce up-to-date geographic
prolonged wet winters, flooding can occur from years. information data of the ditch network. •
t he identification of misconnections of
the rifes and the ditch network as they do not surface water sewers to foul sewers (and
have the capacity to drain away the surface Surface water flooding in the Manhood The SWMP assessed flooding data from the vice-versa)
water. Further information is provided in the Peninsula is the result of several factors: WSCC Highways Authority, Southern Water’s
following section on surface water flooding. •
t he low-lying nature of the area with little sewer flooding DG5 Register, key stakeholder •
promoting the use of sustainable drainage
meetings and meetings with local residents. systems (SuDS) to reduce surface water
The management of river and tidal flooding risk gradient to drainage ditches
Based on flooding history, seven high entering the sewers.
is primarily the responsibility of the EA, district •
inadequate capacity of the drainage system priority locations were identified at Birdham Southern Water’s main responsibility is to
and borough councils and internal drainage which is not designed to drain the surface & Westlands, East Wittering & Bracklesham,
boards. The EA has recently completed the provide a resilient wastewater service in the
water flows resulting from extreme rainfall Hunston, Selsey, Sidlesham, Somerley and
Bookers Lane Flood Alleviation Scheme which event of surface water flooding.
•
inconsistency in the ditch network such as West Wittering. A further five medium priority
was partly funded by West Sussex County locations were identified at Crouchers (A286), Sidlesham WTW is in a low risk zone (0.1% to 1%
Council and local residents to reduce the risk of large ditches leading into small pipes or no
Runcton, South Mundham, Stockbridge and annual probability) for surface water flooding.
river flooding of properties in Earnley. Southern pipes at all
West Itchenor. Potential measures were However, some wastewater pumping stations
Water will co-operate with the other relevant •
p oor maintenance of the network that leads identified for each location and actions were such as Memorial Hall, Itchenor are in high risk
authorities in the exercise of their flood and to blockages and collapses of ditches and assigned to key stakeholders. zones (greater than 3.3% annual probability)
coastal erosion risk management functions. culverts which remain unfixed for surface water flooding. Southern Water
The SWMP identified five principles which are will assess flood risk and protect our sites
Southern Water’s main responsibility is to •
impermeable clay soils which lead to key to ensuring the long-term management of from surface water flooding using the same
provide a resilient wastewater service in the ground saturation and results in ponds of the drainage system on the Manhood Peninsula
event of river or tidal flooding. We assess the principles outlined above for river and tidal
surface water.
risk of flooding at our wastewater treatment •
ditch clearance remains the responsibility flooding.
works and pumping stations and whether Surface water can overload combined of riparian (waterside property) owners and
flooding could affect critical assets. If potential sewers which can result in flooding of diluted landowners under the Land Drainage Act of
loss of service is identified and it is cost- wastewater or controlled releases to the river. 1991
beneficial to do so, we can provide permanent Surface water can also inundate foul sewers by
entering the sewer through unsealed manholes •
local communities have a key role to play
flood protection measures such as bunding
in flooded areas. through local flood action groups etc
(retaining walls), flood walls, watertight doors
and raising the height of critical equipment.Page 16 Drainage Strategy – Sidlesham Drainage Strategy – Sidlesham Page 17
Sewer flooding (DG5 Register) Table 7: DG5 Register for the Sidlesham catchment
For each of these flooding clusters, a hydraulic •
Separate surface water from wastewater
modelling assessment has been carried out (eg install separate drainage systems and/
Properties can be flooded internally or Flood Risk Internal External as part of the DAP and the results have been or removal of misconnections)
externally if there is insufficient capacity in the Flooding Flooding
verified by flow monitoring. Properties at risk
sewers to transfer flows caused by surface 2 in 10 Years 0 10 of flooding were assessed using a predictive
•
Put in place sustainable drainage systems
water due to rainfall. After a sewer flooding, model based on a one in 30-year flood scenario •
Adopt other solutions such as infiltration
1 in 10 Years 2 6
we investigate to establish whether it was the and allowed for a 20% increase in extreme reduction
result of an overload of water pressure or other 1 in 20 Years 3 86
rainfall that might reasonably be expected
causes, such as blockages or sewer collapses. Total 5 102 For all flooding problems, surface water
to accompany climate change. An allowance
We then work to identify events caused by separation has been considered as the
for groundwater infiltration of the sewerage
floods beyond our control. If appropriate, first option for flood alleviation. However,
network has also been included in the model.
