Carterton Drainage Strategy - Stage 1 - Initialise/Prepare - Thames Water
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Carterton 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 Carterton Drainage
cities, and its urbanised surrounding environments.
areas; together with the equally complex 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**
2015 - 2017 From 2018
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.
Carterton 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
Carterton 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.
Carterton 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 Carterton 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 been document. Our previous investigations network activities and plans.
developed and recommended by the have identified some sources of inflow,
water industry economic regulator Ofwat, such as the misconnection of surface
and the Environment Agency. water to foul sewers. However, as stated
This will enable us to better understand in the following strategy, we are not
the 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 Carterton 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 Actions underway or planned include:
plan for this area and will provide further (For more information please see Table 4)
details as our plan develops through
Stage 2 to Stage 3 - Option Appraisal. The 1 Stakeholder engagement activities
following drainage actions have already 2 Monitoring of pumping station flows
been undertaken, or are underway, in the 3 Sewer and Manhole surveys
Carterton catchment: 4 Customer surveys
5 Innovative solution analysis.
Actions completed include: (For more
information please see Table 2)
1 CCTV survey
2 Site reconnaissance
3 Hydraulic modelling
4 Depth monitor installation
5 Replacement pumps.
Carterton Drainage Strategy - Stage 1 4
Published version post stakeholder review v1.1
Q3 Are you renovating the sewers in our area?
Answer
As our strategy work continues, we will contributing to, or causing, drainage and
target and repair localised sewer defects flooding issues in the catchment over the
identified through our investigations as coming months.
Q4 What are the improvement plans for Carterton’s
sewage treatment works to manage capacity?
Answer
The Carterton sewage treatment works equivalent, however, there is a current
operates a fully-compliant permanent concern with regards to the capacities of
storm overflow which permits us to the works due to the increase in use of
discharge into local watercourses RAF Brize Norton as a main operational
during storm events. To meet changing base taking operational capabilities and
performance requirements and regulatory servicemen and women from RAF bases
measures the works has undergone that have been closed. Investigations
upgrades over the years. The capacities are taking place to examine what
of the works have been assessed to be requirements if any will be needed to
adequate under normal design flow ensure a robust operation of the site.
conditions for the current population
Q5 How are you planning for future development
in the catchment?
Answer
As per Section 5.3 in the following planning inspectorate and we will continue
Drainage Strategy document, we will to work with all involved stakeholders,
continue to closely monitor development through our stakeholder engagement
applications in the catchment and assess activities, to monitor local plans and
the impact that they may have on the planning applications. We will incorporate
capacity of our operations in the future. current and projected developments into
We are aware that the West Oxfordshire our business planning cycle to ensure that
District Council Local Plan for the period to our service is maintained for customers
2031, is currently being submitted to the throughout the catchment’s development.
Carterton Drainage Strategy - Stage 1 5
Provide information
Published forstakeholder
version post this foot note please
review v1.1
Q6 Are you working with the Highway Authority to
resolve blocked gulleys, sewers and ditches, and with
landowners to reduce field run-off, as both affect
drainage and our sewers?
Answer
In Section 1of the following Drainage We will continue to work closely with
Strategy document, we outline the other the Council and Highways England to
stakeholders who, like us, have important understand the extent to which flood
drainage responsibilities and therefore, waters may be escaping from highway
play an essential role in resolving sewer or land drainage systems; and impacting
flooding issues in this catchment area. the foul sewer network. An update on
As Highways maintenance activities and this issue will be shared with customers
agricultural land maintenance practices and featured in the later stages of this
sit outside of our remit we will work with Drainage Strategy document.
the responsible stakeholders, to highlight
these issues where this is found to have
a major influence and impact on our
sewerage network.
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 winter of 2013/14 tankering
Reduction Plan, as set out in the was utilised in the Carterton catchment,
Environment Agency’s Regulatory Position however temporary pumps were not used.
