Bourton-on -the-Water Drainage Strategy Stage 1 - Initialise/Prepare Introduction
Why sewer flooding Britain’s first sewerage systems were challenges arising in our predominantly This document contains: constructed 150 years ago in the rural catchments in the Thames Valley, Victorian era, and have served us well for Surrey and Kent. • an Introduction to the work generations. The sewer network Thames we are undertaking to Water operates today has been much A number of factors including population alleviate sewer flooding in improved and vastly extended over the growth, less frequent but heavier rainfall, years; yet it remains under increasing the urbanisation of green spaces and our region pressure. changes in agricultural land practices, • a Feedback: Q&A section and utilised machinery, occasionally addressing key questions Everyday our network manages the overwhelm our sewer network. The result from customers and demands of one of the world’s busiest can be unwanted sewer flooding for stakeholders and most densely populated capital customers and our neighbouring natural • the Bourton-on-the-Water cities, and its urbanised surrounding environments. areas; together with the equally complex Drainage Strategy technical document. What can be done and when? Sewer flooding is unacceptable. We have to produce a drainage strategy for our undertaken extensive customer research affected catchments with a primary focus and initiated a programme of work to on our sewerage network. As outlined improve drainage and alleviate sewer in Figure 1, the strategies will develop flooding issues across our region. throughout the 4-stage framework to define how we intend to alleviate sewer We are adopting the good practice flooding or to address growth related Drainage Strategy framework* developed issues in each area sustainably, and by the Environment Agency and Ofwat, economically, over the next few years. the water industry economic regulator, Figure 1 High-level Drainage Strategy framework* and estimated delivery and intervention timeline** From 2018 2015 - 2017 onwards
We are Stage 1 Stage 2 Stage 3 Stage 4 here Initialise/ Risk Options Intervention prepare assesment appraisal
Implementation of Long-term capital 'quick-win' operational investment solutions as appropriate interventions * The detailed 4-stage Drainage Strategy framework can be found in the following Drainage Strategy document. ** The estimated delivery timeline is dependent on factors including weather conditions and is, therefore, open to change. The intervention timeline includes the implementation of ‘quick-win’ operational solutions throughout Stage 2 & 3, and long-term capital investment interventions in Stage 4.
Bourton-on-the-Water Drainage Strategy - Stage 1 1 Published version post stakeholder review v1.1 Who will resolve the sewer flooding?
There are a number of stakeholders land and highways, utilising appropriate who, like us, have important drainage agricultural practices and maintaining responsibilities and therefore, play an private drains. essential role in resolving sewer flooding in our region. These stakeholders include We take full responsibility for resolving all customers, private land owners and the drainage and sewer flooding issues that Environment Agency; to name but a few. fall within our remit. If the causes of sewer We are seeking to work in partnership flooding sit outside of this, we will support with all stakeholders to ensure that the responsible stakeholder to resolve together, we implement and maintain the the issue and to reduce the impact on most effective and sustainable drainage customers. strategies. Figure 2 provides a high-level view of the Just as our responsibilities to improve stakeholders responsible for drainage drainage and alleviate sewer flooding in each catchment area, more detail focus on removing and treating regarding responsibilities can be found wastewater; other stakeholders’ in Section 1 of the following Drainage responsibilities include managing local Strategy document. flood risk on riverbanks, ground water,
Figure 2 Partners with Drainage Responsibilities
Thames Customers Water
Land and Council/ Responsible Riparian Highways Drainage Owners England Partners
Council/Lead Environment Local Flood Agency Authority
Bourton-on-the-Water Drainage Strategy - Stage 1 2 Published version post stakeholder review v1.1 Next steps
Over the coming months we will undertake the following activities as this drainage strategy develops: We will regularly consult • Continue to consult with customers with customers and 2015 - 2017 during this stage and every stage, stakeholders, update and through meetings, communications republish this document and surveys. We have gained important customer feedback during this first throughout this 4-stage Customer stage, which we are using to shape our framework process. Focus Actions activities • Publish strategy documents for comment and contribution, throughout this framework process.
Partnership • Continue to establish partnership Actions working with the regional drainage stakeholders, and agree ongoing consultation processes.
• Define catchment flooding Other Key uncertainties Actions • Prepare flooding risk data • Undertake ongoing repair work to our sewer network, as the strategy develops and our investigations identify reparation work linked to drainage and flooding issues.
Bourton-on-the-Water Drainage Strategy - Stage 1 3 Published version post stakeholder review v1.1 Feedback: Q&A Your questions answered
We are committed to listening to, consulting and collaborating with customers and stakeholders on our sewerage network activities and plans. We have addressed key feedback and questions raised by customers and stakeholders in the Bourton-on-the-Water catchment, and customers affected by flooding throughout the region, in this Feedback Q&A section. As far as possible, and as is relevant to Stage 1 in the framework process, we have incorporated feedback from customers and stakeholders into our network strategy development. More detail can be found throughout the following Drainage Strategy document.
Q1 Will undertaking this Drainage Strategy framework We are committed to process defer essential work in our area? listening to, consulting and Answer collaborating with customers It is essential for us to complete this in Section 7 of the Drainage Strategy and stakeholders on our sewerage drainage strategy process, which has document. Our previous investigations network activities and plans. been developed and recommended by have identified some sources of inflow, the water industry economic regulator such as the misconnection of surface Ofwat, and the Environment Agency. This water to foul sewers. However, as stated will enable us to better understand the in the following strategy, we are not root cause of the sewer flooding issues yet able to say how much this impacts affecting the catchment, before any major on flooding and will update this as our investment can be considered. However, plan develops through Stage 2 to Stage we may carry out some repair works as this 3 –Option Appraisal. Therefore, this more strategy develops, in the event that our detailed approach is required to ensure investigations identify faults or problems that the most effective and sustainable with the sewerage network that are highly drainage strategies are implemented in likely to have caused flooding, as outlined the Bourton-on-the-Water catchment. Q2 What drainage actions are you undertaking in our area, and when will they be happening? Answer As stated above, we are developing our 5. CCTV investigations plan for this area and will provide further 6. Sewer repairs details as our plan develops through Stage 7. Sewer flooding clean-up 2 to Stage 3 – Option Appraisal. The 8. Manhole ‘Lift and Look’ surveys following drainage actions have recently 9. Depth monitor installation been undertaken, or are underway, in the 10. Repairs to rising main. Bourton-on-the-Water catchment: Actions underway or planned include: Actions completed include: (For more (For more information please see Table 4) information please see Table 2) 1. Permanent monitoring of sewer levels 1. Pump replacement 2. Permanent flow monitoring of 2. Maintenance of flows using tankering pumping station 3. Sewer cleaning, descales and blockage 3. Customer surveys clearing 4. Misconnection surveys 4. Manhole sealing 5. Consider innovative solutions.
Bourton-on-the-Water Drainage Strategy - Stage 1 4 Published version post stakeholder review v1.1 Q3 Are you renovating the sewers in our area? Answer We will renovate sewers which have been improvements to the sewerage network damaged, either as they have aged or in the Bourton-on-the-Water catchment. through other streetworks activities. Our As our strategy work continues, we will CCTV investigations and manhole ‘Lift target and repair localised sewer defects and Look’ surveys sought to identify identified through our investigations as possible defects and assess the structural contributing to, or causing, drainage and and service grade of the sewer network. flooding issues in the catchment over the As per Question 2 above, Table 4 in the coming months. drainage strategy sets out the proposed Q4 What are the improvement plans for Bourton-on-the-Water’s sewage treatment works to manage capacity? Answer The Bourton-on-the-Water sewage The capacities of the sewage treatment treatment works operates a fully- works has been assessed to be more compliant permanent storm overflow than adequate under normal design flow which permits us to discharge into local conditions. Therefore, we do not plan to watercourses during storm events. . To enhance the treatment capability of the meet changing performance requirements sewage treatment works at this time. and regulatory measures the works has undergone upgrades over the years. Q5 How are you planning for future development in the catchment? Answer As per Section 5.3 in the following We will continue to work with all involved Drainage Strategy document, we will stakeholders through our stakeholder continue to closely monitor development engagement activities, to monitor local applications in the catchment and assess plans and planning applications and the impact that they may have on the to incorporate current and projected capacity of our operations in the future. developments into our business planning Our existing sewage treatment works cycle, to ensure that our service is currently has the capacity to manage maintained for customers throughout the all of the development applications catchment’s development. submitted and projected for the catchment, as outlined in the latest Cotswold District Council Development Plan.
