Embed Size (px)
Citation preview
Coventry Surface Water Management Plan
Revision History
Revision Author and date Checked by and date
First issue MJ. 2nd Apr 2015 NT. 2nd Apr 2015
Second Issue OM 12th Nov 2015 NT 12th Nov 2015
Final Issue OM 10th Feb 2016 NT 10th Feb 2016
Executive Summary
Coventry has been affected by surface water flooding during storm events several times since 1998.
Current surface water hazard mapping shows 10,600 homes to be at risk of surface water flooding.
The Preliminary Flood Risk Assessment undertaken in 2010 brought together many sources of
information to form a single flood hazard map. This flood hazard mapping is updated periodically when
new reports are received or when new modelled data is available from the Environment Agency.
The Surface Water Management Plan (SWMP) is a document that will set out the long term plan for
reducing the risk of surface water flooding throughout the city. The SWMP includes methodology for the
identification and assessment of risk and options for implementation of risk reduction methods.
The SWMP was written by the Flood Risk Management and Drainage Team and reviewed by the
Environment Agency and Severn Trent Water Ltd as the Risk Management Authorities in the region.
This SWMP has identified that more information is needed on flood risk. The options considered by the
SWMP include improvements to the existing hazard mapping. The options also include the collation of
more data in the Flood Risk Asset Register and the hydraulic study of the Bablake Ward, as this
undeveloped catchment could have the greatest impact on future flooding, if substantially developed in
the future. Review of existing and the collation of more data will result in a robust catchment wide
approach to surface water flood risk management. This data will expand the currently modelled
catchments information.
Table of Contents
1 Introduction ................................................................................................................................. 1
1.1 Surface Water Management Plan .................................................................................... 1
1.2 Flooding ........................................................................................................................... 1
1.3 Background to the project ................................................................................................ 2
1.4 Definition of Surface Water Flooding ............................................................................... 3
1.5 Timescales for the SWMP ............................................................................................... 3
2 SWMP Production Process ........................................................................................................ 4
2.1 SWMP Production Phases............................................................................................... 4
2.1.1 Preparation .................................................................................................................. 4
2.1.2 Risk Assessment ......................................................................................................... 4
2.1.3 Options ........................................................................................................................ 4
2.1.4 Implementation and review .......................................................................................... 4
2.2 Progress on Each Phase of the Project ........................................................................... 6
2.2.1 Preparation .................................................................................................................. 6
2.2.2 Risk Assessment ......................................................................................................... 6
2.2.2.1 Challenges ............................................................................................................... 6
2.2.2.2 Extent of Risk ........................................................................................................... 7
2.2.3 Options ........................................................................................................................ 7
2.2.4 Implementation and Review......................................................................................... 8
2.2.4.1 Action Plan for Implementation ................................................................................ 8
2.2.4.2 Implementation Progress to Date ............................................................................ 8
2.2.4.3 Review ..................................................................................................................... 8
3 Preparation Stage ....................................................................................................................... 9
3.1 Identify the need for a SWMP study ................................................................................ 9
3.2 Scope of the study ........................................................................................................... 9
3.2.1 Areas to be covered ..................................................................................................... 9
3.2.2 Aims and objectives ................................................................................................... 10
3.3 Partners ......................................................................................................................... 10
3.3.1 Severn Trent Water Ltd. ............................................................................................ 10
3.3.2 Environment Agency .................................................................................................. 11
3.4 Data Sources ................................................................................................................. 11
4 Assessment of the Risk ............................................................................................................ 14
4.1 Introduction to the Risk Assessment Process ................................................................ 14
4.1.1 Areas Considered ...................................................................................................... 14
4.1.2 Importance of storm duration and intensity ................................................................ 14
4.2 Fluvial Flood Risk .......................................................................................................... 15
4.2.1 Main Rivers Response to Rainfall .............................................................................. 15
4.2.2 Watercourses Response to Rainfall ........................................................................... 15
4.3 Flood Risk from Underground Conduits and Sewers ..................................................... 16
4.3.1 Severn Trent Water Ltd. Sewers ................................................................................ 16
4.3.2 Highway Drainage ..................................................................................................... 17
4.4 Private Drainage Systems ............................................................................................. 17
4.5 Groundwater Flooding Risks.......................................................................................... 17
4.6 Surface Water flooding risks .......................................................................................... 18
4.7 Specific areas of flood risk ............................................................................................. 18
4.7.1 Overview .................................................................................................................... 18
4.7.2 Site 1 – City Centre .................................................................................................... 18
4.7.3 Site 2 - Sherbourne Fields ......................................................................................... 18
4.7.4 Site 3 - Kingfield Road ............................................................................................... 18
4.7.5 Site 4 - Bennetts Road ............................................................................................... 19
4.7.6 Site 5 - Hen Lane ....................................................................................................... 19
4.7.7 Site 6 - Duggins Lane ................................................................................................ 20
5 Current and Future Actions ....................................................................................................... 21
5.1 Introduction .................................................................................................................... 21
5.2 Ongoing Actions ............................................................................................................ 21
5.2.1 Engage with professional partners to develop flood alleviation schemes .................. 21
5.2.2 Planning and Development Controls ......................................................................... 21
5.2.3 Asset Register ........................................................................................................... 22
5.2.4 Planned maintenance ................................................................................................ 22
5.2.5 Highway Maintenance ............................................................................................... 23
5.2.6 Reactive maintenance and emergency response ...................................................... 23
5.2.7 Hazard Mapping ........................................................................................................ 23
5.2.8 Significant Drainage Problems ................................................................................... 24
5.2.9 Flood Investigation and Allocation of Responsibility .................................................. 24
5.2.10 Floods Register & Database ...................................................................................... 25
5.3 Future Actions................................................................................................................ 25
5.3.1 Bablake Ward Hydraulic Study .................................................................................. 25
5.3.2 SuDS Policy ............................................................................................................... 26
5.3.3 Local SuDS Technical Guidance ............................................................................... 27
5.3.4 Community Engagement Plan ................................................................................... 27
5.3.5 SuDS Retrofit ............................................................................................................. 27
6 Implementation ......................................................................................................................... 28
6.1 Action Plan .................................................................................................................... 28
6.2 Surface Water Management Concept ............................................................................ 28
7 Conditions on Development ...................................................................................................... 31
7.1 Discharge from a Development ..................................................................................... 31
7.2 Design Principles ........................................................................................................... 31
8 Staffing and Resourcing ........................................................................................................... 33
8.1 Capacity Building to Enhance the LLFA Team Capabilities ........................................... 33
Appendix A – Source Path Receptor Methodology ................................................................................ 35
Source Path-Receptor Methodology for assessing flood risk ................................................... 35
Appendix B – Flood Risk Assessment Requirements ............................................................................ 37
All applications .......................................................................................................................... 37
Site Details ............................................................................................................................... 37
Explanation of how the drainage hierarchy has been followed. ................................................ 37
Where to find data on Flood Risks ............................................................................................ 37
Outline applications .................................................................................................................. 37
Flood Risk to the Site ............................................................................................................... 37
Mitigation of the Flood Risk ...................................................................................................... 38
Surface Water Design for the Site ............................................................................................ 38
Explanation of likely forms of SuDS for the site ........................................................................ 38
Full, Detailed, Discharge of Conditions or Reserved Matters ................................................... 38
Flood Risk to the Site ............................................................................................................... 38
Mitigation of Flood Risk to the Site ........................................................................................... 39
Surface Water Design for the Site ............................................................................................ 39
Detailed Drainage Plans for the proposed development. .......................................................... 40
Calculations of current runoff from site ..................................................................................... 40
Calculations of proposed runoff from site ................................................................................. 40
Calculations of proposed discharge from site ........................................................................... 40
Full explanation of the forms of SuDS used on the site ............................................................ 40
Residual Risk. ........................................................................................................................... 41
Phasing..................................................................................................................................... 41
Appendix c – Hierarchy of Documents ................................................................................................... 42
Glossary ................................................................................................................................................ 44
List of Figures
Figure 1- SWMP Wheel ........................................................................................................................... 5
Figure 2 - Phases of a SWMP ................................................................................................................. 6
Figure 3 - Main river network in Coventry ................................................................................................ 9
Figure 4 - River flooding, Butt Lane ....................................................................................................... 15
Figure 5 - Flooding on Duggins Lane ..................................................................................................... 16
Figure 6 - Bablake Study Area ............................................................................................................... 26
Page | 1
1 Introduction
Under the Floods and Water Management Act 20101 and Flood Risk Regulations (2009)2, Local
Authorities such as County Councils and Unitary Authorities have new responsibilities for a leadership
role in local flood risk management. This is a new role for upper tier Local Authorities and an extended
role for Unitary Authorities, as Lead Local Flood Authority (LLFA) under the Act and includes the
requirement to “develop, maintain, apply and monitor a strategy for local flood risk management in its
area (a “local flood risk management strategy”)3. Production of a Surface Water Management Plan
(SWMP) is a key supporting document to deliver flood risk management and to fulfil this statutory role.
1.1 Surface Water Management Plan
The definition, within the Technical Guidance, of a SWMP is: “A framework through which key local
partners with responsibility for surface water and drainage in their area work together to understand the
causes of surface water flooding and agree the most cost effective way of managing surface water
flood risk4”. This is important as the SWMP requires regular liaison with both Severn Trent Water Ltd
and the Environment Agency to share information and work in partnership to meet the requirements of
the SWMP. The integrated approach to working with professional partners has greatly benefitted the
outcomes from the SWMP and has served to strengthen links between the City Council and its partner
RMAs.
1.2 Flooding
For the purposes of the SWMP, “flooding” is defined as the inundation of property and infrastructure, by
water, that causes damage or nuisance to property or infrastructure. This definition includes the
occupied areas of a dwelling but excludes garden and garage spaces.
