LAND OFF SIR ISAAC NEWTON DRIVE
WYBERTON, BOSTON, LINCOLNSHIRE PE21 7SG
FLOOD RISK ASSESSMENT
FOR
CYDEN HOMES
April 2013
Our Ref: RCEF24009-02 R
RPS Health, Safety & Environment
Unit 12 Watersedge Business Park
Modwen Rd, Salford Quays
M5 3EZ
RCEF24009-02 R – Land off Sir Isaac Newton Drive
April 2013
Report Status:
Final
Project Reference:
RCEF24009
Name:
Signature:
Report Author:
Ruth Abbott
Technical Reviewer:
Neil Bagley
Date:
19/04/2013
This report has been prepared in the RPS Group Quality Management System to British Standard EN ISO 9001:2008
RPS Health, Safety & Environment is part of the RPS Group Plc with around 5,000 staff based at over 85 offices located throughout the
UK, Ireland and the Netherlands and in the USA, Canada, the Russian Federation, Australia, Malaysia, Singapore and Abu Dhabi. RPS
offers an unparalleled range of commercially focused services relating to property and land due-diligence, site development and geoenvironmental investigations (including liability reviews, planning feasibility, EIAs and flood risk, energy & sustainability assessments).
RPS Health, Safety & Environment (London office) is certified to Environmental Management Standard ISO 14001.
RCEF24009-02 R – Land off Sir Isaac Newton Drive
April 2013
RCEF24009-02 R – Land off Sir Isaac Newton Drive
April 2013
CONTENTS
PAGE
1
INTRODUCTION......................................................................................................................................5
2
PLANNING POLICY CONTEXT..............................................................................................................6
3
CONSULTATION.....................................................................................................................................8
4
SITE DESCRIPTION..............................................................................................................................10
5
PROPOSED DEVELOPMENT ..............................................................................................................11
6
HYDROLOGICAL SETTING .................................................................................................................12
7
HYDROGEOLOGICAL SETTING .........................................................................................................14
8
EXISTING DRAINAGE / WATER MAINS .............................................................................................15
9
FLOOD RISK AND MITIGATION ..........................................................................................................16
10
SURFACE WATER MANAGEMENT ....................................................................................................21
11
SEQUENTUIAL TEST AND EXCEPTION TEST ..................................................................................24
12
SUMMARY AND CONCLUSIONS ........................................................................................................25
FIGURES
1
SITE LOCATION PLAN
2
ENVIRONMENT AGENCY FLOOD MAP
APPENDICES
A
ENVIRONMENT AGENCY FLOODING DATA
B
IDB DRAINAGE PLAN
C
ANGLIAN WATER PRE-DEVELOPMENT CONSULTATION
D
TOPOGRAPHIC SURVEY
E
PROPOSED DEVELOPMENT PLAN
F
CONCEPTUAL DRAINAGE SCHEME
RCEF24009-02 R – Land off Sir Isaac Newton Drive
December 2012
4
1
INTRODUCTION
RPS was commissioned to undertake a Flood Risk Assessment of a site off Sir Isaac Newton Drive,
Wyberton to support the planning application in relation to the proposed residential development of the
site.
The aim of the Flood Risk Assessment is to outline the potential for the site to be impacted by
flooding, the impacts of the proposed development on flooding in the vicinity of the site, and the
proposed measures which could be incorporated into the development to mitigate the identified risk.
The report has been produced in accordance with the guidance detailed in the National Planning
Policy Framework (NPPF). Reference has also been made to the CIRIA SUDS manual (C697), the
Boston Borough Council Strategic Flood Risk Assessment (SFRA), and the Lincolnshire Coastal
Study.
This report has been produced in consultation with the Partnership and Strategic Overview Team at
the Environment Agency, Lincolnshire Emergency Planning Team and the Black Sluice Internal
Drainage Board (IDB).
This report is not intended to provide formal details of the final drainage design for the development. It
presents an indicative surface water drainage scheme which provides information regarding the
capabilities of the proposed development to meet the requirements of the NPPF and the IDB.
The desk study was undertaken by reference to information provided / published by the following
bodies:
•
Environment Agency
•
Centre for Ecology and Hydrology
•
British Geological Survey
•
Ordnance Survey
•
Anglian Water
•
Black Sluice Internal Drainage Board
RCEF24009-02 R – Land off Sir Isaac Newton Drive
December 2012
5
2
PLANNING POLICY CONTEXT
2.1
Local Planning Policy
Boston Borough Council’s planning policy currently comprises the saved Policies from the adopted
Boston Borough Local Plan (April 1999). The Local Plan contains the following Policies relating to
flood risk and drainage:
G3 Foul And Surface Water Disposal: Planning permission will not be granted for development
where the proposed means of disposing of the resultant foul and surface water are unsatisfactory.
This Policy states that developers must make it clear in their planning applications how they propose
to deal with the foul and surface water associated with their development. Foul drainage must be to a
foul sewer whenever possible. The Borough Council will be advised by the Environment Agency,
Internal Drainage Boards and Anglian Water as to whether these proposals are satisfactory.