Bourton-on-the-Water Drainage Strategy - Stage 1 5 Published version post stakeholder review v1.1 Q6 Are you working with the Highway Authority to resolve blocked gullies, sewers and ditches, and with landowners to reduce field run-off, as both affect drainage and our sewers? Answer We have undertaken work to clean, As Highways maintenance activities and descale and remove blockages from agricultural land maintenance practices sections of the Bourton-on-the-Water sit outside of our remit we will work with sewerage network as the needs were the responsible stakeholders, to highlight identified. However, the customer these issues where this is found to have feedback we have received specifically a major influence and impact on our regarding sewer maintenance by the sewerage network. “Highways” and to agricultural practices by landowners, relates to highway drainage We will continue to work closely with and to land drainage issues rather than to the Council and Highways England to foul sewers maintenance. understand the extent to which flood waters may be escaping from highway In Section 1 of the following Drainage or land drainage systems; and impacting Strategy document, we outline the other the foul sewer network. An update on stakeholders who, like us, have important this issue will be shared with customers drainage responsibilities and therefore, and featured in the later stages of this play an essential role in resolving sewer Drainage Strategy document. flooding issues in this catchment area.
Q7 Is an Infiltration Reduction Plan (IRP), required for this catchment? Answer Ensuring that our drainage strategies fully Following extreme weather conditions meet the requirements of an Infiltration during the winters of 2012/13 and Reduction Plan, as set out in the 2013/14 tankering was utilised in the Environment Agency’s Regulatory Position Bourton-on- the-Water catchment, Statement, is a fundamental consideration however temporary pumps were not used. in their development. Therefore, to In the event that temporary overflows are maintain our service to customers during required, as stated above, we will describe future wet weather events, if we need their location and the circumstances under to discharge to watercourses through which we would use them. Together with temporary overflows, a dedicated section plans to reduce infiltration, this drainage will be included in each affected strategy, strategy would then fully meet the providing details of their location and requirements of an Infiltration Reduction intended use. This section will be included Plan. and /or revised when each strategy is updated.
Bourton-on-the-Water Drainage Strategy - Stage 1 6 Published version post stakeholder review v1.1 Q8 Why are costs a consideration within your Wastewater outcomes? Answer The service we provide is the most As a regulated company we have to fundamental of all – at the heart of daily carefully balance service and cost in order life for the 15 million customers we serve. to keep customer bills affordable, whilst Getting it right is our focus every day, and delivering our outcomes and customer we never forget it is paid for by customers. service commitments.
Q9 How are you ensuring that our local pumping stations are operating effectively? Answer The pumping stations in the catchment As we move through this 4-stage are supported by 24 hour diagnostic framework process and further develop monitoring so that we can tightly control our drainage strategy for this catchment, its operation through a flow of real-time we will review the operational control information. Based on this performance options of this station, particularly during data we can respond quickly through wet weather, carefully avoiding increasing site visits by our engineers, and both the risk of sewer flooding in doing so. project and rectify potential issues before they occur. The pumps at the principal Bourton-on-the-Water pumping station were replaced with more modern pumps in 2012.
Q10 Are growth and urban creep minor factors in these rural catchments? Answer When compared against the rest of the applications. Similarly urban creep, and Thames Water Operational Area, the in particular misconnection of surface urban creep rate for Bourton-on-the-Water water and change of land use, can have is just above average, but not as high as a significant impact on sewer flooding; suburban areas around central London particularly when permeable areas such and major towns. as grass are replaced with hard-standings and driveways. Relatively small population increases in these smaller rural catchments can More growth and urban creep information be influential on sewer flows, hence the can be found in Section 5.1 of the need for us to closely monitor planning following Drainage Strategy document.
Bourton-on-the-Water Drainage Strategy - Stage 1 7 Published version post stakeholder review v1.1 Q11 Are best practice techniques already used by other water companies being considered? Answer We are constantly reviewing and We are deploying industry best practice improving our business to meet and techniques throughout our Drainage exceed industry standards, to implement Strategy work, and also trialling new best practice and to drive innovation. technology that is innovative within our We lead and participate in a large industry, to achieve the best possible number of industry forums both in the drainage outcomes for customers and UK and worldwide, to share and expand their local environment. our learning; with the ultimate aim of improving services for customers.
Q12 Why are you collecting climate change data rather than ‘climate proofing’ assets? Answer We are committed to responding to continue to assess and collect climate climate change and to reducing our change data and its impact on assets contribution to it by reducing emissions across our region, to ensure that we are in accordance with government policy. fully informed and can prioritise our plans, Our voluntary target is to achieve a targeted actions and investments. For challenging 20 per cent reduction in more information please see the Climate emissions (compared to 1990 levels), Change section on the Homepage of our for our Scope 1 and 2 emissions*. We website.
*Scope 1 emissions refer to greenhouse gas emissions associated with the operation of our assets. Scope 2 emissions are emissions associated with the use of grid electricity.
Q13 What is the impact on local rivers of overflow points? Answer During extreme weather conditions river levels are high, and therefore sewage foul sewers may become overwhelmed dilution rates are also high. Additionally, through a combination of surface water or we are also investigating deploying mobile ground water, resulting in a much diluted biological filters and screening to prevent sewage. The impact on local rivers is litter and other matter from entering dependent on the nature and size of the local rivers. If during the development of river, and on the overflow. our drainage strategy we consider that temporary overflow points are necessary To reduce the environmental impact in the local network, we will update the on local watercourses we will only use Drainage Strategy document to reflect this overflow points when groundwater and position.
Bourton-on-the-Water Drainage Strategy - Stage 1 8 Published version post stakeholder review v1.1 Bourton-on -the-Water Drainage Strategy Technical Document Stage 1: Initialise / Prepare
Table of Contents
About this document 11 Executive summary 13 1 Thames Water and drainage 15 1.1 Our statutory responsibilities 15 1.2 Working in partnership with other stakeholders 16 2 Catchment description 18 2.1 Geology and topography 18 2.2 Sewage treatment works 18 2.3 Foul sewers 19 2.4 Surface water sewers 21 3 Long-term outcomes 22 3.1 Asset health 23 3.2 Properties and public areas protected from flooding 24 3.3 River water quality meets customers’ expectations 24 and regulatory requirements 4 Current issues 25 4.1 Recent wet weather events 25 4.2 Our operational response 27 4.3 Investigations and activities completed to date 29 4.4 Actions carried out by drainage partners 31 5 Future challenges 32 5.1 Urban creep 32 5.2 Climate change 33 5.3 Population growth and new development 34 6 Strategy development 36 7 Preferred strategy and plan 37 8 Temporary overflows 38 Appendix A – Glossary of terms 39 Appendix B – Supporting figures and photographs 40 List of Tables
Table 1 Wastewater outcomes 22 Table 2 Investigations and activities completed 29 Table 3 Actions by other stakeholders to prevent flooding 31 Table 4 Activities planned and ongoing to enable strategy development 36 Table 5 Activities identified in preferred plan to date 37 List of Figures
Figure 1 The Drainage Strategy Framework 11 Figure 2 Bourton-on-the-Water priority sub-catchments 14 Figure 3 Stakeholder responsibilities for drainage 16 Figure 4 Bourton-on-the Water foul sewerage catchment schematic 20 Figure 5 Bourton-on-the-Water key performance issues 26 Figure 6 Bourton-on-the-Water sewage treatment works treated flows and River Windrush flows 28 Figure 7 Urban creep rates in the Thames Water region 32 Figure 8 Locations assessed for increased rainfall intensity by 2080 33
Bourton-on-the-Water Drainage Strategy - Stage 1 10 Published version post stakeholder review v1.1 About this document
Based on customer research Undertaking extensive customer research The water industry economic regulator, on alleviating sewer flooding issues within has been a fundamental step in our Ofwat, defines ‘outcomes’ as “High-level our region, through effective, economic business plan preparation for 2015-20. objectives that company actions, activities and sustainable drainage. This document Our research findings have informed and achievements are intended to help describes the strategy that we will follow our business planning activities, and deliver..[they] represent what customers in delivering our long-term customer contributed to the development of a set of and society value”. As a company, we are outcomes for drainage, specifically in the long-term customer ‘outcomes’. committed to achieving our customer Bourton-on-the-Water catchment, in a outcomes, a number of which are focussed sustainable and economic manner.
Approved approach We have adopted the Drainage Strategy focus on the sewerage network, and not the activities that we plan to undertake Framework1 outlined in Figure 1 below, the performance of sewage treatment to address current issues and future developed by the Environment Agency works. The Bourton-on-the-Water drainage challenges facing the catchment, and the and Ofwat. It identifies 4 key stages to strategy is currently at the first stage of data that we need to gather to complete producing a good-practice drainage this framework - the Initialise/Prepare the risk assessment and options appraisal strategy. Drainage strategies typically stage. In this document, we describe stages.
