flood risk assessment in connection with a planning - Breckland ...

Flood Risk Assessment Tesco Supermarket, Swaffham
The approximate required storage volumes (should off site attenuation
and discharge SUDS system be adopted) have been calculated using
the WINDES software package.
The calculations are reproduced for reference in Appendix F.
4.2.1.2 Infiltration rates
Borehole infiltration tests were conducted by May Gurney during a site
investigation for the adjacent commercial park in 1996 (report ref
41001, June 1996).
These give approximate soil infiltration rates into chalk at a depth of 6m
below ground level of approximately 8.4 x 10 -5 . Further porosity tests
are planned following planning approval to re-affirm these values.
4.2.2 Groundwater Flood Risk
Groundwater flooding can occur when groundwater levels rise above
ground levels at the site. The underlying solid geology (Chalk) is known
to be a factor in areas/cases of groundwater flooding, especially in low
lying Chalk valleys and areas were „winterbourne‟ type springs and
groundwater are known to cause flooding during the winter.
However, in this area the Chalk is known to be capped with a depth of
low permeability glacial deposits, and the Chalk groundwater beneath
is not known to be artesian. No records of previous groundwater
flooding in the area have been noted.
Groundwater flooding is therefore not considered to be a significant
risk.
4.2.3 Incapacity of drains or sewer flooding;
There are several references in the SFRA for the area to surface water
drainage incapacity and sewer flooding.
Providing that any off site surface water discharges are restricted to the
run-off rates set out in Appendix F and section 4.2.1 above, then
incapacity of drainage is unlikely to be an issue.
The proposed foul water discharge rates from the proposed
development will need to be established and the rates approved by the
local water authority/sewage undertaker.
The risk of flooding to the site from this source is considered low.
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Jubb Consulting Engineers Limited February 2012
P9632-G300C-Rep-120209-MH