G4 Safeguarding the Water Environment: Planning permission will not be granted for developments
which will have an adverse effect on the water environment, or the quality of surface or ground water.
This Policy states that contamination of surface and groundwater must be prevented. Rivers, other
watercourses, ponds and wetlands are worthy of protection and, in the interests of ensuring that
development is sustainable, the Borough Council will not normally permit proposals which will harm
these features. The Borough Council will be advised by the Environment Agency on these matters.
2.2
Strategic Flood Risk Assessment
The Boston Borough Council Strategic Flood Risk Assessment (SFRA) 2010 identifies and maps flood
risk from all sources at a borough-wide scale as well as providing guidance on producing site specific
FRAs. Relevant information from the SFRA has been reproduced throughout this Flood Risk
Assessment report.
2.3
Lincolnshire Coastal Study
The Lincolnshire Coastal Study (2010) (‘the Study’) aims to produce and evaluate a set of long-term
options for the sustainable spatial development of Lincolnshire’s coastal communities.
The site is located within Flood Zone 3 as it is within an area where the hazard rating in the event of a
breach during an extreme flood event is ‘danger for most’ and ‘danger for all’ (also referred to in the
Study as the ‘orange’ and ‘red’ zones respectively). The Study advises that, “Exceptionally,
development to meet local housing need may continue in the red, orange and yellow zones although
RCEF24009-02 R – Land off Sir Isaac Newton Drive
December 2012
6
flood risk must be mitigated through flood resilient design and emergency planning.” This statement
applies to the proposed development site as the site is within a wider area where the Council have
advised (letter dated 10/10/12; enquiry ref PENQ/12/1865) that there is an absence of a 5 year
housing supply and there are also no sequentially preferable sites available within the Borough.
The Study notes that, “The key to delivery of buildings in [the red and orange] zones is therefore the
implementation of resilience and emergency planning measures appropriate to the flood hazard zone
and type of development.”
In addition, the Study advises, “The focus of housing design (to meet local need only) in the red,
orange and yellow zones must be protection of life of its residents in a flood event. In these flood
hazard zones, it is unlikely that it will be possible to design or build houses which are totally resistant
to flood waters and some loss of property must be accepted. It is also unlikely that in the event of a
flood occurring, the emergency services will be able to evacuate the area for a number of days. It is
thus essential that properties in these zones are designed so that people can safely remain in them for
a number of days following a flood.”
Relevant information from the Study, including advice on appropriate mitigation measures within the
residual tidal flood risk zones, has been reproduced throughout this Flood Risk Assessment report.
RCEF24009-02 R – Land off Sir Isaac Newton Drive
December 2012
7
3
CONSULTATION
The Flood Risk Assessment has been produced in consultation with the Partnership and Strategic
Overview Team at the Environment Agency. The Environment Agency responded to a request for preth
application advice (letter dated 4 October 2012); the main points are summarised below:
•
Development should meet the agreed the principles of the Lincolnshire Coastal Study and the
site to be discussed at the Lincolnshire Flood Risk Management Framework Strategy Group.
•
Given the nature of the hazard associated with the residual (breach) tidal risk to the site, it is
not suitable for single storey development.
•
Proposed development must be:
o
At least two-storey
o
Minimum finished floor levels of 1 m above ground level
o
Flood resilient construction to a height of 300 mm above the predicted flood level
o
Demountable defences to 600 mm above finished floor levels
o
Accompanied by a Flood Warning and Evacuation Plan.
Subsequent telephone conversations with the Environment Agency (Debbie Morris, Flood and Coastal
Risk Management Officer) have clarified this advice in the context of the specific site and proposed
development.
Flood data from the Environment Agency has been requested (Environment Agency Product 8;
included as Appendix A). Maps showing the depth and velocity of flooding as well as the hazard rating
based on modelling of present day and future breach and overtopping scenarios have been provided.
The site is located within the Black Sluice Internal Drainage Board (IDB) District. The IDB have been
consulted (telephone conversations with Ian Watts) and their advice regarding the proposed
development is summarised below:
•
The drain adjacent to the southern boundary of the site is under the ownership of the IDB (the
other drains within the vicinity of the site are under ownership by other parties as described in
section 8 - Existing Drainage / Water Mains)
•
Easement required off the IDB drain is 9 m (although this can be reduced to 6 m to a fence;
this was done on the adjacent site with house no. 41)
•
Consent from the IDB is required in order to pipe or in-fill any drain. If the drain through the
middle of the site is redundant and serves no drainage role for off-site locations, then in-filling
would be acceptable. If this drain does form a conveyance route as part of a wider drainage
network for the wider area, it would need to be piped within the proposed development.
RCEF24009-02 R – Land off Sir Isaac Newton Drive
December 2012
8
•
If discharging surface water to the IDB drain, restriction to Greenfield rates is required. The
IDB’s acceptable Greenfield rate is based on pump capacity and is set at 1.4 l/s/ha. Any rate
above this would require a developer contribution fee.