Figure 1 The Drainage Strategy Framework
Initialise Risk Options Intervention /prepare assesment appraisal Establish Aligned partnership & Consulatation Consult on delivery & consulatation on risk issues options collaborative process solutions
Define Quantify Plan for Innovative & uncertainties uncertainties uncertainties sustainable
Prepare Demonstrate Live & risk data Quantify whole life risks cost & benefit visiable
Partnership Uncertainty Risk based Whole life cost Live Innovative & & benefits process sustainable
1 http://www.ofwat.gov.uk/future/sustainable/drainage/rpt_com201305drainagestrategy.pdf
Bourton-on-the-Water Drainage Strategy - Stage 1 11 Published version post stakeholder review v1.1 Consultation and publication We will update and republish this fieldwork has been collected and analysed. customers and stakeholders. We will also document to provide the results of our make the Drainage Strategy documents risk assessment, options appraisal and our Throughout this process we will available on the Drainage Strategies selected strategy for intervention, once attend local flood forums for ongoing webpage of our website. data from instrumentation and other communication and consultation with Meeting the Infiltration Reduction Plan (IRP) To ensure that this Drainage Strategy groundwater surcharged sewers, we have over time, where it has been identified meets the requirements of an Infiltration included a section in this document which as a root cause of sewer flooding. Please Reduction Plan, as set out in the defines if, how and when we propose to see Section 8 in the Drainage Strategy Environment Agency’s Regulatory Position operate temporary overflows. This is in document below. Statement on discharges made from addition to our plans to reduce infiltration
Bourton-on-the-Water Drainage Strategy - Stage 1 11 Bourton-on-the-Water Drainage Strategy - Stage 1 12 Published version post stakeholder review v1.1 Published version post stakeholder review v1.1 Executive summary
In recent years the foul sewerage system in authorities (RMAs), to cooperate regarding We have therefore developed a set of the Bourton-on-the-Water catchment has flood risk. In our role as a RMA, Thames company outcomes that we are committed become overwhelmed in some locations, Water will work with Gloucestershire County to working towards over the next 5 years following prolonged and heavy rainfall Council as lead local flood authority, and beyond. The outcomes relevant to the and raised groundwater levels. This has Cotswold District Council as Planning Bourton-on-the-Water drainage strategy resulted in certain properties suffering from Authority and the Environment Agency to are: sewer flooding and restricted toilet use. The ensure that a collaborative approach can sewerage system is identified on the public be developed to address the problems. • Asset health - a composite range sewer records as being a separate foul of measures against which we will system, rather than a combined system, In response, this Drainage Strategy follows manage the health of our sewerage and should therefore only be accepting foul the Environment Agency and Ofwat’s network drainage rather than the combination of 4-stage framework. The Bourton-on-the- • Properties and public areas protected foul and surface water, however, there are Water strategy is currently at Stage 1 from sewer flooding a limited number of public surface water (Initialise/Prepare). We describe in this • River water quality meets customers’ sewers in the area. document the actions that we plan to carry expectations and regulatory out to complete the risk assessment and requirements. We believe that the foul system has options appraisal stages. We will update surcharged because of a combination of and republish this document once this work This drainage strategy must also address groundwater infiltration, surface water has been completed. future challenges to the Bourton-on-the- run-off from saturated fields, surface water Water catchment. We assess these to be: inundation from highways and public In preparing our company business spaces, surface water misconnections and plan for the 5 year period 2015 to 2020 • Climate change – analysis of the flooding from the River Windrush. we have listened very carefully to the latest data suggests that rainfall could views of customers. Beyond being able become 15% more intensive by 2080 The root causes of sewer surcharges are to maintain the current service that we increasing the likelihood of flooding. therefore numerous and the resolution provide, customers have told us that they Longer wetter winters may also mean of the issues complex, requiring all would like to see a reduction in instances groundwater levels are high more stakeholders responsible for drainage in of sewer flooding and odour nuisance and often; this could also exacerbate fluvial the catchment to work together to resolve an improvement in river water quality. flooding from local watercourses them. The Floods and Water Management Our research indicates that customers are • Urban creep – paving over of front Act 2010 places a responsibility on lead willing to pay for these improvements to gardens and loss of green space results local flood authorities (LLFAs), to manage service ; a summary of our related customer in more strain on the sewerage network flood risk from surface and groundwater, research can be found on our website via when it rains heavily. Modelling we plus a duty on all risk management the hyperlink below. have undertaken suggests urban creep
2 http://www.thameswater.co.uk/cr/Howwedobusiness/Engagingwithourstakeholders/Publicconsultationresearch/index.html
Bourton-on-the-Water Drainage Strategy - Stage 1 13 Published version post stakeholder review v1.1 rates in Bourton-on-the-Water are Figure 2 Bourton-on-the-Water priority sub-catchments just above average for the Thames Operational Area • Population growth – the population in the South East is set to grow rapidly. A number of possible developments are identified in Bourton-on-the-Water and we will continue to track these and any other emerging applications for this catchment arising in the future.
Our strategy is to understand the relative impact on this catchment of overland flow from saturated fields, groundwater infiltration and surface water misconnections and how this effects the performance that our network has in respect of flooding. We will then try to identify cost beneficial solutions to reduce the risk of sewer flooding using customer willingness to pay research. We may carry out sewer rehabilitation works as the strategy develops in the event that our investigations identify faults or problems with the sewerage network that are highly likely to have contributed to flooding.
Our next steps are to continue to collect real-time flow information from the permanent depth monitors we installed at the end of last year, and to collect sewer flooding information from customers. The depth monitors will remain in place as The extent of the catchment is outline in red. we move through this 4-stage framework and develop our plans. The priority sub- catchments we are monitoring are outlined in Figure 2.
Bourton-on-the-Water Drainage Strategy - Stage 1 14 Published version post stakeholder review v1.1 1 Thames Water and drainage
1.1 Our statutory responsibilities
Thames Water is a regulated Water and (Ofwat). The original 1991 Act has been legislation, including European Directives. Sewerage Company. We supply water to amended by further legislation in recent The Water Framework Directive 9 million customers in London and the years, transferring some drains and sewers establishes a strategic approach to Thames Valley and provide wastewater that were hitherto in private ownership to managing the water environment, services to 15 million customers across an Thames Water’s responsibility3. which the Environment Agency achieves area that stretches from Gloucestershire through River Basin Management Plans to Essex. We operate 108,000km of sewer Other recent pieces of legislation relevant and setting environmental objectives through which an average of more than to this Drainage Strategy are the Flood for groundwater and surface water. 4.4bn litres of wastewater is collected & Water Management Act (2010) and The environment is also protected and treated every day at our 350 sewage the Water Act (2014). These set out new from adverse effects of discharges of treatment works. responsibilities for Thames Water to urban wastewater through the Urban manage flood risk in partnership with local Wastewater Treatment Directive, which The primary legislation that sets out councils and the Environment Agency, requires us to improve and extend the our role and responsibilities is the Water with more emphasis on Sustainable sewerage system according to section 94 Industry Act (1991), which describes the Drainage Systems (SuDS), such as swales of the Water Industry Act (1991). duties and services that we are responsible and permeable paving to mimic natural for and the powers that we have to drainage. A comprehensive and detailed list of all connect, operate, maintain and extend legislation relevant to Thames Water can the sewerage network. We are regulated Thames Water also has a statutory be found in the ‘statement of obligations’ by the Water Services Regulation Authority obligation to comply with environmental published by Defra4.
3 See http://www.thameswater.co.uk/help-and-advice/8654.htm for more information. 4 See https://www.gov.uk/government/publications/statement-of-obligations.
Bourton-on-the-Water Drainage Strategy - Stage 1 15 Published version post stakeholder review v1.1 1.2 Working in partnership with other stakeholders
Other stakeholders responsible for managing various forms of drainage need to work together with us to reduce the risk of flooding. Each has specific responsibilities as summarised in Figure 3 below.
