University of Northampton - Northampton Borough Council

Transcription

University of Northampton
Document: GIH015-RPT-039 Version: 1
Northampton University Relocation
Flood Risk Assessment
September 2013
University of Northampton
Northampton University Relocation
Flood Risk Assessment
September 2013
Halcrow Group Limited
Red Hill House, 227 London Road, Worcester WR5 2JG
tel 01905 361 361 fax 01905 361 362
halcrow.com
Halcrow Group Limited is a CH2M HILL company
Halcrow Group Limited has prepared this report in accordance with
the instructions of the client, for the client’s sole and specific use.
Any other persons who use any information contained herein do so at their own risk.
© Halcrow Group Limited 2013
Document history
This document has been issued and amended as follows:
Version
Date
Description
Created by
Verified by
Approved by
0
August
2013
Final Issue
RB
DPW
DPW
1
September
2013
Revised as per comments
received from Environment
Agency on 16 September 2013
and response provided by
Halcrow on 24 September 2013.
RB
BV
DPW
Contents
1
Introduction
1
1.1
1.2
1.3
Introduction
Available Information
Report Structure
1
1
1
2
Development Description and Location
3
2.1
Development Location and Description
3
3
Definition of Flood Hazard
4
3.1
3.2
3.3
3.4
Existing Drainage Characteristics
Sources of Flooding and Existing Flood Defences
Surface Water Drainage and overland flow routes
Other sources of Flooding
4
6
7
7
4
Probability of Flooding
8
4.1
4.2
4.3
4.4
4.5
4.6
Environment Agency modelled River Nene flood extents and
levels
Speed with which fluvial flooding might occure
SFRA Level 1 (February 2009)
Upton Flood Attenuation Area
Surface Water Flooding
5
Climate Change
15
6
The Development Proposals
16
6.1
6.2
6.3
6.4
Waterside Campus Site Layout
Drainage Strategy
Overland Flow Routes
Adoption and Maintenance of the surface water drainage and
SuDS
7
Flood Risk Management
7.1
7.2
7.3
7.4
7.5
7.6
Fluvial Flooding
Structures which may influence local hydraulics
Surface water flooding
Other sources of flooding
Maintenance of sustainable drainage systems
Maintenance Access for River Nene and Hardingstone Dyke
8
10
10
12
14
14
16
17
17
17
18
18
19
20
20
20
21
8
Off Site Impacts
8.1
8.2
8.3
8.4
Fluvial flooding
Breach of Northampton flood defences
Fluvial Morphology
9
Residual Risks
9.1
9.2
9.3
Breach of Northampton Flood Defences
Surface water drainage exceedance flows
Flood warning and evacuation
10
EA Strategic Review and Update
27
10.1
Compliance with EA Strategic Review and Update
27
11
Conclusions
31
12
References
33
22
22
24
24
24
25
25
26
26
Appendices
Appendix A
NPPF Sequential Test for the proposed
Relocation of the University of Northampton
Appendix B – SFRA Level 2 Figures D-6 and D-7
Appendix C – Technical Note: University of Northampton
Relocation breach analysis modelling
Appendix D – Data received from the Environment Agency
Appendix E – Site cross sections pre and post development
and details of proposed road, River Nene Road
Bridge and foot/cycle bridge
Appendix F – Outline Foul, Surface Water Drainage and SuDS
Strategy and associated calculations
Figures
1
Introduction
1.1
Introduction
This flood risk assessment supports the University of Northampton
Relocation to Waterside Campus planning application, which proposes
development of the Waterside Campus site (also known as the Avon and
Nunn Mills Roads development sites) as a University Campus.
1.2
Available Information
Key information considered when preparing this flood risk assessment
includes:
•
•
•
•
•
•
•
•
•
•
1.3
National Planning policy Framework (NPPF) and Technical Guidance to
the NPPF
PPS25 (now superseded by NPPF – but NPPF retains much of PPS25)
Current Environment Agency modelled River Nene flood levels and
flood extents
West Northamptonshire SFRA Level 1 (Feb 2009)
Northampton SFRA Level 2 (Feb 2010)
Northampton Waterside Campus development design
Environment Agency breach analysis results for Northampton and
further breach analysis undertaken by University of Northampton for the
University Relocation to Waterside Campus planning application.
Hydraulic modelling and results undertaken by the Homes and
Communities Agency for the Ransome Road development site planning
application
Hydraulic modelling and results undertaken by the West
Northamptonshire Development Corporation for the Nunn Mills Road
upgrade/extension planning application
Other references are included in Section 12.
Report Structure
As the NPPF retains key aspects of PPS25, for ease of reference to PPS25
requirements, Sections 2 to 9 of this report are structured according to the
Flood Risk Assessment Checklist provided in the PPS25 Practice Guide.
Compliance with the Environment Agency’s Strategic Review of Development
and Flood Risk: Nene Catchment Northampton and Upstream and its update is in
Section 10. Conclusions are in Section 11 and references are listed in Section
12.
Appendix A demonstrates that the proposed University of Northampton
Relocation to Waterside Campus development passes the NPPF Sequential
Test. The SFRA Level 2 Figures D-6 and D-7, including breach analysis
results relevant to the University of Northampton Relocation site, are
reproduced in Appendix B. A technical note detailing breach analysis
undertaken for the University of Northampton Relocation proposed
Waterside Campus site is in Appendix C. Appendix D includes information
received from the Environment Agency to support this flood risk
assessment. Appendix E contains plan and long section drawings of the
proposed new access road and River Nene bridge and details of the
proposed culvert and typical proposed road cross sections. Information
relating to the proposed sustainable drainage systems is in Appendix F:
Outline Foul, Surface Water Drainage and SuDS Strategy and associated
calculations.
2
Development Description and Location
2.1
Development Location and Description
The location of the proposed Waterside Campus development is shown in
Figure 1. The proposed development is located approximately 1 km to the
east of Northampton town centre. The planning application boundary
includes:
•
•
•
•
•
the Avon/Nunn Mills development site south of the River Nene
the existing Nunn Mills Road and its junction with Bedford Road
land to the east of the existing Nunn Mills Road River Nene crossing to
allow for a new River Nene crossing
land to allow for a new foot/cycle bridge connecting the north-west
corner of the Avon/Nunn Mills development site to Northampton
land to allow for an electricity substation in the south-east corner of the
application site
The application boundary crosses the River Nene and Nene navigation and
borders Hardingstone Dyke.
The proposed development layout is shown in Figure 2. The development
includes:
•
•
•
•
•
•
•
•
•
•
•
•
Academic facilities
Student accommodation
Social and sports/leisure buildings
Sports pitches
Car parking
Commercial office buildings
Hotel
Energy centre
Access roads, footpaths and cycle-ways
New river Nene road/foot/cycle bridge
New River Nene footbridge/cycle bridge
No works are proposed on Delapre Brook
Whilst the proposed new road will not, as part of the University
development proposal, extend southwards into the Ransome Road
development, the masterplan enables a future “Green Link” facilitating Bus,
Pedestrian and Cycle Movement with minimal levels of through traffic to
enable delivery of strategic development in the Waterside: Avon/Nunn
Mills/Ransome Road area of Northampton, identified in the Northampton
Central Area Action Plan January 2013.
The NPPF vulnerability classification of the proposed development is
considered as follows:
•
•
•
Essential infrastructure
- proposed road upgrade and extension (the proposed road is
required for the planned strategic development in the Waterside
area)
- energy centre
Less vulnerable
- academic facilities
- social and sports/leisure buildings
- sports pitches
- car parking
- commercial office buildings
- access roads, paths and cycle-ways
More vulnerable
- student accommodation
- hotel
A NPPF Sequential Test for the proposed University of Northampton
Waterside Campus development is included in Appendix A.
3
Definition of Flood Hazard
3.1
Existing Drainage Characteristics
The Waterside Campus development site location is described in Section 2.1.
The principal hydrological features in the vicinity of the proposed
development are:
•
•
•
•
•
River Nene
Hardingstone Dyke
Delapre Brook
Delapre Lake
Drainage ditch along the southern edge of the railway line to the south
of Hardingstone Dyke
The Kislingbury and Brampton branches of the River Nene converge about
250m upstream of South Bridge where the combined flow in the river is
gauged. South Bridge is a key gauging station and the flood levels here are
often used as an indicator of the flooding situation in Northampton.
Downstream of South Bridge, the flow in the River Nene is controlled by a
set of hydraulic structures at the southern end of Beckett’s Park. Water
levels in the main river channel are controlled by Nunn Mills Sluice which is
situated immediately upstream of the Nunn Mills Road bridge. The sluice
consists of three parallel vertical steel lifting gates.
Upstream of the Nunn Mill Sluice, there are three bypass channels on the left
bank of the Nene which convey water from the higher level main channel to
a lower level channel. Immediately upstream of the Nunn Mills Sluice, the
first bypass channel is controlled by Red House Sluice which is a modern
structure with a single tilting gate. Upstream of that, the second bypass
channel is controlled by Calvesholme Weir which is an old iron tilting-gate
weir. The third channel is the navigation channel which consists of
Northampton Town Lock. The three bypass channels join up on the northern
side of Beckett’s Park upstream of Nunn Mills Road and converge with the
main channel a further 100m downstream of Nunn Mills Sluice.
A further 1500m downstream, the next control structure on the Nene is the
Rush Mills Sluice.
Hardingstone Dyke is a catchwater drain which runs parallel to the Nene on
its southern side. It intercepts flow from minor tributaries along the
southern side of the valley between Cotton End and Little Houghton where
it discharges into the river. Hardingstone Dyke is classified as a Main River
and splits into two branches. The first branch continues in a straight line as
far west as Old South Bridge Road. The second branch, which is connected
to the first branch by a culvert just east of Nunn Mills Road crossing runs
along the northern side of the railway line. At its upstream end at Cotton
End Road, it is fed by a pair of surface water sewers.
Delapre Brook, which is also sometimes referred to as Ransome Road Drain
or Hardingstone Drain, is classified as Main River and runs for a distance of
900m along the northern boundary of Delapre Park from London Road
(A508) to Delapre Lake into which it discharges. Delapre Lake drains into
Hardingstone Dyke upstream of the A45 road crossing, via a concrete weir
into an open channel followed by approximately 25m of 750mm diameter
pipe. Delapre Brook is classified as main river including through Delapre
Lake, which is approximately 750m long and would provide significant
attenuation of Delapre Brook flood flows.
There is a railway drainage ditch south of the railway line adjacent to
Hardingstone Dyke, draining west-east. This collects local surface water
drainage only and eventually drains to Hardingstone Dyke, north of Delapre
Lake. The Ransome Road development proposals (Homes and Communities
Agency, 2010) include extending the railway ditch westwards to collect
surface water runoff from the developed site (Appendix C). A minor drain
receiving local surface water drainage is culverted under the Nunn Mills
Road alignment, approximately 20m north of the Hardingstone Dyke
crossing, and this becomes open channel east of Nunn Mills Road.
3.2
Sources of Flooding and Existing Flood Defences
The principal source of flooding for the Waterside Campus development site
is from the River Nene. Flood defences in the vicinity of the Waterside
Campus site are mapped and described in Appendix D.
South of the River Nene, the Northampton Flood Alleviation Scheme
includes an earthen embankment along part of the Avon Cosmetics site
riverbank, designed to provide a 1 in 200 year level of protection. Flood
defences upstream of the site have also been improved and the flow paths of
the 1998 flood, during which the River Nene burst its banks upstream of
South Bridge and water came over Cotton End Road, have now been
blocked (Northampton flood defences were designed to provide a 200-year
standard of protection and are reported in NFCDD to be in good condition).
More extreme events would result in overtopping of the defences and would
affect the site.
North of the River Nene, flood defences directly downstream of Nunn Mills
Road comprise of a flood bank designed to a 200 year standard of protection.
Upstream of Nunn Mills Road, land to the north of the river in Beckett’s
Park rises away from the river and the extent of any flooding in Beckett’s
Park is limited by the topography.
The impact on the Waterside Campus site of breaching of Northampton
flood defences is considered in Section 9.1.
Two sewers draining part of Cotton End discharge into Hardingstone Dyke
at the west end of Thomas Chapman Grove. There is no history of flooding
in the development site from Hardingstone Dyke and its capacity is
significantly more than the sewer that discharges into it. Hardingstone
Dyke discharges into the River Nene at Clifford Hill, more than 4 km
downstream of Nunn Mills Road and flood-locking of the Dyke can be
disregarded.
