Download Addendum to Northampton University Relocation Flood Risk

TECHNICAL MEMORANDUM
Addendum to Northampton University Relocation Flood
Risk Assessment
PREPARED FOR:
Northampton University
COPY TO:
PREPARED BY:
DATE:
PROJECT NUMBER:
DOCUMENT NO:
STATUS:D1
RB, FL
21 October 2014
481537
CH2-00-00-TN-Z-003
REVISION:P1
Introduction
An outline planning application (N/2013/912) was approved in December 2013 for the proposed
relocation of Northampton University to the Nunn Mills Road / Avon development site in Northampton.
A Flood Risk Assessment was submitted as part of this application and this was supported by hydraulic
modelling. The FRA and associated hydraulic modelling were reviewed by the Environment Agency as
part of the planning consultation.
Since outline planning approval was gained in December 2013, the proposal design has been developed
in more detail. The current Masterplan is shown in Figure 1.
Design changes that have potential to influence flood risk are:
•
•
•
Revised site ground levels within the proposed Northampton university Waterside Campus extent
Revised River Nene road bridge design
A design is now available for proposed pedestrian/cycle bridge crossing the River Nene (previously
design constraints were specified)
The FRA addendum has been specifically prepared to assess the emerging proposals and changes to the
masterplan in the context of the outline planning submission. The impact of the changes on flood risk
have been assessed by further hydraulic modelling.
Drainage Strategy
The Academic Buildings and Sports Hub comply with the drainage strategy approved as the outline
planning submission and will not have any additional impacts.
.
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Figure 1: Current masterplan
Design changes and their model representation
Proposed new River Nene road crossing
The approved and revised River Nene road bridge designs are in Figures A1 and A2 respectively in Annex
A. Since the initial bridge design more detailed information has been made available regarding the
locations of underground utilities infrastructure in the vicinity of the proposed road bridge. To provide
adequate maintenance access for the infrastructure the bridge piers have been rotated compared to the
approved design. The revised pier design also includes an additional piling footprint of approximately
0.5m on the water side of the piers. These changes result in small (compared to channel width) localised
gains and losses in channel width.
Model cross sections were updated in the ISIS 1D hydraulic model domain at the River Nene bridge
location to account for the revised channel profile specified in the bridge design.
The road bridge was represented in the ISIS 1D model with an ISIS arched bridge unit (with calibration
coefficient 1.0, skew angle 0 degrees relative to channel flow direction). As before, the new road was
represented in the 2D domain (both north and south of the proposed new bridge) by adjusting modelled
ground levels in the 2D domain along the road alignment.
Proposed new River Nene pedestrian/cycle crossing
Figures A3 and A4 in Annex A show the proposed River Nene pedestrian/cycle crossing.
The proposed new River Nene pedestrian/cycle crossing has a soffit level above the modelled 1000 year
with climate change allowance food level except for where it lands north of the River Nene in the River
Nene floodplain. The bridge has therefore been represented by adjusting modelled ground levels in the
2D domain where the bridge footprint is below the modelled 1000-year with climate change allowance
flood level.
Revised Northampton University site ground levels
Since the outline planning application was approved in December 2013, the Northampton University site
Masterplan and landscape design has been developed in more detail. This has resulted in adjustments to
the proposed site levels. As for the approved design, with the revised proposed site levels the site is
excluded from the 200-year with climate change allowance fluvial flood extent. Revised design proposed
site cross sections are shown in Figures A5 to A8 in Annex A.
The revised Northampton University site ground levels were represented in the 2D model domain by
adjusting modelled ground levels (i.e. 2D grid levels) according to the revised design.
Hydraulic model simulations and assessment of results
Model simulations
The following model scenarios were run:
A version of the FRA hydraulic model was run for the post development case including the design
changes described above. Here the post development case includes both the Northampton University
development and the Ransome Road development (represented as in the 2013 FRA modelling) as this
considers a ‘worse’ development case than just the Northampton University development.