Figure 3 Stakeholder responsibilities for drainage
Surface and ground Private surface water water flooding; and foul drains: Lead Local Flood Customers Highways flooding; Authority and Highway Authority land owners Highways England River flooding: Riparian owners and the Environment Agency
Surface water sewers: Thames Water Foul water sewers: Thames Water
Bourton-on-the-Water Drainage Strategy - Stage 1 16 Published version post stakeholder review v1.1 Thames Water responsibility under the Flood & Water Customers We are responsible for removing and Management Act for managing the local Customers own, and are responsible for, treating wastewater which includes flood risk from groundwater and surface the maintenance of private drains within the foul sewers, and in some areas the water runoff e.g. local watercourses and the curtilage of their property, which did 6 combined sewers that are in some of culverts . They work with landowners not transfer to Thames Water ownership in the older large urbanised areas such as to maintain privately owned ditches, October 20117. London5. We also manage and maintain drainage and watercourses, keeping them clear of blockages. They are surface water sewers where they exist, Riparian Owners these will typically discharge to a also responsible for managing the Riparian Owners are landowners who own watercourse or river. In some cases, the risk of groundwater flooding, both land with watercourses or land adjacent cause of sewer flooding may not fall under inside and outside of properties. Water to watercourse (ie road side ditches). our responsibility. In these circumstances, from these local authority gullies and The responsibility for the operation we will explain what we can do to help drains and privately owned ditches can and maintenance of ditches, local and continue supporting the relevant also impact Thames Water’s sewers, watercourses and general land drainage authorities or third parties to reduce the therefore, we work with all responsible lies mostly with riparian owners. impact for our customers. stakeholders to resolve the excess flow. Cotswold District Council is the Planning Authority responsible for approving new Environment Agency development, but equally may have Land Owners The Environment Agency is responsible for responsibility for ensuring maintenance Land owners include farmers and both main rivers and part of its remit includes of watercourses; particularly on council- residential and commercial customers, but monitoring and informing the levels of owned land. includes trusts etc. They are responsible ground and river water. The Environment for ensuring the adequate drainage of Agency also investigates pollution their land, such that it is not a nuisance to incidents and monitors the quality of the Highway Authority others. water in rivers. Gloucestershire County Council is the Highway Authority and is responsible for highway maintenance and highway Lead Local Flood Authority drainage and for clearing roadside gullies. and District Council Highways England is responsible for the Gloucestershire County Council is the drainage of motorways and certain trunk Lead Local Flood Authority and has the roads.
5 Thames Water is responsible for the collection and treatment of commercial and domestic sewage. Typically this will be the foul sewerage. Domestic or commercial roof and paved drainage will often go to a soakaway or directly to a water course/river, which if so is not the responsibility of Thames Water. 6 Some local watercourses and/or culverts are termed as ‘Riparian’ meaning that a land owner, possibly adjoining or owning the land containing the watercourse/culvert is responsible for the maintenance and free-flowing of the watercourse/culvert. 7 See http://www.thameswater.co.uk/help-and-advice/8654.htm for more information.
Bourton-on-the-Water Drainage Strategy - Stage 1 17 Published version post stakeholder review v1.1 2 Catchment description
2.1 Geology and topography
The Bourton-on-the-Water sewerage and mudstone. There is a small area of create localised high groundwater levels catchment is located approximately 20km sand and gravel to the south of Bourton- within the catchment and increase the risk west of Cheltenham and serves Bourton- on-the-Water and a band of clay, silt and of induced infiltration. According to the on-the-Water along with the villages of sand that runs up to the north east. The Environment Agency, the current ecological Clapton-on-the-Hill, Great Rissington, Little geological make-up of this area shows status of the River Dickler, the River Eye and Rissington, Wyck Rissington, Sherborne, mostly highly permeable soils within the the River Windrush is ‘Good’8. Upper Slaughter and Lower Slaughter as catchment areas, and following the River outlined in Figure 2. Windrush, River Eye and River Dikler. The Appendix B includes maps showing surrounding area is composed of clayey the geology and fluvial, pluvial and The geology of the catchment is generally soils with an impermeable layer at shallow groundwater flood risk areas in the made up of limestone, sandstone, siltstone depth. This impermeable layer is likely to catchment.
2.2 Sewage treatment works
The Bourton-on-the-Water sewage to the 1920s and would have been The capacity of the sewage works has treatment works serves the town of constructed by the local authority been assessed to be adequate under Bourton-on-the-Water and the villages incumbent at the time. Foul sewage was normal design flow conditions for the of Lower Slaughter, Upper Slaughter originally treated at a sewage treatment current population. However, given the and Sherbourne via a number of sewage works at the south of the town, near Lower likely growth for the catchment area, pumping stations and gravity-fed sewers. Marsh Farm, but the flows were diverted we will be carrying out a study over the The works serves a population equivalent in the 1950s to the current sewage course of this current Business Plan period of circa 6,430 with a dry weather flow treatment works at the east of the town, (2015 to 2020), to determine how the consent of 3,366 m3/day, but typically via the Bourton-on-the-Water sewage growth may impact on the performance treating a daily flow of up to 7,808m3/day pumping station. The sewerage system of the sewage works. An implementation during wet weather periods. The treated has grown further in the intervening programme will be undertaken based on effluent discharges to the River Dikler and period necessitating enhancements to the resulting recommendations from the then on to the River Windrush. It includes the sewage treatment works to treat the study, should this be required. a storm tank to handle excess flows increased flows. The most recent major above the flow to full treatment during improvement was carried out at the works storms, which returns the flows to the filter in 2001 due to changes in performance distribution chamber for treatment. criteria. The foul sewerage system dates back
8 Environment Agency website, interactive map, Basin Management Plans.
Bourton-on-the-Water Drainage Strategy - Stage 1 17 Bourton-on-the-Water Drainage Strategy - Stage 1 18 Published version post stakeholder review v1.1 Published version post stakeholder review v1.1 2.3 Foul sewers
The catchment is served by a foul water an appropriate gradient to maintain a a number of the sewers in the area are gravity sewerage system, which includes a satisfactory self-cleansing characteristic. cast iron, which is typically used in areas number of sewage pumping stations that The capacity of the sewers is typically with high water tables, as the jointing pump flows from the outlying villages. set to cater for a maximum of six times techniques significantly reduces the risk of The sewerage system is identified on the Dry Weather Flow and a 10% allowance infiltration. Clay pipework has a very long public sewer records as being a separate is included for infiltration8. In terms service life, but sometimes the joint seals foul system, rather than a combined of capacity a 225mm diameter sewer deteriorate over time. system designed to cater for both foul and laid at a gradient of 1 in 225 will have surface water, however, there are a limited sufficient capacity to cater for the foul The earlier pipes were laid on bedding number of public surface water sewers in sewage from 1,000 to 1,500 houses or material such as pea shingle with the the area. 3,000 to 4,500 people. Problems in sewers trenches likely to have been backfilled with smaller than 300mm diameter tend to ‘as dug’ material. More recent drains and The earliest foul sewers in the catchment be a result of blockages in the pipes. sewers, i.e. since the 1980s, are typically are believed to date from the 1920s. However, occasionally surface water can surrounded with pea shingle. This protects Flows were diverted in the 1950s to the be misconnected into the foul sewerage the pipe but also acts as a good conduit current sewage treatment works to the network – problems then arise when it for groundwater. The layout of the village east of the town, via the Bourton-on-the- rains heavily. suggests that most properties are likely to Water sewage pumping station, but land have their own foul drains that connect at the former sewage treatment works In Bourton-on-the-Water, the foul sewers directly into the public sewer. The private is used as a storm irrigation area. The typically range from 150mm to 525mm in foul water drains within the property sewerage system in the intervening period diameter, and include a 900mm diameter boundaries in Bourton-on-the-Water are has grown to serve additional areas and offline storm attenuation tank sewer to the responsibility of the property owners, new developments, including first time the south of Risington Road upstream where they are not shared. sewerage schemes in outlying villages of the Bourton-on-the-Water sewage such as Upper and Lower Slaughter and pumping station. Wyke Rissington constructed by the North Cotswold Rural District Council in Materials used in the construction of the 1970’s. A schematic diagram of the the sewerage system are typical of the catchment is shown in Figure 4 below. time, with vitrified clay pipework for smaller diameters, concrete pipes for Sewer design criteria ensures the larger diameters and brick and concrete appropriate sizing and laying of pipes at manholes. However, surveys indicate that
8 Dry Weather Flow is the term given to the average flow rate observed over a 24 hour period and based on Sewers for Adoption, the industry standard, includes an allowance for infiltration of 10% of the calculated flow rate.