The IDB have provided a plan showing the layout and ownership of the drains within the vicinity of the
site. This plan is shown in Appendix B.
The public sewer network within the vicinity of the site is operated by Anglian Water. A pre-planning
assessment has been obtained from Anglian Water (included in Appendix C). Anglian Water have
advised that, “The preferred method of surface water disposal is through the use of Sustainable
Drainage Systems (SuDS). If this is not practical then surface water should be discharged to a
watercourse. Only once these options have been fully explored can the use of a surface water sewer
be considered.”
The site is within the administrative boundary of Boston Borough Council. The Council responded to a
th
request for pre-application advice (letter dated 10 October 2012); the main points are summarised
below:
•
The Council have confirmed that it is unable to demonstrate a five year supply of housing land
therefore in terms of the sequential test, the absence of a 5 year supply indicates that there
are no sequentially preferable sites available.
•
th
The Environment Agency’s advice (as given in their pre-application response letter dated 4
October 2012) should be followed.
The Council have been consulted regarding the requirement for a Flood Warning and Evacuation Plan
to be produced for the site. The Council have advised (in an e-mail from Rachael Vamplew, Planning
th
Information Officer, dated 14
December 2012) that providing the Flood Risk Assessment is
acceptable in all other respects the requirement for a Flood Warning and Evacuation Plan can be
ensured through Condition.
The advice and information received during consultation with the Environment Agency, Internal
Drainage Board, Anglian Water, and the Council had been used to inform the design of the proposed
development.
RCEF24009-02 R – Land off Sir Isaac Newton Drive
December 2012
9
4
SITE DESCRIPTION
4.1
Site Description
The site is located at National Grid Reference 532672 342317 and occupies an area of approximately
1.4 hectares. A site location plan is provided in Figure 1.
The site is currently vegetated land covered with grass and small trees and is therefore wholly
permeable.
A drain flows west to east through the middle of the site. Drains also flow parallel to the southern and
northern boundaries of the site.
Vehicular access to site is via Sir Isaac Newton Drive to the east of the site. A public footpath crosses
onto the site from land to the immediate south.
4.2
Surrounding Land Uses
The site is bound to the immediate west by a strip of grassland which buffers the site from the A16.
Beyond the A16 is an area comprising mixed land use (residential, industrial and recreational). To the
immediate east and north of the site are existing residential estates. A drain flows adjacent to the
southern boundary of the site; beyond this there is an area of grassland.
The Wash is located approximately 4 km south east of the site at its nearest point. The Wash is a
designated Special Area of Conservation (SAC), Special Protection Area (SPA), Site of Special
Scientific Interest (SSSI) and Ramsar site. The extensive network of open surface water drains which
serve the area in which the site is located eventually discharge to The Wash.
4.3
Topography
The topographic survey identifies that the majority of the site is relatively flat with levels typically
ranging between 2.4 m AOD and 2.7 m AOD. Levels on Sir Isaac Newton Drive to the immediate east
of the site are also within this range. There is a linear bund close to the western boundary of the site.
Levels on this feature rise up to approximately 4.95 m AOD. The two dykes on the site have bed levels
of typically 1.2 m AOD to 1.6 m AOD. The topographic survey is located in Appendix D.
RCEF24009-02 R – Land off Sir Isaac Newton Drive
December 2012
10
5
PROPOSED DEVELOPMENT
It is proposed to develop the site with 33 residential properties with associated back gardens, garages
and access roads. Development plans are shown in Appendix E.
Vehicular access will remain via Sir Isaac Newton Drive and the public footpath crossing through the
south west corner of the site will be maintained.
The proposed development is within an area which is at a residual risk of flooding in the event of a
breach or overtopping of defences. The Lincolnshire Coastal Study and Environment Agency in this
area have specific recommendations for housing design to mitigate this risk. Such mitigation
measures have been built into the proposed development design as follows:
•
Finished floor levels will be 1 m above ground levels.
•
Properties will be two storey, with no sleeping accommodation on ground floor and the top
storey with bedroom and bathroom.
•
Demountable defences to a height of 1.6 m above ground level (i.e. 600 mm above finished
floor levels) will be implemented on the properties.
•
Flood resilient construction methods will be implemented. These will include: Accessible roof
spaces; ensuring access to all spaces to permit drying and cleaning; provision of concrete
floors as standard; raising items which can be damaged above projected flood level; raising
electrical wiring above predicted flood level.
The site is presently wholly permeable. The proposed development will result in an increase in
2
impermeable area of 6,584 m .
The proposed use of the site is classified as ‘more vulnerable’ within the NPPF.
A conceptual drainage layout has been designed for the site and is described in Section 10 and
included as drawings in Appendix F. Two potential solutions have been considered: one comprising
unlimited discharge to the IDB drain to the south of the site (this would incur a contribution fee in
accordance with the IDB guidelines); the other comprising restricted discharge to the IDB drain with
on-site surface water attenuation.