The model was run for the following cases:
•
200-year return period modelled flood with climate change allowance – to assess the impact of the
proposed development on flood risk elsewhere (assessed against the base case, taken to be predevelopment of the proposed Northampton University Relocation Waterside Campus and preconstruction of Upton FAA, as described in the approved Northampton University Relocation outline
planning application FRA, Section 8.1), and confirm flood risk on the proposed Northampton
University development will not be higher than for the approved design. As was done for the
approved design, base case and post development model results were compared for the cases with
and without a breach of Northampton flood defences.
ADDENDUM TO NORTHAMPTON UNIVERSITY RELOCATION FLOOD RISK ASSESSMENT
•
1000-year return period modelled flood with climate change allowance – to confirm the proposed
new road will provide safe access during extreme flooding.
Model results
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
Northampton University proposed Waterside Campus development are listed in Table 1 for the case that
there is no breach of Northampton flood defences.
The model results in Table 1 show that post-development flood levels in the River Nene upstream and
downstream of the proposed Waterside Campus development, and at the new bridge location, are
lower than pre-development as a result of the mitigation provided by Upton FAA. Figure B7 shows an
insignificant change in hazard score upstream of the proposed new River Nene bridge.
Table 1: Comparison of pre and post development 200-year flood levels in the ID model extent
Peak level (mAOD) and difference compared to pre-development case (m) for
modelled 200-year with climate change allowance flood
Location
ISIS ID
Pre-development (before
model node development of Waterside
Campus, Upton FAA and
Ransome Road)
Post-development (after
development of Waterside
Campus, Upton FAA and
Ransome Road) – revised
design
Post-development (after
development of Waterside
Campus, Upton FAA and
Ransome Road) - previous
results for approved
design
River Nene:
Southbridge
a7486b1
59.62
59.56 (-0.054)
59.56 (-0.057)
River Nene: upstream
end of Waterside
Campus site
a7455
58.74
58.72 (-0.018)
58.72 (-0.018)
River Nene: approx.
160m upstream of
Nunn Mills sluice
a7424a
58.46
58.44 (-0.016)
58.45 (-0.015)
Upstream of River Nene a7416b1
bridge
57.63
57.63 (-0.007)
57.65 (+0.019)
Upstream of Nunn Mills a7408as
sluice gates
58.32
58.30 (-0.012)
58.31 (-0.009)
Upstream of new River
Nene bridge
a7403
57.63
57.62 (-0.009)
57.65 (+0.020)
Downstream of new
River Nene bridge
a7403i2
57.61
57.55 (-0.060)
57.58 (-0.037)
Annex B contains updated model results figures as listed in Table 1 (versions for the approved designs
were included in the Northampton University Relocation FRA Appendix C). Proposed site cross sections
previously included in the Northampton University Relocation FRA are updated in Annex A as Figures A5
to A8.
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Table 2: Appendix B figures
Figure in Annex B
Equivalent figure in approved FRA
Appendix C
Description
B1
C1
200yr return period flood with climate change allowance. Modelled
flood depth, velocity and hazard score grids. Pre development - no
breach of Northampton flood defences.
B2
C9
200yr return period flood with climate change allowance. Modelled
flood depth, velocity and hazard score grids. Pre development with
a breach of Northampton flood defences.
B3
C4
200yr return period flood with climate change allowance. Modelled
flood depth, velocity and hazard score grids. Post development with
a breach of Northampton flood defences.
B4
C6
1000yr return period flood with climate change allowance.
Modelled flood depth, velocity and hazard score grids. Post
development without a breach of Northampton flood defences.
B5
C8
200yr return period flood with climate change allowance. Numerical
difference between modelled flood depth, velocity and hazard score
grids (difference between the base case and post development case)
with a breach of Northampton flood defences.
B6
C11
200yr return period flood with climate change allowance. Modelled
flood depth, velocity and hazard score grids. Post development
without a breach of Northampton flood defences.