The costs from repair or recovery after flooding include:
• Repairing damage to buildings and other property due to inundation and contamination by
floodwater
• Repairs to infrastructure damaged by the floods including highways, telecoms, electrical
services, gas supply, water supply and sewerage services
• Temporary accommodation for the victims of flooding.
• The wider community costs of disruption to employers, people unable to attend their workplace
or the recovery time to resume normal business activities.
1 http://www.defra.gov.uk/environment/flooding/policy/fwmb/key-docs.htm Flood & Water Management Act
2010
2 Statutory Instrument No. 3042 2009
3 Section 9 of the Flood & Water Management Act 2010.
4 http://www.defra.gov.uk/environment/flooding/manage/surfacewater/index.htm
Page | 2
A consequence of the damage or disruption is often calculated as a cost and this value is used in
economic appraisals of options to remedy or reduce the risk of flooding. The inundation of gardens and
areas that do not suffer a great degree of damage as a result of the flooding incident are not considered
as a serious flooding problem unless the water may pass through these areas or more vulnerable
receptor.
The degree of vulnerability in the context of flooding for new development is defined within Table 2 of
National Planning Policy Framework Technical Guidance5. The table provides a useful way of
classifying the vulnerability of properties at risk of flooding and is primarily used for the assessment of
proposed new development at the planning application stage and it also provides background
information for the SWMP.
1.3 Background to the project
Historic flooding events in April 1998 (Midlands), November 2000 (Midlands), January 2005 (Carlisle),
June/July 2007 (Midlands and Yorkshire), September 2008 (Morpeth) resulted in several thousand
properties being inundated due to fluvial, pluvial and sewer flooding – in some cases from more than
one source. In response to these events, the Government commissioned a series of reviews and
reports to examine the flood risk management procedures and the level of protection to vulnerable
communities. A number of reports and guidance documents have been published including ‘Learning to
Live with Rivers’6, the ‘Pitt Review’7 and NPPF8 that recognised the need for improved surface water
and flood risk management, also the need for new development to be designed to minimise the future
flood risks both to the development itself and also to other 3rd party properties where the risk may be
increased as a consequence of the development taking place. In particular the Pitt Review identified a
need for integrated surface water management throughout the UK to address the previous lack of a
coordinated approach to the reduction of flood risk and management of surface water.
In 2008 and 2009 DEFRA commissioned Integrated Urban Drainage (IUD)9 studies to examine the
interactions between surface water, sewers and fluvial drainage systems in fifteen urban areas in
England that had suffered from flooding to some extent in recent years. These ‘IUD Pilots’ were
informative in helping to identify good practice approaches and contributed to the development of the
‘Living Draft’ SWMP Guidance. A recurring theme in the IUD studies was the interaction between
different drainage systems and also the complexities of urban flooding mechanisms. Following on from
the IUD studies, six ‘first edition’ Surface Water Management Plans were produced to test the draft
SWMP guidance (published in February 2009). The SWMPs were produced for Gloucestershire, Hull,
Leeds, Richmond and Kingston, Thatcham and Warrington and helped to refine the technical guidance
and highlight difficulties and challenges in producing an effective flood risk assessment and
5 https://www.gov.uk/government/publications/national-planning-policy-framework--2
6 Learning to Live with Rivers – Institution of Civil Engineers Nov 2001
7 The Pitt Review – Sir Michael Pitt 2008
8 National Planning Policy Framework Technical Guidance 2012
9 http://www.defra.gov.uk/environment/flooding/manage/surfacewater/urbanrisk.htm
Page | 3
management plan. The final technical guidance was published in March 2010 and forms the basis of all
the SWMPs being produced across England at the present time.
Coventry City Council was awarded funding to produce a SWMP as one of 77 local authorities receiving
a share of £15 million to produce a SWMP on the basis of the cumulative number of properties deemed
to be at risk from surface water flooding. The number of properties at risk from flooding in Coventry is
10,600.
1.4 Definition of Surface Water Flooding
Surface water flooding has been defined for the purposes of this exercise as flooding from sewers,
drains, small watercourses, ditches and groundwater that occurs during heavy rainfall. It includes:
• Pluvial flooding; flooding as a result of high intensity rainfall when water is ponding or flowing
over the ground surface (surface runoff) before it enters the underground drainage network or
watercourse, or cannot enter it because the network is full to capacity.
• Sewer flooding; flooding which occurs when the capacity of underground systems is exceeded,
resulting in flooding inside and outside of buildings. Normal discharge of sewers and drains
through outfalls may be impeded by high water levels in receiving waters.
• Flooding from small open-channel and culverted urban watercourses which receive most of
their flow from inside the urban area.
• Overland flows from the urban/rural fringe entering the built-up area, including overland flows
from groundwater springs.
1.5 Timescales for the SWMP
The SWMP will be published in 2015 and will be regarded as a living document. As a policy the SWMP
will be reviewed and reissued every 3 years.
Page | 4
2 SWMP Production Process
The SWMP production process comprises four main parts as shown on the ‘wheel’ diagram (shown in
figure 1 and reproduced from the SWMP Technical Guidance March 2010). The stages are detailed
below:
2.1 SWMP Production Phases
2.1.1 Preparation
• Identify the need for a SWMP study
• Establish Partnerships – identify who should be involved and establish roles and
responsibilities.
• Scope the SWMP study
2.1.2 Risk Assessment
• Undertake a strategic assessment – collate information and undertake assessment.
• Undertake intermediate assessment – collate information, undertake assessment and
determine whether a more detailed assessment is required.
• Undertake detailed assessment – select modelling approach, develop models and determine
present and future flood risk.
• Map surface water flood risk and communicate this to relevant parties.
2.1.3 Options
• Identify and short list measures,
• Assess options – identify type of assessment to be carried out, assess the options and agree
on the preferred option(s).
2.1.4 Implementation and review
• Prepare Action Plan, review and publish.
• Implement and review Action Plan.
Page | 5
Figure 1- SWMP Wheel
Page | 6
Figure 2 - Phases of a SWMP
2.2 Progress on Each Phase of the Project
2.2.1 Preparation
This phase is complete and utilised the existing cooperative relationships the City Council has with both
the Environment Agency and Severn Trent Water Ltd as the main Risk Management Authority (RMA)
partners in the process. A project plan was agreed in February 2010 with clear ambitious aims and
objectives that were designed to lead to a greater level of understanding of the surface water flood risks
faced by Coventry City and also the scope for managing these in the short, medium and long term.
2.2.2 Risk Assessment
2.2.2.1 Challenges
Asset Information
During the early stages of the project it became apparent that the risk assessment stage would be a
particular challenge as the level of information available for some drainage assets within Coventry was
insufficient to enable detailed risk assessments. The asset information relating to the condition,
capacity and dimensions of the drainage systems operated and maintained by Severn Trent Water Ltd
is generally good, although there are gaps in this knowledge following the vestment of lateral drains
with Severn Trent Water. Less is known about the details, condition and capacity of highway drainage,
culverted watercourses and ordinary open watercourse systems regulated by the city council. During
the project and as an on-going action the Flood Risk Management and Drainage Team continue to
collate asset condition information for culverts and associated watercourses. A comprehensive
programme of work is needed to strengthen and extend the asset database and also to identify
shortfalls in performance and reduce the risk of flooding, where necessary. It is expected that this work
will continue as part of a long term programme on a risk and resource capacity basis.
Page | 7
Flooding Event Records
Historic data was sourced and collated by Coventry University for this project. This data will be
enhanced in the future as part of the plan to enable more information to be captured and more
comprehensive records.
Additionally the Act places a requirement on LLFAs to objectively investigate flooding incidences, when
they are notified of these; therefore beyond the scope of the specific targeted data collation project,
flood risk asset data will improve.
Topographical survey information
The nature of surface water flooding and in particular, overland pluvial flows, means that the accuracy
of topographical survey information is vital in order to correctly identify properties at risk of flooding.
Producing overland flow models for relatively shallow (<50mm), fast-moving flows of water requires a
great deal of accurate survey and site investigation information together with site walkovers to assess
the flow paths and the degree of risk. Aerial survey information exists for a large proportion of Coventry
however the accuracy of this is limited to approximately 100mm tolerance. Linking the aerial surveys
with ground-based topographical surveys referenced to OS benchmarks can improve the accuracy
however this is an extensive undertaking and a long term goal. Despite this challenge, some risk
assessment is nevertheless possible although the information derived at this stage should be read in
context until such time as it can be verified with more accurate level data.
2.2.2.2 Extent of Risk
The Preliminary Flood Risk Assessment (PFRA) identified the risk of flooding from surface water
flooding. This PFRA requires more detailed analysis to include the effects of local drainage systems to
develop site specific risk assessment and this is a long term goal to achieve. The sources are sewer
flooding, overland flows, highway flooding and flooding from ordinary watercourses.
Overall, it has been estimated that around 10,600 properties may be at risk from surface water flooding
alone, neglecting the additional risks from large fluvial systems. Some of the risk arises from
interactions between sewer and large fluvial systems as the discharge of combined sewer overflows
and surface water outfalls into watercourses are compromised when water levels in the receiving
watercourse are too high leading to a surcharged outfall and resulting in increased risk of flooding in the
lower reaches of the sewer network.
There are further issues within the city where residents are limited in their choice of insurers or face
high premiums and excesses due to flood risk. Work is being undertaken nationally by the Environment
Agency and National Flood Forum to enable residents to be insured.
2.2.3 Options
The SWMP is focussed on a more detailed assessment of flood risk from surface water flooding as well
as practical and efficient management. The aims at the outset of the project included both structural
and non-structural objectives. Structural objectives include physical tangible work ‘on the ground’ to
manage and reduce the risk of flooding, where possible. Non-structural options will include undertaking
more detailed assessments of risk, hydraulic studies and improved mapping and asset information. This
Page | 8
work has commenced with improvements to the intake arrangements to some culverted watercourses
and vegetation clearance to further investigate flood risk areas and improve access and inspection of
flood risk management assets. Much work remains to be done, however the action plan in this
document details the elements of work, timescales, anticipated staffing resource and cost implications
for this work.