Detailed drainage design will be undertaken post-planning
application.
RCEF24009-02 R – Land off Sir Isaac Newton Drive
December 2012
11
6
HYDROLOGICAL SETTING
6.1
Nearby Watercourses
Reference to Ordnance Survey Mapping indicates that the nearest Main Watercourse to the site is
The Haven which is located approximately 400 m to the north of the site at its nearest point. The
Haven is a tidal tributary of The Wash, the coastal water off the Lincolnshire coast. The coastline is
located approximately 7 km east of the site.
Two drains flow within the site boundary; one through the centre of the site and one adjacent to the
northern boundary. Both flow in an easterly direction through the site. The drain in the centre of the
site is approximately 1.6 m wide and is open within the site boundary. It is reported from site that no
inlet or outlet has been identified for this drain and therefore it is not considered to be linked to the
wider drainage network. A further drain flows in an easterly direction to the immediate south of the
southern boundary of the site. The layout and ownership of the drains within the vicinity of the site are
shown on the plan provided by the IDB which is shown in Appendix A.
The drains within and immediately adjacent to the site are part of an extensive drainage network which
serve the wider area in which the site is located. This drainage network eventually discharges to The
Wash.
6.2
Flood Risk Classification
Flood risk data for the site has been obtained from the Environment Agency Indicative Flood Map
website (reproduced in Figure 2) and Environment Agency Product 8 (included as Appendix B).
The site is located with the Environment Agency’s tidal flood zone 3 which is classified as being at a
greater than 1 in 200 (0.5%) annual probability of tidal flooding in the absence of any defences. The
site is, however, located within an area benefitting from tidal defences. The tidal defences protecting
this site are located along the banks of The Haven and consist of earth embankments and concrete.
The Environment Agency have advised that the defences are considered to be in a fair condition and
provide protection against a flood event with a 1 in 150 (0.67%) chance of occurring in any year. The
defences are regularly inspected by the Environment Agency to ensure that any potential defects are
identified early.
As the site is served by flood defences, the actual flood risk to the site is a residual risk in the event of
a breach in the defences or overtopping. Maps showing the depth and velocity of flooding as well as
the hazard rating based on modelling of present day and future breach and overtopping scenarios
have been provided. These are included as Appendix B and summarised in the table below:
RCEF24009-02 R – Land off Sir Isaac Newton Drive
December 2012
12
Scenario
Year
2006
Breach
Event
200
1000
2115
2006
Overtopping
2115
200
1000
200
1000
Hazard rating (proportion of site
within that rating – bold text
indicates majority of site)
Low hazard (10%)
Danger for most (30%)
Danger for all (60%)
Danger for most (35%)
Danger for all (65%)
Low hazard (10%)
Danger for all (90%)
Danger for most (10%)
Danger for all (90%)
0.0-0.3 (10%)
0.5-1.0 (70%)
1.0 - 2.0 (20%)
0.5-1.0 (50%)
1.0 - 2.0 (50%)
0.5-1.0 (10%)
1.0 - 2.0 (90%)
0.5-1.0 (10%)
1.0 - 2.0 (90%)
site not affected
site not affected
Velocity
(m/s)
0.3-1.0
0.3-1.1
0.3-1.2
0.3-1.3
200
Low hazard (30%)
Danger for some (30%)
Danger for most (40%)
0.0 - 0.25 (10% )
0.25 - 0.50 (40%)
0.50-1.0 (50%)
0.0-0.3
1000
Low hazard (10%)
Danger for some (10%)
Danger for most (80%)
0.0 - 0.25 (10%)
0.25 - 0.50 (10%)
0.50-1.1 (80%)
0.0-0.4
RCEF24009-02 R – Land off Sir Isaac Newton Drive
December 2012
Depth (m) (proportion of
site within that depth bold text indicates
majority of site)
13
7
HYDROGEOLOGICAL SETTING
Reference to the British Geological Survey online mapping (1:50,000 scale) indicates that the site is
underlain by tidal flat deposits – clay and silts, described as, “Normally a consolidated soft silty clay,
with layers of sand, gravel and peat. Characteristically low relief; from the tidal zone.”
The bedrock beneath the site comprises Ampthill Clay Formation, which is described as, “Mudstone,
mainly smooth or slightly silty, pale to medium grey with argillaceous limestone (cementstone)
nodules; some rhythmic alternations of dark grey mudstone in the lower part; topmost beds are
typically pale grey marls with cementstone.”
The Cranfield university on-line Soilscapes map suggests that the soils beneath the site are, “Loamy
and clayey soils of coastal flats with naturally high groundwater.”
A Site Investigation has not been carried out on the site to date. Investigations undertaken during the
development of the adjacent site identified that soakaways are not suitable in this area.
Reference to the Environment Agency’s online groundwater Source Protection Zone maps indicates
that the site is not located within a groundwater Source Protection Zone.