properties are added to the DG5 Register DAP flooding needs and options the Sidlesham catchment is predominantly
Asset surveys and impermeable area surveys
which records properties at risk of flooding separately drained and infiltration removal
Our drainage area plan (DAP) for the Sidlesham have been commissioned for this assessment.
under a classification based on the number of is the recurring first step in addressing many
catchment (Southern Water/MWH, 2015) is For each flooding cluster, a position statement of the needs. Infiltration reduction would be
times they are likely to be flooded in a 10 or
primarily focused on a hydraulic appraisal of the identifies the flooding mechanisms and highly beneficial to the performance of the
20-year period.
drainage network. It includes an assessment of provides a high-level review of the current sewer network and will be likely to reduce the
The DG5 Register is managed by Southern properties that are at risk of internal or external preferred solutions based on the following need to expand capacity or introduce storage
Water and is regularly reviewed and updated. flooding due to hydraulic overload and are short-term and long-term options: schemes designed to address flooding and
Further information may allow us to remove currently on the DG5 Register. growth needs in the catchment. Costs will be
properties from the register or to downgrade •
D o nothing
developed to enable a full appraisal to be made
The properties and locations at risk of flooding
the risk of sewer flooding by transferring the •
I solation – disconnection of properties of all the options.
in the Sidlesham catchment have been grouped
property from, for example, a two in 10 to a one from the public sewer network. Flows are
into nine priority flooding clusters The Sidlesham DAP action plan in appendix B
in 10-year register or from the one in 10 to one transferred to offline storage and then
in 20-year register. •
Selsey provides a summary of the flooding issues in
pumped to the public sewer.
the catchment, proposed schemes to resolve
Figure 5 shows that there have been two •
M emorial Hall, Itchenor •
G et the best from the existing system (eg the flooding, and the current status of these
internal flooding incidents due to ‘hydraulic make improvements to the pumping station) schemes.
overloading’ (excessive water) in the Sidlesham •
Church Road, East Wittering
•
Re-direct flows to parts of the network with Further long-term strategies are considered in
catchment over the last five years. In 2012–13 •
H ighleigh sufficient capacity
and 2013–14, there were a high number of section 7.10 and appendix A.
external flooding incidents due to hydraulic •
East Bracklesham •
Provide additional storage within the
network Sewer flooding (other causes)
overloading which were related to rainfall. •
Almodington
There have been significantly fewer external •
I ncrease the size of storm tanks at the Around 85% of internal flooding incidents are
flooding incidents in the last three years. •
West Wittering
due to other causes such as sewer blockages,
treatment works
Within the Sidlesham catchment, there are •
Rookery Lane
•
E xpand the existing sewer network
sewer collapses, or equipment failure. Figure
6 shows a peak in 2012–13 in the number of
currently five properties on the DG5 Register •
Pinks Lane, Birdham •
I ncrease the size of existing pumping external flooding incidents due to other causes
which are at risk of internal flooding and 102
stations or provide new ones which have reduced significantly in the last four
locations at risk of external flooding with a
return period categorisation shown in table 7. •
Provide or make changes to the combined years.