Bourton-on-the-Water Drainage Strategy - Stage 1 19 Published version post stakeholder review v1.1 Figure 4 Bourton-on-the Water foul sewerage catchment schematic
Bourton-on-the-Water Drainage Strategy - Stage 1 19 Bourton-on-the-Water Drainage Strategy - Stage 1 20 Published version post stakeholder review v1.1 Published version post stakeholder review v1.1 2.4 Surface water sewers
There are a few public surface water ditches and minor watercourses that Highway drainage typically discharges to sewers within Bourton-on-the-Water itself. are intended to take surface water from the roadside ditches. Owing to the high The surface water from the majority of roads and public spaces in the area. As local beauty of the area the ditches tend properties is likely to drain to soakaways per Section 1.2 the responsibility for the to be well maintained with clearance of or to the River Windrush. Soakaways operation and maintenance of these vegetation and debris occurring. However, can only function satisfactorily when ditches, local watercourses and general the area does have an active groundwater ground conditions allow soakage and land drainage is mostly with riparian table and as a result groundwater springs may be completely ineffective at times owners. Gloucestershire County Council do occur from time to time, which can when groundwater levels are high. In as lead local flood authority has overall lead to localised land drainage issues. some areas, we have seen examples of responsibility for managing groundwater. The extent of highway drainage is customers draining surface water through The responsibility for the maintenance lies uncertain, but it is likely that highway their foul drains when their soakaways with the riparian owners. surface water run-off discharges directly do not work. This exacerbates capacity to roadside ditches, some of which problems for other customers connected The Environment Agency has the duty will act as soakaways. Gloucestershire further downstream in the sewerage and the authority to ensure that the River County Council is responsible for highway network. Windrush, River Dickler, and the River drainage and culverts crossing the Eye are maintained appropriately. The highway. The rest of the catchment is mostly rural responsibility for maintenance lies with and incorporates a network of roadside the riparian owners.
Bourton-on-the-Water Drainage Strategy - Stage 1 21 Published version post stakeholder review v1.1 3 Long-term outcomes
We have listened very carefully to the views service that we provide, customers have In response to this, we have developed of customers before developing our plan for told us that they would like to see a 4 company outcomes and 11 service the Asset Management Period 6 (AMP6) reduction in instances of sewer flooding outcomes for our wastewater service that regulatory period. Between 2009 and 2013 and odour nuisance and an improvement we are committed to working towards over we carried out over 50 separate customer in river water quality. These are areas where the next 5 years and beyond, further details research and engagement activities. some customers are prepared to see, and can be found in Table 1 below and on our pay for, an improvement in the current level website9. Beyond being able to maintain the current of service.
Table 1 Wastewater outcomes
Company Wastewater service Why is this service outcome outcome outcome chosen We will provide a safe and reliable Asset health: maintaining our assets to We must ensure an appropriate balance wastewater service that complies ensure we can provide a safe and between reducing costs today and not with all necessary standards and reliable service in the long-term. compromising our future service. is available when our customers require it. Properties and public areas protected Flooding is one of the worst service from flooding. failures for customers.
Resilient sewage treatment service We need to be able to provide service that minimises the impact of extreme against a variety of pressures such as events on river water quality. climate change and population growth.
Our customers and stakeholders Do the basics excellently by getting This service outcome ensures our can trust us, we are easy to do things right first time. wholesale activity is completely aligned business with and we care. to our objective to improve our Service Incentive Mechanism (SIM) scoring.
We will provide the level of Reduced dependence on energy from Reducing dependence on energy from customer service our customers the grid. the grid is one of a range of measures require, in the most economic and across our entire plan to keep costs down efficient manner, to ensure that to an affordable level for customers. bills are no more than necessary.
We will limit our impact on the Minimising our carbon footprint. There is an expectation from society that environment and achieve a socially we will play our part in reducing carbon responsible, sustainable business emissions. for future generations, including reducing levels of leakage.
9 See http://www.thameswater.co.uk/tw/common/downloads/about%20us%20-%20corporate%20responsibility/ AMP6_-_Outcomes_Reporting_Policy.pdf for more information.
Bourton-on-the-Water Drainage Strategy - Stage 1 22 Published version post stakeholder review v1.1 Company Wastewater service Why is this service outcome outcome outcome chosen River water quality meets customers’ We must meet environmental expectations and regulatory regulations, and river quality is a visible requirements. indicator to citizens of our environmental stewardship. Satisfactory sludge disposal. Sludge is a resource that we should manage effectively to keep bills down. Corporate responsibility. We will act as a responsible company, meeting expectations from wider society. Reduced odour from wastewater Odour is a problem for some of our operations. customers. We must meet environmental Compliance with new environmental regulations, and river quality is a visible regulations. indicator to citizens of our environmental stewardship.
Below we provide more information about our asset health, properties and public areas protected from flooding and river water quality outcomes, as these are relevant to the Bourton-on-the-Water drainage strategy.
3.1 Asset health
Our Asset health performance service. It includes sewer collapses, commitment encompasses a composite blockages, unconsented category 1 to range of measures against which we 3 pollution incidents and properties will manage the health of our sewerage internally flooded due to operational network. This commitment underpins our problems (such as blockages, collapses or outcome of a safe and reliable wastewater equipment failures).
Bourton-on-the-Water Drainage Strategy - Stage 1 23 Published version post stakeholder review v1.1 3.2 Properties and public areas protected from flooding
There are two performance commitments groundwater levels are high following routing the flow through sustainable that underpin the delivery of this service prolonged periods of wet weather). Our drainage measures such as water butts, outcome: customer research indicates that our permeable paving, rain gardens and sewer flooding programme will deliver green roofs. We aim to retrofit over 1. We commit to protecting properties £20m of benefit to customers every 20 hectares of sustainable drainage from flooding due to rainfall. We year by 2020. measures by 2020. We may also apply estimate that our plan for 2015-20 2. We commit to reducing the risk of this commitment to areas where the will result in over 2,100 properties sewer flooding and pollution from network was designed to take foul flow being alleviated from internal flooding, combined sewers (i.e. those that convey only, but investigation shows that a external flooding and also from both foul and surface water) by slowing substantial amount of surface water is restricted toilet use. (for example when down surface water run-off and re- in the foul sewer.
3.3 River water quality meets customers’ expectations and regulatory requirements
We have a performance commitment equipment and also following heavy to reduce the number of pollution rainfall when our sewers have insufficient incidents as a result of discharges from capacity to cope with the flow. All our sewerage network and treatment pollution incidents are reported to the works. Pollution can occur as a result Environment Agency’s National Incident of blockages, collapses or failure of our Recording System (NIRS).
Bourton-on-the-Water Drainage Strategy - Stage 1 24 Published version post stakeholder review v1.1 4 Current issues
4.1 Recent wet weather events
The foul sewerage system in Bourton- observed with respect to the sewerage During these events, other sources of on-the-Water has become overwhelmed network (see Figure 5 below): flooding have also been observed: for weeks and even months at a time in recent years following prolonged heavy • surcharging sewers causing spills out of • highway drainage overwhelmed rainfall. This has been associated with many public manholes and restricted causing highway flooding significant sewer flooding. Based on site toilet use, including: • the River Windrush overwhelmed reconnaissance that we have carried out • Rissington Road near the Bourton– causing fluvial flooding of properties we believe that the system has surcharged on-the-Water sewage pumping • land drainage issues with water running due to a combination of groundwater station off fields and onto highways. infiltration, surface water run-off from • Roman Way upstream of the saturated fields, surface water inundation Roman Way sewage pumping To reduce the risk of flooding, we from highways and public spaces, surface station understand that some customers have water misconnections and flooding from • Lower Slaughter upstream of the had to pump floodwaters onto the the River Windrush. Lower Slaughter sewage pumping highway or protect their properties with station sandbags. We are confident that this is a • Park Farm. comprehensive list of factors that have • pollution caused by surcharging sewers caused flooding. • burst rising main – Lower Slaughter The following incidents have been sewage pumping station.
Bourton-on-the-Water Drainage Strategy - Stage 1 25 Published version post stakeholder review v1.1 Figure 5 Bourton-on-the-Water key performance issues
1 – Lower Slaughter flooding and RTU 2 – Lower Slaughter rising main bursts 3 – Park Farm 4 – Roman Way 5 – Rissington Road
SPS - Sewage pumping station
Bourton-on-the-Water Drainage Strategy - Stage 1 25 Bourton-on-the-Water Drainage Strategy - Stage 1 26 Published version post stakeholder review v1.1 Published version post stakeholder review v1.1 4.2 Our operational response
We have deployed tankers to Bourton-on- drainage strategy develops. There may be Figure 6 compares the treated flows at the-Water and Lower Slaughter sewage circumstances that might necessitate this Bourton-on-the-Water sewage treatment pumping stations during the winters practice. works with river flows recorded at the of 2012/13 and 2013/14 to manage Environment Agency gauging station in excess flows and maintain service to Even if flows can be contained within the the area. It can be seen from this graph our customers. We acknowledge that sewer network, excessive flows arriving that periods of high flow to the sewage customers find tankering noisy and at the treatment works may not always treatment works correlate well with high disruptive. To date we have not installed be given full treatment prior to discharge flow in the river. However, high river flows temporary pumps to discharge excess to the River Windrush. The use of such do not necessarily mean that infiltration flows to neighbouring watercourses storm sewage overflows, including the levels will be high, because pluvial and during prolonged wet weather, but we will storm treatment land, is accepted by our fluvial flooding will also correlate with high investigate the benefits of doing so as this regulators, subject to conditions. river flows.
