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Concept Design Long Term Storage 7.4.14 Defining the area of development that contributes to runoff Concept Design
The area generating increased runoff is the
SuDS design seeks to mimic the natural
The area of development may change during
developed area of the site, and comprises:
the design process, but it is important to
losses that occur across natural catchments.
The volume of post development runoff
have an initial estimate of the amount of
Roofs and hard surfaces (roads, car parks,
storage, to inform the layout of the SuDS
should match that of the natural catchment.
paving, etc.) proposed for the site.
design.
Reduction in development runoff volume can
There is no industry standard for setting the
be achieved by:
green space). In calculations allow for the
location’s estimated greenfield runoff rate.
■ ■ rainwater re-use (harvesting) rate of runoff from permeable areas (e.g.
interception losses
Hard surfaces generate increased runoff, and
■ long-term storage.
determine the volumes to be managed.
Where rain harvesting is provided, 50% of the
harvest volume can be offset against volume
losses where demand exceeds yield. This is a SuDS components such as permeable
general rule of thumb which is stated within pavements provide interception losses. roofs
BS8515. Long- term storage can also be incorporated
into the pavement design and they can be used car parks
for rainwater harvesting in certain situations,
paving
47 48
Previously developed land (Brownfield sites)
paths
Approach 1 and Approach 2 also apply to A relaxation on outflow controls or the extent car parks
management of rate and volume of runoff of storage required will only be permitted
from previously developed sites. LPAs will with the express agreement of the LPA and roads
request runoff from these sites to be reduced LLFA at an early stage of the project. This
to greenfield runoff rates. should be discussed at the Pre-Application
stage.
Design Note:
The percentage of rainfall that occurs as runoff from a surface is called the ‘coefficient of
volumetric runoff’ (Cv). Water & Sewerage Companies (Thames Water) use Sewers for
Design Note:
Adoption Ed7 (p.55) which recommends a Cv of 1.0 (100%) from all hard surfaces.
Storage volumes derived at the Concept Design stage may differ from those calculated at the
Detail Design stage. Storage volumes derived at Concept Design stage should be Cv’s of 0.95 from roofs and 0.9 from paved areas would be considered by the LLFA as part
approximate, in order to demonstrate that the scheme is sensibly proportioned. of Technical Assessment, where SuDS are not being adopted by Thames Water.
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