sewer overflow
Flooding incidents
Figure due
5. Flooding toto
due hydraulic
hydraulicoverloading
overloading ininthe
theSidlesham
Sidlesham catchment
catchment Figure 6. Flooding due to other causes in the Sidlesham catchment
Flooding incidents due to other causes in the Sidlesham catchment
Number of flooding incidents due to other causes
Number of flooding incidents due to hydraulic
80 1200 80
overloading (public + S105A sewers)
Internal Flooding External Flooding Regional Rainfall (mm) Internal Flooding External Flooding
70 1050 70
Regional rainfall (mm)
60 900
(public + S105A sewers)
60
50 750
50
40 600
40
30 450
20 300 30
10 150 20
0 0 10
2012–13 2013–14 2014–15 2015–16 2016–17
0
2012–13 2013–14 2014–15 2015–16 2016–17Page 18 Drainage Strategy – Sidlesham Drainage Strategy – Sidlesham Page 19
5. Wastewater services in the
Sidlesham catchment
The wastewater flows by gravity through 5.2 Wastewater catchment In Selsey, there are combined sewer overflows
5.1 Wastewater collection smaller drains and sewers to large diameter (CSO) at East Beach Road, Crablands and
and treatment process map and schematic Church Road. If the flows exceed the capacity
trunk sewers that carry wastewater flows from
whole villages or suburbs of towns. Where Southern Water collects and treats wastewater of the network in these locations then there
Southern Water recycles wastewater from
required, the wastewater is pumped from a from the Sidlesham catchment on the Manhood will be a controlled release of excess diluted
almost two million homes in Kent, Sussex,
low level to a higher level using pumps at a Peninsula. Figure 8 shows how wastewater screened flows to a watercourse, so that
Hampshire and the Isle of Wight. Every day we
wastewater pumping station (WPS). Figure flows to the treatment works from the sewerage properties are protected from flooding.
treat and recycle an average of 717 million litres
7 provides a simple diagram showing how network. These overflows are approved by the EA’s
of wastewater at our 365 treatment works after
the wastewater is collected, screened and permit team and none are currently classed
it has been pumped through a sewer network The Sidlesham catchment has a total of
processed at a wastewater treatment works as ‘unsatisfactory’ by the EA (Southern Water/
39,600 kilometres long. approximately 167km of public sewers which
before the treated water is returned to rivers or MWH, 2015).
primarily comprise foul sewers that transfer
Wastewater comes from water used in homes, the sea. Further information on the wastewater waste flows only. Less than 1% of the network Southern Water also manages approximately
businesses and factories as well as some of the process can be found at southernwater.co.uk/ is combined sewers which transfer waste and 26km of separate surface water public
surface water run-off from rain falling on roofs the-wastewater-process. surface water flows together in the same pipe. sewers mainly in Selsey and a small network
or roads which ends up in our sewer network.
in Bracklesham. The surface water sewers in
There are 46 wastewater pumping stations
Selsey flow into a pond at East Beach before
in the Sidlesham catchment which have
discharging into the sea. Private systems
36km of rising mains associated with them.
Figure 7. The wastewater treatment process for surface water drainage may exist but are
This is a relatively high number of pumping
unrecorded and may drain into soakaways or
stations for the size of catchment and they
potentially discharge, as illegal connections,
Homes and factories generally operate in sequence, pumping flow
to the foul sewerage network.
from one station sub-catchment to another.