RCEF24009-02 R – Land off Sir Isaac Newton Drive
December 2012
14
8
EXISTING DRAINAGE / WATER MAINS
The site is presently undeveloped and is therefore surface water generated on the site is likely to soak
into the ground or flow over the surface in an uncontrolled manner, following the topographic gradient
of the site towards the drainage ditches in the vicinity of the site (described below). Some flows may
potentially pond within topographic low points within the site.
Two drains flow within the site boundary; one through the centre of the site and one adjacent to the
northern boundary. Both flow in an easterly direction through the site. The drain in the centre of the
site is approximately 1.6 m wide and is open within the site boundary. It is reported from site that no
inlet or outlet has been identified for this drain and therefore it is not considered to be linked to the
wider drainage network. A further drain flows in an easterly direction to the immediate south of the
southern boundary of the site. The layout and ownership of the drains within the vicinity of the site are
shown on the plan provided by the IDB which is shown in Appendix A. The two drains flowing within
the site boundary are likely to be privately owned. The drain flowing alongside the southern boundary
of the site is under the ownership of the Black Sluice IDB.
The drains within and immediately adjacent to the site are part of an extensive drainage network which
serves the wider area in which the site is located. This drainage network eventually discharges to The
Wash.
Anglian Water sewer and clean water plans are provided in Appendix A. There are presently no public
surface or foul water sewers on the site itself. The existing residential estate to the immediate east of
the site is served by the Anglian Water sewer network. The nearest manhole to the site is within the
length of Sir Isaac Newton Drive to which it is proposed to connect the road for the proposed
development. This is surface water manhole 7351 on the Anglian Water plans. This manhole has a
cover level of 2.557 m AOD and an invert level of 0.907 m AOD (i.e. depth to invert is 1.65 m).
Reference to clean water network plans provided by Anglian Water identifies that there are presently
no water mains on the site itself. The existing residential estate to the immediate east of the site is
served by the Anglian Water potable water network.
RCEF24009-02 R – Land off Sir Isaac Newton Drive
December 2012
15
9
FLOOD RISK AND MITIGATION
The key sources of flooding that could potentially impact the site are discussed below:
9.1
Fluvial / Tidal Flooding
Flood risk data for the site has been obtained from the Environment Agency Indicative Flood Map
website (reproduced in Figure 2) and Environment Agency Product 8 (included as Appendix B).
The site is located with the Environment Agency’s tidal flood zone 3 which is classified as being at a
greater than 1 in 200 (0.5%) annual probability of tidal flooding in the absence of any defences. The
site is, however, located within an area benefitting from tidal defences. The tidal defences protecting
this site are located along the banks of The Haven and consist of earth embankments and concrete.
The Environment Agency have advised that the defences are considered to be in a fair condition and
provide protection against a flood event with a 1 in 150 (0.67%) chance of occurring in any year. The
defences are regularly inspected by the Environment Agency to ensure that any potential defects are
identified early.
As the site is served by flood defences, the actual flood risk to the site is a residual risk in the event of
a breach in the defences or overtopping. Maps showing the depth and velocity of flooding as well as
the hazard rating based on modelling of present day and future breach and overtopping scenarios
have been provided. These are included as Appendix B and summarised in the table below:
Scenario
Year
2006
Breach
Event
200
1000
2115
2006
Overtopping
2115
200
1000
200
1000
Hazard rating (proportion of site
– bold text indicates majority of
site)
Low hazard (10%)
Danger for most (40%)
Danger for all (60%)
Danger for most (35%)
Danger for all (65%)
Low hazard (10%)
Danger for all (90%)
Danger for most (10%)
Danger for all (90%)
0.0-0.3 (10%)
0.5-1.0 (70%)
1.0 - 2.0 (20%)
0.5-1.0 (50%)
1.0 - 2.0 (50%)
0.5-1.0 (10%)
1.0 - 2.0 (90%)
0.5-1.0 (10%)
1.0 - 2.0 (90%)
site not affected
site not affected
Velocity
(m/s)
0.3-1.0
0.3-1.1
0.3-1.2
0.3-1.3
200
Low hazard (30%)
Danger for some (30%)
Danger for most (40%)
0.0 - 0.25 (10% )
0.25 - 0.50 (40%)
0.50-1.0 (50%)
0.0-0.3
1000
Low hazard (10%)
Danger for some (10%)
Danger for most (80%)
0.0 - 0.25 (10%)
0.25 - 0.50 (10%)
0.50-1.1 (80%)
0.0-0.4
RCEF24009-02 R – Land off Sir Isaac Newton Drive
December 2012
Depth (m) (proportion of
site - bold text indicates
majority of site)
16
Given the nature and extent of the residual flood risk to the site and surrounding area and road
network, as shown on the breach hazard maps (Appendix B) dry access/egress for the site during a
breach event is unlikely to be achievable. This is acknowledged in the Lincolnshire Coastal Study
which advises, “The focus of housing design (to meet local need only) in the red, orange and yellow
zones must be protection of life of its residents in a flood event. In these flood hazard zones, it is
unlikely that… in the event of a flood occurring, the emergency services will be able to evacuate the
area for a number of days. It is thus essential that properties in these zones are designed so that
people can safely remain in them for a number of days following a flood.”
