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

Flood Risk Assessment Tesco Supermarket, Swaffham
2.7 Soils
According to the Wallingford Procedure Winter Rainfall Acceptance
Potential (WRAP) map, the site has a low acceptance potential (WRAP
Class 4), with an associated „SOIL‟ classification of 0.3. This SOIL
parameter indicates that, on an annual basis, approximately 30% of
rainfall does not infiltrate into the underlying ground and runoff as
surface water.
The standard percentage runoff derived using the hydrology of soils
types (SPRHOST) provides a measure of the volumetric characteristic
of the runoff response to rainfall. SPRHOST was obtained for the site
from the Flood Estimation Handbook (FEH) CD-ROM for the
catchment. The catchment was selected as it covers a small area
approximately 1.19km 2 . An SPRHOST value of 11.19 was obtained,
indicating that approximately 11% of rainfall runoff as surface water in
this catchment, An SPRHOST value of 11% corresponds with a SOIL
value of 0.45. This is the value for the whole catchment.
As discussed above the soil survey identifies the site to be underlain
wholly by the Denbigh 1 – 541j soil type.
Using the FEH volume 4, a specific assessment of the SPR for the site
can be calculated from the 29 index soil classes using the percentages
which make up the Denbigh 1 – 541j soil type. The SPR is estimated
from HOST soil class fractions, the table below showing how the soil
beneath the site is divided up and pro-rata‟d to give a site specific SPR.
Soil Type HOST HOST Class SPR value for Totals
Classes Percentage HOST Class
Denbigh 1 – 541j Class 1 50.00% 2% 1.0%
Class 5 30% 14.50% 4.35%
Class 16 20.00% 29.20% 5.84%
TOTAL
Site SPR =
11.19%
An SPR value of 11% corresponds with a SOIL value of 0.11. This is
the value for the site only. There is a significant difference in the WRAP
soil parameter, the SPRHOST and the SPR for the site. As the
SPRHOST and SPR values are derived using 29 soil classes as
opposed to the five defined on the WRAP map, it is generally
considered that SPR values provide a more accurate representation of
soil characteristics and variation in runoff between soil types. The most
conservative value, SPR, has therefore been adopted in the estimation
of greenfield runoff rates from the proposed site.
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Jubb Consulting Engineers Limited February 2012
P9632-G300C-Rep-120209-MH