Counters Creek storm relief sewer Why we need the Counters Creek storm relief sewer The problem The sewer network we operate today has been much improved and vastly extended over the years and remains in good condition, but it is under increasing pressure from climate change, population growth, and the paving over of green spaces that provide natural drainage. Many areas of outer London have been developed with separate sewers to take foul waste and rainwater. In much of inner London and some other urban areas, the sewer system has always been combined, meaning that foul sewage from kitchens and toilets mixes with rainfall. This means that, during a heavy storm, the flow in the sewer is much greater and can reach maximum capacity. Our sewers are designed to cope with the majority of storms but occasionally rainfall can be so heavy that it overwhelms the system, causing sewer flooding. This is a particular issue for our Counters Creek sewerage catchment in west London. Sewer flooding is one of the worst experiences our customers can face and it is unacceptable. It causes significant social, economic and environmental impacts to residents, businesses and communities.
Why we need the Counters Creek storm relief sewer Our statutory responsibilities Thames Water is the statutory sewerage undertaker for the London Area. We have a statutory duty under the Water Industry Act 1991 to provide, improve and extend a system of public sewers, and to effectively deal with the contents of those sewers. The duty is enforceable by the Secretary of State and the industry s financial regulator, Ofwat. We have a statutory duty to address and resolve the problems of sewer flooding experienced in the Counters Creek catchment area. The Counters Creek catchment Counters Creek is one of the lost rivers of London and is situated on the boundary of the London Borough of Hammersmith and Fulham and the Royal Borough of Kensington and Chelsea. This former river and its catchment is now part of the sewerage network, draining surface water from buildings and roads, as well as foul water from toilets, bathrooms and kitchens. There are over 1700 properties across parts of the London Borough of Hammersmith and Fulham and Royal Borough of Kensington and Chelsea that are subject to sewer flooding. We believe that the number of properties subject to flooding exceeds this figure, as we are still receiving new reports of flooding and our modelling assessment predicts many more properties are at risk of sewer flooding in these boroughs. In some areas customers have been flooded up to six times in recent years. There is a large cluster of flooded properties that sit within the boundaries of both boroughs and the properties share many similar characteristics. They are generally located in low lying areas, where there are no local water courses and there is a very high density of properties with basements. The storms of 2007 After the storms of 2007, which severely tested our sewerage network, we completed a detailed review of the flooding that affected many local properties in the Counters Creek area. The catchment is large and served by a number of interconnecting sewer systems developed over many years. There are approximately 44,000 properties with basements in the Counters Creek area, of which some 30,000 lie within the London Borough of Hammersmith and Fulham and the Royal Borough of Kensington and Chelsea. Many of these basements were converted following the construction of the Thames Barrier which reduced the risk of river flooding in the early 1980s. Key factors contributing to sewer flooding There are several key factors contributing to sewer flooding in the Counters Creek area which make it unique in the Thames Water region. A significant proportion of properties have basements. The basements were generally built down to the same level as the sewers. The area has the highest density of properties with basements per square kilometre in the UK and basement developments are continuing. The catchment has lost a great deal of open space over the last 40 years and now has a high coverage of impermeable surfaces (e.g. buildings and roads), resulting in more rainwater entering the sewer network. There are no other local watercourses in the area to divert surface water so all storm waters have to be taken away by the sewer system. The entire sewer network in the Counters Creek catchment takes both wastewater from properties and rainwater water, making it prone to flooding following heavy rain. 2 Counters Creek storm relief sewer
