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Detailed Design 9.5.5.2 Attenuation and long term storage

Approach 1

For Approach 1, some runoff must be

retained on site for a longer period after
attenuation storage has emptied to mitigate
for the increased runoff volume generated by
the development. (NSTS S4)

There are a number of ways to reduce and
manage the volume of runoff generated by
development as follows:

■ Rain harvesting - Where it can be demonstrated that the harvesting system will be in use for
the majority of time and demand exceeds supply, 50% of the rain harvesting volume can be
offset against the long-term storage volume requirements. (BS 8515:2009)

■ Natural Losses – For SuDS components which provide natural losses a 5mm reduction can
be applied to rainfall depths to account for interception losses. To demonstrate potential for
sufficient interception losses, a ratio of ‘SuDS space’ to ‘developed area’ of 1:4 would be
considered acceptable by LPAs. Where SuDS components are unlined, some infiltration may
occur even if rates are very low. These additional losses can be offset against the long-term
storage volume requirements.
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■ Separate area of storage - A separate area of storage can be provided. There are no set
procedures on how frequently long term storage is utilised.

It is prudent for areas which serve other long term storage basins within pond
purposes such as carparks or playing fields complexes the frequency of fill may be much

not to be inundated on a regular basis. more regular - i.e. they will be inundated for
rainfall events less than 1 in 30 year.
The 1 in 30 year event is suggested as the
point at which these areas would be first Outflow from Long Term storage area should
utilised for storage. be via infiltration or a controlled discharge
rate of 2 l/s/ha.
In other locations such as raingardens and

Design Note:
Infiltration tests where low rates of infiltration are anticipated may have to be specified over a
period greater than 24 hours

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