Two large sewers of 750mm and 600mm diameter draining Far Cotton
discharge into Delapre Brook which in turn discharges into Delapre Lake
and onwards to Hardingstone Dyke. Anecdotal evidence suggests that due
to fly tipping and obstructions of culverts in the drain, localised flooding
occasionally occurs on the right bank in Delapre Park. However, this is not
regarded as a serious issue if obstructions from the drain are cleared. The
development site is not affected as adjacent land north of Delapre Brook is at
elevated levels. As Delapre Brook is a tributary of Hardingstone Dyke, but
separated from it by Delapre Lake, flood-locking of the drain may be
disregarded.
The railway ditch, south of the railway adjacent to Hardingstone Dyke and
draining west-east, collects local surface water drainage only and so is not
considered a significant source of flooding.
3.3
Surface Water Drainage and overland flow routes
The existing surface water drainage for the application site is described in
Section 5.1 of the Outline Foul, Surface Water Drainage and SuDS Strategy
in Appendix F.
Existing overland flow routes are shown in Figure 3.
3.4
Other sources of Flooding
The site is not considered to be at risk of groundwater flooding or tidal
flooding.
4
Probability of Flooding
4.1
Environment Agency modelled River Nene flood extents and
levels
4.1.1
Flood extents
Figure 4 shows historic flood outlines for the 1947 and 1998 and floods. The
proposed Waterside Campus site is shown to be within the 1947 flood extent
and partly with the 1998 flood extents. The 1998 event was thought to have a
return period between 75 and 150 years and was the most severe event in
recent memory. However, since the 1998 event, flood defences have been
constructed in Northampton designed with a 200-year standard of
protection.
The current Environment Agency Flood Zones 2 and 3 are shown in Figure 5
as the Area at risk of Flooding from Rivers or the Sea and Extreme Flood Outline
respectively. These correspond directly to the NPPF Zones which do not
account for flood defences. Figure 5 also shows Areas Benefitting from
Defences.
The Environment Agency modelled flood extents are shown in Map 5 in
Appendix D. The modelling allows for flood defences and overtopping of
the defences where modelled river levels are higher than the defence levels.
The modelling includes the recently constructed Upton Flood Attenuation
Area.
The proposed Waterside Campus site is shown in Figure 5 to be mostly in
the Environment Agency Flood Zone 3 with some areas in Zones 2 and 1
(Zone 1 is the area outside Zones 2 and 3). However, the Environment
Agency Flood Zones do not allow for flood defences and Map 5 in Appendix
D shows the proposed development to be outside the modelled 100-year
flood extent (accounting for flood defences) except for the River Nene
corridor and parts of the Beckett’s Park river island.
4.1.2
Environment Agency breach modelling results
The Environment Agency breach modelling results included in Appendix D
show the Waterside Campus development site to be at risk of a breach of
Northampton flood defences. The figures show significant flooding on the
Waterside Campus development site following a breach of Northampton
flood defences during the 200-year return period flood with climate change
allowance, including areas with Defra flood hazard classification Danger for
Most both upstream and downstream of Nunn Mills Road. The proposed
Waterside Campus development removes this residual flood risk through
land raising with mitigation provided by Upton Flood Attenuation area
(Section 7.1). Breach modelling has been undertaken to assess residual flood
risk for the proposed Waterside Campus development, reported in Section 9
and Appendix C.
4.1.3
Modelled flood levels
For this flood risk assessment hydraulic modelling has been undertaken to
assess the impact of the proposed development on flood risk including the
consequences of a breach of Northampton flood defences. The modelling for
this was developed from the current version of the Environment Agency’s
Upper Nene Catchment Strategic Model (2013). The Environment Agency
Upper Nene model represents the current situation, including the Upton
Flood Attenuation Area upstream of Northampton. Environment Agency
modelled flood levels (taken from data provided for this FRA – see
Appendix 4.1.2) in the vicinity of the proposed Waterside Campus site are
listed in Table 2.
Location in River
Model
Node
Modelled Flood Levels (mAOD)
200-year with
climate change
allowance
1000-year with climate
change allowance
Upstream end of Waterside Campus
site
a7455
58.72
59.05
Approx. 100m upstream of Nunn Mills
sluice gates
a7436a
58.37
58.68
Approx. 50m upstream of Nunn Mills
sluice gates
a7414a
58.12
58.47
Location of proposed new River Nene
bridge
a7403
57.86
58.20
Downstream end of Waterside
Campus site
a7365
57.69
57.99
Hardingstone Dyke at upstream end
of Waterside Campus site
sec003
56.91
58.67
Hardingstone Dyke at upstream end
of Nunn Mills Road culvert
sec07u
56.51
58.39
Hardingstone Dyke at downstream
end of Waterside Campus site
sec019
56.28
57.83
Table 2: Modelled flood levels in the River Nene and Hardingstone Dyke near the
Waterside Campus site
4.2
Speed with which fluvial flooding might occur
The proposed Waterside Campus development is located behind existing
flood defences (or above River Nene flood levels at the bridge crossings).
Northampton flood defences were designed to provide a standard of
protection of 200 years. It is highly unlikely that this rare event would not be
preceded by a considerable lead in time to provide adequate flood warning.
In the Upper Nene Catchment, the time to peak at Southbridge, upstream of
the proposed Northampton University Relocation site, is approximately 36
hours in the Brampton Nene and 32 hours in the Kislingbury Nene. This is
based on the Environment Agency simulated 200-year with climate change
allowance design flood in Northampton.
The impacts of a breach of Northampton defences on the proposed
Waterside Campus development have been investigated by hydraulic
modelling. This modelling is reported in Appendix C. For the modelled
breach scenarios, breach flood water does not reach the Waterside Campus
site.
4.3
The West Northamptonshire SFRA Level 1 was completed in February 2009.
When assessing flood risk at planned development sites in Northampton,
the West Northamptonshire SFRA Level 1 summarises the assessment made
in the Northampton Borough Council SFRA (2004). This acknowledges the
effect of flood defences by classifying the part of the Waterside Campus
proposed development site in the defended floodplain as PPG25 Zone 2
(equivalent to NPPF Zone 2) rather than Zone 3. However the Environment
Agency Flood Zones are defined ignoring the presence of defences and the
defended floodplain is classified as Flood Zone 3.
The proposed University of Northampton Waterside Campus (i.e. all of the
proposed University of Northampton development excluding the new
road/road bridge, the foot/cycle bridge and the energy centre) is classified as
less vulnerable and more vulnerable development (Section 2.1) and is shown in
Figure 5 to be all within Flood Zone 3a (defended River Nene floodplain).
NPPF considers less vulnerable development to be appropriate in Flood Zone
3a. For the more vulnerable development (comprising student
accommodation and hotel) the NPPF Exception Test is required to be
passed. Table 3 details how the proposed Waterside Campus development
passes the Exception Test.
Where the proposed new road crosses the River Nene the development site
is in Flood Zone 3b. NPPF considers the essential infrastructure and water
compatible development classes to be appropriate in Flood Zone 3b and
essential infrastructure in this zone should pass the Exception Test. The proposed
new road is considered essential transport infrastructure, providing
access/egress for development planned in the Waterside area (including the
proposed University of Northampton Waterside Campus development and
the Ransome Road development site. Table 3 details how the proposed new
road essential infrastructure passes the Exception Test.
Appendix A demonstrates that the proposed Waterside Campus site passes
the NPPF Sequential Test.
NPPF Exception Test Requirement
How Passed
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.
The proposed relocation of University of
Northampton to Waterside Campus
contributes to the regeneration of urban
Northampton.
The proposed new road as part of the
University development proposal, extends
southwards towards the Ransome Road
development and the masterplan enables a
future “Green Link” facilitating Bus,
Pedestrian and Cycle Movement with
minimal levels of through traffic to
Ransome Road
The Waterside Campus brownfield site is
identified as a strategic development site
in the Northampton Level 2 SFRA (i.e. the
Nunn Mills site) and the proposed use of
the site for relocation of University of
Northampton is considered appropriate in
the Northampton Central Area Action
Plan.
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.
This FRA report demonstrates that the
development will be safe without
increasing flood risk elsewhere. Upton
Flood Attenuation Area (FAA) provides
strategic mitigation for development
planned in and around Northampton,
including the proposed Waterside
Campus development. Upton FAA
reduces flood risk throughout
Northampton over a significantly wider
area than the proposed Waterside Campus
site.
Table 3: NPPF Exception Test for proposed Waterside Campus development
4.4
The Northampton SFRA Level 2 was completed in February 2010. This
assesses residual flood risk behind Northampton flood defences and
includes the impacts of modelled breaches on Northampton during a 200year return period flood. The potential breaches considered to have most
impact on flood risk at the proposed Waterside Campus site (in terms of
highest depths, velocities and hazard scores) are a breach of the Carlsberg
sluice, approximately 80m upstream of Southbridge (SFRA Level 2 Breach 5)
and a breach of flood defence embankments south of the River Nene
upstream of Nunn Mills Road (SFRA Level 2 Breach 6).
The SFRA Level 2 breach modelling considers breaching of Northampton
flood defences during the 200-year return period flood only (without an
allowance for climate change). However, the NPPF requires the impacts of
climate change on flood risk to be considered. Figures D-6 and D-7 of the
SFRA Level 2 are reproduced in Appendix B. The SFRA Level 2 results show
no flooding on the Nunn Mills Road site as a result of Breach 5. For Breach 6,
there is modelled flooding on the Waterside Campus site upstream of the
existing Nunn Mills Road.
The Environment Agency has undertaken breach modelling for
Northampton including modelling the consequences of a breach during the
200-year return period with climate change allowance flood. These results
inform this FRA and are developed further to consider the impacts of the
proposed Waterside Campus development on the consequences of a breach
of Northampton defences (Section 9.1 and Appendix C). The proposed
University of Northampton Relocation development includes raising ground
levels such that a breach of Northampton flood defences along the northern
edge of the site is no longer considered a significant risk, since the proposed
levels adjacent to the flood defences are above the 1000-year with climate
change allowance modelled flood level (and the raised levels extend into the
site rather than form raised embankments).
Table 4 includes the policies and recommendations for the Nunn Mills Road
site (which corresponds to the proposed Waterside Campus site) presented
in Table 5-2 of the SFRA Level 2 and how these are addressed in this Flood
Risk Assessment.
Area (as defined
in SFRA Level 2)
SFRA Level 2 Table 5-2 policies and recommendations
How addressed in this FRA
Nunn Mills Road
The area affected by a breach in embankment at location 6 is limited
to the north of the area, to the west of Nunn Mills
The proposed University of Northampton Relocation development
includes raising ground levels such that a breach of Northampton
flood defences along the northern edge of the site (including the SFRA
Breach 6) is no longer considered a significant risk, since the proposed
levels adjacent to the flood defences are above the 1000-year with
climate change allowance modelled flood level (and the raised levels
extend into the site rather than form raised embankments). The breach
modelling undertaken for this study assesses a breach directly
upstream of South Bridge, at Carlsberg Sluice, during a 200-year
return period with climate change allowance flood in the River Nene.
Road and flooding is not extensive. Generally a breach would
present ‘Danger for Some’, with some pockets of deeper,
faster flowing water presenting ‘Danger for Most’.
The proposed Waterside Campus development is designed to be safe
from flooding during a breach of Northampton flood defences and the
new road will provide safe access/ egress routes following a breach of
Northampton defences (Section 7.2, Section 9 and Appendix D).
Any future proposed development should be rolled back from the
watercourse. More vulnerable and highly vulnerable
development should be located to the south of the area, with less
vulnerable and water compatible development being
more appropriate towards the north of the area.
Due to the orientation of the site in relation to the River Nene, a
breach could occur at any point along the defences. This is
important to note at site-specific FRA stage as only a single breach
has been considered as part of the Level 2 SFRA.
The proposed Waterside Campus development is considered Essential
Infrastructure, Less Vulnerable and More Vulnerable. Following the
proposed land raising on the Waterside Campus site the risk of a
breach of the River Nene flood defences along the northern edge of
the proposed campus is removed.
Following the proposed land raising as part of the Waterside Campus
development, the most likely location for a breach that could impact
the Waterside Campus site is considered to be at Carlsberg sluice,
upstream of Southbridge. A breach at this location has been modelled,
with a breach commencing at the peak river level during the design
200-year with climate change allowance flood.