The options appraisal is about getting the correct option for improving the level of flood defence or
surface water management, based on objective criteria and a robust decision making process. As
described earlier, the need for robust risk assessment information is a prerequisite to being able to
perform an objective appraisal of options for the management of risk, therefore the emphasis for the
City Council remains on the continued collation of asset data.
2.2.4 Implementation and Review
2.2.4.1 Action Plan for Implementation
The final sections of this document will set out the action plan going forward. This action plan is split
into sections to identify short, medium and long term targets for both structural and non-structural
actions as an outcome of the SWMP.
2.2.4.2 Implementation Progress to Date
The implementation process for the SWMP has already commenced with plans for improvements in
communication links with Risk Management partners and other teams within Coventry City Council. The
implementation of the action plan is, dependent on the allocation of resources and funding to support
the actions. Additional funding will be needed to conduct surveys and inspections, and carry out
maintenance and improvement works to flood defence and drainage infrastructure within Coventry.
2.2.4.3 Review
An important part of the SWMP is the periodic review of progress and assesses effectiveness and cost
benefit of outcomes. It is considered that the SWMP should be monitored by the Flood Risk
Management Group to both objectively assess the performance and sanction actions.
Page | 9
3 Preparation Stage
3.1 Identify the need for a SWMP study
Coventry City Council’s Preliminary Flood Risk Assessment identified areas at high risk from surface
water flooding, as well as locations requiring hydraulic analysis. The SWMP study provides an in-depth
evidence base in relation to the flooding issues in these areas. Furthermore, the large amount of new
development planned throughout Coventry will present a challenge to the existing drainage
infrastructure. It is important that these challenges are managed and that opportunities are taken to
improve water issues, in terms of both quantity and quality.
3.2 Scope of the study
3.2.1 Areas to be covered
Figure 3 - Main river network in Coventry
Coventry City's administrative area is approximately 99km2, and is bisected by the River Sherbourne
flowing northwest to southeast and the River Sowe flowing from northeast to southwest around the City
(figure 3). The Canley Brook and other tributaries feed into these two main rivers in the south of the City
area.
Page | 10
3.2.2 Aims and objectives
Aims:
• To gain a comprehensive understanding of the flood risk, from surface water and other
sources, in Coventry
• To establish a collaborative working relationship among partners and stakeholders
• To understand the impact of current and future developments on flood risk and drainage
provisions, and to subsequently agree planning mitigation measures
• For the study to further the Council’s understanding of flood risk in the city, and therefore meet
their duties as LLFA.
Objectives:
• Investigate the various sources of flooding which could affect Coventry, taking into account the
impacts of other variables such as climate change, population change and increasing
development
• To identify and prioritise specific areas that are at greatest risk of surface water flooding, in
locations with significant drainage problems,
• Undertake regular communication with partners and stakeholders to raise awareness of the
flood risks throughout Coventry, and the ways in which flooding can be mitigated
• To develop robust conditions and comments on planning applications, ensuring that existing
and future flood risk is managed, along with sustainable drainage solutions
• To establish links between other emerging strategic policies in Coventry.
3.3 Partners
Our local flood risk management authority partners in the region are Severn Trent Water Ltd and the
Environment Agency. At present these two organisations form the primary source of flood risk
information available to us.
3.3.1 Severn Trent Water Ltd.
Severn Trent Water Ltd has a duty to manage the adopted public sewer assets within the Coventry City
area and these include foul, combined and surface water sewers. The performance of the sewer
systems is monitored by Severn Trent Water Ltd and they aim to reduce the risk of sewer flooding
where this has been reported. Their regulatory body, OFWAT, places emphasis on reducing the
incidence of sewer flooding rather than all public sewers achieving a particular performance standard.
The present standard for public sewer design can be found within Sewers for Adoption. Severn Trent
Water Ltd has a responsibility to provide, improve and extend the system of public sewers to ensure
that the area is, and continues to be, effectually drained, in accordance with Section 94 of the Water
Industry Act 1991. Severn Trent Water Ltd record incidents of flooding on a sewer flood risk register,
which is a modified form of the DG5 register.
As part of an on-going commitment by all flood risk management partners, Severn Trent Water Ltd will
be working closely with Coventry City Council to reduce risk resulting from the sewer network.
Page | 11
The sewer flooding register is not a log of past sewer flooding events, but is an ever changing register
of current known flooding issues that have not yet been resolved. Entries on the register are removed
when a STW scheme has been completed to alleviate flooding (usually up to a 1 in 40 year return
period standard of protection) or investigations suggest it is neither a sewer flooding problem nor a
sewer hydraulic problem. The entries on the register are deemed to be caused by the sewer being
overloaded due to a rainfall event. If it is a flooding due to a structural or blockage problem on the
sewer, then these are not recorded on this register. (Referred to as ‘flooding other causes). More recent
entries are subject to an initial investigation to determine the probable cause. Historic entries on the
register are subject to regular reviews.
3.3.2 Environment Agency
The Environment Agency is responsible for taking a strategic overview of the management of all
sources of flooding and coastal erosion. It has operational responsibility for managing the risk of
flooding from main rivers, reservoirs, estuaries and the sea, as well as being a coastal erosion risk
management authority. The Environment Agency is therefore a key partner in the SWMP process and
in the mitigation of flood risk. The assessment and management of the fluvial flood risk associated with
Main Rivers is specifically excluded from the SWMP however the interaction between these and the
other surface water drainage systems requires that their behaviour is understood and taken into
account in assessing the overall flood risk. In this regard, Severn Trent Water Ltd and the Environment
Agency liaise regarding river water levels and behaviour of both fluvial and sewer systems at the points
where they interact in order to improve the accuracy of the models that they are using.
3.4 Data Sources
A decision was taken at the commencement of the SWMP to base the outputs on a Geographic
Information System platform. This would enable tis indicative flood risk and asset database information
to be readily available to assist multi discipline Council functions. In this respect the data sources have
been collated and transferred to the GIS database and will be updated on a periodic basis.
Page | 12
Data source Data description including confidence level.
Coverage Updates
Environment Agency Fluvial flood risk maps. Flood Zones 2 & 3.
100% Every 3 months
Environment Agency Historical risk maps Available for the main rivers
Every 3 months
Environment Agency Pluvial flood risk maps – assess flooding scenarios as a result of rainfall with the following chance of occurring in any given year:
• 1 in 30
• 1 in 100
• 1 in 1000
They also provide extent, depth, velocity and hazard data for each flooding scenario
City wide Maps date from April 2013. These are referred to in EA documentation as the ‘updated Flood Map for Surface Water’ (uFMfSW)
Environment Agency The National Receptor Dataset – a spatial dataset which contains a number of layers categorised into themes of buildings, transport, utilities, land use, agriculture, heritage, environment and miscellaneous
City wide Maps date from 2011
Environment Agency Surface elevation data
and DTM from LIDAR
surveys.
80% of City Areas have been flown within last 5 years.
Severn Trent Water
Ltd
Records of flooding
incidents that they are
aware of (postcode
level only)
City Boundary The sewer flooding register is an ever-changing register of current known flooding issues that have not yet been resolved.
Severn Trent Water
Ltd
Drainage asset
database covering
pipes, manholes etc
under STW assets
City wide Correct at October 2013
Page | 13
Coventry City Council Flooding records –
highway, garden and
domestic known
flooding incidents
City Boundary
Approx. 100%
complete
Live data
Coventry City Council GIS data including
historical maps
showing areas pre-
development,
ordnance survey base
mapping,
for intakes, headwalls
and culverted
watercourses
City Boundary
approx. 75%
complete
Live data
Page | 14
4 Assessment of the Risk
4.1 Introduction to the Risk Assessment Process
4.1.1 Areas Considered
As part of the initial work on the SWMP, the project team considered the flood risks to the City and
evaluated the vulnerability in broad terms to each risk. The current Flood Risk Hazard Mapping is held
on a Geographic Information System within the Flood Risk Management and Drainage Team. The list is
not exclusive and it is expected that the flood risk areas will be refined in future years as the modelling
and forecasting improves as an outcome of the SWMP.
4.1.2 Importance of storm duration and intensity
Important variables in this are the rainfall intensity, duration of the storm and the response time of the
catchment.
• Longer duration rainfall events on average have lower intensity than the shorter events.
• The longer a rainfall event is, the greater the catchment area that may feed runoff water into a
particular point downstream. In the case of Coventry, with the exception of the River Sowe,
Canley Brook and River Sherbourne catchments, the drainage areas are considered to be
small. Therefore the potential flows in the downstream reaches of the small catchments are
correspondingly small, in the order of a few cubic meters per second (m3/s) rather than 10s of
m3/s.
• In this respect, prolonged steady rainfall is not a particular risk to most of the City as the flows
are small and the drainage infrastructure is on the whole able to cope with these flows.
• Conversely, shorter more intense storms are a particular concern as the rainfall intensities
combined with the short flow paths and short times of concentration through the urban areas
can lead to interaction between drainage systems and water from a number of discrete
catchments arriving in the same area simultaneously. This phenomenon has led to flooding on
a number of occasions in recent years.
• There are a few exceptions to this general observation, however these are localised. Areas
within the flood plain of the large fluvial sources (Rivers Sowe, Sherbourne and Canley Brook)
that lie adjacent to large capacity sewers or culverted watercourses are more often affected.
