The PSD is the maximum allowable post-development discharge from a site for the selected discharge design storm and is estimated on the basis that flows within the downstream stormwater drainage system will not be increased. PSD is dependent on the following criteria:
- The time of concentration of the catchment to its outlet, or a point of concern either within or downstream of the catchment.
- The position of the site, time-wise from the uppermost reach of the catchment.
- The original or adopted ARI of the public drainage system within the catchment and rainfall data
- The area of the development site.
- The proportion of impervious area of the development site.
- The type of OSD storage facility.
- The extent of development or redevelopment within the catchment.
- Local and/or regional drainage policies.
The Permissible Site Discharge (PSD) for the site in l/s is given by (Equation 19.1 of MSMA, 2000):
The factors a and b are different for above-ground and below-ground storages due to differences in storage geometry and outflow characteristics.
For above-ground storage:
For below-ground storage:
tc = Peak flow time of concentration from the top of the catchment to a designated outlet or point of concern (min)
tcs = peak flow time of concentration from the top of the catchment to the development site (min)
Qa = the peak post-development flow from the site for the discharge design storm with a duration equal to tc (l/s)
Qp = the peak pre-development flow from the site for the discharge design storm with a duration equal to tc (l/s).
Editor :- Dilah – 13 Feb 14