Extract from FLOOD RISK AND DRAINAGE ASSESSMENT FOR SHELL  HAYLE AT CARWIN RISE ,  HAYLE , CORNWALL report

3.0 FLOOD RISK ASSESSMENT

3.1 Assessment Approach

This assessment aims to minimise any impact of the development on flood risk through
adequate consideration of flooding issues and the inclusion of appropriate mitigation.

3.2 Fluvial Flood Risk

The development area, because of its proximity to a main river, is considered to be at
risk of fluvial flooding despite the presence of flood defences in the form of bunds that
have been constructed along both banks of the main river.

The map showing the local main rivers and other watercourses in the area is provided
in Appendix G.

3.3 Tidal Flood Risk

The development is approximately 1.5km from the sea at a ground level of 8.5m AOD.
The site benefits from Flood Defences which are located approximately 0.5km
downstream of the site as shown on the plan on page 7 of the Groundsure Report in
Appendix E. The development is not considered to be at risk of tidal flooding. Extreme
sea level T200 is 4.57m and climate change for a 60 year design life is 0.552m giving a
tide level of 5.122m AOD which is approximately 3.4m below the site ground level.

3.4 Surface Water Flood Risk

There are two existing dedicated drainage systems on the site that collect surface
water. The two systems drain to the existing soakaways. The JBA surface water flood
maps show the small forecourt area to the north of the sales building to be at a low to
moderate risk of flooding from a 1 in 200 year rainfall event (between 0.1 and 0.3 in a
200 year rainfall event as shown on the modelling output provided on page 19 in the
Groundsure Report (Appendix E)). The JBA modelling may not take account of
narrow Flood Defences and so the predicted flood level is likely to be less because of
the presence of the raised banks to the main river to the northeast of the site. The 5m
cell sizes used in the JBA modelling may mean the effects of narrow flood defences

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may not fully model surface water flow paths, therefore, it is likely the JBA model
represents a worst case. Environment Agency Risk of flooding from surface water
(Depth) for the 3.3%, 1% and 0.1% AEP events is represented in figures in Appendix I.
These indicate no flooding for the 3.3% event, flooding between 0.15 and .3m deep
for the 1% event and flooding of similar depths but over a greater area for the 0.1%
event.

Taking the above into account, the forecourt area is considered to be at risk from
surface water flooding from storms that exceed the design rainfall events. The
exceedance flow path should be incorporated into the detailed design to mitigate
these risks.

A surface water drainage rationale is presented in Section 5 of this report.

3.5 Groundwater Flood Risk

The BGS rates the susceptibility to groundwater flooding within 50m of the site as
potential for groundwater flooding to occur at surface. The EA maps highlighting the
protection zones from groundwater sources indicate potential for groundwater
flooding and thus the site is considered to be at risk. Further investigations will be
undertaken at the design stage to determine the extent and depth of groundwater.

3.6 Sewers Flood Risk

Reviewing Cornwall Council’s Preliminary Flood Risk Assessment Report, this area of
Hayle is not shown to have incidences of sewers flooding at the site. The existing sewer
system is isolated from the tide influences and the surface water system serving Hayle.
Where existing site drainage is planned to be re-used, its capacity should be assessed
in order to demonstrate that the final system can adequately collect the design
storms up to 1 in 100 year plus 40% for climate change.
The development area is not considered to be at risk from sewers flooding.

3.7 Reservoir, Canals or Other Flood Risk from Artificial Sources

The EA website shows no risk to the development area and the site is not considered
to be at risk from reservoir, canals or other artificial sources of flooding.

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3.8 Pluvial Flood Risk

All developments could potentially be affected by pluvial flooding with the possibility
of this increasing flood risk in other areas. The surface water drainage system for the
proposed development will be designed to ensure that an appropriate level of
performance is provided, minimising the risk associated with surface water flooding.

A surface water drainage rationale is presented in Section 5 of this report.

4.0 SEQUENTIAL TEST AND EXCEPTION TEST

The proposed development is located within Flood Zone 1, Low Probability. Therefore,
the proposed development is considered to pass the Sequential Test which aims to
steer new development towards the lowest risk zones. The proposed development
land use type is considered suitable for location within Flood Zone 1. Consequently,
the proposed development will not need to pass the Exception Test before planning
permission can be granted.

via http://planning.cornwall.gov.uk/online-applications/files/02250338EDA831...