Table 4: SFRA Level 2 policies and recommendations for the Waterside Campus development site
4.5
Upton Flood Attenuation Area
Upton Flood Attenuation Area, upstream of Northampton, was implemented by the
Homes and Communities Agency as strategic infrastructure to mitigate increased
flood risk due to planned development in and around Northampton, including the
development of the proposed Waterside Campus site.
4.6
Surface Water Flooding
Existing rates and volumes of runoff generated by the site, used in designing the
proposed drainage scheme, are included in Appendix F.
5
Climate Change
The NPPF requires the assessment of flood risk to account for the impacts of
predicted climate change.
To account for predicted climate change, following current NPPF guidance, an
additional 20% has been added to modelled fluvial inflows in the hydraulic
modelling used within this assessment.
The drainage design for the proposed development will be carried out with an
allowance for climate change in accordance with Table 5 of the “Technical Guidance
to the National Planning Policy Framework” March 2012. The climate change
allowance is dependent upon its design life and for a university development, a
design life up to 2085 is appropriate with an associated 20% increase to peak rainfall
intensity for determining attenuation requirements. A residential development
would normally attract a design life of up to 2115 and a 30% climate change
allowance, however it is considered that as the 1500 residential units are for
university accommodation, are integral to the university development and dependant
on it, it is appropriate to consider them as part of the university in terms of design life
with the associated 20% climate change allowance.
6
The Development Proposals
6.1
Waterside Campus Site Layout
The proposed Waterside Campus development site layout is shown in Figure 2. The
proposed scheme includes:
•
•
•
•
•
•
•
•
•
•
•
•
Academic facilities
Student accommodation
Social and sports/leisure buildings
Sports pitches
Car parking
Commercial office buildings
Hotel
Energy centre
Access roads, footpaths and cycle-ways
New river Nene road/foot/cycle bridge
New River Nene footbridge/cycle bridge
No works are proposed on Delapre Brook
The proposed development is considered to include the NPPF vulnerability
classifications essential infrastructure, less vulnerable and more vulnerable (Section 2.1).
The development site is mostly within Flood Zone 3a with some Flood Zone 1, 2 and
3b. Section 4.3 considers the Waterside Campus site flood zones and appropriate
development.
A NPPF Sequential Test for the proposed University of Northampton Waterside
Campus development is included in Appendix A.
Cross sections showing the Waterside Campus pre- and post-development are in
Appendix E. These show that for the proposed Waterside Campus ground levels are
raised above the 200-year with climate change flood level with a breach of
Northampton flood defences. Ground levels are not raised in the vicinity of the
existing engine shed, which is a listed building, however the engine shed is outside of
the 200-year with climate change flood level with a breach of Northampton flood
defences.
A plan and long section of the proposed new road and new River Nene bridge are in
Appendix E. The proposed new road and River Nene road crossing is designed to
provide safe access/egress during the 1000-year with climate change allowance River
Nene flood, whilst not increasing flood risk elsewhere for a breach of Northampton
flood defences during the 200-year with climate change allowance flood in the River
Nene. The soffit level of the new bridge is design to provide 3m clearance above the
normal water level for navigation and so the soffit is significantly higher than the
1000-year with climate change allowance modelled flood level.
The design for proposed foot/cycle bridge has not yet been finalised but it will be a
clear span with soffit levels above the 1000-year return period with climate change
allowance flood level (at least 3m above normal water level to meet navigation
requirements) and designed to have no negative impact on flood risk compared to
the base case model (i.e. Upton Flood Attenuation Area provides mitigation). The
outline design of the proposed foot/cycle bridge is shown in appendix E.
The proposed Ransome Road development FRA (Homes and Communities Agency,
2010) includes extending the existing drainage ditch running west-east along the
southern side of the railway crossed by the proposed Nunn Mills Road. The Ransome
Road FRA specified a 10m wide channel width for this ditch to convey breach flood
flows, and this channel width is therefore retained in the modelled representation of
the proposed Ransome Road development during this proposed Waterside Campus
development flood risk assessment.
6.2
Drainage Strategy
The Surface Water Drainage and SuDS Strategy for the proposed development are
detailed in Section 5 of Appendix F: Outline Foul, Surface Water Drainage and SuDS
Strategy.
6.3
Overland Flow Routes
For storms exceeding the drainage system capacity surplus runoff will be routed
overland as shown in Figure 6.
In line with the design criteria set out in Section 5.2 of Appendix F, exceedance flows
will not flood proposed and retained buildings for storm events with a return period
bigger than in 200 years plus 20% climate change. In accordance with best practice
(CIRIA C635 Designing for Exceedance), flood water from the storm events that
exceed the design storm of 1 in 200 years plus 20% climate change will be channelled
towards the lower areas like the car parks, the pond and then discharge into
Hardingstone Dyke or River Nene. Further details are in Appendix F.
6.4
Adoption and Maintenance of the surface water drainage and SuDS
The proposed adoption and maintenance arrangements for the development surface
water drainage and Sustainable Drainage Systems (SuDS) are in Section 7 of
Appendix F: Outline Foul, Surface Water Drainage and SuDS Strategy.
The drainage system within the Avon site (excluding Avon Building) will be private
and will have to be maintained by a private management company. However, the
proposed surface water drainage system and SuDS within Nunn Mills site can be
offered for adoption to Anglian Water under the Section 104 Agreement of the Water
Industry Act based on the assumption that commercial buildings will be owned by
different parties/people.
If piped networks are to be offered for adoption to Anglian Water, these will be
subject to Easement Agreements along the proposed sewers. If SuDS are offered for
adoption this process will involve land ownership transfer from future land owner to
Anglian Water. The extent of this will have to be determined through liaisons with
Anglian Water.
The drainage in the adopted highway will be maintained by the Local Highway
Authority as part of its routine maintenance programme to ensure operation of the
system.
7
Flood Risk Management
7.1
Fluvial Flooding
The Waterside Campus development site is mostly in the River Nene defended
floodplain, protected from flooding by Northampton flood defences. The defences
were designed with a 200-year designed standard of protection. The proposed
Waterside Campus development will not compromise the effectiveness of the existing
flood defences. Those parts of the development within the active floodplain are the
proposed new River Nene bridge and part of the new road within the River Nene
Flood Zone 3b and the part of the proposed new foot/cycle bridge within Flood Zone
3b.
Construction of Upton Flood Attenuation Area (FAA) upstream of Northampton was
completed in 2009. Upton FAA was implemented by the Homes and Communities
Agency ahead of development in and around Northampton to mitigate any
associated increase in flood risk. Upton FAA is designed to reduce flood levels in the
River Nene in Northampton with a modelled reduction in peak flood level at
Southbridge, for the 200-year with climate change allowance flood, by approximately
350mm (source: Upton FAA Flood Risk Assessment).
The proposed new road is designed to provide a safe access route for planned
development in the area during the modelled 1000-year with climate change River
Nene flood. Modelled post-development peak flood depths, velocities and hazard
scores within the 2D hydraulic model domain, during the 1000-year return period
with climate change allowance River Nene flood, are shown in Figures C5 and C6
(Appendix C) for the cases without and with development of the Ransome Road site
respectively. Figures C7 and C8 show that the proposed new road provides a safe
route within the 2D hydraulic model domain for the cases without and with
development of the Ransome Road site, i.e. the maximum hazard score along the
proposed new access road is ‘very low’ as classified in Defra project report FD2320.
The new River Nene bridge is represented in the 1D model domain. Table 5 list
modelled peak 1000-year return period with climate change allowance flood levels in
the River Nene in the vicinity of the new River Nene bridge for the cases that Nunn
Mills Road is constructed both with and without the planned Ransome Road
development. For the proposed new road lying within the 1D hydraulic model
domain, the design road levels are a minimum of 60.6 mAOD and significantly
higher than the modelled 1000-year with climate change allowance peak flood levels
in Table 5.
To meet River Nene navigation requirements, the soffit level of the proposed new
foot/cycle bridge will be a minimum of 3m above the normal River Nene water of
57.2mAOD level (i.e. a minimum of 60.2mAOD). This is significantly higher than the
modelled 1000-year with climate change allowance peak flood levels at the foot/cycle
bridge location in Table 5.
The proposed development includes land raising within the University of
Northampton Waterside Campus to remove the risk associated with a breach of
Northampton flood defences. The extent of land raising and site cross sections are
shown in Appendix E. The proposed land raising increases ground levels adjacent to
the River Nene flood defences to be above the 1000-year return period with climate
change modelled flood levels. There is no land raising adjacent to the existing engine
shed as it is a listed building. However, post-development, the whole of the proposed
University of Northampton Waterside Campus will be outside of the modelled 200year return period with climate change allowance flood extent (Figures C2 and C3 in
Appendix C). Finished floor levels will be a minimum of 300mm above the modelled
200-year return period with climate change allowance flood level (including with a
breach of Northampton flood defences).
Flood risk due to breaching of Northampton flood defences is discussed as a residual
risk in Section 9.1 and Appendix C.
Modelled post-development 1000-year with climate
change allowance peak flood level (mAOD)
Without development of
the Ransome Road site
With development of the
Ransome Road site
Location of new
foot/cycle bridge
(model node a7445)
57.90
57.89
Upstream of new
River Nene bridge
(model node a7403)
58.62
58.62
Downstream of new
River Nene bridge
(model node a7403i2)
57.89
57.88
Location
Table 5: Modelled peak 1000-year return period with climate change allowance flood levels in
the River Nene
7.2
Structures which may influence local hydraulics
There are no structures planned for the proposed development which will adversely
influence local hydraulics.
The proposed new River Nene bridge will span the River Nene with no piers and
have a soffit level above the 1000-year return period with climate change allowance
flood level (Section 7.1). Hydraulic modelling indicates the new bridge has a marginal
impact directly upstream where there is a slight and localised increase in modelled
floodplain depths (modelled depth increase between 0mm and 20mm) directly
upstream of the proposed new bridge. This is considered an insignificant impact as
the level increase is marginal and only affects the floodplain adjacent to a car park which remains dry during the modelled 1000-year with climate change allowance
flood for the post-development case (Figures C5 and C6 in Appendix C), and the
Waterside Campus development site, for which there will be land raising above the
1000-year with climate change flood level adjacent to the River Nene.
Upton Flood attenuation area provides mitigation for the new road and bridge.
The proposed foot/cycle bridge is at a preliminary design stage. The bridge will have
a soffit level above the modelled 1000-year return period with climate change
allowance flood level and will be designed to have no adverse impact on flood risk
elsewhere.
The proposed development will include improvement works where possible to the
Hardingstone Dyke, which will be developed in detail with the Environment Agency
as part of the Reserved Matters applications as the detailed design develops.
Whilst the proposed new road does not continue as far south as the Hardingstone
Dyke crossing, the new road has been designed so that it can be extended to provide
access to the Ransome Road development site. The hydraulic modelling for this flood
risk assessment assumes that when extended, the Ransome Road site access road will
include a 2.7m wide Hardingstone Dyke culvert, and the two channels south of
Hardingstone Dyke will be crossed with 10m wide culverts (these are required to
convey breach flood flows).
The proposed Ransome Road development FRA (Homes and Communities Agency,
2010) includes extending the existing drainage ditch running west-east along the
southern side of the railway. The Ransome Road FRA specified a 10m wide channel
width for this ditch to convey breach flood flows, and this channel width is therefore
retained in the representation of the Ransome Road development in this flood risk
assessment.
The Ransome Road development will need its own FRA and ground re-modelling,
but the FRA for the Waterside development does not place any more onerous
requirements on this site.
7.3
Current and proposed surface water drainage arrangements are described in
Appendix F. The proposed surface water drainage strategy limits outflows from the
proposed development to pre-development rates for storms up to a 200-year return
period with allowance for climate change. For larger storms surplus runoff will be
routed over land as shown in Figure 6.
7.4
Other sources of flooding
The site is located upstream of the limit of tidal influence on the River Nene and there
is no risk of tidal flooding. The site is not considered to be at risk of groundwater
flooding.
7.5
Maintenance of sustainable drainage systems
The proposed adoption and maintenance arrangements for the development surface
water drainage and Sustainable Drainage Systems (SuDS) are in Section 7 of
Appendix F: Outline Foul, Surface Water Drainage and SuDS Strategy.
proposed surface water drainage system and SuDS within Nunn Mills site can be
this process will involve land ownership transfer from future land owner to Anglian
Water. The extent of this will have to be determined through liaisons with Anglian
Water.
system.