One reason for highlighting these at this stage is that the flood risk reduction measures for
these areas are generally more complex due to multiple inflows. In addition to the multiple
sewer/overflow connections there are numerous small private outfalls, highway drainage
outfalls and confluences with small culverted watercourses.
Page | 15
4.2 Fluvial Flood Risk
Figure 4 - River flooding, Butt Lane
4.2.1 Main Rivers Response to Rainfall
The flood risk from Main River in response to rainfall is areas alongside the banks and in low lying
areas adjacent to the watercourses. The extent of these areas tends to vary in proportion to the size of
the parent watercourse and the anticipated flood flows. The River Sherbourne floodplain is extensive
and properties within this may be considered at risk of flooding if they are at a similar level to the banks
of the river. The flood defences alongside the larger rivers limit the extent of the flooding that might
occur as even if they are overtopped or breached, there is a finite amount of water that would flow into
the flood-cell area behind the failure point of the defences. The risk from the main rivers is discussed in
detail within the Coventry Strategic Flood Risk Assessment (SFRA) and the PFRA. The performance of
the river defences for these watercourses is not discussed in detail within the SWMP report however
the flood risk areas are held within the SWMP GIS model and may be examined as needed.
Flood risk from Main Rivers can be broken down into 3 broad categories. These are also known as the
Flood Zones.
4.2.2 Watercourses Response to Rainfall
There are a number of small watercourses that enter the main rivers that exhibit a response to rainfall
and increase floodrisk. Unlike the larger watercourses, the flood risk tends to be concentrated around
throttle points including:
• intakes/trash screens upstream of culverted sections,
Page | 16
• small bridges and restrictions in the watercourses due to vegetation growth,
• fly-tipping,
• ad-hoc culverting by Riparian owners, often without knowledge of the capacity requirements.
The watercourses require maintenance on a regular basis to minimise the flood risk as a response to
surface water inflows, however budgetary constraints preclude maintenance by Local Authorities.
Riparian owners are being encouraged to exercise their duties under the Land Drainage Act to ensure
maintenance needs are met.
4.3 Flood Risk from Underground Conduits and Sewers
Figure 5 - Flooding on Duggins Lane
4.3.1 Severn Trent Water Ltd. Sewers
Coventry has an extensive network of sewers that are separated into foul, combined and surface water
sewers. Sewer flooding risks arise from:
• Rainfall events of a severity that exceeds the design performance criteria.
• Older sewer systems that do not meet the performance requirements.
• Blockages and damage that may occur to a sewer system.
• High water levels in watercourses preventing outfalls from functioning correctly with
consequent flooding in the upstream sewer system.
• “Urban creep” is a term used to describe the increase in impermeable areas connected to the
sewers due to infilling of green spaces in urban areas and also the construction of additional
Page | 17
driveways, building extensions, conservatories, garages and patios. In localised areas this can
add 25% or more to the areas draining to the sewers and poses a serious risk to properties
downstream.
4.3.2 Highway Drainage
Coventry has approximately 44,500 road gullies with connections into Severn Trent Water Ltd owned
surface water or combined sewers, Coventry owned separate highway drainage systems or directly to
watercourses. Flooding from the Highway is often caused by small localised problems such as a
collapse or blockage. These are dealt with, on the whole, by Severn Trent Water Ltd where the issue is
associated with their asset through reactive jetting and engineering works. When an issue causes
flooding from highway drainage the City Council undertake cleansing and repair works on a priority
basis.
The performance of the highway drainage system often determines whether properties adjacent to
highway flood in heavy rainfall events. The highway drainage system may be compromised in a number
of ways that increases the flood risk as follows:
• Gully grates becoming blocked by leaf litter and debris or the gully chamber filling with silt from
the road surface.
• The gully pipework connections may fail either due to collapse or root ingress.
• The highway drainage system is designed to drain the highway areas only. In some instances it
has been determined that frontage developments off the highway slope towards the
carriageway but do not have any intercepting drainage system to prevent flows entering the
dedicated highway drainage system. In some cases this can cause overloading of this highway
drainage. Under Section 163 of the Highways Act 198010, the Local Authority may take action
to prevent water flowing directly onto the surface of the highway.
4.4 Private Drainage Systems
These include both small domestic systems and large networks serving industrial and commercial
estates. In general, flooding incidents within private properties by a private drainage system have not
been reported to the City Council therefore the risk of these systems to other 3rd party properties is not
known. It is nevertheless possible that derelict properties or poor maintenance may compromise the
efficiency of these systems and lead to flooding both on and off the site. Based upon service requests
received the scale of this problem is thought to be small.
4.5 Groundwater Flooding Risks
City wide groundwater information is being sought from a number of sources to build up an
understanding of the wider risk. Isolated incidents of groundwater flooding are investigated on a priority
and risk basis. The city was subject to historic mining which has an influence on groundwater levels.
10 http://www.legislation.gov.uk/ukpga/1980/66/section/100
Page | 18
4.6 Surface Water flooding risks
Owing to the uniform and relatively flat topography in the majority of the city, there are few areas in
which very high velocity flows of surface water have been reported. The central city area is bisected by
the River Sherbourne of which considerable lengths are in culverts. Extreme rainfall may exceed
hydraulic design parameters for some culverts and small bridges. In such cases pluvial flooding
(primary element of surface water flooding) is considered to exacerbate fluvial flood risk. Where
watercourses have been culverted, particularly in the centre of Coventry, the route of these
watercourses has been extensively built over. In many cases, infrastructure has been constructed
across the natural flow paths creating barriers which will increase the risk of potential deep ponding.
This could cause significant damage to property and possibly a risk to livelihood and wellbeing in the
event of extreme weather conditions.
The flat nature of the terrain in many areas also means that such areas can be susceptible to
widespread shallow ponding if the surface runoff cannot drain away easily. Whilst unlikely to pose a
serious risk, these areas have the potential to cause minor flooding to very large numbers of properties,
particularly if building threshold levels are low.
4.7 Specific areas of flood risk
4.7.1 Overview
The Coventry PFRA identified the following sites to be “…areas within Coventry which are most
susceptible to surface water flooding.” Since the publication of the PFRA work has progressed to better
understand the flood risk at these locations and this data has been included here.
4.7.2 Site 1 – City Centre
City Centre is the heart of Coventry City with high densities of businesses and offices. It generally has
no major flow paths that could result in deep, fast flowing water but the existence of undulating terrain,
basements, under story car park means that during intense rainfall, some shallow ponding at those
underground zones might occur. Widespread flooding could result in significant damages to business
and private properties.
4.7.3 Site 2 - Sherbourne Fields
Sherbourne Fields School is located adjacent to a floodplain area of River Sherbourne. It is at the lower
end of a gentle sloping hill within a residential area. Tiverton School and Kingsbury Road experience
sewer flooding in 2001.
4.7.4 Site 3 - Kingfield Road
Coventry City Council flood records shows that there have been more than 6 historical flood events at
74-86 Kingfield Road since 1980.
Flooding occurs in the highway and in extreme circumstances can enter private property. Investigations
suggest that the water is arriving from multiple sources.
• The Springfield Brook culvert runs across the highway at this location.
Page | 19
• The Severn Trent Water Surface Water sewer network has 2 discharges into the Springfield
Brook Culvert.
• The topography of the surrounding area channels surface water onto the highway at this
location.
The exact mechanism of the flooding is unclear and is likely to be due to the interaction of all three
sources of flow. Further study is required in partnership with Severn Trent Water to better understand
this flooding.
4.7.5 Site 4 - Bennetts Road
Coventry City Council flood record shows that there have been four historical flood events at Watery
Lane and Penny Park Lane since 1980.
Penny Park Lane and the school are located within lower end catchment zone with flat gradient. At this
area, relatively shallow depths of surface water are likely to cause traffic problems and pedestrian
access for school children could become difficult.
Flooding also occurs on Bennetts Road 200m north of the junction with Fivefield Road. This flooding is
caused by the poor repair of a culvert running under private properties and the highway. To reduce the
impact of surface water on the problem works were undertaken in 2014/15 to improve the highway
drainage to the north of the affected site. However repairs to the culvert are required to reduce the risk
of flooding further.
It is believed that there is a historical connection from the Bennetts Road Culvert to Watery Lane
however it is unclear if this connection still exists across Prologis Park.
4.7.6 Site 5 - Hen Lane
Coventry City Council flood record shows that there have been 5 historical highway flood events at Hen
Lane since 1980. The flooding takes the form of an accumulation of water underneath the railway
bridge close to the junction with Winding House Lane.
The Wyblynd Brook runs along Hen Lane at this location and is the receptor for all surface water
drainage. It is likely that the 900mm culvert is running at full capacity in high intensity rainfall events. It
is unclear whether the main cause of the flooding is surcharge from the culvert, surcharge from public
sewers unable to discharge to the culvert or inefficient highway drainage causing a build-up of water on
the surface.
Due to the moderate to steep gradient at Hen Lane, during intense rainfall events, surface water flowing
through Hen Lane at high velocities towards the bridge and roundabout could be a risk factor. Any flood
events at the bridge are likely because of traffic delays if vehicles have to slow down through standing
water, which causes congestion and increase risk of Road Traffic Accidents (RTA).
The culvert is being modelled in 2015 as part of the SFRA update and this model will inform the way
forward at this site.
Page | 20
4.7.7 Site 6 - Duggins Lane
In 2008, there was a recorded flood event in which 25 properties were affected. Flood records shows
surface water accumulated at the junction of Duggins Lane/ Station Avenue. Surface water is collected
into a both sides of lane with few road gullies. There is an electricity substation is located between 31
and 33 Duggins Lane.
Due to the potential damage caused to private residents and the critical infrastructure of the substation
a scheme of flood mitigation works was undertaken in 2014/15. Thresholds to the properties were
raised and extra highway drainage provided to offer enhanced protection when flooding occurs. The
site will continue to be monitored.