The site is not within a fluvial floodplain therefore there will be no increase in flood risk through loss of
flood storage area and flood compensation is not required for loss of floodplain storage.
Given the size of the proposed development in relation to the area potentially impacted during a
breach event, the significance of any impact of new buildings on flood flows and flood risk to
surrounding areas is considered to be negligible.
Proposed Mitigation
The proposed mitigation for the proposed development has been informed through consultation with
the Environment Agency and in accordance with the recommendations of the Lincolnshire Coastal
Study. These measures are as follows:
•
Finished floor levels will be 1 m above ground levels.
•
Properties will be two storey, with no sleeping accommodation on ground floor and the top
storey with bedroom and bathroom.
•
Demountable defences to a height of 1.6 m above ground level (i.e. 600 mm above finished
floor levels) will be implemented on the properties.
•
Accessible roof spaces designed to accommodate people in an emergency will be
implemented;
•
Access to all spaces will be available to permit drying and cleaning;
•
Provision of concrete floors as standard;
•
Raising items which can be damaged above projected flood level; and
•
Raising electrical wiring above predicted flood level.
A Flood Warning and Evacuation Plan will be prepared prior to the occupation of the development.
The Council have confirmed that this requirement can be secured through Condition.
RCEF24009-02 R – Land off Sir Isaac Newton Drive
December 2012
17
These mitigation measures are to minimise the potential damage to the buildings during an extreme
breach event and ensure that residents safety during such events is at an acceptably high level. In the
event of an extreme tidal breach event, residents will be able to safely remain in the upper floors / roof
areas of their property until water recedes or emergency service arrive to rescue. Should damage
occur to property during an extreme breach event, the expense and speed of the subsequent clean-up
will be reduced through the resilience measures outlined above.
9.2
Flooding from sewers
Sewer flooding can occur during periods of heavy rainfall when a sewer becomes blocked or is of
inadequate capacity. The public sewers within the vicinity of the site are described in Section 8. There
are presently no sewers on the site itself; the nearest public sewer network is that serving the
residential estate to the immediate east of the site.
In the event of surcharging of the public sewers on the adjacent site to the east, the resulting flows are
likely to flow in an easterly direction along the road following the topographic gradient. This direction of
flow is away from the site and therefore it is considered unlikely that the site would be affected by this
source of flooding.
The proposed foul discharge rate for the proposed surface water drainage network on site to the
existing sewer will be agreed with Anglian Water to ensure that there is capacity to receive discharge
from the site without significantly increasing flood risk. The proposed surface water drainage system
for the development does not discharge surface water to the public sewer network therefore the
proposed development will not contribute to increased flooding of this nature.
Proposed Mitigation
Whilst the risk of sewer flooding has been identified as low, the proposed raising of floor levels to 1 m
above ground level will nevertheless prevent egress of flood waters from this source affecting the
buildings within the proposed development.
9.3
Surface water flooding (overland flow)
This can occur during intense rainfall events, when water cannot soak into the ground or enter
drainage systems. The northern and southern boundary of the site are defined by drains therefore it is
likely that any overland flows from either of these directions would be intercepted by the drains and
would therefore not migrate onto the site. The western boundary of the site is adjacent to the A16
which would form a barrier to any overland flows approaching the site from the west. The land to the
east of the site is generally lower than the site therefore the direction of overland flow generated to the
east of the site is likely to be away from the site.
RCEF24009-02 R – Land off Sir Isaac Newton Drive
December 2012
18
Surface water flooding from on-site sources is considered in Section 10 of this report.
Proposed Mitigation
Whilst the risk of flooding from overland flow from off-site sources has been identified as low, the
proposed raising of floor levels to 1 m above ground level will nevertheless prevent egress of flood
waters from this source affecting the buildings within the proposed development.
9.4
Groundwater flooding
This can occur in low-lying areas when groundwater levels rise above surface levels, or within
underground structures. BGS mapping suggests that the site is underlain by tidal flat deposits – clay
and silts, overlying bedrock comprising the Ampthill Clay Formation. The Cranfield university on-line
Soilscapes map suggests that the soils beneath the site are, “Loamy and clayey soils of coastal flats
with naturally high groundwater.”
Given the reported geological conditions, there is potential for shallow groundwater to be present on
the site. However, the proposed residential buildings on the site do not comprise basement levels and
have finished floor levels set at 1 m above ground levels, therefore the risk of groundwater flooding is
considered to be low.
Proposed Mitigation
The proposed residential buildings on the site do not comprise basement levels and have finished
floor levels set at 1 m above ground levels.
9.5
Other Sources
There is a limited risk of flooding occurring as a result of a break in a water main. The locations of the
water mains in the immediate vicinity of the site are described in Section 8. There are presently no
water mains on the site itself; the nearest public clean water network is that serving the residential
estate to the immediate east of the site.