Bourton-on-the-Water Drainage Strategy - Stage 1 27 Published version post stakeholder review v1.1 Figure 6 Bourton-on-the-Water sewage treatment works treated flows and River Windrush flows
Bourton-on-the-Water Drainage Strategy - Stage 1 28 Published version post stakeholder review v1.1 4.3 Investigations and activities completed to date
Table 2, below, details the investigations and actions that we have completed in recent years within the Bourton-on-the-Water catchment. These form the extent of our current understanding of issues within the catchment. Table 2 Investigations and activities completed Activity Purpose Date Outcome complete Pump Replacement Bourton-on-the-Water sewage pumping February Maintain ‘asset health’. station, both pumps replaced. 2012 Maintenance of Tankering has been used to maintain levels October 2012 Service maintained during wet weather. flows of service during autumn 2012. Sewer Descale 3 sewer descales carried out at Greenlea March 2013 Improve flow through pipework. between May 2012 and March 2013. Repairs to rising Restore pumping capability of the Lower 2013/2014 Main repaired on 3 occasions. main Slaughter sewage pumping station. Manhole Sealing 2 manholes investigated and sealed at January 2014 Stop infiltration into the manholes. Greenlea on 08/10/2013 and 15/01/2014. CCTV investigations Assess structural and service grade of the February 9 CCTV investigations carried out between sewerage network and if possible detect 2014 June 2012 and February 2014 including two sources of infiltration. cases in Station Road of 240m and 430m to aid planned maintenance. July 2014 240m of main sewer in Rissington Road, near sewage pumping station. September 640m of main sewer in fields to south of 2014 Rissington Road. Sewer Flooding Clean-ups carried out after sewer flooding March 2014 7 clean-up activities undertaken between clean-up events to ensure public health and safety. November 2012 and March 2014 including one large event on Rissington Road, completed 19/03/2014. Sewer Blockages 107 blockages cleared between 04/04/2012 March 2014 Improve flow through pipework and reducing cleared and 22/03/2014 – clearing of blockages as flooding problems. they are reported. Sewer Cleaning 3 sewers cleaned: 100m on Lansdown, 85m March 2014 Maintain ‘asset health’. on Station Road and 144m on Moore Road – all 150mm pipes. Sewer and manhole Ascertain sewer and manhole condition and March 2014 Survey included 5km of sewer in Bourton-on- surveys evidence of infiltration. the-Water and Lower Slaughter.
Bourton-on-the-Water Drainage Strategy - Stage 1 29 Published version post stakeholder review v1.1 Activity Purpose Date Outcome complete Manhole ‘Lift and Undertaken on all accessible public June 2014 No obvious defects identified – one length of Look’ surveys manholes in the village to identify any sewer with higher than expected flows obvious points of inflow. identified for further investigation in wet weather, when infiltration will be observable. Permanent Installation of permanent depth monitors Installed Nov Use the information to identify additional monitoring of sewer into the foul sewers at strategic locations in 2014 actions for inclusion in the drainage strategy levels Bourton-on-the-Water and Lower Slaughter. for Bourton-on-the-Water. Share information Analyse the recorded depths and compare with other agencies. with other catchment variables, such as rainfall events and changes in groundwater levels. Sewer repairs Sewer lining of 165m of 150mm diameter April 2015 Stop infiltration into the public sewers pipework to prevent ingress of groundwater. (private sewers and drains were not included in the scope of the project).
In summary, following previous concerns extent of the surcharging and flooding further survey and investigation is required that the sewerage network suffered subsequently experienced. This may be to identify sources of inflow and their excessive infiltration, considerable action due to our earlier work focussing only resolution. Permanent depth monitors has been taken over a number of years on public sewers within our ownership, were installed in the catchment in 2014, to identify the potential sources. It is and not private sewers and drains, or we will continue to monitor this data as clear that small scale rehabilitation and inundation from surface water ponding or our strategy develops through this 4-stage existing surveys may have been locally misconnected properties. framework process. successful in identifying and preventing some sources of inflow, but they have Survey works have been carried out in not significantly impacted on the the catchment over recent years and
Bourton-on-the-Water Drainage Strategy - Stage 1 30 Published version post stakeholder review v1.1 4.4 Activities carried out by drainage partners
Table 3 below, details the activities carried out by other stakeholders with drainage responsibilities within the Bourton-on-the-Water catchment, to reduce the risk of flooding in the area. For more detail on the other organisations responsible for managing various forms of drainage within the catchment, please see Section 1 of this drainage strategy document. Table 3 Actions by other stakeholders to prevent flooding
Activity Purpose Impact on sewerage
Routine maintenance Ensure free flow of river and ditches. Less risk of surface water flooding and inundation of River Windrush, into the foul sewers and hence less risk of sewer watercourses and local flooding and pollution incidents. ditches. Routine maintenance Ensure adequate surface water drainage from Less risk of surface water flooding and inundation of private surface properties. into the foul sewers and hence less risk of sewer water drainage and flooding and pollution incidents. soakaways. Routine maintenance Ensure adequate highway drainage. Less risk of surface water flooding and inundation of highway drainage. into the foul sewers and hence less risk of sewer flooding and pollution incidents. Routine maintenance Ensure effective land drainage. Less risk of surface water flooding and inundation of land drainage. into the foul sewers and hence less risk of sewer flooding and pollution incidents.
Strategy for infiltration Consider a strategy for reducing infiltration into Less risk of groundwater flooding and infiltration into through private drains*. the sewer network via private drains if the private drains and hence less risk of sewer flooding permanent monitoring identifies this as a and pollution incidents. significant cause for concern. Monitoring and control Local Authority Building Control to ensure private Less risk of groundwater flooding and infiltration into of construction drainage is fit for purpose. private drains and hence less risk of sewer flooding standards for private and pollution incidents. drains. Sharing of information. Agencies to share information to ensure Identification of most cost beneficial solutions and collaborative approach to groundwater quicker resolution of issues. infiltration, surface water inundation, pluvial and fluvial flooding. Use forums as appropriate, i.e. Cotswold Flood Action Group.
*Thames Water does not have powers to compel customers to repair defective private drains at their cost. At this stage, we do not have the data to show how significant infiltration from private drains is within the Bourton-on-the-Water catchment, but we will develop an appropriate strategy when this information becomes available. We note that local authorities are only able to instigate action under Section 59 of the Building Act where evidence is provided of a defective private drain.
Bourton-on-the-Water Drainage Strategy - Stage 1 31 Published version post stakeholder review v1.1 5 Future challenges
In 2011, Ofwat commissioned Mott MacDonald to look at factors likely to affect sewerage networks in the future. The report ‘Future impacts on sewer systems in England and Wales’ (June 2011)10 looked at the likely relative impact of climate change, population growth and impermeable areas up to around 2040. In preparing our plan for 2015-2020, we have also carried out research into these factors across the Thames Water region. We summarise our findings for the Bourton-on-the-Water catchment in this section. 5.1 Urban creep
Urban creep is defined as the carried out a statistical analysis and built compared against the rest of the Thames transformation of a catchment by the a model to predict the rate of urban creep Water region, Bourton-on-the-Water is paving over of previously permeable for the entire Thames Water region, taking just above average, but not as high as areas, and includes extensions to existing account of factors such as property age, suburban areas around central London properties and other land use changes. land use, demographics such as family and major towns. Whilst the immediate Rather than surface water soaking into sizes and financial income, need and issues in Bourton-on-the-Water appear to the ground when it rains heavily, more available space. We found that affluent be strongly related to groundwater and water runs off into the sewerage network suburban areas with detached and semi- pluvial/fluvial inundation, we will continue and can cause the sewers to surcharge detached properties, where families have to monitor change in impermeable area and flood. It is therefore important to young children, are most likely to have as the strategy continues to develop. If understand the rate at which urban creep high urban creep rates. we observe an increase in urban creep, we is occurring. will raise the issue with Gloucester County The results for the Thames Region Council who is responsible for managing We have studied aerial photography and are presented in Figure 7 below. The surface water. We may then also look to satellite imagery across 11 catchments urban creep rate for Bourton-on-the- retrofit sustainable drainage measures across the Thames Water region using Water is 0.097%. In other words, this (such as permeable paving and water data from two periods in the late 1990s is the increase in impermeable area per butts), in the area to counter the increased and mid-2000s to determine the rate at year as a percentage of the total area run-off following rainfall, to reduce the risk which urban creep is occurring. We then connected to the sewerage network. When of flooding.