Statement, is a fundamental consideration In the event that temporary overflows are
in their development. Therefore, to required, as stated above, we will describe
maintain our service to customers during their location and the circumstances under
future wet weather events, if we need which we would use them. Together with
to discharge to watercourses through plans to reduce infiltration, this drainage
temporary overflows, a dedicated section strategy would then fully meet the
will be included in each affected strategy, requirements of an Infiltration
providing details of their location and Reduction Plan.
intended use. This section will be included
and /or revised when each strategy
is updated.
Carterton 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 stations are supported by 24 hour which has significantly improved the
diagnostic monitoring so that we can station’s operation and prevented further
tightly control their operation through a flooding during the wet winter of 2013/14.
flow of real-time information. Based on
this performance data we can respond As we move through this 4-stage
quickly through site visits by our engineers framework process and further develop
and rectify potential issues. our drainage strategy for this catchment,
we will review the operational control
In the past flood water caused operational options of this station, particularly during
issues at Halton Road pumping station. wet weather, carefully avoiding increasing
These pumps were replaced in 2013, the risk of sewer flooding in doing so.
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 region, the urban creep in particular misconnection of surface
rate for Carterton is slightly above average water and change of land use, can have
for the Thames Water Operational Area, a significant impact on sewer flooding;
but not as high as suburban areas around particularly when permeable areas such
central London 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.
Carterton 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 We 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 our Climate
emissions (compared to 1990 levels), Change section on the Homepage of
for our Scope 1 and 2 emissions*. our 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
overflow points when groundwater and this position.
Carterton Drainage Strategy - Stage 1 8
Published version post stakeholder review v1.1
Carterton 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 28
4.3 Investigations and activities completed to date 29
4.4 Activities carried out by drainage partners 30
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 35
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 30
Table 4 Activities planned and ongoing to enable strategy development 35
List of Figures
Figure 1 The Drainage Strategy Framework 11
Figure 2 Carterton priority sub-catchment 14
Figure 3 Stakeholder responsibilities for drainage 16
Figure 4 Carterton catchment schematic 20
Figure 5 Carterton sewage treatment works treated flows and groundwater levels 26
Figure 6 Halton Road depth monitor record 27
Figure 7 Urban creep rates in the Thames Water Region 32
Figure 8 Locations assessed for increased rainfall intensity by 2080 33
Carterton Drainage Strategy - Stage 1 10
Published version post stakeholder review v1.1About 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 and society value”. As a company, we are outcomes for drainage, specifically in the
of long-term customer ‘outcomes’. committed to achieving our customer Carterton catchment, in a sustainable and
outcomes, a number of which are focussed economic manner.
Approved approach
We have adopted the Drainage Strategy focus on the sewerage network, and not activities that we plan to undertake
framework outlined in Figure 1 below, the performance of sewage treatment to address current issues and future
developed by the Environment Agency works. The Carterton drainage strategy challenges facing the catchment, and the
and Ofwat. It identifies 4 key stages to is currently at the first stage of this data that we need to gather to complete
producing a good-practice drainage framework - the Initialise/Prepare stage. the risk assessment and options
strategy. Drainage strategies typically In this document, we describe the appraisal 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
Carterton Drainage Strategy - Stage 1 11
Published version post stakeholder review v1.1Consultation and publication
We will update and republish this fieldwork has been collected and analysed. We will also make the Drainage Strategy
document to provide the results of our Throughout this process we will documents available on the Drainage
risk assessment, options appraisal and our attend local flood forums for ongoing Strategies webpage of our website.
selected strategy for intervention, once communication and consultation with
data from instrumentation and other customers and stakeholders.