Page | 21
5 Current and Future Actions
5.1 Introduction
The work to date by the City Council has produced drainage schemes and reduced the flood risk to
many properties. The work undertaken through the Capital Works Programme includes improvements
to highway drainage, improvements to land drainage and enforcing residents’ Riparian responsibilities.
The transfer of Consenting for Works on Ordinary Watercourses in 2012 also enabled the Flood Risk
Management and Drainage team to gain much tighter controls on the discharges to watercourses within
the city.
Data has also been gathered through better flood incident recording and asset inspections. This data
has enabled the GIS based Flood Hazard Mapping to be improved. Investigations into flood events
have also improved the knowledge of Flood Risk throughout the city.
Many of these actions are detailed below with additional actions identified for implementation.
5.2 Ongoing Actions
5.2.1 Engage with professional partners to develop flood alleviation schemes
The annual capital works programme is delivering the drainage repairs within the City area. This
programme of works is an on-going list of prioritised projects deliverable according to annual funding
allocations available. There is also a joint venture between Coventry City Council, Coventry University
and the wider partners under the title of CLuSteReD. This collaborative working could influence regional
funding to bring forward projects identified within the Ripple Effect Project.
5.2.2 Planning and Development Controls
The SFRA produced by Coventry City Council in 2008 outlined some key requirements for the
management of surface water. These requirements place an emphasis on new developments and
redevelopments to manage the surface water locally and not increase the risk to properties either on
site or downstream. The requirements are outlined below and expanded to enable future developments
to be more sustainable and have a lower impact on existing drainage systems and resulting flood risk.
These will inform the Drainage Supplementary Planning Document to be developed following the
publication of the Local Flood Risk Management Strategy.
• All sites (including Flood Zone 1) will be considered as Qbar greenfield, even those that were
previously developed.
• All flood flows will need to be controlled through SuDS to Qbar, for all storm events up to the
100 year + climate change.
• All SuDS features must deliver improvements in water quality as well as reduced peak flow
volumes
• All developments and redevelopments must seek to not culvert watercourses within site and
where possible restore culverts to a natural water course status.
• All new developments will require an appropriately scaled Flood Risk Assessment.
Page | 22
5.2.3 Asset Register
A programme of surveying all the significant drainage assets will be undertaken to identify assets and
locate them on the mapped database. The survey programme is necessarily limited by the access to
the drainage assets as follows:
• Permission to access land outside the ownership of the City Council will be required
• Vegetation may need to be cleared in order to provide adequate and safe access to the asset
• For assets under carriageways, access to these must include suitable traffic management
• Debris and accumulated silt may need to be removed prior to undertaking CCTV surveys of
highway drains and culverted watercourses.
There are a substantial number of drainage assets which have not been inspected in detail for some
time; therefore it is likely that a programme of enabling work will be required to permit surveying to take
place.
5.2.4 Planned maintenance
The philosophy behind the planned maintenance regime should be to minimise the flood risk from all
sources. At the present time, there are many discrete flood risk areas shown on the ‘updated Flood
Map for Surface Water’ (uFMfSW) however many of these lie within private properties and the flood
paths are outside the control of the City Council. Conversely the flood risk to many other areas might be
reduced through a planned maintenance regime that interrupts the source-pathway-receptor
mechanism, preventing flooding of a property from a particular source.
The most practical and efficient ways in which Coventry City Council can act to manage the flooding
risk at the present level and/or reduce this where possible are:
• Encouraging a reduction on the run-off from development areas,
• Ensuring the maintenance of appropriate flood relief pathways and intercepting features to
convey floodwater away from vulnerable properties.
Within budgetary constraints, some work has been carried out over recent years however this has
tended to be reactive work in response to complaints rather than being based on an overarching flood
risk management strategy. The aim of the SWMP is to guide the work away from being predominantly
reactive and instead move to minimise intervention and deliver intelligent and directed maintenance.
Some examples of how this would be achieved are:
• Using the asset records and survey information, where available, to schedule work to manage
intakes, outfalls, embankments and other flood defence features with a priority list being
established based upon the consequences of failure or under-performance of an asset. This
work should include debris clearance, vegetation management and general inspection work.
• Planned maintenance to substantially repair or replace failing assets including intakes, outfalls,
repairing flood defence bunds and walls. It is envisaged that the improvements to flood defence
assets could also include measures to enable remote monitoring of performance, such as
water level gauges with a telemetry link to a mobile phone or a central control. These devices
Page | 23
can be used to identify when a critical trash screen is becoming blocked or where the onset of
‘out of bank’ flows is imminent. The one off cost implications of the installations would limit the
number to a few sites per financial year with the priority order based on the flood risk
consequences.
5.2.5 Highway Maintenance
The clearing of the gullies needs to be efficient and well planned.
• Gully cleansing in alignment with the public sewer cleansing schedules of Severn Trent Water
Ltd as the principle Surface Water Sewer asset owner. This will allow a better co-ordinated
maintenance approach.
• Co-ordinated cleansing of gullies with street cleansing to reduce the frequency of blockages
from leaf litter and other debris.
• Investigate issues when gullies become blocked to establish the theme and nature of the
problem and if it requires further intervention by another partner.
5.2.6 Reactive maintenance and emergency response
The responsibility as LLFA requires the City Council to actively manage the flood risk within Coventry
and some of this work would inevitably involves reacting to forecasts of severe rainfall that may affect
the flood risk to properties or critical infrastructure. The Flood Risk Management & Drainage team
currently does not operate or maintain a 24-hour callout facility. The city council does however have an
out of hour’s response service to assist residents subject to flooding from watercourses and highways.
Historically, reported incidents also include collapsed highway culverts, blocked trash screens and
grills, property flooding and debris in watercourses.
Emergency reactive works includes:
• Check and clear trash screens in advance of forecast heavy rainfall.
• Responding to reports of pollution incidents in watercourses.
• Responding to reports of fly-tipping affecting a drainage or flood-defence asset including
removal of the debris.
• Coordination of other parties to manage a flooding incident.
• Surveys of flooded areas.
• Works to investigate the cause of the flooding incident and suggest mitigation measures to
reduce the likelihood of recurrence.
Further information on the response of the City Council is outlined in the Flood Risk Response Policy.
This document also includes the Sandbag Policy. The sandbag policy outlines the circumstances for
the City Council to distribute sandbags to residents.
5.2.7 Hazard Mapping
Coventry City Council through the PFRA process now has a mapped database which shows flood risk
from different sources. This data is based on a national mapping exercise undertaken by the
Environment Agency. These flood maps do not take into account local drainage systems however. The
Page | 24
capacity and performance of the drainage assets should be assessed to investigate and ascertain
where flooding might occur either because surface water cannot enter the drainage network or where
water leaves the network due to a reduction in capacity and localised blockage scenarios. The amount
of flooding at the various surface water network junctions may then be considered in its own right to
assess whether this accumulates or flows overland towards vulnerable properties. The hydraulic
assessment would almost certainly require the use of detailed ground models and contemporary flood
risk simulation software therefore, it is likely that this phase of work where internal resources are limited
this would be undertaken by external consultants.
Utilising flow monitors and localised rainfall data, areas of risk can be mapped and linked to rainfall
data. This will allow predictions of where flooding could occur during an event. This will act as an early
warning system to allow the Council to respond with targeted clearance and inspections.
5.2.8 Significant Drainage Problems
Using indicative mapping showing predicted surface water flooding issues, the Flood Risk Management
and Drainage Team will be able to assist in the identification of areas with significant drainage
problems. These are areas outside the fluvial Flood Zones 2 and 3 which are at risk of flooding from
other sources. Through the identification of these areas, it will be necessary for developers to produce
appropriately scaled Flood Risk Assessments which include the potential surface water, sewer and land
drainage issues at the site. Identifying areas with significant drainage problems will also highlight inlet
structures and culverts which are critical in reducing local flood risk.
5.2.9 Flood Investigation and Allocation of Responsibility
Coventry City Council as LLFA investigates all flood incidents reported on a priority basis. These
investigations follow a set format. The investigations start with initial desktop studies of the area
flooded. Following these, site investigations are conducted and partners are informed and engaged as
appropriate. Once the evidence has been gathered, responsibility for the flooding is assigned to
relevant stakeholders. The City Council can conduct formal investigations in line with S19 of the Flood
and Water Management Act when the incident exceeds the thresholds below:
The LLFA is not obliged under the Flood and Water Management Act to resolve the flooding, however
they will investigate the cause and assign responsibility to any relevant authority or individual. The
LLFA will seek to conduct informal investigations on all flood events where considered appropriate.
Significant harmful consequences’
defined as greater than…
Description
200 people or
Flooded to a depth of 0.3m during a rainfall event
with a 1 in 200 chance of occurring (or 0.5%)
20 businesses or
1 critical service
Page | 25
These informal investigations will be shared with the other relevant Risk Management Authorities and
their resulting action plans will be shared with the LLFA.
Upon learning of a flood event within Coventry, the Investigating Officer will follow an investigation
process whereby it will be determined whether a formal in informal investigation should be carried out,
taking into account the available resources and significance of the event.
It is important to note that this is a technical assessment and that it is for the relevant responsible body
or persons to assess any recommendations in terms of their legal obligation, resource implications,
priority and cost/benefit analysis of undertaking such actions.
Following significant widespread flooding in the City, where a number of incidents meet the thresholds
for investigation, the investigations will be undertaken on a priority basis. This will take into
consideration factors such as the number of properties affected, the extent, depth and duration of the
flooding, the history of flooding, and the impact on infrastructure such as roads.
The formal flood investigation report will describe the flood incident and aim to determine any
contributing factors. The report will explain the roles and responsibilities of those involved, and provide
recommendations for future actions. It is important to note that it is for the relevant responsible body or
persons to assess each recommendation in terms of the legal obligation, resource implications, priority
and cost/benefit analysis of undertaking such action. The recommendations may be included within the
Action Plan linked to the Local Flood Risk Management Strategy or in the relevant risk management
authority’s future work programmes, as appropriate.