Investigating the extent of flooding and underlying causes We have learned a significant amount about the sewerage network in the Counters Creek catchment since the widespread flooding in July 2007. Since then, we have completed detailed research of both the number of customers affected and the underlying causes. The purpose of this research was to: Identify the number of properties affected by sewer flooding Understand the causes of flooding to customers basements Identify alternative solutions that reduce the risk of flooding more sustainably for the future Design a programme of work that offers the most protection to customers, but which is still affordable We have completed over 3,000 door-to-door surveys taking physical measurements of basements and sewer level and ground level assessments. Sewer Figure 1: How basement flooding occurs. Main causes of flooding Top floor Ground floor Basement We identified the main causes of flooding to be: a. Deep basements: Some property basement levels were found to be very close or at the same level as the sewer. So, as soon as the flow levels rise during heavy rainfall, the flows back up the connection pipework into the basement areas, as shown in Figure 1. b. Inadequate capacity in the sewers: This causes water levels within the sewer network to rise when it rains. This rise can result in flows backing up the connection pipework serving the properties and then into the basement areas. c. Large flows from the upper catchment: the deep storm relief sewers fill up following rainfall from the flows coming from the upper areas of the catchment, leaving insufficient capacity to relieve the sewer network in the lower catchment (Hammersmith and Fulham and Kensington and Chelsea) Independent research In 2008 we began our investigation to understand the extent of basement sewer flooding in the Counters Creek catchment area of west London. We established an Independent Advisory Group of industry experts to assess and scrutinise our work to investigate and develop a sustainable and long-term solution. The group concluded that a combination of measures was necessary to alleviate flood risk in the Counters Creek catchment. We have engaged with the Independent Advisory Group throughout the development of our proposals and the group is supportive of our approach and the proposed solution of constructing a storm relief sewer. Appraisal of initial options We assessed the following options in terms of effectiveness and cost: Isolation Temporarily disconnect the property from the sewer network during storms (e.g. Flooding Local Improvement Programme (FLIP) devices) Flow transfer Divert flow to another part of the system with capacity System optimisation Real time control of sewer levels Storage Where the storm flows are stored in a large underground chamber and pumped back into the existing sewer system after storm has passed Upsizing Replacing the existing overloaded sewers with larger diameter pipework Counters Creek storm relief sewer 3
Overflow Construction of chambers on the existing overloaded sewers to allow the excess flows to overflow into the River Thames Sustainable Drainage Systems (SuDS) Permeable paving, water butts etc. This process highlighted where certain options would not work locally (e.g. they moved the flooding elsewhere), but would still be effective at a regional level. In addition to the hydraulic performance of each option, we considered how viable they would be in terms of construction practicalities and their associated costs. A key part of the process was the investigation of the potential for sustainable drainage systems (SuDS) across the Counters Creek catchment to reduce surface water runoff and flood risk. Using a Geographical Information System (GIS) assessment, we identified different impermeable areas (such as roofs, driveways, roads, car parks). We assessed the potential for a range of different SuDS measures, including diversion of downpipes to gardens, diversion of highway run-off to natural land, green roofs, permeable paving and water butts. The range of options considered demonstrated that a number of the options proved to be very expensive to reduce sewer flooding risk and no single solution could deal with the problem by itself. In addition, we realised that we simply could not help a number of properties without using a catchment-wide solution to intercept and transfer storm flows. We therefore investigated the construction of a new storm relief sewer. The proposed solutions Through this work we determined that the most cost beneficial solution would be a programme of flood alleviation measures including the following: Sustainable drainage systems (SuDS) We are piloting different sustainable drainage systems in three streets locally, with the aim of slowing down the rate at which rainwater enters the sewer network when it rains heavily. This involves installing permeable road surfaces and underground rainwater storage. Hydraulic investigations have shown that the volumes of storm flows in the sewer network are so significant that reasonable application of SuDS would not resolve the flooding problem. However, we agree that SuDS should be developed and encouraged wherever possible and beneficial, through collaborative working between all the stakeholders involved, in order to reduce the risk of future increases in impermeable areas. We have not trialled the retrofitting of SuDS solutions into densely populated urban areas before. We will use key outputs from these SuDS pilots, such as how well the different types of SuDS hold back rainwater from entering the sewers under different rainfall conditions, to inform future SuDS schemes across our region. A programme of local sewer improvements For some of the sewer