Viridor
Waste Treatment Plant, Lostock Gralam
Appendix 9.1 Flood Risk Assessment – Appendix 2
Appendix 2 – Hydraulic Modelling Report
1
INTRODUCTION
The Wade Brook flows from east to west along the southern boundary of the site and is
characterised on both sides by a series stepped banks and plateaus which are heavily
vegetated. The watercourse is contained within a channel formed by the lower of
these steps. A series of intermediate plateaus provide additional capacity for the
watercourse.
This report provides detail of a rudimentary hydraulic modelling exercise. The aim of
the study was to more accurately determine flood extents along the watercourse within
the vicinity of the site, for a range of annual probabilities of occurrence.
The model extends from the far easterly (upstream) extent of the site where the brook
exits a culvert located beneath the industrial site, downstream (by approximately 400m)
to a second culvert/bridge structure.
Due to the location and setting of the Wade Brook it has not been possible to
undertake detailed cross sections of the watercourse. Historical information has,
therefore, been utilised, along with a detailed topographical survey of the site itself in
the production of the hydraulic model.
In total, 5 cross sections have been modelled at intervals where historical data is most
complete and able to offer an appropriate level of detailed topographical information.
There are no structures along the watercourse within the extents of the model, other
than the downstream culvert/bridge which has been shown to act as a throttle and,
therefore, has the effect of protecting the areas downstream from it.
2
TOPOGRAPHICAL SURVEY
Two main sources of topographical data have been used in the production of the
model:
1. Historical data from existing site survey plans, which date from the mid 1970’s
when the existing development was constructed;
2. A site topographical survey of the site (April 2009).
The historical data has been compared with the topographical survey and the levels
shown on both plans have a suitably small discrepancy to justify the use of the
historical data in compiling the model cross sections.
LE10104
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September 2009
Viridor
Waste Treatment Plant, Lostock Gralam
Appendix 9.1 Flood Risk Assessment – Appendix 2
3
MODELLING APPROACH
Hydraulic modelling has been undertaken using the Environment Agency approved,
DHI MIKE11 modelling software. Flood Estimation Handbook (FEH) statistical data
has been used to estimate flows for the 1 in 100 year and 1 in 1000 year rainfall event.
The 1 in 100 year flow has then been increased by 20% to allow for climate change
during the lifetime of the development, as stated in PPS25 and the (former) Vale Royal
Borough Council Strategic Flood Risk Assessment.
A basic hydraulic model has been produced for the watercourse, incorporating the
culvert/bridge downstream of the site. The extent of the model is shown on Figure 2.1
below, and on Drawing No. LE10104/EIA039.
Figure 2.1 – Extent of the Hydraulic Modelling Exercise
There are no gauged records for the site and the Wade Brook does not have any
permanent flow gauging stations. The hydrological inflows were estimated using the
Flood Estimation Handbook (FEH) (Institute of Hydrology, 1999) which is the standard
UK methodology for un-gauged locations. Catchment descriptors for the Wade Brook
were extracted from the FEH CD-ROM and are summarized below in Table 2.1. The
National Grid Reference (NGR) for the site, from where the catchment descriptors are
taken, is 367950, 374100.
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September 2009
Viridor
Waste Treatment Plant, Lostock Gralam
Appendix 9.1 Flood Risk Assessment – Appendix 2
Table 2.1 – Catchment Descriptors
CATCHMENT DESCRIPTOR
Value
AREA
41.27
BFIHOST
0.515
DPLBAR
12.8
FARL
0.969
PROPWET
0.38
SAAR
782
SPRHOST
31.9
URBEX1990
0.0178
Peak flows associated with the critical rainfall levels of 1 in 100 year, 1 in 100 year
+20% and 1 in 1000 year were then calculated using the revitalised FEH rainfall runoff
method. These are summarised in Table 2.2
Table 2.2 – Peak Flows
4
RAINFALL RETURN PERIOD
PEAK FLOW
(years)
RATE (m3/s)
100
15.9
100+20%
19.1
1000
26.7
BASELINE HYDRAULIC MODEL BUILD
Model extents and cross sections are shown on Figure 2.1 and on Drawing No.’s
LE10104/EIA039 and LE10104/EIA040, respectively.