In the event of a burst in the public clean water network on the adjacent site to the east, the resulting
flows are likely to flow in an easterly direction along the road following the topographic gradient. This
direction of flow is away from the site and therefore it is considered unlikely that the site would be
affected by this source of flooding.
Reference to the Environment Agency risk of flooding from reservoirs map identifies that the site is not
at risk of flooding from this source.
RCEF24009-02 R – Land off Sir Isaac Newton Drive
December 2012
19
The risk of flooding associated with reservoirs, canals and other artificial structures is considered to be
low given the absence of any such structures in the site vicinity. There will be no increase in flood risk
from sources such as water mains, canals or other artificial water bodies as a result of the
development.
Proposed Mitigation
Whilst the risk of flooding from other sources has been identified as low, the proposed raising of floor
levels to 1 m above ground level will nevertheless prevent egress of flood waters from these sources
affecting the buildings within the proposed development.
9.6
Event Exceedence and Residual Risk
The mitigation measures proposed as part of the development scheme are considered appropriate to
mitigate against any residual risks or event exceedence scenarios.
RCEF24009-02 R – Land off Sir Isaac Newton Drive
December 2012
20
10 SURFACE WATER MANAGEMENT
10.1 Introduction
The existing surface water drainage network in the vicinity of the site is described in Section 8.
Potential receptors for surface water generated on the site are listed below, along with an appraisal of
their suitability.
Anglian Water public surface water sewer network
Anglian Water have advised that connection into their network would only be acceptable if it could be
demonstrated that discharge of water to ground via infiltration or to a watercourse is unsuitable. Given
that discharge to the IDB drain to the south is considered to be a viable option at this stage,
connection to the Anglian Water sewer network is not the preferred option at this stage.
On-site drain close to northern boundary of site
This drain is not under IDB ownership and is probably privately owned or under shared maintenance.
It is not a preferred option as the receiving watercourse for the surface water drainage scheme due to
uncertainty of ownership and potential difficulties in gravity drainage due to relative site levels.
On-site drain through centre of site
This drain is not under IDB ownership. It is reported from site that no inlet or outlet has been identified
for this drain and therefore it is not considered to be linked to the wider drainage network. It is
therefore not a preferred option as the receiving watercourse for the surface water drainage scheme.
Drain flowing alongside the southern boundary of the site
This drain is under IDB ownership. The IDB have advised that there are two potential approaches to
discharging into this watercourse:
i)
Discharge at Greenfield rate. The IDB’s acceptable Greenfield rate is based on pump capacity
and is set at 1.4 l/s/ha.
ii)
Discharge at a rate higher than Greenfield and incur a developer contribution fee.
Discharge to the IDB drain is the preferred option for the proposed surface water drainage scheme.
Both of the above approaches have been investigated and are presented within this Section and
drawings included as Appendix E. Both schemes are considered to be feasible and a decision on the
RCEF24009-02 R – Land off Sir Isaac Newton Drive
December 2012
21
final preferred solution to be implemented within the proposed development will be made once the
proposed development has obtained Planning Approval.
10.2 Proposed Conceptual Surface Water Scheme – Option 1: Restricted discharge to IDB drain.
This option comprises restricting discharge to the IDB drain to the IDB Greenfield rate of 1.4 l/s/ha.
The site is 1.4 hectares in size, therefore the allowable rate of discharge for the proposed
development is 1.96 l/s. In order to achieve this rate and ensure the 100 year plus climate event storm
is accommodated on site, on-site attenuation will need to be provided. The proposed conceptual
surface water drainage scheme solution is presented in Appendix E. The required attenuation volume
has been calculated using Micro Drainage WinDes based on the increased impermeable area of 6,584
2
m and a climate change allowance of 30% in line with NPPF guidelines. The volume of required
3
attenuation is approximately 500 m .
The proposed scheme provides attenuation in the form of oversized pipes. In order to provide some
additional attenuation and water quality improvement of the run-off, porous paving has been
maximised throughout the site. This is considered to be the most feasible SUDS technique for use on
the site, based on the SUDS appraisal summarised in the table below:
SUDS technique
Suitability for site
Swales, detention
There is insufficient public open space within the proposed development layout in
basins and ponds
which to implement these SUDS features.
Soakaways
Given the reported ground conditions beneath the site (tidal flat deposits – clay
and silts underlain by the Ampthill Clay Formation) and based on investigations
undertaken during the development of the adjacent site, soakaways are not
considered to be suitable in this area.
Green Roofs
Green roofs are not proposed within the proposed development plan.
Porous Paving
The smaller access roads within the site are considered to be suitable locations in
which to locate porous paving.
10.3 Proposed Conceptual Surface Water Scheme – Option 2: Unrestricted discharge to IDB drain.
The IDB offer the option of allowing discharge onto their drainage network at a rate higher than the
Greenfield rate if a contribution fee is paid. The fee works on a sliding scale, based on Greenfield rate
of 1.4 litres per second per hectare incurring zero charge, up to Impervious area rate of 100 litres per
2
second per hectare incurring a full charge (£6.90 +VAT per impervious m of development).