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
Carterton Drainage Strategy - Stage 1 12
Published version post stakeholder review v1.1Executive summary
In recent years the foul sewerage system the catchment to work together to resolve to maintain the current service that we
in the Carterton catchment has become them. The Floods and Water Management provide, customers have told us that they
overwhelmed in some locations, following Act 2010 places a responsibility on lead would like to see a reduction in instances
prolonged and heavy rainfall and raised local flood authorities (LLFAs), to manage of sewer flooding and odour nuisance and
groundwater levels. This has resulted in flood risk from surface and groundwater, an improvement in river water quality.
certain properties suffering from sewer plus a duty on all risk management Our research indicates that customers
flooding and restricted toilet use. This authorities (RMAs), to cooperate regarding are willing to pay for these improvements
has particularly impacted the part of the flood risk. In our role as a RMA, Thames to service 2; a summary of our related
catchment served by the Halton Road Water will work with West Oxfordshire customer research can be found on our
sewage pumping station. County Council, West Oxfordshire District website via the hyperlink below.
Council and the Environment Agency to
We believe that the system surcharged ensure that a collaborative approach can We have therefore developed a set
and flooded because of a combination be developed to address the problems. of company outcomes that we are
of groundwater infiltration, surface water committed to working towards over the
run-off from saturated fields, surface water In response, this drainage strategy follows next 5 years and beyond. The outcomes
inundation from highways and public the Environment Agency and Ofwat’s relevant to the Carterton drainage
spaces, surface water misconnections, and 4-stage framework. The Carterton strategy strategy are:
river water inundation from the Shill Brook. is currently at Stage 1 (Initialise/Prepare).
Flood water also caused operational We describe in this document the actions • Asset health - a composite range of
problems at Halton Road pumping station that we plan to carry out to complete the measures against which we will manage
– the pumps were replaced in 2013, which risk assessment and options appraisal the health of our sewerage network
has significantly improved the situation stages. We will update and republish • Properties and public areas protected
and prevented further flooding during the this document once this work has been from sewer flooding
wet winter of 2013/14. completed. • River water quality meets customer’s
expectations and regulatory
The root causes of sewer surcharges are In preparing our company business requirements.
therefore numerous and the resolution plan for the 5 year period 2015 to 2020
of the issues complex, requiring all we have listened very carefully to the
stakeholders responsible for drainage in views of customers. Beyond being able
2
http://www.thameswater.co.uk/cr/Howwedobusiness/Engagingwithourstakeholders/Publicconsultationresearch/index.html
Carterton Drainage Strategy - Stage 1 13
Published version post stakeholder review v1.1This drainage strategy must also address the South East is set to grow rapidly. In parallel, we are developing solutions to
future challenges to the Carterton A number of possible developments accommodate the proposed development
catchment. We assess these to be: are identified around Carterton, and we to the east of Carterton. We have not
will continue to track these and any identified any other interim maintenance
• Climate change – analysis of the latest other emerging applications for this activities or rehabilitation works at this
data suggests that rainfall could catchment arising in the future. stage.
become 15% more intensive by 2080
increasing the likelihood of flooding. Our strategy is to understand the Our next steps are to continue to collect
Longer wetter winters may also mean relative impact on this catchment of real-time flow information from the
groundwater levels are high more often; overland flow from saturated fields, permanent depth monitors we installed
this could also exacerbate fluvial groundwater infiltration and surface water in 2013, and to collect sewer flooding
flooding from local watercourses misconnections; and then to identify information from customers. The depth
• Urban creep – paving over of front cost beneficial solutions to reduce the monitors will remain in place as we
gardens and loss of green space results risk of sewer flooding using customer move through this 4-stage framework
in more strain on the sewerage network willingness to pay research. We may carry and develop our plans. Figure 2 below,
when it rains heavily. Modelling we out sewer rehabilitation works as the highlights the priority sub-catchments we
have undertaken suggests urban creep strategy develops, in the event that our are monitoring and focussing on in the
rates in Carterton are about average for investigations identify faults or problems Carterton catchment.
the Thames Operational Area with the sewerage network that are highly
• Population growth – the population in likely to have contributed to flooding.