7.6
Maintenance Access for River Nene and Hardingstone Dyke
The River Nene and Hardingstone Dyke are classified as Main River and so are
maintained by the Environment Agency. The proposed Waterside Campus
development therefore allows a minimum of 5m access strips to maintain the River
Nene and Hardingstone Dyke.
8
Off Site Impacts
8.1
Fluvial flooding
The proposed Waterside Campus development is mostly in the River Nene defended
floodplain, protected from flooding by Northampton flood defences. The defences
were designed to provide a 200-year standard of protection. Those parts of the
development within the active floodplain are the proposed new River Nene road
bridge and the proposed foot/cycle bridge.
The proposed new River Nene bridge is designed to have negligible impact on flood
flows and levels.
Construction of Upton Flood Attenuation Area (FAA) upstream of Northampton was
completed in 2009. Upton FAA was implemented by the Homes and Communities
Agency ahead of development in and around Northampton to mitigate any
associated increase in flood risk. This includes mitigation of increased flood risk
associated with the Waterside Campus development. When assessing the impact of
the proposed Waterside Campus on flood risk the pre-development case is therefore
taken to be pre-development of Waterside Campus and pre-construction of Upton
FAA. The post-development case is assessed for the cases with and without the
planned Ransome Road development as the Waterside Campus development may
take place before or after the planned Ransome Road development.
Upton FAA is designed to reduce flood levels in the River Nene in Northampton with
a modelled reduction in peak flood level at Southbridge, for the 200-year with climate
change allowance flood, by approximately 350mm (source: Upton FAA Flood Risk
Assessment).
Elements of the proposed Waterside design requiring mitigation are:
•
•
•
Displaced defended floodplain potential storage due to land raising within the
Waterside Campus area
Displaced active and defended floodplain storage due to construction of the
proposed new road connecting the Waterside Campus development to Bedford
Road
Displaced defended floodplain potential storage due to construction of the
proposed new foot/cycle bridge
The impacts of the proposed development are assessed by comparing predevelopment and post-development;
•
•
flood levels in the River Nene 1D model extent and;
gridded flood depths, velocities and hazard scores in the 2D model extent
(defended floodplain).
A comparison of pre- and post-development modelled 200-year return period with
climate change allowance flood levels in the 1D model extent at key nodes upstream
and downstream of the proposed Waterside Campus development are listed in Table
6 for the case that there is no breach of Northampton flood defences. The model
results in Table 6 show that post-development flood levels in the River Nene
upstream and downstream of the proposed Waterside Campus development are
lower than pre-development as a result of Upton FAA. The new bridge has a
marginal impact directly upstream where there is a slight and localised increase in
modelled floodplain depths (modelled depth increase between 0mm and 20mm)
directly upstream of the proposed new bridge. This is considered an insignificant
impact as the level increase is marginal and only affects the floodplain adjacent to a
car park - which remains dry during the modelled 1000-year with climate change
allowance flood for the post-development case (Figures C5 and C6 in Appendix C),
and the Waterside Campus development site, for which there will be land raising
above the 1000-year with climate change flood level adjacent to the River Nene.
Peak level (mAOD) and diff. Compared to pre-devl. case (m) for
modelled200-year with climate change allowance flood
Location
River Nene:
Southbridge
ISIS ID
model
node
Pre-development
(before development of
Waterside Campus,
Upton FAA and
Ransome Road)
Post-development A
Post-development B
(after development of
Waterside Campus
and Upton FAA but
before development of
Ransome Road)
(after development of
Waterside Campus,
Upton FAA but and
Ransome Road)
a7486b1
59.62
59.55 (-0.06)
59.56 (-0.06)
River Nene:
upstream end of
Waterside Campus
site
a7455
58.74
58.72 (-0.02)
58.72 (-0.02)
River Nene:
approx. 160m
upstream of Nunn
Mills sluice
a7424a
58.46
58.46 (-0.02)
58.45 (-0.02)
Upstream of River
Nene bridge
a7416b1
57.63
57.65 (+0.02)
57.65 (+0.02)
Upstream of Nunn
Mills sluice gates
a7408as
58.32
58.31 (-0.01)
58.31 (-0.01)
Upstream of new
River Nene bridge
a7403
57.63
57.65 (+0.02)
57.65 (+0.02)
a7403i2
57.61
57.58 (-0.04)
57.58 (-0.04)
Downstream of new
River Nene bridge
Table 6: Comparison of pre and post development 200-year flood levels in the ID model extent
The defended floodplain in the vicinity of the proposed Waterside Campus
development is represented within the 2D hydraulic model extent. The impact of the
proposed Waterside Campus development on flooding in the defended floodplain
during a breach of Northampton flood defences is discussed in Sections 8.2, 9.1 and
Appendix C.
8.2
Breach of Northampton flood defences
The proposed Waterside Campus development includes displacement of defended
floodplain potential storage due to raising of ground levels within the development
site. Whilst these raised levels are in the defended floodplain there is a residual risk
that Northampton flood defences are breached or overtop. The proposed defended
floodplain displacement results in a loss of potential defended floodplain storage.
The mitigation against increased flood risk during a breach of Northampton flood
defences is provided by Upton Flood Attenuation Area, which reduces River Nene
flood levels and so reduces flows over a breach for a given design return period. This
mitigates for the loss of defended floodplain storage is detailed in Section 9.1 and
Appendix C.
8.3
The proposed surface water drainage strategy (Sections 6.2 and 6.3) specifies on-site
attenuation of surface water flows limited to be less than pre-development rates for
storms up to a 200-year with climate change return period. Surface water flows from
the developed site will therefore be lower than those of the existing site.
For storms exceeding the drainage capacity surplus flow would be directed overland
as described in Section 6.4 and Figure 6.
8.4
Fluvial Morphology
The proposed Waterside Campus development will not impact fluvial morphology
since the proposed new River Nene bridge and foot/cycle bridge have negligible
impact on local hydraulics.
9
Residual Risks
9.1
Breach of Northampton Flood Defences
The Waterside Campus development site is mostly in the defended River Nene
floodplain and there is a residual flood risk associated with a breach or over-topping
of Northampton flood defences.
Upton Flood Attenuation Area was implemented upstream of Northampton by the
Homes and Communities Agency to mitigate the impacts of development in and
around Northampton, including the proposed Waterside Campus development. The
impact of the proposed development on the consequences of a breach of
Northampton defences, and the mitigation provided by Upton FAA, has been
assessed by hydraulic modelling.
This breach analysis modelling is reported in Appendix C which concludes that:
•
•
•
•
•
The Environment Agency Upper Nene Strategic 1D-2D Model (2013) has been
developed by University of Northampton in 2013 to assess the impacts of the
planned Waterside Campus development on flood risk during a breach of
Northampton flood defences. This model has been developed further to assess
the impact of the proposed Waterside Campus development on flood risk during
a breach of Northampton flood defences and during the 1000-year with climate
change allowance River Nene flood.
The implementation of Upton Flood Attenuation Area has wide spread benefits in
terms of reduced flood risk throughout Northampton.
The breach modelling undertaken shows that the proposed Waterside Campus
development does not result in increased flood risk elsewhere during a breach of
Northampton flood defences occurring at the peak level of the 200-year with
climate change allowance River Nene flood, except for a marginal impact directly
upstream of the proposed new River Nene bridge where there is a slight and
localised increase in modelled depths (modelled increase between 0m and 20mm)
directly upstream of the proposed new bridge. This is considered an insignificant
impact as the level increase is marginal and only affects the floodplain adjacent to
a car park - which remains dry during the modelled 1000-year with climate change
allowance flood for the post-development case (Figures C5 and C6 in Appendix
C), and the Waterside Campus development site, for which there will be land
raising above the 1000-year with climate change flood level adjacent to the River
Nene.
The proposed new access road will provide safe access/egress to the planned
Waterside Campus development (and later to the panned Ransome Road
development) during a modelled 1000-year return period flood with climate
change allowance.
The modelling presented demonstrates that Upton FAA provides the required
mitigation against increased residual flood risk due to the proposed Waterside
Campus development.
9.2
Surface water drainage exceedance flows
For storms exceeding the surface water drainage capacity surplus runoff would be
routed over land as detailed in Section 6.4 and Figure 6.
9.3
Flood warning and evacuation
The proposed new road within the Waterside Campus development is designed to
provide emergency access/egress for planned development in the area including the
Waterside Campus development site and, when implemented, the Ransome Road
development site. The proposed new road will provide safe access/egress during the
modelled 1000-year with climate change allowance River Nene flood and during a
breach of Northampton flood defences during a modelled 200-year with climate
change allowance River Nene flood.
The proposed Waterside campus development is close to the existing Environment
Agency flood warning siren at the Ransome Road/ London Road junction.
10
EA Strategic Review and Update
10.1
Compliance with EA Strategic Review and Update
Tables 7 and 8 demonstrate compliance of the proposed Waterside Campus
development with the Environment Agency’s Strategic Review of Development and
Flood Risk: Nene Catchment Northampton and Upstream (1999) and its update (2004).
Table 7: Strategic Review policies and principles for development in the Upper Nene
catchment
Policies
No.
1
Policy followed by response
Development will not be permitted if it would either alone, or in conjunction with other developments, be
likely to:
• Be itself at risk from flooding;
• inhibit the capacity of the floodplain to store water
• impede the flow of flood water; and
• otherwise increase the flood risk.
The proposed Waterside Campus development includes land raising to remove the site from the 200-year
return period with climate change allowance modelled flood extent, including with a breach of
Northampton flood defences. Finished floor levels will be a minimum of 300mm above the modelled 200year return period with climate change allowance modelled flood level. The proposed new access road
and River Nene bridge are designed with an elevation to provides a safe access/egress route during the
1000-year with climate change allowance flood in the River Nene.
Section 9.1 and Appendix C describe breach modelling undertaken to demonstrate that Upton Flood
Attenuation Area mitigates for increased residual risk due to displacement of floodplain storage by the
proposed development.
The proposed development will include improvement works where possible to the Hardingstone Dyke,
which will be developed in detail with the Environment Agency as part of the Reserved Matters
applications as the detailed design develops.
The proposed Ransome Road development FRA (Homes and Communities Agency, 2010) includes
extending the existing drainage ditch running west-east along the southern side of the railway crossed by
the proposed Nunn Mills Road. The Ransome Road FRA specified a 10m wide channel width for this
ditch to convey breach flood flows, and this channel width is therefore retained when representing the
Ransome Road development in the proposed Waterside Campus development FRA. The Ransome Road
development will need its own FRA and ground re-modelling, but the FRA for the Waterside
development does not place any more onerous requirements on this site.
Modelled flood levels in the River Nene are lower post-development than pre-development upstream
and downstream of the proposed development (Table 6).
Whilst modelled flood levels are lower post-development upstream and downstream of the proposed
Waterside Campus development, the new River Nene bridge results in a slight and localised increase in
modelled 200-year return period with climate change allowance flood levels (between 0mm to 20mm)
directly upstream of the proposed bridge, reducing to zero approximately 100m upstream in the northern
river branch (50m upstream of the existing River Nene bridge) and 50m upstream in the southern branch
(at Nunn Mills sluice). This is considered an insignificant impact as the level increase is marginal and
only affects the floodplain adjacent to a car park - which remains dry during the modelled 1000-year with
climate change allowance flood for the post-development case (Figures C5 and C6 in Appendix C), and
the Waterside Campus development site, for which there will be land raising above the 1000-year with
climate change flood level adjacent to the River Nene.
The proposed development incorporates SUDS surface water drainage systems which are designed to
have discharge rates less than pre-development for storms up to a 200-year return period with climate
Policies
No.
Policy followed by response
change allowance.
2
Proposals which do increase flood risk, either alone or in conjunction with other development, will be
resisted unless appropriate mitigation measures, including suitable maintenance arrangements, are
provided, that comply with the requirements of these policies and principles as part of the overall
development.
Section 9.1 and Appendix C describe breach modelling undertaken to demonstrate that Upton Flood
Attenuation Area mitigates for increased residual risk due to displacement of floodplain storage in the
defended floodplain.
modelled flood levels (by 0 to 20mm) directly upstream of the proposed bridge, reducing to zero
approximately 100m upstream in the northern river branch (50m upstream of the existing River Nene
bridge) and 50m upstream in the southern branch (at Nunn Mills sluice). This is considered an
insignificant impact as the level increase is marginal and only affects the floodplain adjacent to a car park
- which remains dry during the modelled 1000-year with climate change allowance flood for the postdevelopment case (Figures C5 and C6 in Appendix C), and the Waterside Campus development site, for
which there will be land raising above the 1000-year with climate change flood level adjacent to the River
Nene.
