25 October 2004
WATER QUALITY CALCULATIONS. There are a variety of strategies and
methods available to provide water quality control. One conventional approach is to
capture a certain volume of runoff and hold it in a detention pond to allow pollutants to
settle out of the water. A common regulatory requirement is to store the first 13 mm
(0.5 in) of runoff from impervious areas (e.g. roofs, pavement or walks).45 Based on
this requirement and the fact that there are 0.85 ha (2.1 acres) of impervious area in the
proposed development, 111 m3 (0.09 acre-feet) of water quality storage is needed.
Since this is less than the total retention storage requirement of 321 m2 (0.26 acre-feet,)
the water quality storage volume is already contained in the proposed design.
Many LID components use the biological, chemical and physical processes of
plant and soil interactions to filter and treat pollutants. The effectiveness of these
components can be measured in terms of a relative reduction in pollutant concentration
or a reduction in the total mass of the pollutant that reaches the receiving waters
annually. For this method, the reduction is based on the removal efficiency and flow
rates rather than a storage volume. A detailed analysis of the combined effectiveness
of the LID components will demonstrate, in some cases, that a storage volume for water
quality is not necessary.
10-2.10 CONCLUSION. This case study has shown how LID can be incorporated
into the design of a residential housing development. The use of LID practices has
eliminated the need for a traditional stormwater detention pond, thereby reducing the
disturbance to existing forested area. The retention of this forested buffer will in turn
reduce impacts to the wetland and receiving waters. The need for piped stormwater
conveyances has been eliminated. The LID approach has the added benefit of
improving the aesthetics of the development and can provide opportunities for
community involvement in the protection and maintenance of the local environment.
Novotny and Olem, 1994.