Figure 7 Urban creep rates in the Thames Water region
Bourton-on-the-Water
10 Mott MacDonald, Future impacts on sewer systems in England and Wales, June 2011.
Bourton-on-the-Water Drainage Strategy - Stage 1 31 Bourton-on-the-Water Drainage Strategy - Stage 1 32 Published version post stakeholder review v1.1 Published version post stakeholder review v1.1 5.2 Climate change
We have analysed the 2009 UK Climate were selected to give a representative The nearest catchment to Bourton-on- Projections (UKCP09) to determine the sample of inner London, outer London and the-Water that was analysed for climate likely increase in rainfall intensity due to more rural areas in the Thames Valley. A change was Cirencester. The results show climate change in 15 catchments across number of these catchments are also areas a central estimate of an increase of 15% our region. More intensive rainfall in which experience sewerage related issues in rainfall by 2080, but in some scenarios the future will increase the peak flow like pollution, flooding and urban creep. this could be as high as 26% or as low in sewerage networks and with it the as 7% as per Figure 8 below. We will likelihood of sewer flooding. We assessed different combinations of ensure that our strategy takes account of emission scenarios and climate change these potential increased peak flows as it Fifteen catchments across our region11 percentiles for each of the 15 catchments. develops.
Figure 8 Locations assessed for increased rainfall intensity by 2080
Key Thames Locations Percentiles Bourton-on-the-Water High (90th) Medium (50th) Low (10th) County boundary Motorway Urban areas
Increased rainfall intensity may not be likely to become more frequent in future. the only consequence of climate change. As the recent experience of prolonged UKCP09 data also suggests that the UK is rainfall and high groundwater levels have likely to experience longer wetter winters been shown to be the principal factors, in future. Further research is needed to this research will be very significant understand whether high groundwater in informing any risk assessment and levels, such as those observed in the appraisal of costs and benefits of winters of 2012/13 and 2013/14 are solutions.
11 Atkins Thames UKCP09 Rainfall Intensity Assessment Revised Report, October 2012.
Bourton-on-the-Water Drainage Strategy - Stage 1 33 Published version post stakeholder review v1.1 5.3 Population growth and new development
We use a combination of top-down and capacity to accommodate the increase sustainable drainage systems are put bottom-up information to ensure that in flow. Where it does not, we propose in place, unless demonstrated to be our forecast of population and new planning conditions for consideration inappropriate. This requires that when development is as robust as possible to by the Planning Authority, although we considering planning applications, local keep costs down, in order to minimise the encourage developers to contact us as planning authorities should consult the bill impact of any investment that may be early as possible in the planning process to relevant Lead Local Flood Authority necessary. avoid this. (County Council or Unitary Authority), on the management of surface water, Our forecast of the number of new The independent review into the causes to satisfy themselves that the proposed households is taken directly from Experian of the 2007 floods (The Pitt Review), standards of operation are appropriate data. We have used the ‘Plan-Based’ concluded Sustainable Drainage Systems and ensure, through the use of planning projection which uses information (commonly known as SuDS), are an conditions or planning obligations, that provided by local authorities about effective way to reduce the risk of ‘flash- there are clear arrangements in place for planned numbers of new dwellings in their flooding’ which occurs when rainwater ongoing maintenance over the lifetime of respective areas. During the period 2015 rapidly flows into the public sewerage and the development. to 2020 we expect to see an increase in drainage systems, causing overloading new development across that Thames and back-up of water. Typically, SuDS In the case of Bourton-on-the -Water, Water region and are forecasting a total slow the rate of surface water run-off whilst SuDS might help to reduce the of 263,000 new connections to the entry into the drainage system and risk of flooding following rainfall when sewerage network during this time. improve the percolating feature, ie groundwater levels are low (i.e. typically rainfall recharging the groundwater during summer months), they may not Our Development Tracker System is used system, thus mimicking natural drainage be as effective in reducing the flood to track developer enquiries through processes. In April 2015, the Government risk when groundwater levels are high the planning process to construction. made changes to the planning process (typically during the winter months). We When we are contacted by a developer, effecting planning policies and decisions will therefore take account of the potential we typically carry out preliminary on planning applications of 10 dwellings influence of groundwater when we come modelling to determine whether our or more (or equivalent non-residential to assess any options as part of this network or treatment works has the or mixed development), to ensure that drainage strategy framework process.
Bourton-on-the-Water Drainage Strategy - Stage 1 33 Bourton-on-the-Water Drainage Strategy - Stage 1 34 Published version post stakeholder review v1.1 Published version post stakeholder review v1.1 Cotswold District Council is currently • 148 dwelling, Land to north of Roman A key element of our assessments will developing its Local Plan for the period to Way, Bourton-on-the-Water be to establish the extent to which 2031. Using sources of data that include • 63 Units, land parcel north of Bourton these developments may be significant the Cotswold District Council Preferred Industrial Park Bourton Industrial Park in the context of challenges currently Development Strategy (PDS May 2013), • 26 dwellings, land between Sandy Lane experienced in the catchment. This the Strategic Land Housing Availability Court and Southgate Court, Sandy Lane assessment work will be undertaken Assessment, and third party planning Court, Upper Rissington. and findings shared in an update to this enquiries, potential development sites we Strategy document. are currently tracking include: Other applications exist but relate to developments in which detailed numbers • 100 dwellings, Station Road, Bourton-on- have not been confirmed, or are single the-Water properties. We will continue to monitor • 20 dwellings, Pulhams Coaches, Station future plans for the catchment through Road, Bourton-on-the-Water our stakeholder engagement work.
Bourton-on-the-Water Drainage Strategy - Stage 1 35 Published version post stakeholder review v1.1 6 Strategy development
The Drainage Strategy for the Bourton-on-the-Water catchment is currently at Stage 1 (Initialise/prepare), of the 4-stage framework process. The following activities are planned and ongoing, in order to carry out our risk assessment and development of strategy options. Table 4 Activities planned and ongoing to enable strategy development
Activity Purpose Date Outcome planned Permanent Continue monitoring sewer depth monitors Ongoing Use information to identify additional monitoring of installed last year. Plan is for monitors to actions for inclusion in the drainage sewer levels remain in situ for at least 5 years and to strategy for Bourton-on-the-Water. Share capture the next wet weather event as a information with other agencies. minimum. Analyse the recorded depths and compare with other catchment variables, such as rainfall events and changes in groundwater levels. Permanent flow Enhance existing monitoring with the From Use information to identify additional monitoring of installation of permanent flow monitors on winter actions for inclusion in the drainage pumping station rising mains. Bourton-on-the-Water, Roman 2015 strategy for Bourton-on-the-Water. And Way and Lower Slaughter sewage pumping monitor performance of pumping station stations through future events Customer surveys Validate the historical records of flooding and From Use information to help test the cost restricted toilet use in the catchment to winter benefit of options to improve drainage and enable a detailed benefits assessment of 2015 reduce the risk of sewer flooding in potential further intervention options that Bourton-on-the-Water. could be implemented by Thames Water Pilot trials of As part of our wider approach to managing Ongoing Service may be restored for customers mobile treatment high groundwater levels, we have trialled the without the need for tankering. plant use of biological filters within other catchments. Our findings will inform and may influence our strategy plans for Bourton-on-the-Water. If successful, these could be used to abstract dilute sewage from surcharged sewers and discharge it safely to a watercourse. Misconnection Carry out visual inspection of properties to From A better understanding of the contribution surveys determine the extent of roof drainage and winter that misconnections make to sewer other surface water drainage that discharges 2015 flooding in the area. into the foul sewer network. Update drainage Improve the drainage strategy based on the 2017 Risk assessment, options appraisal and strategy initial results from the permanent monitoring, preferred strategy to be completed, subject customer surveys, misconnection surveys and to capturing weather events through feedback from stakeholders. monitoring and surveys. Consider innovative Identify quicker / cheaper / collaborative Ongoing Enhanced toolkit available to reduce the risk solutions options that improve the benefit to cost ratio of sewer flooding and then apply this once in order to keep customers’ bills down to data becomes available. prioritise investment to ensure greatest benefit to customers.