The proposed conceptual surface water drainage scheme solution for this option (presented in
Appendix E) demonstrates a discharge rate from the site totalling 90 l/s which is equivalent to 64 l/s/ha
RCEF24009-02 R – Land off Sir Isaac Newton Drive
December 2012
22
given the size of the site. Based on the sliding scale described above, this rate would incur a charge of
£4.39 +VAT per impervious m
2
of development. The impermeable area within the proposed
2
development is 6,584 m , therefore this potential drainage option would incur a total contribution fee of
£28,903.76 +VAT.
In order to provide water quality improvement of the run-off, porous paving has been maximised
throughout the site.
10.4 Event Exceedence
The proposed indicative surface water drainage concept provides underground storage up to the 1 in
100 year plus climate change event. In an event exceeding this magnitude, detailed drainage design
will identify mitigation measures to ensure that the resulting above-ground flooding will be confined to
temporary shallow flooding of the on-site road network and will not affect the buildings on site or
significantly increase flood risk to off-site locations.
RCEF24009-02 R – Land off Sir Isaac Newton Drive
December 2012
23
11 SEQUENTUIAL TEST AND EXCEPTION TEST
11.1 Sequential test
The NPPF requires the Local Authority to apply the Sequential Test in consideration of new
development. The aim of the Test is to steer new development to areas at the lowest probability of
flooding. The Council have advised (letter dated 10/10/12; enquiry ref PENQ/12/1865) that, “in terms
of the Sequential test, clearly, the absence of a 5 year supply indicates that there are also no
sequentially preferable sites available.”. It is therefore considered that the site meets the Sequential
Test.
11.2 The Exception Test
The NPPF advises that [‘more vulnerable’] development can be considered appropriate in Flood Zone
3a, following satisfactory application of the Exception Test. The Exception Test aims to ensure that
more vulnerable property types are not allocated to areas at high risk of flooding. For the Exception
Test to be passed:
a.
it must be demonstrated that the development provides wider sustainability benefits
to the community that outweigh flood risk, informed by a Strategic Flood Risk
Assessment where one has been prepared;
b.
a site-specific flood risk assessment must demonstrate that the development will be
safe for its lifetime taking account of the vulnerability of its users, without increasing
flood risk elsewhere, and, where possible, will reduce flood risk overall.
With reference to point (a) above, the Council have advised (letter dated 10/10/12; enquiry ref
PENQ/12/1865) that there is an absence of a 5 year housing supply within the Borough. The proposed
development will therefore provide a benefit to the community through helping to meet the housing
shortage in the area.
With reference to point (b) above, this Flood Risk Assessment demonstrates that the development will
be safe, without increasing flood risk elsewhere.
It is considered that the development passes the Exception Test.
RCEF24009-02 R – Land off Sir Isaac Newton Drive
December 2012
24
12 SUMMARY AND CONCLUSIONS
The aim of the Flood Risk Assessment is to outline the potential for the site to be impacted by
flooding, the potential impacts of the development on flooding both onsite and in the vicinity, and the
proposed measures which can be incorporated into the development to mitigate the identified risks.
The report has been produced in accordance with the guidance detailed in the National Planning
Policy Framework (NPPF). Reference has also been made to the CIRIA SUDS manual (C697), the
Boston Borough Council Strategic Flood Risk Assessment (SFRA), and the Lincolnshire Coastal
Study.
This report has been produced in consultation with the Partnership and Strategic Overview Team at
the Environment Agency, Lincolnshire Emergency Planning Team and the Black Sluice Internal
Drainage Board (IDB).
The potential flood risks to the site, and the measures proposed to mitigate the identified risks, are
summarised in the table below:
Identified
Risk
Source of flooding
L
M
Residual
risk
Mitigation proposed
H
L
M
H
Fluvial
The proposed development
incorporates the recommended
Tidal
mitigation measures of the EA and
the Lincolnshire Coastal Study
(2010).
Restricted run-off from the site to
Greenfield rate (or higher with a
developer contribution fee to the
Surface Water
IDB) and provide a SUDS based
attenuation scheme to
accommodate the 100 year plus
climate change storm event on the
site.
Groundwater
RCEF24009-02 R – Land off Sir Isaac Newton Drive
December 2012
25
Other Sources (e.g. reservoirs,
water mains)
The site is located within an area which is considered to be at a relatively high residual risk of tidal
flooding in the event of a breach of the flood defences. The potential impact of a breach is fairly well
understood due to Environment Agency modelling studies that have been undertaken in the region.
The Lincolnshire Coastal Study (2010) recognises the nature of the tidal flood risk to the area and
provides useful guidance on the mitigation measures that should be implemented in proposed
developments within the area. Through adopting these recommended mitigation measures, and
implementing the advice obtained through consultation with the Environment Agency and Council, the
proposed development would be safe, without increasing flood risk elsewhere.
RCEF24009-02 R – Land off Sir Isaac Newton Drive
December 2012
26