B7
C13
200yr return period flood with climate change allowance. Numerical
difference between modelled flood depth, velocity and hazard score
grids (difference between the base case and post development case)
without a breach of Northampton flood defences.
Figures 2 and 3 show modelled peak flood levels and depths in the vicinity of the pedestrian/cycle bridge
footprint in the River Nene northern floodplain (elsewhere the bridge is above the 1000 year return
period with climate change allowance modelled flood level and so results are not shown). Figures 2 and
3 show that the proposed pedestrian/cycle bridge deck top level is above the modelled 200 year with
climate change allowance flood level and surrounding flood depths are modest (less than 100mm). For
the 1000 year with climate change allowance flood level, model results show there may just be some
marginal inundation of the proposed pedestrian/cycle bridge only at its most northern extent where it
lands in the northern River Nene floodplain. Figures B5 and B7 show that modelled flood risk in the
vicinity of the pedestrian/cycle bridge footprint in the River Nene northern floodplain is generally lower
for the post-development case than the pre-development base case, for the cases with and without a
breach of Northampton flood defences respectively.
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Figure 2: Modelled 200-year with climate change allowance flood, post-development with no breach of
Northampton flood defences. Modelled flood depths (upper map) and levels (lower map) in the vicinity
of the proposed pedestrian/cycle bridge footprint in the River Nene floodplain. The proposed bridge
alignment and deck top levels are also shown.
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Figure 3: Modelled 1000-year with climate change allowance flood, post-development with no breach of
Northampton flood defences. Modelled flood depths (upper map) and levels (lower map) in the vicinity
of the proposed pedestrian/cycle bridge footprint in the River Nene floodplain. The proposed bridge
alignment and deck top levels are also shown.
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Discussion of results
The proposed design revisions are minor and conclusions drawn from examination of Figures B1 to B6
are the same as those drawn in the approved FRA, as below:
Figure B5 shows that, for an occurrence of a breach of Northampton flood defences, the mitigation
provided by Upton FAA results in a widespread reduction in flood risk hazard score and depth. For the
post-development case (post Northampton University site and Ransome Road site development), in the
vicinity of the proposed Waterside Campus development, Figure B5 shows a reduction in flood depth,
hazard and velocity. For the post-development case Figure B5 shows 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 B5 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 B5 also shows an increase in hazard score on Ransome
Road where the hazard score increases from dry (no hazard score) to Low (comparing Figures B2 and
B3). 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 B5 and B7 show an insignificant change in hazard score upstream of the proposed new River
Nene bridge.
Figure B5 shows a localised increase in flood extent (compared to the pre-development case) within the
Northampton University development site, west of the Avon Cosmetics building and adjacent to the
River Nene, for the modelled 200-year with climate change flood. This is due to proposed re-profiling of
ground levels in this area, adjacent to the River Nene. Re-profiled ground levels at the locations of the
proposed buildings further south (Figure 1, Masterplan) are above the modelled 200-year with climate
change allowance flood level.
Figure B4 shows 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 case. Figure B4 shows 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 case.
Figure B7 shows that Upton Flood Attenuation Area mitigates for the proposed Waterside Campus
development for the case without a breach of Northampton flood defences. For the post-development
case, there are localised increases in flood risk associated with the assumed outline Ransome Road
development design as discussed above.
Conclusions
Hydraulic modelling has been undertaken to assess the impact on flood risk of the revisions to the
Northampton University Relocation design presented in the approved outline planning application
(N/2013/912, December 2013). This has been discussed with the EA prior to commencing modelling
works.
Hydraulic model results demonstrate that, with the proposed design revisions, model results derived for
the revised design are not significantly different to those of the approved design and the conclusions of
the approved FRA still hold. That is, the proposed Waterside Campus development will not be at risk of
flooding and will not increase the risk of flooding elsewhere.
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The hydraulic model is to be submitted directly to the EA following submission of the Academic Buildings
and Sports Hub reserved matters application.
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ANNEX A
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ANNEX B
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