The proposed development site will incorporate SUDS surface water drainage systems which are
designed to limit runoff rates to pre-development rates for storms up to a 200-year return period with
climate change allowance. Section 7.6 and Appendix F provide information relating to maintenance of the
proposed sustainable drainage systems (SUDS).
3
Development proposals must incorporate source control techniques and sustainable urban drainage
systems.
The proposed development incorporates SUDS surface water drainage systems.
4
Development proposals that would result in the restoration and or enhancement of the floodplain or
contribute to a reduction in surface water runoff will be encouraged.
The proposed development site would incorporate SUDS surface water drainage systems which are
designed to limit runoff rates to pre-development rates for storms up to a 200-year return period with
climate change allowance.
5
Development proposals must bring about an overall reduction in flood risk.
Surface water runoff from the development site would have runoff rates less than or equal to predevelopment rates for storms up to a 200-year return period with climate change allowance. There would
therefore be a reduction in flood risk.
The Upton Flood Attenuation Area scheme results in lowered flood levels throughout Northampton.
approximately 100m upstream in the northern river branch (50m upstream of the existing River Nene
bridge) and 50m upstream in the southern branch (at Nunn Mills sluice). This is considered an
insignificant impact as the level increase is marginal and only affects the floodplain adjacent to a car park
- which remains dry during the modelled 1000-year with climate change allowance flood for the postdevelopment case (Figures C5 and C6 in Appendix C), and the Waterside Campus development site, for
which there will be land raising above the 1000-year with climate change flood level adjacent to the River
Nene.
Principles
No.
Principle followed by response
1
Development will be directed away from Greenfield sites in the floodplain to prevent any further erosion of
its flood flow and flood storage capacity, which would increase flood risk.
The Waterside Campus development site is a brownfield site.
2
Any permitted development outside the floodplain will need to include mitigation action against its effects
up to at least the same level of storm event governing the existing flood defence standard, this will prevent
any future erosion of that standard.
Complies. Runoff form the development site limited to pre-development runoff conditions for storms up to a
200-year return period with climate change allowance.
3
Any redevelopment of brownfield sites within the floodplain will only be permitted if there is no increase in
flood risk to people and property, and the development maintains or improves river flood defences to at least
the accepted defence standard, and if any works required to prevent any future erosion of that standard are
carried out where it is relevant to the site.
Complies. Runoff from the development site will be limited to pre-development runoff conditions. Benefits
in terms of reduced flood levels are realised throughout Northampton by adoption of the Upton Flood
Attenuation Area scheme.
approximately 100m upstream in the northern river branch (50m upstream of the existing River Nene bridge)
and 50m upstream in the southern branch (at Nunn Mills sluice). This is considered an insignificant impact
as the level increase is marginal and only affects the floodplain adjacent to a car park - which remains dry
during the modelled 1000-year with climate change allowance flood for the post-development case (Figures
C5 and C6 in Appendix C), and the Waterside Campus development site, for which there will be land raising
above the 1000-year with climate change flood level adjacent to the River Nene.
4
New development must contribute towards a reduction in flood risk. This may be achieved by extra
mitigation measures or by contributing to improved floodplain flow or floodplain storage to achieve a
progressive reduction to flood risk.
Development of the site will improve flood storage through provision of SUDS. Upton Flood Attenuation
Area reduces flood levels in Northampton.
5
Appropriate long-term maintenance agreements in respect of all flood risk reduction infrastructure including
flood defences, attenuation lagoons, structures and facilities etc. must be completed prior to construction, in
order that the long-term effectiveness of these facilities is ensured.
The proposed adoption and maintenance arrangements for the proposed development surface water
drainage and Sustainable Drainage Systems (SuDS) are in Section 7 of Appendix F: Outline Foul, Surface
Water Drainage and SuDS Strategy.
6
A formal Flood Risk Impact Assessment of the potential impacts of any proposed development, detailing
flood risk reduction measures to be incorporated, will be required, prior to development taking place.
This FRA addresses the issues.
7
All flood risk reduction measures should be completed and operationally effective prior to development
commencing.
Upton Flood Attenuation Area mitigates for the displacement of floodplain storage and is already in place.
8
In seeking to promote a reduction in flood risk a strategic approach which will both reduce flood risks and
provide opportunities for environmental enhancements, should be promoted.
The proposed development will incorporate SUDS surface water drainage systems which reduce flood risk
by limiting runoff flows from the site.
The Upton Flood Attenuation Area scheme benefits a much wider area in Northampton.
The site is allocated in an up to date development plan document (the NCAAP) which is informed by a
Strategic Flood Risk Assessment.
There are a number of significant sustainability benefits arising from redeveloping this brownfield site.
Not least, the remediation and biodiversity improvements are considered to be of significant
environmental benefit to the area. These significant benefits are considered to outweigh flood risk
(especially given that the site is now subject to flood defences).
2. 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.
This document has been prepared by Halcrow and demonstrates that this part of the test has been
passed.
Conclusion
This Sequential Test has been undertaken in order to support the University of Northampton’s
planning application for the development of new campus at the Avon/Nunn Mills site, Northampton.
It is clear from undertaking this Sequential Test that there are no reasonably available alternative sites
to that at Ransome Road/Nunn Mills within the University’s search area (i.e. within close proximity to
Northampton town centre).
This is an allocated site that needs to be developed. The accompanying Flood Risk Assessment has
demonstrated how flood risks will be managed so that the development remains safe throughout its
lifetime.
Both elements of the Exception Test are also considered to be passed.
Appendix B
SFRA Level 2 Figures D-6 and D-7
Information Requirement
Response
Compliance with PPG25
This document complies with current planning policy, NPPF.
Statement of compliance with Strategic
review and update
Strategic Review:
Table 7 details compliance with the Strategic Review.
Strategic Review update:
The proposed development will incorporate SUDS surface drainage
systems which will result in limited runoff flows from the site and
would therefore contribute towards reduced flood levels in
Northampton.
Programme of compensation for
permanent loss of floodplain
Potential loss of defended floodplain is mitigated by Upton Flood
Attenuation Area.
Modification of model to incorporate
impacts and mitigation
See Section 9.1 and Appendix C.
Legal agreements for future maintenance
The proposed adoption and maintenance arrangements for the
proposed development surface water drainage and Sustainable
Drainage Systems (SuDS) are in Section 7 of Appendix F: Outline
Foul, Surface Water Drainage and SuDS Strategy.
Statements on contribution to sustainable
communities objectives e.g.
improvements to infrastructure
The proposed development will incorporate SUDS surface water
drainage systems. The proposed new road within the development
is essential infrastructure to enable planned brownfield site
regeneration in Northampton.
Table 8: Compliance with Strategic Review Update
11
Conclusions
The following conclusions can be drawn from this Flood Risk Assessment report:
•
This NPPF compliant flood risk assessment demonstrates that the proposed
Waterside Campus development will not be at risk of flooding and will not
increase the risk of flooding elsewhere.
•
Whilst modelled flood levels are lower post-development upstream and
downstream of the proposed Waterside Campus development, the new River
Nene bridge results in a slight and localised increase in modelled flood levels
(0 to 20mm) directly upstream of the proposed bridge, reducing to zero
approximately 100m upstream in the northern river branch (50m upstream of
the existing River Nene bridge) and 50m upstream in the southern branch (at
Nunn Mills sluice). This is considered an insignificant impact as the level
increase is marginal and only affects the floodplain adjacent to a car park which remains dry during the modelled 1000-year with climate change
allowance flood for the post-development case (Figures C5 and C6 in
Appendix C), and the Waterside Campus development site, for which there
will be land raising above the 1000-year with climate change flood level
adjacent to the River Nene.
•
The current 200-year level of protection provided by Northampton Flood
Defences will not be affected by the proposed development.
•
The surface water drainage strategy for the proposed development will employ
sustainable drainage systems (SUDS) designed to limit surface water flows to
pre-development rates for storms up to a 200-year return period with climate
change allowance. For storms exceeding the drainage capacity surplus runoff
will be directed over land as described in Section 6.4.
•
The proposed adoption and maintenance arrangements for the development
surface water drainage and Sustainable Drainage Systems (SuDS) are in Section
7 of Appendix F: Outline Foul, Surface Water Drainage and SuDS Strategy.
proposed surface water drainage system and SuDS within the Nunn Mills site can be
this process will involve land ownership transfer from future land owner to Anglian
Water. The extent of this will have to be determined through liaisons with Anglian
Water.
system.
•
The loss of defended floodplain due to the proposed Waterside Campus
development is mitigated by Upton Flood Attenuation Area, which was
constructed ahead of development in and around Northampton as strategic
infrastructure to mitigate the impacts of development on flood risk.
•
Benefits, in terms of reduced flood levels resulting form implementing Upton
Flood Attenuation Area, extend over a much larger area in Northampton and
are not confined to the development sites linked to the scheme.
•
A minimum of 5m maintenance easement will be provided adjacent to the
River Nene and Hardingstone Dyke.
•
The new access road will provide safe access/egress to the planned
development during a modelled 1000-year return period flood with climate
change allowance.
•
The Waterside Campus site is located close to the existing Environment
Agency flood warning siren at the Ransome Road/ London Road junction.
12
References
NPPF: National Planning Policy Framework and technical Guidance, Communities
and Local Government 2012
PPS25: Development and Flood Risk, Communities and Local Government 2006
Northampton Central Area Action Plan, Northampton Borough Council 2013
Ransome Road Development Site Flood Risk Assessment, Halcrow 2010 (approved
FRA of planning application reference 06/0016/OUTWNN)
Nunn Mills Road Extension/Upgrade Flood Risk Assessment, Halcrow 2011
(approved FRA of planning application reference 11/0014/FULWNN)
Strategic Review of Development and Flood Risk Nene Catchment Northampton and
Upstream, EA 1999
Update to the Strategic Review of Development and Flood Risk Nene Catchment
Northampton and Upstream, EA 2004
West Northamptonshire Strategic Flood Risk Assessment Level 1, Scott Wilson 2009
West Northamptonshire Strategic Flood Risk Assessment Level 2, Scott Wilson 2010
FD2320: Flood Risk Assessment Guidance for New Development: Phase 2, Defra 2005
FD2321 Flood Risks to People – Guidance Document, Defra 2005
Preliminary rainfall-Runoff Management for Developments R&D Technical Report,
Defra 2005
Upton Flood Attenuation Area Phase 3 Reserved Matters Submission, Flood Risk
Assessment, English Partnerships July 2007 (Planning application N/2007/0967)
Appendix B – SFRA Level 2 Figures D-6 and D-7
Appendix A
NPPF Sequential Test for the proposed Relocation of
the University of Northampton
Appendix A
NPPF Sequential Test for the proposed
Relocation of the University of Northampton
Sequential Test and Exception Test
University of Northampton Relocation to Avon/Nunn Mills site
Introduction
The University of Northampton is seeking to move to the Avon/Nunn Mills site. The reasons for this
are set out in Appendix 1 to this document (and elsewhere in the planning application). In short, in
order to maximise its prospects for the future the University needs to consolidate its existing split
campus operations to a single site. The University has also taken the decision that its new site should
be close to (at least within reasonable walking distance of) Northampton town centre.
This sequential test has been undertaken in order to support the planning application for the
development of the Avon/Nunn Mills site for a new campus plus commercial development. The
purpose of a sequential test is to steer new development into areas that are at the lowest probability
of flooding, Flood Zone 1. Where there are no reasonably available sites in this zone, reasonably
available sites should be considered in Flood Zone 2.
Guidance on the Sequential Test is contained within the NPPF and its accompanying technical
guidance. Paragraph 101 of the NPPF states that The aim of the Sequential Test “is to steer new
development to areas with the lowest probability of flooding. Development should not be allocated or
permitted if there are reasonably available sites appropriate for the proposed development in areas
with a lower probability of flooding. The Strategic Flood Risk Assessment will provide the basis for
applying this test. A sequential approach should be used in areas known to be at risk from any
form of flooding”
Paragraph 102 states that If, following application of the Sequential Test, it is not possible, consistent
with wider sustainability objectives, for the development to be located in zones with a lower probability
of flooding, the Exception Test can be applied if appropriate. For the Exception Test to be passed:
•
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; and
•
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.
Both elements of the test will have to be passed for development to be allocated or permitted.