flooding problems in this area a local sewer enhancement is required. We need to improve a number of smaller local sewers in the area to ensure they have capacity to cope with heavy rainfall. Once we have a proposed work programme we will consult with the communities that may be affected. Installation of anti-flooding devices Installation of Flooding Local Improvement Programme (FLIP) devices at some of the properties that have suffered sewer flooding. These devices are designed to pump sewage and rainwater from the drains in basements to the local sewers in the road, and prevent sewage from the sewer entering the property. Counters Creek storm relief sewer To remove one of the key underlying causes of flooding, we determined that flows in the existing larger storm relief sewers and trunk sewers had to be collected at key pinch points to allow the existing systems to drain freely. We identified that a strategic, catchment wide solution was needed to pick up, drain and transport the storm flows away from the clusters of properties at risk of flooding. This ability to collect and transfer storm flows is a key element of any overall solution. The other solutions, on their own or in combination, will not enable us to tackle the sewer flooding problem. This established the need for a new Counters Creek storm relief sewer as part of the above package of measures to alleviate sewer flooding within the catchment. Counters Creek storm relief sewer 4
Benefits of the Counters Creek storm relief sewer By constructing the new storm relief sewer and lowering the flow levels during storms, we would be able to provide relief to the existing sewers. By transferring flows to the new storm relief sewer, we would remove existing hydraulic constraints or pinch points and reduce the risk of flooding for a large number of properties. In the future, both climate change with the potential to impact rainfall rates, and new development with the potential to increase the amount of foul water entering the drainage system, could increase the risk of sewers overloading. To address these we are actively working with councils and developers to maximise the potential for new development to incorporate sustainable urban drainage. We are also constructing pilot schemes across London to retrofit sustainable drainage in an urban environment and then extending the lessons learned to retrofitting sustainable drainage into existing developments where possible. These measures are aimed to not only address climate change by reducing the rainwater entering the sewer network, but create additional capacity for foul flows from future developments. It is essential that we fulfil our statutory duty and ensure that the sewerage system operates effectively. By introducing the storm relief sewer, alongside our wider flood alleviation work and sustainable drainage, we will be putting in place a sewer network that will be fit for purpose for years to come. Shepherd s Bush Westfield Shopping Centre 5 Stamford Brook CHISWICK Ravenscourt Park Goldhawk Road HAMMERSMITH 4 3 Olympia London West Kensington Barons Court HAMMERSMITH AND FULHAM KENSINGTON AND CHELSEA 2 Earls Court Earls Court Exhibiton Centre High Street Kensington KENSINGTON Gloucester Road South Kensington Proposed storm relief sewer 1 Cremorne Wharf Depot, Lots Road, Chelsea 2 Land adjacent to Talgarth Road and Trevanion Road, Hammersmith 3 Maclise Road car park and woodland, adjacent to Kensington BARNES Olympia Station 4 A section of the car park at Tesco Superstore, Shepherd s Bush Road 5 Holland Villas Road at its junction with Lower Addison Gardens Parsons Green Fulham Broadway Stamford Bridge 1 Counters Creek storm relief sewer 5
Related documents The following documents are available at our drop-in sessions and to download from the document library on our website - www.thameswater.co.uk/counterscreek Phase 1 consultation and engagement process Feedback from our phase 1 consultation and engagement process Phase 1 Consultation and Engagement Process Feedback Report Project information Being a good neighbour Environment Getting planning permission Have your say How we chose our preferred sites How we would build the proposed storm relief sewer Transport Timing Tunnel drive strategy Site information Cremorne Wharf Depot, Lots Road, Chelsea Land adjacent to Talgarth Road and Trevanion Road, Hammersmith Maclise Road car park and woodland adjacent to Kensington Olympia Station Holland Villas Road at its junction with Lower Addison Gardens Tesco Superstore car park, Shepherd s Bush Road Site Suitability Reports Cremorne Wharf Depot, Lots Road, Chelsea Land adjacent to Talgarth Road and Trevanion Road, Hammersmith Maclise Road car park and woodland adjacent to Kensington Olympia Station Holland Villas Road at its junction with Lower Addison Gardens Tesco Superstore car park, Shepherd s Bush Road Project contact details: Web: www.thameswater.co.uk/counterscreek Email: consultations@thameswater.co.uk Phone: 0800 316 9800, option 2 Ref: BB968577 Postal address: Counters Creek sewer flooding alleviation scheme consultation, Freepost RTHE-CJYYCXCJ, 7 St Peter Street, Winchester SO23 8BW