Figure 2 Carterton priority sub-catchments
Priority sub-
catchments
Non priority sub-
catchments
Pumping mains
Pumping station
Halton Road
pumping station
Carterton sewage
treatment works
The extent of the catchment is outlined in red.
Carterton Drainage Strategy - Stage 1 14
Published version post stakeholder review v1.11 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 responsibility 3. 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 Defra 4.
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.
Carterton Drainage Strategy - Stage 1 15
Published version post stakeholder review v1.11.2 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
Carterton Drainage Strategy - Stage 1 16
Published version post stakeholder review v1.1Thames Water Management Act for managing the local responsible for the drainage of motorways
We are responsible for removing and flood risk from groundwater and surface and certain trunk roads.
treating wastewater which includes the water runoff e.g. local watercourses and
culverts 6. They work with landowners
foul sewers and surface water sewers in
to maintain privately owned ditches,
Customers
some areas 5, and the combined sewers Customers own, and are responsible for,
that are in some of the older large drainage and watercourses, keeping
the maintenance of private drains within
urbanised areas such as London . In some them clear of blockages. They are also
the curtilage of their property, which did
cases, the cause of sewer flooding may responsible for managing the risk of
not transfer to Thames Water ownership in
not fall under our responsibility. In these groundwater flooding, both inside and
October 2011 7.
circumstances, we will explain what we outside of properties. Water from these
can do to help and continue supporting local authority gullies and drains and
the relevant authorities or third parties to privately owned ditches can also impact Riparian Owners
reduce the impact for customers. Thames Water’s sewers, therefore, we Riparian Owners are landowners who own
work with all responsible stakeholders land with watercourses or land adjacent
to resolve the excess flow. The District to watercourse (ie road side ditches).
Environment Agency Council tends to be the local Planning The responsibility for the operation
The Environment Agency is responsible for Authority responsible for approving new and maintenance of ditches, local
main rivers and part of its remit includes development, but equally may have watercourses and general land drainage
monitoring and informing the levels of responsibility for ensuring maintenance lies mostly with riparian owners.
ground and river water. The Environment of watercourses; particularly on council-
Agency also investigates pollution owned land.
incidents and monitors the quality of the
Land Owners
water in rivers. Land owners include farmers and both
Highway Authority residential and commercial customers, but
The Highway Authority is generally the includes trusts etc. They are responsible for
Lead Local Flood Authority County Council (or Unitary Authority), and ensuring the adequate drainage of their
and District Council is responsible for highway maintenance land, such that it is not a nuisance
Lead local flood authorities have the and highway drainage and for clearing to others.
responsibility under the Flood & Water roadside gullies. Highways England is
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.
Carterton Drainage Strategy - Stage 1 17
Published version post stakeholder review v1.12 Catchment description
2.1 Geology and topography
The Carterton catchment is located in The geology is generally made Appendix B includes maps showing
West Oxfordshire, less than 20 miles to up of limestone bedrock and the the geology and fluvial, pluvial and
the west of Oxford and approximately 20 geotechnical make-up of the catchment groundwater flood risk areas in the
miles north east of Swindon. It includes is predominantly very permeable soils. catchment.
the villages of Alvescot, Filkins, Shilton, This means that the catchment is prone
Bradwell Village and part of RAF Brize to significant seasonal fluctuations in According to the Environment Agency,
Norton, as well as Carterton itself. groundwater levels, with the added the current ecological status of the Shill
likelihood of rainfall-induced infiltration 8 Brook as ‘Good’ 9.
owing to its permeable soils.
2.2 Sewage treatment works
The Carterton sewage treatment works discharging to the Shill Brook. Rural District Council, together with the
serves the whole of Carterton, Alvescot, sewerage system, as a first-time sewerage
Filkins, Shilton, Bradwell Village and part of The works includes storm tanks to handle scheme.