The key documents upon which this Sequential Test have been based are:
•
Northampton Central Area Action Plan
•
The West Northamptonshire Strategic Flood Risk Assessment Final Level 1 Report – Volume
1
•
Northampton Level 2 Strategic Flood Risk Assessment Living Document
Proposal
A full description of the Avon/Nunn Mills site is contained withing the accompanying planning
application and EIA. In short, it is a brownfield site formerly used (to the west of Nunn Mills Road) as
Avon’s manufacturing and distribution centre and (to the east of Nunn Mills Road) as a power station.
Avon’s headquarters building remains within the site (albeit excluded from the University’s planning
application). The planning application site itself has something of an unkempt appearance, containing
large areas of hardstanding, piles of materials and vacant buildings. The site is bounded to the north
by the River Nene and to the south by Hardingstone Dyke and the former Northampton to Bedford
railway line (now disused).
Again a full description of the development is contained within the accompanying planning application
and EIA. The University proposes to construct is academic buildings fronting the River Nene and
around a diagonal corridor from the north west to the south central of the site. Student
accommodation will be focussed in the south western corner of the site. The east part of the site is
proposed for commercial development (office development by a commercial organisation independent
of the University), the University’s administration and management functions, plus a hotel, leisure
facility and sports pitch. Car parking is generally at grade at a number of locations around the site.
Road access is proposed via a new bridge from Bedford Road. A pedestrian bridge is also proposed
into the north west corner of the site from Becket’s Park.
The Sequential Test in the Context of the University Proposal
With regard to the Table 2: Flood risk vulnerability classification of the NPPF technical guidance, the
proposal is considered to partly fall within the ‘more vulnerable’ category (university and student
accommodation) and the ‘less vulnerable’ category (office and commercial development, plus
ancillary university development).
The primary flood risk source for this development area is the River Nene, which lies to the north of
the site. However, the whole of the area is now defended by the recently completed Northampton
Flood defences, designed to provide protection against a 1 in 200 years return period flood event ad
Upton Flood Attenuation Area upstream of Northampton. It is understood that the development area,
despite being in the Nene floodplain, is classified as Flood Risk Category 2 and 3a.
As part of the site is a ‘more vulnerable use’ falls within Flood Risk Category 3a, Table 3 of the NPPF
Technical Guide states that the Exception Test must be carried out.
Planning Policy Background
The Avon Nunn Mills site is allocated in the Northampton Central Area Action Plan 2013 (NCAAP).
(This allocation also includes the Ransome Road site to the south of the former railway line.) Whilst
the site is allocated for 2000 dwellings and 16000 sq m of office space, paragraph 6.68 of the NCAAP
refers to the University and makes clear that educational use on part of the site would be acceptable
in principle. The NCAAP also identifies (at para 6.74) that extensive works to increase flood
attenuation capacity upstream, west of Upton Valley Way has been undertaken to allow the Avon /
Nunn Mills/ Ransome Road developments to proceed.
The NCAAP and is predecessor documents were developed from a sound evidence base that
included much work on flood risk:
-
The West Northamptonshire Strategic Flood Risk Assessment Final Level 1 Report – Volume
1 February 2009 sets out the general approach to development in areas of West
Northamptonshire affected by flooding.
-
The Northampton Level 2 Strategic Flood Risk Assessment Living Document refers to the
Nunn Mills Road area and states “Any future proposed development should be rolled back
from the watercourse. More vulnerable and highly vulnerable development should be located
to the south of the area, with less vulnerable and water compatible development being more
appropriate towards the north of the area. Due to the orientation of the site in relation to the
River Nene, a breach could occur at any point along the defences. This is important to note at
site-specific FRA stage as only a single breachhas been considered as part of the Level 2
SFRA.” Table B-1 of this document highlights that NBC accept that “’More Vulnerable’
development is likely.
Methodology
NPPF paragraph 104 states that “for individual developments on sites allocated in development plans
through the Sequential Test, applicants need not apply the Sequential Test.”
Appendix C
Technical Note: University of Northampton
Relocation breach analysis & Modelling Figures
The site is allocated for residential and office development. the NCAAP makes clear that educational
use on part of the site would be acceptable in principle. This allocation was made in full knowledge of
the SFRAs set out above.
However, the Environment Agency has stated that a Sequential Test is required because the
proposed development (education) differs from the actual allocation within the NCAAP.
The proposed methodology for the Sequential Test is based upon the University’s requirements for a
new site and the general national planning requirements for major development. The key criteria is as
follows:
-
Site size of at least 20 ha to accommodate University campus
The site should be a sustainable site, close to Northampton town centre
Development should be in accordance with the Local Development Framework
The site should be brownfield
The University needs to open for the 2018/19 academic year
The site should be within or in close proximity to the Enterprise Zone
The University’s clear requirement, and one of the key reasons for its move, is to locate to a site
within closer proximity to the town centre. Accordingly, this Sequential Test can only realistically
consider sites within the central area of Northampton. If a site was not available close to the town
centre, then the University would be highly unlikely to relocate.
Accordingly, the Sequential Test considers sites within the Northampton Central Area Action Plan.
The key development sites are:
-
Bus Interchange/Fishmarket
Grosvenor Centre
Abington Street East
Castle Station
St Johns
Angel Street
Bridge Street
Upper Mounts/Great Russell Street
Spring Boroughs
The Waterside: Brampton Branch/St Peters Way
The Waterside: Southbridge West
The Waterside: Becket’s Park
The Waterside: Nene Meadows
Market Square
Drapery
Freeschool Street
Former Royal Mail Sorting Office
Telephone Exchange, Spring Gardens
Site Assessment
The scale of site required by the University inevitably has a significant effect upon the site
assessment. The University needs to find a campus that can accommodate over 50,000 sq m of
development. There needs to be open space in and around the campus to create areas for informal
recreation. It also has a preference for expansion space plus a co-location with commercial business
development. For these reasons the minimum site size is considered to be 20 ha.
Of the NCAAP sites, the Bus Interchange/Fishmarket, Abington Street East, St Johns, Angel Street,
Bridge Street, Upper Mounts/Great Russell Street Spring Boroughs, The Waterside: Southbridge
West, Market Square, Drapery, Freeschool Street, Former Royal Mail Sorting Office and Telephone
Exchange, Spring Gardens are all far to small to be considered as a realistic redevelopment site.
The Grosvenor Centre site is larger but is preferred use is retail development. This is the key retail
area within Northampton and it is considered that redevelopment for any use other than retail is highly
unlikely and not likely to be supported in planning policy terms.
Castle Station is smaller than the University would prefer (c12 ha). Planning permission has been
granted for the redevelopment of the station and development is underway. For these reasons the site
is not considered to be available.
The Waterside: Brampton Branch/St Peters Way is a larger site. It has been considered by the
University but deliverability given the contamination issues is considered to be questionable. The site
is anyway within an area at risk of flooding.
The Waterside: Becket’s Park is essentially a Greenfield park. There is a small potential development
site on the former railway embankment, but this would be far too small for the Univeristy’s purposes. It
is considered highly unlikely that planning permission would be granted for redevelopment of a public
park. The site is anyway within an area at risk of flooding.
The Waterside: Nene Meadows is essentially an open, greenfield amenity area. It is considered highly
unlikely that planning permission would be granted for redevelopment of this largely greenfield site.
The site is anyway within an area at risk of flooding.
Therefore only the Avon/Nunn Mills site meets the University’s requirements for the development.
Exception Test
1. 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
There are a large number of wider sustainability benefits arising from the proposal. The relocation of
the University to a more sustainable (town centre) site is a key aspiration supported by the Borough
Council and West Northampton Development Corporation. The University is a key employer within the
town and a key contributor towards the vitality of Northampton as a whole. The retention of a strong
University within the town and in a more sustainable location is considered to have a major beneficial
impact upon the social and economic well being of the town centre.
As set out elsewhere in the planning application, the University needs to pursue this relocation in
order to remain competitive. The University considers that to ‘do nothing’ is not an option. To retain a
University within the town is considered to enhance the viability of Northampton, which in turn
enhances the sustainability of the entire Borough and wider County.
The Avon/Nunn Mills site has remained undeveloped for years. It is contaminated, with poor road
access (the existing access bridges have a weight restriction). The existing power station needs to be
relocated and new services provided across the site. The site overall has the potential to become an
environmental issue for the town if it is not developed.
The overall development proposal will remediate the site, with a programme of works to be agreed
with the EA, which is a clear sustainability benefit. A number of biodiversity improvements are also
proposed.
The site is allocated for development and lies within the Enterprise Zone. It is understood that all the
key stakeholders, including the Borough Council, WNDC, the County Council and the Enterprise Zone
support the development. It is also understood that the Environment Agency supports the
redevelopment in principle.
An additional benefit of the University’s development is that it will create development plots for
commercial business development. These will benefit from the overall remediation programme, the
new access and the campus environment. It is highly unlikely that they could be delivered alone. The
commercial business development will clearly create employment opportunities for the town and
surrounding area and within the Enterprise Zone.
Appendix C – Technical Note: University of
Northampton Relocation breach analysis & Modelling
Figures
Halcrow Group Limited
Burderop Park, Swindon, Wiltshire SN4 0QD
tel 01793 812479 fax 01793 812089
halcrow.com
Technical note
Project Northampton University Relocation FRA
Subject Hydraulic Modelling
Author Feifei Leighton/Robert Bird
Date
Ref
15 August 2013
203239
Prepared by
Date
Checked by
Date
Approved by
Date
Halcrow Group Limited is a CH2M HILL company
Northampton University Relocation FRA: Hydraulic modelling
1
Page 2 of 21
Introduction
This technical note is an appendix to the flood risk assessment for the proposed relocation of
Northampton University to the Northampton Waterside Campus (also known as the Avon/Nunn Mills
Road site) and reports hydraulic modelling undertaken to assess the residual flood risk associated with a
breach of Northampton flood defences, and the impact of a 1000-year flood with climate change
allowance on the proposed development.
The proposed development layout is shown in Figure 1.1. The proposed development site lies mostly in
the defended River Nene flood plain in Northampton. The development involves:
•
Raising site levels above the modelled 200-year return period with climate change allowance
flood level as indicated in Figure 3.4 – and for some areas above the 1000-year return period with
climate change allowance flood level .
Construction of university buildings and facilities, and commercial buildings, as shown in the
proposed Masterplan in Figure 1.1.
Construction of a new road crossing the development site connecting Bedford Road to Ransome
Road as shown in Figure 1.1. The new road will include a new bridge crossing the River Nene
and openings to accommodate existing drainage channels in the vicinity of the disuse railway at
the southern edge of the proposed Northampton University Waterside Campus site.
New foot/cycle bridge
•
•
•
The impact of the proposed development on the consequences of a breach of Northampton flood
defences has been assessed by hydraulic modelling. The locations of simulated breaches were originally
agreed in consultation with the Environment Agency to be locations 1 and 2 shown in Figure 1.2.
However, the proposed land raising of the site is at a level above the modelled River Nene 200-year with
climate change allowance flood level. Breaches at these locations would therefore not occur and so have
not been simulated. In additional to the breach locations agreed with the Environment Agency, a breach
has been simulated at the Carlsberg sluice gate upstream of Southbridge. A breach at this location was
simulated for the Ransome Road development site FRA and the same breach parameters have been
adopted for this assessment (i.e. same breach width, depth and with breach occurring at time of peak
river level).
The modelling has the following objectives:
•
To assess the impact of the proposed development on flood risk elsewhere including following a
breach of Northampton flood defences, during a 200-year with climate change allowance flood in
the River Nene, both with and without implementation of the approved outline design for the
development of the Ransome Road site (Ransome Road Flood Risk Assessment, Homes and
Communities Agency 2010).
•
To show that the proposed new road provides safe access and egress (low flood risk category in
Defra R&D Technical Report FD2320/TR2 and the May 2008 EA/HR Wallingford supplementary
guidance note) along its length during a 1000-yr with climate change flood event (without a
breach of Northampton flood defences), both with and without implementation of the approved
outline design for the development of the Ransome Road site (Ransome Road Flood Risk
Assessment, Homes and Communities Agency 2010).