Definition of internal flooding: For these purposes, 'living accommodation' means to domestic dwellings
but NOT gardens, outbuildings such as sheds, garages etc.
Definition of close proximity: Where it is reasonable to assume that the affected properties were flooded
from the same source or interaction of sources.
5.2.10 Floods Register & Database
As identified earlier in this document the logging of flood reports has been inconsistent. To ensure that
all floods are logged in the same manner the Flood Risk Management and Drainage Team will be
working to develop a form for data collection for these occasions. Strengthening the existing data
capture systems to identify themes and issues either for Coventry City Council or other Risk
Management Authorities to address. The team will also log the results of any investigations in GIS so
that the reports can be mapped by source.
5.3 Future Actions
5.3.1 Bablake Ward Hydraulic Study
It is likely that over the next few years a large amount of new development will occur in the North West
of the City area (figure 6), as it is an area identified for future economic growth for both business and
residential usage. Whilst development in this area is crucial to the sustainability of the City, it poses an
issue for flood risk and surface water management as the topography falls towards the City. A study will
be required of this area to inform a policy for all planning applications in this area. Managed
Page | 26
urbanisation can cause an increase in the flood risk downstream. Mitigation works employed on new
developments should be monitored closely to ensure adequate functionality. This study will need to be
scoped in order to ensure it identifies the surface water flood risk this development will create and
should identify strategic flood risk and surface water management solutions to facilitate sustainable
development.
Figure 6 - Bablake Study Area
5.3.2 SuDS Policy
As an outcome of the Sustainable Drainage consultation on 18th December 2014, the ministerial
decision was that Local Planning Authorities will approve sustainable drainage solutions similar to all
other elements of building projects. The LLFA will be a statutory consultee on the management of
surface water for major development and non-statutory consultee and technical advisor to the Local
Planning Authority for minor development. This will include assessment of drainage solutions allowing
for the promotion of SuDS to reduce flood risk.
Page | 27
5.3.3 Local SuDS Technical Guidance
The implementation of SuDS will be critical for the future to deliver managed run off from all new and
re-development sites. This will ensure that no new or re-developments increase flood risk to the site, off
site and may mitigate existing flood risk issues. The SuDS will also help deliver watercourse
improvements through improved water quality, quantity, potential habitat improvements and
enhancement of amenity value. This will ensure the City complies with the Water Framework Directive.
Coventry City Council’s SuDS Technical Guidance will inform developers and will include the technical
specifications that are required for a system to be adopted by the City Council. The Guidance will
outline maintenance requirements and the funding.
5.3.4 Community Engagement Plan
A plan is being developed to allow communities to engage with the Council and partners to promote
community engagement. This work will increase the wider knowledge of Riparian ownership issues and
promote self-resilience. It will also provide an opportunity for the Flood Risk Management and Drainage
Team to engage in a data collecting exercise to build the flood register and improve the hazard
mapping.
5.3.5 SuDS Retrofit
SuDS should be retrofitted throughout the city where possible. The City Council will be looking to
deliver programmes of retrofit throughout the city. These retrofit programmes will ensure that where
there are known flooding or system capacity issues the flows are managed down both within highway
and within development. This flow management should include the use of water storage to balance
flows and reduce the peak discharges from domestic and commercial properties. It is anticipated that
there will be opportunities for collaboration with Severn Trent Water on a case-by-case basis.
Page | 28
6 Implementation
6.1 Action Plan
The action plan is an outcome of the previous chapter, and highlights which stakeholders will be
involved in the various options. A number of actions are ongoing, alongside other strategic plans.
6.2 Surface Water Management Concept
The concept for the management of surface water will follow the hierarchy of risk decisions set out in
the Surface Water Management Pyramid below:
Option Timescale Stakeholders
Asset Register On-going programme of CCTV surveys and logging of
assets
CCC
Planned
Maintenance For
Highways Drainage
On-going capital works programme including city wide
gulley repairs
CCC
Reactive
Maintenance For
Highways Drainage
On-going commitment to react to flood incidences either by
clearing screens or delivering sandbags where possible
CCC
Hazard Mapping A living document supported through data gathering and
improvements in flood models. The Hazard mapping will
develop greater confidence levels in data.
CCC
EA
Critical Drainage
Problem Areas
A living document highlighting areas with critical drainage
problems as identified by the on-going Hazard Mapping
work and other data sources
CCC
EA
SuDS Policy The LLFA will be a statutory consultee on the management
of surface water as part of the planning process. This will
include assessment of drainage solutions allowing for the
promotion of SuDS to reduce flood risk.
CCC
SuDS Technical
Guidance
This will be produced once the National Standards have
been approved and will include local information and
requirements.
CCC
Community
Engagement Plan
This plan is intended to publication in 2015 and will form a
strategy for public engagement for subsequent years.
CCC
Flood Register The flood register continues to be populated and data
gathered for future events will be more detailed.
CCC
Bablake Ward
Hydraulic Study
It is planned that this study will be scoped and started in
2015. This study will then influence all developments
following its completion.
CCC
EA
Funding Bids Partners will work together to deliver robust business
cases to bid for joint funding to develop and deliver Flood
Risk Management Schemes.
CCC
EA
STW
WWT
Page | 29
• Flow Reduction
Reducing the flows arriving at, or leaving, the site will cause the flood risk to be reduced. If a
new development upstream of a known flooding hotspot is caused to reduce the discharge
from its site the flood risk downstream is decreased.
• Diversion
By routing watercourses and flood flow routes safely past development the residual flood risk
can be minimized. By routing flows to open greenspace damages to properties can be reduced.
Flows can also be diverted utilizing hard infrastructure such as footways and carriageways.
This offers a level of protection to dwellings and property by diverting flows away.
• Storage
By storing the flood flow volumes in purpose built areas the risk of flooding to properties is
reduced. Open storage ponds and surface drainage features have secondary advantages to
boost biodiversity and improve habitats too.
• Exceedance
Planning for system exceedance is a vital part of flood risk assessment. When a system fails
the flows often result in damage to property. This damage can be avoided by proper modelling
of exceedance flow routing. New developments can utilize carriageway and footway features to
divert flows away from dwellings and into public open spaces in extreme events.
• Protection
Property level protection details can reduce the impact flooding has on homeowners by
preventing property damage. In situations where none of the above can be applied a last line of
defense is often to install property level protection. This can vary from door and air brick guards
to passive systems such as doors with built in flood barriers.
• Conveyance
Flow Reduction
Conveyance
Diversion
Exceedance
Protection
Storage
Reducing the total flow discharged from a site
Re route flows around the site to greenspace
Store flows to reduce flood risk through storage areas
Route exceedance flows utilising infrastrcture
Protecting properties from flooding when it occurs
Convey flows away through enlarging channels or sewers
Page | 30
By enlarging the watercourses or sewers the flood flows can be removed from an area where it
causes flooding. This flow may then cause additional flooding elsewhere so should only be
used in extreme cases where no other options are applicable.
This methodology applies to the management of surface water by design for new development and
interventions for surface water management for existing development.
At its highest level in the hierarchy, surface water will be managed at source to reduce discharge and
the risk associated with surface water flooding. As intervention decisions pass down the hierarchy, risk
is increased and inverse benefits emerge. It should be noted that the lowest level in the hierarchy will
be as a last resort as there is an elevated risk of surface water flooding being conveyed to others
downstream.
The Surface Water Management Pyramid forms the basis of surface water management within the city
of Coventry.
Page | 31
7 Conditions on Development
As previously stated, in its role as LLFA, Coventry City Council is a statutory consultee for major
planning applications. The LLFA can also provide advice on minor development on a non-statutory
basis, as the cumulative effect of a large number of minor developments in an area could have a
significant effect on surface water drainage.
The LLFA have devised a suite of conditions on development which are imposed on a site-specific
basis. These conditions aim to reduce the flood risk to, as well as risk emerging from, the development
and ensure that future development is sustainable and resilient to flood risk from any source. The
provision of SuDS is a necessity, with focus upon reduction of peak and total discharge. The LLFA
requires drainage discharge rates of Qbar greenfield minus 20% for developed and formerly developed
sites. The design must also consider, and provide for, surface water exceedance flows appropriately
routed to and through the development. The conditions not only seek to balance water quantity, but
deliver improvements in water quality for compliance with the Water Framework Directive. The use of
technologies such as green roofs and water reuse systems are strongly encouraged.
In addition to conditions applied by the LLFA, further conditions may be applied by the EA where main
rivers are affected, and other statutory consultees within their Risk Management Authority remit.
7.1 Discharge from a Development
Rainwater runoff from a drainage system shall discharge to one of the following listed in order of
priority.
1 An appropriate soakaway or some other surface infiltration system, or where not
reasonably practicable;
2 Reduced peak and total discharge to a watercourse, or where not reasonably
practicable;
3 Reduced peak and total discharge to the public sewer network.
7.2 Design Principles
All developments will need to adhere to the design standards below
• Watercourses have a 5m way leave, from the top on bank on either side, to allow access for
future maintenance. This way leave applies to the sides of culverted watercourses.
• Pockets of green infrastructure are to be utilised to minimise the total area of hard surfacing
within development sites.
• Where an area is susceptible to any form of flood risk, building floor slab levels will need to be
raised to a level agreed with the LLFA in writing, alongside the employment of flood resilient
construction. In areas at risk of fluvial flooding from Main River, the EA recommend that
Page | 32
finished floor levels are set no lower than 600mm above the 1 in 100 year plus climate change
flood level.
• Cellular storage is to be designed in such a way that flow surcharges into a continuous storage
void, silting of the storage void is minimised through design and the storage void is
maintainable.