RAF Brize Norton, as outlined in Figure 4 excess flows above the flow to full
below, and serves a population equivalent treatment, these store he flows and pump The capacities of the works have been
of 16,700 including considerable trade them back to the inlet for treatment when assessed to be adequate under normal
effluent emanates from RAF Brize Norton the storm flows subside but can discharge design flow conditions for the current
airfield. It treats a typical daily dry weather to the Shill Brook via a screen if they population equivalent. However, there is a
flow of 3,949m3/day which can increase become full. During extended wet periods, current concern regarding the capacities
up to a full treatment flow capacity treated flows at the sewage treatment of the works being impacted by RAF Brize
of 11,652m3/day. Flows arrive at the works can be in excess of 15,000 m3/day, Norton being increasingly used as a main
inlet works via the pumping mains and which is around 5 times greater than the operational base, for servicemen and
a gravity sewer from RAF Brize Norton. expected maximum daily dry weather flow women from recently closed RAF bases.
These flows are screened then pass of 3,950m3/day. Investigations are being undertaken to
through a grit chamber and on to primary assess the requirements, if any, that are
settlement tanks. The settled sewage The sewage treatment works is believed needed to ensure that the operation of
then gravitates to biological filters and to date from the 1950/60’s and is likely this site remains robust.
humus tanks, with the treated effluent to have been implemented by Witney
8
Rainfall Induced Infiltration is the term given to sewer infiltration that occurs as a result of rainfall percolating into the ground
impacting the sewer on route to recharging the groundwater table.
9
Environment Agency website, interactive map, Basin Management Plans.
Carterton Drainage Strategy - Stage 1 18
Published version post stakeholder review v1.12.3 Foul sewers
The Halton Road sewage pumping In terms of design capacity, a 225mm • vitrified clay pipework for smaller
station drains foul water from Shilton, diameter sewer laid at a gradient of 1 in diameter pipes with concrete used for
the northern section of RAF Brize Norton 150 will have sufficient capacity to collect the large diameters
and the majority of Carterton. This covers foul sewage from around 1,500 houses, • brick and concrete manholes.
an area of some 260 hectares with a which based on average occupancy
population of approximately 10,000 rates equates to 4,500 people. Problems The pipework can have a very long
(although this varies with activities at in sewers with diameters of less than service life, but sometimes the joint seals
the airbase). 300mm tend to be as a result of blockages deteriorate over time. The 1960s pipes
in the pipes. However, occasionally surface were typically laid on bedding material
The system drains predominately under water can be misconnected into the foul such as pea shingle, with the trenches
gravity to a series of sewage pumping sewerage network, problems then arise likely to have been backfilled with ‘as
stations lifting flows from low lying areas when it rains heavily. dug’ excavated material. More recent
towards the larger Halton Road sewage drains and sewers, i.e. since the 1980s,
pumping station. Halton Road pumps The public sewers draining to the Halton are typically surrounded with pea shingle.
flows to an inverted siphon in Milestone Road sewage pumping station range from This protects the pipe but can also act as a
Road, which in turn discharges into 150mm diameter in the upper reaches of good conduit for groundwater.
Carterton sewage treatment works. When the catchment to 450mm diameter near The layout of the catchment suggests
flows are high, the siphon can overflow to the pumping station. The capacities of that most properties are likely to have
the sewerage network in west Carterton, the sewers have been assessed to be more their own foul drains (as opposed to
which drains to Carterton sewage than adequate under normal dry weather shared drains), that connect directly into
pumping station. conditions. the public sewer. The private foul water
drains within the property boundaries
Sewer design criteria ensures the The current drainage system is believed in Carterton are the responsibility of the
appropriate sizing and laying of pipes at to have been constructed by the Witney property owners, where they are not
an appropriate gradient to maintain a Rural District Council in the late 1960s. shared.
satisfactory self-cleansing characteristic. Materials used in the construction of the
The capacity of sewers is typically set to sewerage system are:
cater for six times Dry Weather Flow with
a 10% allowance for infiltration 10.
10
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.