The performance of the pumping stations is In most villages there is a positive highway
monitored remotely through a telemetry link. drainage system via gullies and pipes which
Wastewater flows in sewers
Surface water Figure 8. Sidlesham Wastewater Catchment
Arrives at wastewater
treatment works
Screening Grit removal Primary Aerobic biological Final
comprising Sedimentation treatment settling
debris such Organic waste Harmful bacteria is destroyed
as wet wipes sinks to the bottom. by good bacteria in aerated
Remaining water is water tanks
taken from top
Solids are Composting
recycled
Sludge
Combined heat Anaerobic digestion
and power plant of sludge by bacteria
Methane produced Treated wastewater
during sludge process is returned to the environment
burned for heat and Sludge
electricity generation
Soil conditioner
for agriculture
MethanePage 20 Drainage Strategy – Sidlesham Drainage Strategy – Sidlesham Page 21
drain into the ditch network, and surface water Rising mains contain wastewater that is pumped Wastewater treatment works compliance
flows which run into the rifes. West Sussex 5.3 Wastewater service performance under pressure from our wastewater pumping with permits
County Council is responsible for managing We routinely monitor, analyse and report the stations. A burst will often result in pollution of
and maintaining the highway drainage network. performance of our wastewater sewerage the environment or flooding. There have been As part of the wastewater treatment process,
networks and treatment processes to enable two rising mains bursts per year on average we must comply with permits issued by
Sidlesham Wastewater Treatment Works the EA to release treated wastewater into
us and our regulators to assess the service over the last five years in the Sidlesham
serves approximately 10,500 properties in the watercourses. For 2010–15, the Sidlesham
provided to our customers and the impact of catchment.
villages of Almodington, Birdham, Bracklesham, Wastewater Treatment Works was compliant
Selsey, Sidlesham, West Itchenor, East and our activities on the environment.
Pollution incidents with its permits.
West Wittering. The treatment works is located Sewer blockages
approximately 2km south-west of Sidlesham. Pollution incidents due to a failure of our
Every year there are thousands of avoidable wastewater assets are reported to the EA.
The inlet to the works receives wastewater blockages in our sewers caused by people The severity of the pollution is agreed and
flows from pumped rising mains located at flushing the wrong things down the toilet or categorised as 1 (major), 2 (significant), 3 (minor)
Crablands Selsey WPS, East Beach Selsey by pouring fat, oil and grease down the sink. or 4 (no pollution).
WPS, Almodington Lane Earnley WPS, Chalk Sewer blockages can result in flooding to
Lane Sidlesham South WPS and Manhood Lane Figure 10 shows that there have been no
customers’ properties or pollution. serious pollution incidents (Categories 1 and
Sidlesham WPS. Flow is discharged into the
inlet screen chamber where screens remove Figure 9 shows that the number of blockages 2) in the Sidlesham catchment over the last
non-biological items such as wet wipes. During recorded per km of public sewer in the five years. Following a peak in 2013, there has
storms, excessive flow is diverted into two Sidlesham catchment has generally been close been a significant drop in Category 3 pollution
storm storage tanks which have an overflow to the Southern Water regional average. In incidents in the last three years.
should the storm tanks be filled to capacity. the Sidlesham catchment, 85% of blockages
The wastewater is pumped from the inlet occurred in Selsey with a high proportion on
Figure 10. Pollution incidents in the Sidlesham catchment
works to primary settlement tanks. This is the High Street. We use high-powered water Pollution incidents in the Sidlesham catchment
followed by biological treatment to break down jets to clear blockages and ensure our sewers 10
organic matter and further treatment to reduce are running freely. In 2015, we launched our Category 1 Category 2 Category 3
9
nitrogen. ‘Keep it Clear’ campaign which involves teams
visiting ‘blockage hotspot’ areas to educate
Number of pollution incidents
8
The final treated effluent is discharged into the customers on how to safely dispose of items.
Broad Rife which, in turn, discharges into the 7
sea to the west of Selsey. Sewer collapses and rising main bursts 6
In some locations on the Manhood Peninsula, The Sidlesham DAP (Southern Water/MWH, 5
there are private sewage treatment plants or 2015) has identified seven sewer collapses 4
septic tanks which are not owned or maintained of public sewers in the Sidlesham catchment
by Southern Water. over the last five years. We have an ongoing 3
programme to replace or refurbish ageing 2
sewers at high risk of collapse.
1
0
Figure 9. Blockages in the Sidlesham catchment 2012 2013 2014 2015 2016
Blockages in the Sidlesham catchment (public sewers only)
1.0
Number of blockages per km of public sewer
Sidlesham SW Regional Average
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
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