Page 90 - Bexley SuDS DESIGN & EVALUATION
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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




           London Borough of Bexley SuDS D & E Guide                                                        © 2018 McCloy Consulting & Robert Bray Associates
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