• Flows and discharges are to be attenuated and managed within individual development plots
where possible to reduce the size requirements of strategic and regional SuDS features. Non-
residential development will require attenuation and the management of total flows and
discharges within plot with more stringent requirements. Additionally greater emphasis will be
placed on the management of water quality.
• The use of dish channels, slot drains or other proprietary channel drainage systems within the
highway is discouraged. Highway drainage must be in the form of gully drainage systems;
however kerb drainage systems can be utilised where expressly agreed in writing with the Lead
Local Flood Authority.
• The use of open air SuDS features should be implemented in all development sites, locally
onsite and as part of a strategic approach. The joint use of space for sustainable drainage
solutions combined with amenity should be maximised. The use of these features will provide
learning opportunities and increase the interaction of people with water as well as managing
discharges and water quality.
• Exceedance flow routing resulting from rainfall and blockage scenarios, including flow direction
and depth, must be mapped and agreed with the Lead Local Flood Authority. The hazard rating
of exceedance flows should also be considered.
• Groundwater levels for development will need to be monitored seasonally to inform the
sustainable drainage solution design.
Page | 33
8 Staffing and Resourcing
8.1 Capacity Building to Enhance the LLFA Team Capabilities
DEFRA have recognized that there are Local Authorities that are not in a position to take on the
responsibilities under the F&WM Act due to a lack of skills, experience, knowledge and capacity within
their organisation. Coventry City Council is fortunate to have an experienced team well versed within
the discipline. In view of the expanding role, and responsibilities of the City Council as LLFA, a
programme of future skill development has commenced to ensure a sustainable and resilient Flood
Risk Management and Drainage team for the future.
Page | 34
Page | 35
Appendix A – Source Path Receptor
Methodology
Source Path-Receptor Methodology for assessing flood risk
The assessment of flood risk is based on a ‘source – path – receptor’ model where:
• The ‘source’ is the catchment of the watercourse that forms the flooding risk therefore an
understanding of the behaviour of this in response to a rainfall event is required. The behaviour
of the catchment is referred to as the hydrology and includes the amount of run-off and rate of
accumulation at a particular point in the catchment. Difficulties that arise in the estimation of the
inflows from the source include the need to estimate the amount of rainfall that is stored where
it falls.
• The ‘path’ represents the flow route of the water between the source and the receptor (see
below). This includes flow through sewers, open and culverted watercourses, along highways
and also across the surface of open areas as ‘overland flows’. Defining the ‘path’ includes a
hydraulic assessment of the available flow routes for the floodwater through conduits and
channels to identify where flooding might occur and also a detailed assessment of the overland
flow routes following flooding from the hydraulic conduit.
• The ‘receptor’ analysis includes collating information on the properties in areas at risk of
inundation and assessing the flood risk to these in detail under a range of rainfall events.
Understanding of the depth of the flooding that might occur is required in order to be able to
evaluate the likely damages costs and the benefit to cost ratio of any scheme proposals to
reduce the flooding risk in the future.
Page | 36
Page | 37
Appendix B – Flood Risk Assessment
Requirements
Level of information needed for Planning in a FRA or Drainage Strategy
All applications
Site Details
What type of development is proposed and where will it be located? Include whether it is new
development, an extension to existing development or change of use etc. Information about the area of
the development site itself and ground conditions (e.g. how much is currently hard standing)
A location plan at an appropriate scale should be submitted with the application. The plan should show
the site outline and other adjacent land under the control of the applicant.
Explanation of how the drainage hierarchy has been followed.
Firstly, to infiltration/soakaway
Secondly, to a watercourse or highway drain (with permission)
Thirdly to a surface water sewer (with permission)
Lastly, to a combined sewer (with permission)
Where to find data on Flood Risks
• Surface water – EA Updated Flood Map for Surface Water (uFMfSW).
• Groundwater – EA Groundwater risk maps.
• Main River flooding – EA indicative flood maps. It is worth noting that these maps only show
flooding from catchments greater than 3km2, so modelling may be required in catchments less
than this.
• Ordinary watercourse flooding – information held by the LLFA.
• Risk of flooding from Canals – information held by Canal and Rivers Trust.
• Risk of flooding from reservoirs – available from EA.
• Sewer flooding – local water company.
• Highway drainage flooding – Highway Authority and Highways England.
Outline applications
Flood Risk to the Site
Assessment of all existing flooding risks to the site. CCC would be interested in groundwater, overland
surface water flows, sewer flooding, infrastructure flooding (from reservoirs/ponds/canals), watercourse
flooding and the risk from the proposed on site surface water drainage system. As well as making
Page | 38
reference to the local SFRA, LFRMS and SWMP, the applicant should contact the Flood Risk and
Drainage Team to discuss potential risks.
Mitigation of the Flood Risk
Brief explanation of how each of these flood risks will be mitigated. This could require detailed
modelling of some sources where significant risk is shown on high level datasets. It might mean
avoiding building on one part of the site where there is known flooding or raising floor levels.
Surface Water Design for the Site
Firstly, a plan showing the existing site, its topography and how it currently drains.
Secondly, a plan showing the proposed site, its proposed topography, how it will drain, and proposed
layout. A fully detailed system is not required at this stage, just evidence that such a system is
achievable and space has been made for water.
Explanation of likely forms of SuDS for the site
This shall include justification for their use and explanation of who will maintain and fund the proposed
system over the lifetime of the development and evidence that access will be physically possible to
carry out that maintenance, without entering others land. Ideally, SuDS features should be located
within public space.
If all of the above points can be clarified adequately, conditions should be agreed on the
application.
Full, Detailed, Discharge of Conditions or Reserved Matters
Flood Risk to the Site
As with an outline application, there must be an assessment of all existing flooding risks to and from the
site.
At the detailed stage, a reasonable assessment should have been made of the actual risk to the site
from the source of flooding.
1. Surface water – if the uFMfSW is considered accurate and not contradicted by local opinion,
this can be used to relocate development away from flood risk. If uFMfSW is disputed, or not
considered accurate, CCC would expect the developer to model the expected flows and use
the results to determine local mitigation.
2. Groundwater – typically a geotechnical report should be submitted.
3. Canals – normally a letter from the Canal and River Trust stating there will be no risk,
otherwise modelling of potential overtopping or breach.
4. Reservoirs – typically using the EA inundation maps to determine local mitigation. If
inundation is disputed, this may require modelling by developer.
5. Sewer – typically a letter or model report from the relevant water company.
6. Main River or ordinary watercourse – the EA have produced indicative floodplain modelling of
most Main Rivers and some ordinary watercourses. If the developer wishes to obtain that
modelling or dispute it, correspondence should be with the EA rather than CCC. The EA will
Page | 39
make decisions on Main River flooding issues. If only approximate modelling is available for
an ordinary watercourse and is considered to be inaccurate or is disputed, the developer
should deal with CCC who may ask the developer to model such flooding accurately to
ensure the development is safe.
Typical outputs from this work should comprise of detailed site layouts with predicted flood areas
superimposed over the development, and detail on flood depths for 30yr, 100yr and 100yr+climate
change events. Further detail may be required on flood velocity, depth and flood hazard (for buildings
and people). There should be qualitative consideration of greater than design events.
Mitigation of Flood Risk to the Site
Explanation of how each of these flood risks will be fully mitigated.
This could require detailed modelling of some sources where significant risk is shown on high level
datasets. It might mean avoiding building on one part of the site where there is known flooding.
Examples of mitigation:
• Setting minimum floor levels of the development.
• Relocating development out of the surface water flood routes and floodplain that affects the site
• Proposed works to improve/divert infrastructure to eliminate risk
• Proposals to route flood flows through a development so they do not adversely affect the
development
• In accordance with Table 3 of Planning Practice Guidance, basements are considered as
highly vulnerable and should not form part of a development located in Flood Zone 3 or Flood
Zone 2 (unless passed by the Exception Test)
• Always setting residential development floor levels 150mm above local ground levels. This
could be more than 150mm where risk requires.
Surface Water Design for the Site
• A plan showing the existing site, its topography and how it currently drains
• A plan showing the proposed site, its proposed topography and how it will drain
• Evidence that the site has an agreed point of discharge.
At the detailed stage, it is expected that:
• If surface water will be infiltrated on site, the developer will need to provide a BRE365
infiltration assessment and detailed calculations for the particular infiltration system.
• If discharge is to an ordinary watercourse, evidence that the system can accept the proposed
flows to an acceptable downstream point without increasing risk to others. Analysis of the
effects if the outfall is likely to be surcharged during flooding events. Consent may be required
from the LLFA for works that affect flows of the receiving watercourse.
• A letter of confirmation from the water company or responsible body (stating their required
discharge rate) if discharge is to a sewer or canal or highway drain, with appropriate signed off
calculation.
Page | 40
Detailed Drainage Plans for the proposed development.
Showing the location of storage within the proposed development and how these relate to submitted
calculations (e.g. including manhole numbers and pipe numbers that are referenced in Microdrainage
reports).
It is likely that separate, more detailed engineering plans will be needed for each of the SuDS and
critical drainage elements. The methods of flow control must be detailed, as should non-conventional
elements such as ponds, swales, permeable paving etc.
Calculations of current runoff from site
At the detailed stage we would expect the applicant to have accurately calculated run off rates for the
existing site for the Qbar rainfall event. They should have looked at a range of rainfall events and
selected the worst case rainfall event in each case.
Calculations of proposed runoff from site
Explanation of the methodology of calculation. This should be at Qbar greenfield rate for all storm
events up to and including the 1 in 100 year + 30% climate change, and should incorporate a minimum
20% reduction to allow for future urban creep.