WATERSIDE CAMPUS
MASTERPLAN
TOWN CENTRE
1
BECKET`S PARK
BEDFORD ROAD
1 NEW FOOT/CYCLE BRIDGE
2 SOCIAL BUILDING
20
MARINA
3 ACADEMIC FACILITIES SPECIALIST SPACE
4 STUDENT RESIDENCES
19
5 LIBRARY/STUDY
6 SHARED TEACHING
RIVER NENE
7 CONFERENCE FACILITY
2
8 HEALTH/FITNESS GYM
3
9 VISITOR RECEPTION/ADMINSPACE
18
4
10 ACADEMIC RESEARCH SPACE
13
13
13
11 ENERGY CENTRE
4
12 HOTEL
13 COMMERCIAL OFFICE
4
17
14 COMMERCIAL PARKING
4
5
6
9
15 SPORTS PAVILION
15
12
16 SPORTS PITCHES
4
14
17 UNIVERSITY PARKING
18 AVON COSMETICS HQ
7
4
4
10
4
14
19 NEW ROAD/FOOT/CYCLE BRIDGE
20 EXISTING BRIDGE ACCESS
17
8
17
11
21 ELECTRICAL SUBSTATION
EXISTING RAILWAY / SUSTAINABLE TRANSPORT CORRIDOR
16
21
Breach location 2:
approx. grid ref:
475965 259865
Page 3 of 21
Breach location 1
(NN17): approx. grid ref:
476145 259859
New Avon HQ building
Figure 1.2: Originally agreed breach locations
2
Scenarios assessed
2.1
Breach scenarios
Following the proposed development, the potential for breaching of the River Nene flood defences along
the northern edge of the development site is removed as ground levels adjacent to the defences will be
raised above the 1000-year with climate change allowance flood level. A breach along the northern edge
of the development site has therefore not been simulated.
During the Ransome Road application FRA (Homes and Communities Agency, 2010), in consultation
with the Environment Agency, the most likely location for a breach that would cause significant flooding
at the Ransome Road site was identified to be at Carlsberg sluice, upstream of Southbridge (Breach 09 in
the Environment Agency’s River Nene breach modelling). The sluice is 4m wide and lies within
Northampton sheet piling flood defences. This assessment considers the impact of the proposed site
design on the consequences of a breach of Carlsberg sluice gate (Breach 09).
Carlsberg sluice (Breach 09) is a structure designed to retain water. Assuming the structure is adequately
maintained the probability of the modelled breach occurring (i.e. failure of the sluice gate during the 200year return period River Nene flood) is therefore considered to be much lower than 0.005 per annum
(much lower than once in 200 years) since the modelled breach requires both a 200-year flood level (with
climate change allowance) and failure of the structure. A breach of Carlsberg sluice (Breach 09) has been
modelled, as in the Ransome Road FRA project and subsequent Nunn Mills Road FRA project, with a 4m
wide breach commencing at the peak river level.
Page 4 of 21
The models were run to assess the consequences of a breach of Northampton defences, during a 200-year
with climate change allowance flood in the River Nene, for Breach 09, for the development scenarios
listed in Table 2.1.
2.2
Development scenarios
Upton Flood Attenuation Area (FAA), upstream of Northampton, was implemented by the Homes and
Communities Agency (formerly English Partnerships) as strategic flood risk mitigation infrastructure to
mitigate flood risk associated with planned development in and around Northampton, including the
proposed Northampton University Waterside Campus site development. Upton FAA was implemented
ahead of the planned development to provide the required mitigation ahead of development.
When assessing the impact of the proposed development on flood risk, the pre-development scenario is
therefore considered to be before implementation of Upton FAA.
The pre-development scenario is before Upton FAA, Ransome Road development and Northampton
University Waterside Campus development. Post-development Scenario A is with the Upton FAA,
without Ransome Road development and with Northampton University Waterside Campus
development, i.e. the situation if the Northampton University Waterside Campus development is
implemented before the Ransome Road Development. Post-development Scenario B is with Upton FAA,
with Ransome Road Development and with the Northampton University Waterside Campus
development, i.e. the final post ‘all development’ scenario.
Development
Scenario
Page 5 of 21
Includes
Upton
FAA?
Includes
Northampton
University
Waterside
Campus
development &
new road
design?
Includes
Ransome Road
development?
Description/Purpose
Predevelopment
Base Case
Pre-development
baseline case
Current
Situation
Pre-development current
situation case
Postdevelopment
Scenario A
Postdevelopment
Scenario B
Required to assess the
impact of the proposed
Northampton University
Relocation development
and the proposed new
road on flooding relative
to the pre-development
baseline case (including
for a breach of
Northampton flood
defences during the
200yr CC flood), and to
show the proposed new
road is passable with no
more than a ‘low’ (green)
flood hazard score
during a 1000yr with
climate change allowance
River Nene flood.
Table 2.1: Development scenarios
2.3
Scenarios to assess safe access/egress along the proposed new road
The post-development A and post-development B scenarios were run for the design 1000-year with
climate change allowance flood, without a breach of Northampton flood defences, to demonstrate that
there would be safe access/egress along the proposed new road during the modelled 1000-year return
period with climate change allowance flood.
Safe access has been defined as a maximum of ‘low’ flood hazard score along the access route.
3
Hydraulic Modelling
3.1
Model Development
3.1.1
Available models and data
Page 6 of 21
The Environment Agency (EA) Upper Nene Strategic 1D-2D Model (2013) has been adapted to investigate
the impact of the proposed Northampton University Waterside Campus development on flood risk.
Other available models and data for this assessment include:
•
•
•
•
3.1.2
Northampton University Waterside Campus topographic survey data (2012)
Nunn Mills Road FRA modelling (WNDC 2011), supporting topographic survey data and reports
Ransome Road FRA modelling (HCA 2010), supporting topographic survey data and reports
Development Masterplan and road design
Representation of pre-Upton Flood Attenuation Area scenarios
The EA Upper Nene Strategic 1D-2D Model (2013)) represents the Upper Nene catchment with Upton Flood
Attenuation Area (FAA) implemented. The pre-Upton FAA model was developed by substituting the EA
model representation in the vicinity of Upton FAA (all within the 1D model domain upstream of
Northampton) with the pre-Upton FAA representation applied in the Ransome Road FRA (2010) and
Nunn Mills Road FRA (2011), for which the modelling was reviewed and approved by the Environment
Agency. The upstream and downstream limits for this substitution are 1D ISIS model nodes kis033d
(upstream limit) and s23a (downstream limit). The connections to spills and reservoirs were checked to
ensure the resulting pre-Upton FAA model has the correct representation. Figures 3.1 and 3.2 show the
pre- and post-Upton FAA 1D model schematisation, with model units within the area subject to model
changes highlighted in red.
•
Page 17 of 21
Ransome Road proposed housing area access road - existing land levels raised by a maximum of
approximately 0.13m to remove low spots and so reduce the flood hazard score for access/egress.
Extension of existing
drainage channel
Land grading
Land raising for access road
Land raising
Localised raising
of Ransome Road
Figure 3.9: Model schematisation of the proposed Ransome Road site land raising
3.1.6
Truncation of Hardingstone Dyke in 1D Model
The model was found to produce unstable model results (oscillations) along the Hardingstone Dyke
branch of the 1D ISIS model, for both the 200-year with climate change allowance and 1000-year with
climate change allowance design floods, caused by large volumes of water flowing across/over the dyke
from the River Nene, which caused problems with the 1d/2d link. This instability could cause inaccurate
model results in both the 1D domain and the 2D domain floodplain representation. Hence, the
representation of Hardingstone Dyke has been truncated in the 1D ISIS model at ISIS node sec016, in
order to give stable water levels and so a better representation of flood risk. The truncated section of
Hardingstone Dyke was not represented in the 2D domain. A similar model stability issue was
highlighted in the Nunn Mills Road Flood Risk Assessment modeling (2011) in which Hardingstone Dyke
was truncated (to ISIS node sec016) for the 1000-year with climate change allowance model runs.
The truncation of Hardingstone Dyke in the 1D model domain without a corresponding representation of
the dyke in the 2D domain is considered conservative in terms of assessing flood risk. For the 1000-year
return period with climate change allowance model runs (required to assess access/egress safety along
the proposed new road) omitting Hardingstone Dyke acts to increase modelled flood levels. For the 200year return period with climate change allowance model runs there is no modelled floodplain flow in the
vicinity of Hardingstone Dyke and so results are not affected by the omission of Hardingstone Dyke
Page 18 of 21
(other than a slightly earlier arrival of Hardingstone Dyke fluvial design flows into the River Nene
downstream). The truncation of Hardingstone Dyke therefore results in a model with stable peak water
levels and a better representation of flood risk.
3.1.7
Breach Representation
A breach of Carlsberg sluice (Breach 09) has been modelled, as in the Ransome Road FRA project, with a
4m wide breach commencing at the peak river level (note different breach timing pre- and postimplementation of Upton FAA). The breaching of Carlsberg sluice (Breach 09) was represented in the
hydraulic model for the Northampton University Waterside Campus assessment exactly as it was in the
Ransome Road FRA modelling. The model uses a 10m surface elevation grid size. In order to represent a
4m breach, a single grid cell (10m length) was lowered so that elevations at the breach point matched
those of immediately adjacent land behind the defence. This land lowering was applied at the time of
peak in-channel water level and occurs instantaneously. A flow constriction unit was then used to
restrict flow passing through the grid cell by 60%, thereby reducing the 10m wide land lowering to a 4m
wide breach flow path. The timing of the breach was set to occur at the peak River Nene level (at ISIS 1d
node a7492) as follows:
3.2
•
T = 35 hour for Pre-development Base Case
•
T = 37 hour for Current Situation, Post-development Scenarios A and B
Model Run Schedule
The following model runs were developed based on the development scenarios described in Section 2.2:
Model Return Period
Development Scenario
200yr CC
no breach
200yr CC
breach 09
1000yr CC
no breach
Pre-development Base Case
Current Situation
Post-development Scenario A
Post-development Scenario B
Table 3.2: Model run schedule
4
Model Results
4.1
Flood Hazard outputs
The flood hazard classification was calculated in TUFLOW (UK flood hazard, ‘ZUK0’) according to Defra
R&D Technical Report FD2320/TR2 and the May 2008 EA/HR Wallingford supplementary guidance note.
The flood hazard rating (HR) was calculated according to the following formula from these reports.
HR = d x (v + 0.5) + DF
(d = depth of flooding, v = velocity of flooding and DF = Debris factor)
Page 7 of 21
Figure 3.1: Pre-Upton FAA 1D model schematisation (model units highlighted in red are within the area subject to
model changes)
Page 8 of 21
Figure 3.2: Post-Upton FAA 1D model schematisation (model units highlighted in red are within the area subject to
model changes)
3.1.3
Representation of the Northampton University Waterside Campus development and
proposed new road
The Waterside Campus development Masterplan is shown in Figure 1.1.
The land within the development site will be raised to levels above the modelled 200-year with climate
change allowance flood level as indicated in Figure 3.3, except at the Avon HQ, the vicinity of the listed
engine shed and the existing electricity substation where levels are to remain the same. Some areas will
actually be raised above the 1000-year with climate change allowance flood level (see results plots in
Figures C1 to C13).
The finished ground levels within the site are specified on a 5mx5m resolution grid which is then
converted to an ascii grid (.txt) format using the Vector Operations tool in ISIS Mapper. This grid was
used to update any change to the ground level that fall onto the 2D surface grids which are specified by
10mx10m cells.
The new road design consists of the construction of a new access road east of the existing Nunn Mills
Road and modifying levels of the existing Bedford Road. To represent the new road design in the model,
design levels specified as points in a .csv file format have been converted to an ascii grid .txt format. The
new road design is represented in the model as a z-shape file with 20-m width along the road centrelines
which are joined by points at the design elevation. The design levels are specified along the road
Page 9 of 21
centrelines at regular intervals (with automatic interpolations in between the points). Figure 3.5 shows
road centrelines of the new road design and Figure 3.6 shows the plan view and long section of the new
road design and the culverts/opening underneath.
The design for proposed foot/cycle bridge has not yet been finalised but it will be a clear span with soffit
levels above the 1000-year return period with climate change allowance flood level (at least 3m above
normal water level to meet navigation requirements) and designed to have no negative impact on flood
risk compared to the base case model (i.e. Upton Flood Attenuation Area provides mitigation). Figure 3.7
shows the design of the proposed foot/cycle bridge.
The proposed buildings shown in Figure 1.1 are represented a local 0.3m increase in ground levels, as has
been dome elsewhere in the EA model. This acknowledges that for higher depths buildings may allow
some conveyance of flood water e.g. through doors and windows.