Carterton Drainage Strategy - Stage 1 19
Published version post stakeholder review v1.1Figure 4 Carterton catchment schematic
Carterton Drainage Strategy - Stage 1 20
Published version post stakeholder review v1.12.4 Surface water sewers
Whilst there are some public surface water their foul drains when their soakaways The responsibility for the maintenance lies
sewers in Carterton these tend to be more do not work. This exacerbates capacity with riparian owners.
prevalent in the newer housing estates, problems for other customers connected
many draining to a 1200mm diameter further downstream in the sewerage Highway drainage typically discharges to
pipe in Monaham Way which discharges network. the roadside ditches. Owing to the high
to a large balancing pond off Norton local beauty of the area the ditches tend
Way. This in turn drains to the east of The catchment is mostly rural and to be well maintained with clearance of
the catchment to a watercourse crossing incorporates a network of roadside vegetation and debris occurring. However,
Brize Norton airfield. Balancing ponds ditches and minor watercourses that are the area does have an active groundwater
are used to store surface water until such intended to drain surface water from table and as a result groundwater springs
time that it can be discharged into local roads and public spaces. As per Section do occur from time to time, which can
watercourses. 1.2 the responsibility for the operation lead to localised land drainage issues.
and maintenance of these ditches,
The surface water from the majority local watercourses and general land The extent of highway drainage is not
of properties is likely to drain to nearby drainage lies mostly with riparian owners. certain, but it is likely that highway run-off
soakaways or to Shill Brook. Soakaways Oxfordshire County Council as lead local discharges direct to the roadside ditches,
can only function satisfactorily when flood authority has overall responsibility some of which will act as soakaways.
ground conditions allow soakage, but for managing groundwater. Oxfordshire County Council is responsible
may be completely ineffective when for the highway drainage and culverts
groundwater levels are high. In some The Environment Agency has the duty crossing the highway.
areas, we have seen examples of and the authority to ensure that the Shill
customers draining surface water through Brook is maintained appropriately.
Carterton Drainage Strategy - Stage 1 21
Published version post stakeholder review v1.13 Long-term outcomes
We have listened very carefully to the Beyond being able to maintain the current In response to this, we have developed
views of customers before developing our service that we provide, customers have 4 company outcomes and 11 service
plan for the Asset Management Period 6 told us that they would like to see a outcomes for our wastewater service that
(AMP6) regulatory period. Between 2009 reduction in instances of sewer flooding we are committed to working towards
and 2013 we carried out over 50 separate and odour nuisance and an improvement over the next 5 years and beyond, further
customer research and engagement in river water quality. These are areas details can be found in Table 1 below and
activities. where customers would like to see and are on our website 11.
prepared to pay for an improvement to
the current level of service.
Table 1 Wastewater outcomes
Company Wastewater service Why is this service
outcome outcome outcome chosen
Asset health: maintaining our assets to We must ensure an appropriate balance
ensure we can provide a safe and between reducing costs today and not
reliable service in the long-term. compromising our future service.
We will provide a safe and reliable
wastewater service that complies Properties and public areas protected Flooding is one of the worst service
with all necessary standards and is from flooding. failures for customers.
available when our customers
require it. Resilient sewage treatment service that We need to be able to provide service
minimises the impact of extreme events against a variety of pressures such as
on river water quality. climate change and population growth.
This service outcome ensures our
Our customers and stakeholders Do the basics excellently by getting wholesale activity is completely aligned
can trust us, we are easy to do things right first time. to our objective to improve our Service
business with and we care. 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 our customers.
bills are no more than necessary.
11
See http://www.thameswater.co.uk/tw/common/downloads/about%20us%20-%20corporate%20responsibility/
AMP6_-_Outcomes_Reporting_Policy.pdf for more information.
Carterton Drainage Strategy - Stage 1 22
Published version post stakeholder review v1.1Company Wastewater service Why is this service
outcome outcome outcome chosen
Minimising our carbon footprint. There is an expectation from society that
we will play our part in reducing carbon
emissions.