We also would require a rough calculation of storage volume required on site for the 100yr plus climate
change case, bearing in mind the controlled discharge rate. Both existing run-off and an estimation of
storage can be produced through the UK SuDS website. http://www.uksuds.com/
Calculations of proposed discharge from site
Explanation of the methodology of calculation. The applicant should use the calculation of current runoff
to arrive at discharge on the following basis:
• Greenfield areas should discharge at a maximum of the Qbar rate minus 20%, where required
by the LLFA, so that the site behaves like the original greenfield and will, in places, reduce the
peak discharge from the site to reduce existing downstream risks.
Based on the existing and proposed discharge cases calculated as above, the applicant should now
have detailed estimates of storage volume required on site for the 100yr plus climate change case.
Full explanation of the forms of SuDS used on the site
Including justification for their use, what flood mitigation, water quality, environmental and social
benefits will be achieved. Modelling of the proposed SuDS system for the site (typically Microdrainage),
showing the behaviour of the site for the main rainfall events as described above and ensuring:
• Typical operation of the system for low rainfall and first-flush events, with indication of how
treatment of surface water will be achieved
• No above ground flooding for any conventional element of the system for the 30yr event
• No flooding from the system to downstream property for the 100yr+climate change event.
Page | 41
Residual Risk.
As well as the consideration of the modelled events above, there should be a qualitative examination of
what would happen when a greater than design event occurs. E.g. if any part of the system fails or is
beaten, that flood water will have flow routes through the site without endangering property.
Phasing
Explanation of how the site will adequately consider flood risk at all stages of the development.
Avoiding interim developed phases that are unprotected.
Phases can only progress if adequate flood mitigation measures are in place for that particular phase.
This should avoid one small phase of the site being allowed to discharge at the calculated rate for a
larger part of the entire development. Adequate flood risk measures for each individual phase should
be able to stand alone, (until the entire site is completed), without themselves being at flood risk and
without increasing flood risk for other parties.
If all of the above issues are addressed, the flood risk proposals should be approved.
Page | 42
Appendix c – Hierarchy of Documents
Page | 43
Page | 44
Glossary
Term Meaning for the purposes of the SWMP
AAP Annual average probability – the chance of a flood event occurring in
any given year. Normally expressed as a percentage. Eg 2% AAP
event means an event with a 2% chance of occurring in a year.
Adopted Sewer A surface water, foul or combined sewer that is maintained by Severn
Trent Water. A developer will often design and construct sewers in
accordance with Sewers for Adoption 7th Edition in order that these
may be adopted by Severn Trent Water.
Attenuation System to reduce the flow and increase the duration of a flood.
Balancing Pond A pond designed to attenuate flows by storing rainwater run-off
during a storm and releasing the water slowly at a controlled rate
over an extended period of time. Also known as a detention basin.
Brownfield Site A piece of land or a site that has previously been developed.
Catchment The area contributing flow or run-off to a particular point on a
watercourse system.
Climate Change Long-term variations in weather patterns, particularly temperature
and rainfall, thought to be a result of an increase in carbon dioxide
emissions.
Combined Sewer A public sewer used to convey both surface water and sewage.
Commuted Sum A single payment made at the beginning of an adoption agreement to
cover maintenance of the drainage system in future years.
Culvert Covered channel or pipe that forms a watercourse below ground
level.
Design Criteria A set of standards agreed by the developer, planners and regulatory
bodies that the proposed system should satisfy.
Design Event A historic or notional regular flood, against which the suitability of a
proposed development is assessed and for which mitigation
measures may be designed.
Design Flood Level The maximum estimated water level during the design event.
Page | 45
Detention Basin A basin in the landscape that is normally dry, except during heavy
rain. Used to store rainwater run-off to attenuate flows. May also
enable infiltration.
Development Works resulting in a change of use or character of a piece of land.
Discharge rate Rate of flow of water out of a pipe system into another drainage
system.
Field or Land Drainage A drainage system to control the water table in agricultural land.
Filter Drain or Filter Trench A linear drain consisting of a trench filled with a permeable material,
often with a permeable pipe in the base of the trench. Used to store
and infiltrate water into the ground. May also act as a conduit to
collect and transfer water through a drainage system.
Filter Strip A vegetated area of gently sloping or flat ground designed to collect
water from impermeable areas and convey it to a filter drain.
Filtration The act of removing particles from fluid by passing it through a filter.
First Flush The initial run-off from a site or catchment following rain. The initial
run-off tends to collect the pollutants on the ground and may be
contaminated.
Flap Valve A simple form of non-return valve, with a hinged flap to prevent
reverse flow from a fluvial system into a piped drainage system.
Flood Defence Infrastructure such as flood walls and embankments to protect an
area against flooding to a specified standard of protection.
Flood Defence Crest Level The top of the walls or embankments, expressed as a level relative to
Ordnance Survey Datum.
Flood Event A flood characterised by its severity.
Flood Risk Assessed by a combination of the flood probability and the potential
consequences of a flood.
Flood Risk Assessment A study to assess the risk of a site or area flooding. Used to assess
the impact a development might have on the site or area's flood risk.
Flow Control Device A mechanical device to limit or manage flow.
Fluvial Flooding Flooding from a river or watercourse.
Freeboard The difference between the flood defence crest level and the
maximum envisaged design flood level.
Page | 46
Greenfield Run-off Rate The rate of runoff of water from a piece of land in an undeveloped or
natural state.
Groundwater Water within the ground - often referred to as the water below the
water table. May exist at a number of different levels underground,
depending on the types of material in the ground. The water table
often lies parallel to the ground surface.
Groundwater Flooding When the water table rises above the surface. A common feature of
this type is a spring line.
Hydrograph A graph showing the variation in water flow in a watercourse.
Impermeable Surface An artificial surface that water can't pass through.
Infiltration The passage of water through the surface and into the ground.
Infiltration Basin A dry basin designed to help infiltrate surface water into the ground.
Infiltration Capacity A characteristic of soil that determines the rate at which water enters
the ground.
Infiltration Trench A trench excavated in permeable ground and filled with permeable
granular material. Used to help the infiltration of water into the
ground.
Land Drain Drain used in agriculture to control to level of the local water table
and reduce waterlogging.
Local Development
Documents
Documents and plans that set out the development strategy for the
Local Planning Authority.
Local Planning Authority Body with responsibility for planning and controlling development
through the planning application system.
Mitigation Measure An aspect of the design of a development that reduces the impact on
the local environment, particularly on the flood risk.
Ordinary Watercourse A watercourse that falls under the control of the local drainage
authority and is neither a private drain nor a main river.
Overland Flow A situation that arises when the ground surface becomes saturated
and can't hold any more rain. The rainwater then collects on the
surface and flows in the direction of the steepest gradient. May result
in pluvial flooding.
Permeable Surface &
Permeable Paving
Material that allows water to pass through gaps between the
constituent materials into the layers below.
Page | 47
Pluvial Flooding Flooding generated when floodwater hasn't entered any watercourse
or sewer system. It is a particular problem in dense urban areas,
although it may occur in rural areas.
Storage Pond A permanently wet feature used to store water in times of heavy
rainfall. Can be home to wildlife.
Rainwater recycling Systems that collect and enable the redistribution or re-use of
rainwater on roofs or pavements. Can include water butts,
underground tanks and pumping systems.
Retention Pond A pond where water stays long enough to allow settlement of
suspended solids and possibly biological treatment of some
pollutants.
Riparian Ownership The ownership of land next to or containing a watercourse. The rights
and responsibilities of the landowners are often referred to as riparian
rights and responsibilities.
Run-off Water flow over-ground to the local drainage system. This occurs if
the ground is impermeable or saturated, or if rain is heavy. It might be
thought of as the remainder of the rainfall that neither get absorbed
into the ground nor evaporates back into the atmosphere.
Sewerage Undertaker The organisation responsible for the maintenance of the sewer
systems and the treatment and disposal of surface water and foul
sewage.
Sewer System The private and public network of drainage used to convey surface
water and foul sewage from roads and buildings.
Soakaway An underground structure into which surface water is conveyed to
allow it to infiltrate into the ground
Source Control The control, attenuation and/or treatment of runoff or pollution near to
its source or origin.
Standard of Protection Refers to the lowest probability flooding at a particular site due to the
extent of the mitigation measures in place. Often referred to as '25,
50 or 100 year protection'.
Sustainable Urban
Drainage Systems
An approach to the management of rainwater to reduce the flood risk
impact of new developments on the surrounding areas.
Swale A shallow linear trench used to convey and store runoff, particularly
from car parks, roads and other paved areas. May also incorporate
infiltration.
Page | 48
Treatment Improving the quality of water by biological, chemical or physical
means.
Urban Creep The increase in impermeable area resulting from planned and
unplanned urban expansion. This includes infill developments and
also small domestic works (extensions, conservatories, drive
widening, hard paving of gardens). Results in
Water Table The level of groundwater in soil and rock below which the ground is
saturated.
Watercourse Any natural or artificial channel that conveys surface water.
Washland An area subjected to frequent flooding at least every 20 years and
used to store, attenuate or convey floodwater.
Wetlands An area where the natural saturation of the ground is the determining
factor for the particular biodiversity of the area.
Whole Life Costing An approach to the accounting of the cost of a particular flood risk
reduction scheme or other system that includes all the costs of the
construction, operation and maintenance and eventual
decommissioning. These costs are usually referenced to a 'present
day' cost to enable the comparison between different alternatives.
![2019 STORMWATER MANAGEMENT PROGRAM PLAN UPDATEci.brier.wa.us › pdf › Brier SWMP 2019 [146728].pdf · the City’s Stormwater Management Program (SWMP). The City’s SWMP is intended](https://img.pdfslide.net/doc/110x75/5f0d8e2f7e708231d43af09f/2019-stormwater-management-program-plan-a-pdf-a-brier-swmp-2019-146728pdf.jpg)