4.1
In channel conceptualisation
The river cross sections have been created using historical topographical surveying
which includes the bank and water levels of the Wade Brook and dimensions of the
culvert/bridge structure at the downstream end.
The vegetation on the banks is currently overgrown, and Manning’s ‘n’ values of 0.035
for the watercourse and 0.06 for the banks have been used. These are considered to
be appropriate for the length of the brook as it flows adjacent to the site.
4.2
Restrictive features
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September 2009
Viridor
Waste Treatment Plant, Lostock Gralam
Appendix 9.1 Flood Risk Assessment – Appendix 2
The culvert/bridge structure downstream of the site is the only feature crossing the
watercourse for the length of the model. This culvert could potentially act as a throttle
and restrict the flow of water downstream of the site. The dimensions of this culvert
are shown on Drawing No. LE10104/EIA040, and a photograph of the culvert is shown
in Figure 2.2. The culvert/bridge forms the downstream boundary of the model.
Figure 2.2 – Photograph of the Culvert/Bridge downstream of the Lostock
Gralam site
4.3
Model simulation and runtime
The same model parameters have been used for all of the model runs. The model
simulation details are shown in Table 2.3.
Table 2.3 – Model Simulation Details
Parameter
4.4
Simulation Type
Fixed timestep
Duration
23
Timestep
55
Initial Conditions
Steady state
Calibration and Verification
It has not been possible to calibrate or verify the model due to lack of measured flow or
level data.
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Viridor
Waste Treatment Plant, Lostock Gralam
Appendix 9.1 Flood Risk Assessment – Appendix 2
There is no record of flooding on the site. Discussions with the existing site occupiers,
Ineos Chlor, have not highlighted any areas on the site which are known to flood and a
site walkover has not revealed any areas which appear to have suffered historical
flooding. Therefore, it is considered that the Wade Brook remained within its banks
during recent rainfall events, including the high rainfall levels experienced during
Summer 2007.
When the site survey was carried out, following a period of light rain, the water level at
the culvert was estimated to be approximately 17.72 mAOD. The model shows that for
a 1 in 1 year rainfall event, the water level at the culvert would be 17.64 mAOD.
Although the exact magnitude of the rainfall event preceding the site survey is not
known, the similar water levels would suggest that the model is accurate within a small
margin of error.
The siting of an electricity pylon and substation immediately adjacent to the
watercourse is also taken as an indication that flood waters would not normally reach
this level.
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September 2009
Viridor
Waste Treatment Plant, Lostock Gralam
Appendix 9.1 Flood Risk Assessment – Appendix 2
5
MODEL RESULTS
The results of the model for a range of annual probabilities of occurrence are shown in
Table 2.4.
Table 2.4 – Modelled Flood Levels
Flood levels (mAOD)
Cross
100 year
100 year + 20%
1000 year
18.09
18.2
18.42
18.51
18.57
18.75
18.87
18.99
19.22
C-C
19.12
19.26
19.53
D-D
19.21
19.36
19.64
section
culvert
A-A
B-B
The flood levels shown in Table 2.4 have been plotted against the site topography to
create a flood zone outline, as shown on Drawing No. LE10104/EIA039. As can be
seen from this drawing, during the 100 year + 20% climate change and the 1000 year
rainfall events, the water level rises above the channel but floodwaters are still
contained within the banks of the watercourse without encroaching onto the site.
6
CONCLUSIONS
The hydraulic modelling exercise for the Wade Brook shows that during the 1 in 100
year +20% climate change rainfall event and the 1 in 1000 year rainfall event, the
watercourse breaches its channel due to the throttling effects caused by the
downstream culvert. However, even though the watercourse breaches its channel it is
still contained within the stepped banks of the watercourse and does not encroach onto
the site itself.
The results of the model show that the site is, therefore, located wholly within Flood
Zone 1, as defined in Planning Policy Statement (PPS) 25, and that the flood waters do
not encroach onto the site for events up to and exceeding the 1 in 1000 year rainfall
event. The site is, therefore, not located within the floodplain and is appropriate for the
proposed development.
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September 2009