Bourton-on-the-Water Drainage Strategy - Stage 1 35 Bourton-on-the-Water Drainage Strategy - Stage 1 36 Published version post stakeholder review v1.1 Published version post stakeholder review v1.1 7 Preferred strategy and plan
We believe that the foul sewerage system the relative impact that each of these proposed development in the catchment. in Bourton-on-the-Water has surcharged factors has on the risk of sewer flooding, and flooded predominantly due to a and then to develop a plan comprising We may carry out some repair works combination of groundwater infiltration, cost beneficial solutions using customer as this strategy develops, in the event surface water run-off from saturated fields, willingness to pay research. In parallel, that our investigations identify faults or surface water inundation from highways, we will assess the extent to which new problems with the sewerage network that public spaces and properties, surface developments may be significant in are highly likely to have caused flooding. water misconnections, and river water the context of challenges currently Table 5 below, details the repair activities overflowing from the River Windrush. experienced and where necessary we will that we have identified to date. Our network strategy is to understand develop solutions to accommodate the
Table 5 Activities identified in preferred plan to date
Activity Purpose Date Outcome planned Replace manhole Covers in areas identified to be at risk of When Reduced inflow of surface water to the foul covers fluvial or pluvial inundation to be replaced identified sewerage system. with low leak covers. Sewer repairs Repair / reline identified defective lengths of When Reduce inflow of groundwater to the foul public sewer. identified sewerage system. Permanent Continued monitoring of depth monitors for Ongoing To identify additional actions for inclusion in monitoring of at least 5 years and to capture the next wet the Drainage Strategy for Bourton-on-the- sewer levels weather event as a minimum. Analyse the Water. recorded depths and compare with other catchment variables, such as rainfall events and changes in groundwater levels.
Our plan will be updated once the Risk Assessment and Options Appraisal sections have been completed in accordance with the Drainage Strategy framework.
Bourton-on-the-Water Drainage Strategy - Stage 1 37 Published version post stakeholder review v1.1 8 Temporary overflows
We have not installed temporary pipework investigating the circumstances under them, in order that this Drainage Strategy, and pumps in the sewerage network which emergency discharges would be together with our plans to reduce during wet weather events in Bourton- required in future, such as the use of infiltration, fully meets the requirements on-the-Water to maintain service, but temporary overflows, to pump out from of an Infiltration Reduction Plan as set out we would consider doing so to prevent the sewerage network through biological in the Environment Agency’s Regulatory the backup of sewerage into customers’ filters to maintain service to customers Position Statement. We will continue to properties and uncontrolled spilling from and prevent homes from flooding. As identify sewer rehabilitation and other the sewer system into the environment. this Drainage Strategy develops, in this permanent works to reduce groundwater section we will describe the location of any infiltration alongside refining the use of As part of the stage 2 risk assessment proposed temporary overflows and the temporary overflows. and stage 3 options appraisal, we will be circumstances under which we would use
Bourton-on-the-Water Drainage Strategy - Stage 1 38 Published version post stakeholder review v1.1 Appendix A Glossary of terms
Term Definition Term Definition
Blockages Obstacles or the build-up of fat and grease, the brickwork or defects in manhole block or obstruct our sewerage pipes. This structures. is normally caused by things which should Inundation Accumulated surface water from rain and/or not be flushed, or poured, into drains and river floodwater that has resulted in localised sewers. flooding, finds its way into the sewerage Combined sewer A pipe conveying the combined rainwater system through manhole covers and drains. and contaminated wastewater from two These may be public or private. or more properties. A combined sewer Lateral drain See definition for Foul drain. is designed to carry wastewater to a Misconnections Property owners have connected rainwater sewage works for treatment but during (surface water to and/or land drainage to our sewers (e.g. roof periods of heavy rainfall or snowmelt, foul water) drainage, paved driveways drains, soakaway the volume in a combined sewer system overflows), and can cause major issues for can exceed the capacity of the sewer the performance of the sewerage system. system or treatment plant. For this reason, Misconnections A plumbing mistake resulting in wastewater combined sewer systems are designed to (foul water to appliances being misconnected to the overflow occasionally and discharge excess surface water) surface water system. wastewater directly to nearby streams, Private sewers See definition for Foul sewer. rivers, or other water courses. Rainfall induced Sewer infiltration that occurs as a result Dry weather The average flow rate observed over a 24 infiltration of rainfall percolating into the ground flow hour period in dry weather and based on impacting the sewer on route to recharging Sewers for Adoption, the industry standard, the groundwater table. includes an allowance for infiltration of 10% Riparian owner If you own land adjoining, above or with a of the calculated flow rate. watercourse running through it, you have Foul drain A pipe conveying the contaminated certain rights and responsibilities. In legal wastewater from a single property. If the terms you are a ‘riparian owner’. If you pipe extends beyond the property boundary, rent the land, you should agree with the the portion of the pipe outside of the owner who will manage these rights and boundary is termed a lateral drain. The responsibilities. portion of the pipe inside the boundary is Soakaway Surface water from a roof and driveway of a private drain. On 1 October 2011 water a property is piped to an underground pit, and sewerage companies in England and usually filled with gravel or similar material. Wales became responsible for lateral drains, Some soakaways are situated within the which were previously the responsibility of boundary of the property. property owners. Private drains remain the Surface water A pipe conveying uncontaminated rainwater responsibility of property owners. drain from a single property. Foul sewer A pipe conveying the sewage from two Surface water A pipe containing uncontaminated or more properties. On 1 October 2011, sewer rainwater from two or more properties. A water and sewerage companies in England surface water sewer is designed to dispose and Wales also became responsible for of rainwater from roofs, driveways, patios, private sewers, which were previously the roads, etc to a local watercourse. responsibility of property owners. A foul Sustainable Measures designed to attenuate and slow sewer is designed to carry contaminated Drainage down surface water before it enters sewers wastewater to a sewage works for Systems (SuDS) to reduce the risk of flooding following treatment. It disposes of wastewater from heavy rainfall. Includes green infrastructure sources including toilets, baths, showers, such as raingardens, green roofs as well as kitchen sinks, washing machines and other measures, such as permeable paving dishwashers. and water butts. Infiltration Groundwater finds its way into the sewerage system (including private drains), via defective pipes or pipe joints and through
Bourton-on-the-Water Drainage Strategy - Stage 1 39 Published version post stakeholder review v1.1 Appendix B Supporting figures and photographs
Figure B1 Bourton-on-the-Water Watercourses This watercourse map information has been sourced from the Environment Agency website. For more detailed flood map information for this catchment, please access the Environment Agency website.
Bourton-on-the-Water Drainage Strategy - Stage 1 40 Published version post stakeholder review v1.1 Figure B2 Bourton-on-the-Water bedrock and drift geology This geological map information has been sourced from the British Geological Survey website. For more detailed geological information for this catchment, please access the British Geological Survey website.
Bourton-on-the-Water Drainage Strategy - Stage 1 41 Published version post stakeholder review v1.1 Figure B3 Bourton-on-the-Water fluvial flood risk based on Environment Agency plans This flood risk map information has been sourced from the Environment Agency website. For more detailed flood map information for this catchment, please access the Environment Agency website.
Nat Defences
Flood Zone 3
Flood Zone 2
Flood Storage Area
Bourton-on-the-Water Drainage Strategy - Stage 1 41 Bourton-on-the-Water Drainage Strategy - Stage 1 42 Published version post stakeholder review v1.1 Published version post stakeholder review v1.1 Figure B4 Bourton-on-the-Water surface water flood risk from Environment Agency plans This flood risk map information has been sourced from the Environment Agency website. For more detailed flood map information for this catchment, please access the Environment Agency website.
30 Yr SW Risk
100 Yr SW Risk
Bourton-on-the-Water Drainage Strategy - Stage 1 43 Published version post stakeholder review v1.1 Figure B4 Groundwater flood risk for Bourton-on-the-Water based on ESI plans This groundwater flood risk map information has been sourced from ESI Ltd.
High Risk
Low Risk
Moderate Risk
ESI Groundwater Flood Risk Map of England and Wales © www.esinternational.com
Bourton-on-the-Water Drainage Strategy - Stage 1 43 Bourton-on-the-Water Drainage Strategy - Stage 1 44 Published version post stakeholder review v1.1 Published version post stakeholder review v1.1 Photographs taken during wet weather of 2013/14
Photo 1 – Flooding off Leasow Lane. Potential Photo 2 – Rissington Road. Potential source of inflows from inundated manholes and consequential river flooding from the River sewer lines. Windrush bursting its banks.
Bourton-on-the-Water Drainage Strategy - Stage 1 45 Published version post stakeholder review v1.1