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.
We will limit our impact on the
environment and achieve a socially Satisfactory sludge disposal. Sludge is a resource that we should
responsible, sustainable business manage effectively to keep bills down.
for future generations, including
reducing levels of leakage. 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.
Compliance with new environmental We must meet environmental
regulations. regulations, and river quality is a visible
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 service outcomes, as these are relevant to the Carterton 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
outcome of a safe and reliable wastewater or equipment failures).
Carterton Drainage Strategy - Stage 1 23
Published version post stakeholder review v1.13.2 Properties and public areas protected from flooding
There are two performance commitments prolonged periods of wet weather). drainage measures such as water butts,
that underpin the delivery of this service Our customer research indicates that permeable paving, rain gardens and
outcome: our sewer flooding programme will green roofs. We aim to retrofit over
deliver £20m of benefit to customers 20 hectares of sustainable drainage
1. We commit to protecting properties every year by 2020. measures by 2020. We may also apply
from flooding due to rainfall. We this commitment to areas where the
estimate that our plan for 2015-20 2. We commit to reducing the risk of network was designed to take foul flow
will result in over 2,100 properties sewer flooding and pollution from only, but investigation shows that a
being alleviated from internal flooding, combined sewers (i.e. those that convey substantial amount of surface water is
external flooding and also from both foul and surface water), by slowing in the foul sewer.
restricted toilet use (for example when down surface water run-off and re-
groundwater levels are high following routing the flow through sustainable
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).
Carterton Drainage Strategy - Stage 1 24
Published version post stakeholder review v1.14 Current issues
4.1 Recent wet weather events
In the winter of 2012/13 the foul a comprehensive list of factors that have • Highway drainage overwhelmed
sewerage system in Carterton became caused flooding. causing highway flooding
overwhelmed for a number of weeks • Shill Brook overwhelmed causing fluvial
following prolonged heavy rainfall. We Fluvial flooding has also proved to be an flooding of properties
believe that the system was surcharged important source of flood water and poses • Land drainage issues with water running
and flooded because of a combination a key threat to properties in the Carterton off fields and onto the highway.
of surface water misconnections, river catchment. Where flood water resides on
water inundation from the Shill Brook in the surface, it can ingress into submerged To reduce the risk of flooding, residents
the Shilton area and subsequent high manholes and cause surcharging of in certain areas have had to pump
flows overwhelming the sewage pumping sewers and flooding further downstream floodwaters onto the highway or protect
station at Halton Road. The Halton Road with the potential for contamination of their properties with sandbags and
pumps were replaced in 2013 and during local watercourses. As far as possible we floodgates.
the following wet winter (2013/14) there will seek to address this problem by sealing
were no significant problems associated manholes and liaising with stakeholders to During extended wet periods, treated
with the foul sewers in the Halton Road find a mutually beneficial solution. flows at the sewage treatment works can
catchment (although the capacity of the be in excess of 15,000 m3/day, which is
pumping station was briefly exceeded During recent events, the following around 5 times greater than the expected
during a major storm event on 24th incidents have been observed with respect maximum daily flow of 3,950m3/day.
December 2013). to the sewerage network: Figure 6 compares the treated flows at
Carterton sewage treatment works with
Future development in the catchment • Restricted toilet use in Shilton groundwater levels recorded at nearby
is likely to put increasing pressure on • External foul flooding from sewage Environment Agency boreholes. This
these pumping stations, highlighting the pumping stations when overwhelmed shows some correlation between ground
importance of working collaboratively with during heavy rainfall. water level and increased flows.
catchment stakeholders to find a future-
proof solution to these issues. During these events, other sources of
We are confident that we have identified flooding have also been observed:
Carterton Drainage Strategy - Stage 1 25
Published version post stakeholder review v1.1Figure 5 Carterton sewage treatment works treated flows and groundwater levels
Figure 6 shows the records depths in the during that period. Records indicate that
main sewer draining to the Halton Road the rainfall during the 2013/14 winter was
pumping station during the winter of significantly greater than the 2012/13
2013/14. winter, but the Halton Road pumping
station was able to accommodate the
The graph indicates that the flow depths flows following the replacement of the
did not approach ground level in the area pumps in 2013.