Figure 3.3 Extent of land raising within the Northampton Waterside Campus
Figure 3.4 New road design centreline and locations of culverts/openings under the road
Page 10 of 21
Page 19 of 21
The debris factor was selected as described in FD2320 and its supplementary guidance note, i.e. DF = 0.5
if d≤0.25m and DF = 1 if d>0.25m or v>2m/s (TUFLOW ‘conservative’ approach). The flood hazard ratings
are classified into the flood hazard categories shown in Table 4.1, and these were reflected in the Flood
Hazard maps.
Flood hazard
Rating
Flood Hazard
Category
Description
< 0.75
Low
Caution - “Flood zone with shallow
flowing water or deep standing water”
0.75 - 1.25
Moderate
Dangerous for some (i.e. children) “Danger: Flood zone with deep or fast
flowing water”
1.25 – 2.0
Significant
Dangerous for most people - “Danger:
flood zone with deep fast flowing water”
> 2.0
Extreme
Dangerous for all - “Extreme danger:
flood zone with deep fast flowing water”
Table 4.1: Flood Hazard Classification from Supplementary Guidance Note.
4.2
Model results maps
Maps have been constructed to show the maximum depth, flood hazard and velocity in the study area.
Flood depth and velocity grids have been coloured so that the variation across the study area can be
clearly seen. Flood hazard grids have been coloured according to the hazard categories in the Defra R&D
Technical Report (FD2320/TR2) and as described in Table 4.1 (green = low hazard, yellow = moderate
hazard, orange = significant hazard and red = extreme hazard).
Modelled maximum depths, velocities and hazard scores for an occurrence of breach 09 during the 200year with climate change River Nene flood are shown in Figures C.1 to C.4 for the Pre-development Base
Case, Current Situation, Post-development A and Post-development B scenarios respectively.
Modelled maximum depths, velocities and hazard scores during the 1000-year with climate change River
Nene flood, without a breach of Northampton flood defences, are shown in Figures C.5 and C.6 for the
Post-development A and Post-development B scenarios respectively. Figures C.5 and C.6 are to
demonstrate that the proposed new road would be operable during a 1000-year return period flood
(rather than for comparison with pre-development scenario model results).
Maps showing the difference in maximum depth, hazard and velocity in the study area during a breach
of Northampton flood defences, as a result of the proposed development, have also been produced. The
differences were calculated by subtracting the post-development results from the pre-development
results. A reduction in value as a result of the Northampton university Relocation development is shown
as a negative change and vice-versa. A reduction in flood risk has been coloured green on the maps and
an increase has been coloured red.
Figures C.7 and C.8 show differences (compared to the pre-development Base Case) for an occurrence of
Breach 09 during the 200-year with climate change allowance River Nene flood for the Post-development
A and Post-development B scenarios.
Page 20 of 21
Model results for the simulated 200-year with climate change allowance River Nene flood without a
breach of Northampton flood defences are shown in Figures C.9 to C.13 as follows:
•
•
•
•
•
4.3
Figure C.9: Maximum flood depths, hazard scores and velocities for the Pre-development Base
Case
Figure C.10: Maximum flood depths, hazard scores and velocities for Post-development A
scenario
Figure C.11: Maximum flood depths, hazard scores and velocities for Post-development B
scenario
Figure C.12: Difference in maximum flood depths, hazard scores and velocities compared to the
Pre-development Base Case for the Post-development A scenario
Figure C.13: Difference in maximum flood depths, hazard scores and velocities compared to the
Pre-development base case for the Post-development B scenario
Interpretation of Results
Figures C7 and C8 show that, for an occurrence of Breach 09, the mitigation provided by Upton FAA
results in a widespread reduction in flood risk hazard score and depth. For the post-development cases A
and B (pre-and post-Ransome Road development), in the vicinity of the proposed Waterside Campus
development, Figures C7 and C8 show a reduction in flood depth, hazard and velocity. For the postdevelopment cases A and B Figures C7 and C8 show a few model pixels with increases in hazard score
along the Delapre Brook alignment compared to the pre-development scenario. However, the modelling
shows an overall reduction in depth, hazard and velocity along Delapre Brook and these localised
increases are considered modelling artefacts due to e.g. model convergence.
Figure C8 shows an increase in hazard score along the breach alleviation routes (extended railway ditch
and land graded towards the extended railway ditch) included in the Ransome Road development
design. These alleviation measures were considered appropriate by the Environment Agency as part of
the Ransome Road flood mitigation design. Figure C8 also shows an increase in hazard score on Ransome
Road where the hazard score increases from dry (no hazard score) to Low (comparing Figures C1 and
C4). This slight increase during a breach scenario is related to the assumed Ransome Road development
outline design (taken from the Homes and Communities Agency Ransome Road FRA modelling, 2010),
which will be refined during the detailed Ransome Road development design to satisfy flood risk
assessment requirements.
Figures C8 and C9 show a single pixel with increased hazard score on the north bank floodplain of the
River Nene between the proposed new River Nene bridge and the existing Nunn Mills Road River Nene
bridge. This is due to a slight and localised increase in modelled depths (modelled increase between 0m
and 10mm) directly upstream of the proposed new bridge. This is considered an insignificant impact as
the level increase is marginal and only occurs in the floodplain adjacent to a car park - which remains dry
during the modelled 1000-year with climate change allowance flood for the post-development case
(Figures C5 and C6).
Figures C5 and C6 show modelled maximum depths, hazard scores and velocities along the proposed
new access road alignment during a modelled 1000-year return period flood with climate change
allowance, for the post-development cases A and B respectively. Figures C5 and C6 show that the
proposed access road would provide safe access/egress (max hazard score ‘very low’) during the
modelled 1000-year return period flood with climate change allowance, for the post-development cases A
and B respectively. The Waterside Campus application does not include the part of the new road south of
the Avon/Nunn Mills development site but this has been included in the modelling to demonstrate that
the required future road extension to the Ransome Road site is viable in terms of flood risk.
Page 14 of 21
Width (m)
Height (m)
Length (m)
Invert Level (mAOD)
Culvert 1
7.7
3.4
23.5
55.0
Culvert 2
10
3.4
20
55.0
Culvert 3
10
3.3
17.5
55.0
Table 3.1 Dimensions and parameters of the three culverts/openings under the new road
The four coefficients are set as those suggested in the TUFLOW User Manual:
3.1.4
•
The height contraction coefficient is set to 0.6 for square edged box culverts.
•
The width contraction coefficient is set to 0.9 for sharp edged box culverts.
•
The general entry loss coefficient as specified by the manufacturer. The value of 0.5
recommended in the TUFLOW User Manual has been used.
•
The exit loss coefficient (normally recommended as) 1.0 has been used.
Representation of Delapre Brook and existing railway drainage ditch
As part of the proposed Ransome Road development FRA modelling, some revisions were made to the
pre-development hydraulic model to improve detail in the vicinity of the proposed Ransome Road
development. The same revisions have been applied to the current Northampton University Relocation
FRA model. These changes are listed below and illustrated in Figure 3.7.
•
•
Representation within the model of Delapre Brook (running from west to east, south of the
proposed development site). The channel was represented using 1D ESTRY sections with
channel cross sections derived from Homes and Communities Agency topographic survey. The
Mencap access bridge which crosses this watercourse was also incorporated, taking bridge details
from drawings submitted as part of the flood risk assessment for the bridge.
Including within the model an existing drainage channel which runs from west to east
immediately south of the railway and along the northern edge of the proposed Ransome Road
development site. The channel was represented by lowering the 2D surface grid elevations by
1m along the channel alignment (with an improvement made to the previous Ransome Road
FRA modelling by using topographic survey data to set the channel bed level rather than
applying a constant 1m lowering of original DTM ground levels along the ditch alignment).
Page 21 of 21
Figures C12 and C13 show that Upton Flood Attenuation Area mitigates for the proposed Waterside
Campus development for the case without a breach of Northampton flood defences. For the Postdevelopment B case, there are localised increases in flood risk associated with the assumed outline
Ransome Road development design as discussed above.
5
Conclusions
The Environment Agency Upper Nene Strategic 1D-2D Model (2013) has been developed by Northampton
University in 2013 to assess the impacts of the planned Waterside Campus development on flood risk
during a breach of Northampton flood defences. This model has been developed further to assess the
impact of the proposed Waterside Campus development on flood risk during a breach of Northampton
flood defences and during the 1000-year with climate change allowance River Nene flood.
The implementation of Upton Flood Attenuation Area has wide spread benefits in terms of reduced flood
risk throughout Northampton.
The breach modelling undertaken shows that the proposed Waterside Campus development does not
result in increased flood risk elsewhere during a breach of Northampton flood defences occurring at the
peak level of the 200-year with climate change allowance River Nene flood, except for a marginal impact
directly upstream of the proposed new River Nene bridge where there is a slight and localised increase in
modelled depths (modelled increase between 0m and 10mm) directly upstream of the proposed new
bridge. This is considered an insignificant impact as the level increase is marginal and only occurs in the
floodplain adjacent to a car park - which remains dry during the modelled 1000-year with climate change
allowance flood for the post-development case (Figures C5 and C6).
The proposed new access road will provide safe access/egress to the planned Waterside Campus
development (and later to the panned Ransome Road development) during a modelled 1000-year return
period flood with climate change allowance.
The modelling presented demonstrates that Upton FAA provides the required mitigation against
increased residual flood risk due to the proposed Waterside Campus development.
Maximum Depth (m)
0 - 0.25
0.25 - 0.5
0.5 - 1.0
1.0 - 2.0
>2.0
1d model area
Land raising
Buildings post development
Proposed road
" Breach 09 location
Maximum Hazard
0 - 0.75
0.75 - 1.25
1.25 - 2.0
>2
1d model area
Land raising
Proposed road
Maximum Velocity (m/s)
0 - 0.25
0.25 - 0.5
0.5 - 1.0
1.0 - 2.0
>2.0
1d model area
Land raising
Proposed road
Figure C1: 200 year CC Breach 09 Pre-development Base Case Scenario (without UFAA, without RRD and without NUR)
Page 15 of 21
Existing drainage
channel
Delapre Brook
Mencap bridge
Figure 3.7: Amendments to model representation to improve local detail
As well as the changes listed above, a small stability patch was added to the current model in the area just
downstream of Delapre Brook. This stability patch increases the material friction factor within the area
defined from default values (0.06 for farm lands, 0.04 for water courses and 0.045 for urban areas) to 0.15,
which removes or reduces instabilities caused by interaction of the 1D and 2D domains. The location of
stability patch is shown in Figure 3.8.
Page 16 of 21
Figure 3.8 Stability patch added at downstream end of Delapre Brook to remove model instabilities
3.1.5
Representation of the Ransome Road development
The proposed Ransome Road development land raising and associated works was represented in the
relevant model scenarios as it was in the Ransome Road FRA project, as detailed below and as illustrated
in Figure 3.9:
•
2D surface grid elevations raised within the areas of proposed land raising.
•
Existing railway drainage channel extended along the northern edge of the site (by lowering
model 2D domain grid cells).
•
Land graded in the north-western corner of the development site to encourage overland flow
towards the drainage channel.
•
Local raising of Ransome Road by a maximum of 0.61m to remove low spots and so reduce the
flood hazard score for access/egress.
Maximum Depth (m)
0 - 0.25
0.25 - 0.5
0.5 - 1.0
1.0 - 2.0
>2.0
1d model area
Land raising
Proposed road
Maximum Hazard
0 - 0.75
0.75 - 1.25
1.25 - 2.0
>2
1d model area
Land raising
Proposed road
0 - 0.25
0.25 - 0.5
0.5 - 1.0
1.0 - 2.0
>2.0
1d model area
Land raising
Proposed road
Figure C2: 200 year CC Breach 09 Pre-development Current Situation Scenario (without UFAA, without RRD and without NUR)
Maximum Depth (m)
0 - 0.25
0.25 - 0.5
0.5 - 1.0
1.0 - 2.0
>2.0
1d model area
Land raising
Proposed road
Maximum Hazard
0 - 0.75
0.75 - 1.25
1.25 - 2.0
>2
1d model area
Land raising
Proposed road
0 - 0.25
0.25 - 0.5
0.5 - 1.0
1.0 - 2.0
>2.0
1d model area
Land raising
Proposed road
Figure C3: 200 year CC Breach 09 Post-development Scenario A (with UFAA, without RRD and with NUR)

University of Northampton - Northampton Borough Council

Transcription

Similar documents

What you have told us

Floodwatch®

p - Garden Commerical Properties

excess flood faq

EM01B Websensor

FL20 CS - Solas Ray Lighting