Carterton Drainage Strategy - Stage 1 26
Published version post stakeholder review v1.1Figure 6 Halton Road depth monitor record
Carterton Drainage Strategy - Stage 1 27
Published version post stakeholder review v1.14.2 Our operational response
To maintain service, tankers were used flooding inside their homes. In Carterton, the sewer system into the environment. As
in the winter of 2012/13. This included tankers were used in some cases to part of the survey works due to commence
cleaning and removal of debris such as clean up pumping stations and outside in winter 2014, we are investigating the
silt and roots from pumping stations properties which suffered from flooding circumstances under which emergency
for Halton Road, Northwood Crescent, during these periods of extreme wet discharges would be required in future.
Alvescot Road and Burford Road. weather. Even if flows can be contained within the
sewer network, excessive flows arriving at
Due to the significant impacts of fluvial To date, we have not installed temporary the treatment works may not always be
and groundwater flooding across our pipework and pumps during wet weather given full treatment prior to discharge to
region during the winter event of 2013/14, events in Carterton to maintain service, Shill Brook. The use of such storm sewage
we decided to mobilise our tanker fleet of but we would consider doing so to prevent overflows is accepted by our regulators,
nearly 100 vehicles to protect customers the backup of sewerage into customers’ subject to conditions.
who were at the greatest risk of sewer properties and uncontrolled spilling from
Carterton Drainage Strategy - Stage 1 28
Published version post stakeholder review v1.14.3 Investigations and activities completed to date
Table 2 below, details the investigations and actions that we have completed in recent years within the Carterton 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
Flooding clean-up Tankers have been used in a few isolated Mar 2012 & Tankers used to remove excess water from
incidents to remove surcharged flow from Dec 2013 surcharged sewers in order to protect
our sewers and to clean up. properties.
Sewer cleaning Sewer and sewage pumping station cleaned Apr 2012 to Maintain ‘asset health’.
(removal of debris, silt and roots) for Halton Aug 2012
Road, Northwood Crescent, Alvescot Road
and Burford Road.
CCTV Survey Ascertain sewer condition and gain evidence Ongoing Short term discharges to reduce impact of
of infiltration and to check all blockages surcharged sewers.
have been removed.
Replacement Replacement of pumps at Halton Road April 2013 Pumps replaced to provide improved
pumps pumping station . performance of sewage pumping station.
Permanent Installation of permanent depth monitor April 2013 Depth monitor installed on the incoming
monitoring of into the foul sewers. We intend the monitor sewer to the Halton Road pumping station.
sewer levels to remain in situ for at least 5 year, and to Data will be used to identify additional
capture the next wet weather event as a actions for inclusion in the drainage
minimum. We will analyse the recorded strategy for Carterton, and information
depths and compare with other catchment shared with other agencies.
variables, such as rainfall events and
changes in groundwater levels.
Hydraulic Investigate the network capacity and Jan 2014 Options for reinforcement of the network
Modelling impact of new development flows. to accommodate the development
explored.
Site Photographic evidence was collected during Feb 2014 Sources of floodwater were documented
Reconnaissance the floods of 2014. and impacts on Thames Water assets were
noted.
In summary, following previous concerns where customers have brought flooding
that the sewerage network suffered issues to our attention, we have reacted
excessive infiltration, considerable effort swiftly to remove flood water through
has been made to better understand the tankering and to clean up
sources of this infiltration and to address any residual pollution.
the impact on our assets. On occasions
Carterton Drainage Strategy - Stage 1 29
Published version post stakeholder review v1.1You can also read
