Detention on development sites has been seen as the solution to problems of established areas where additional development or redevelopment is occurring. Generally, it is not possible, either physically or financially, to progressively enlarge drainage systems as redevelopments that increase impervious areas and runoff rates and volumes occur. Detention and/or retention facilities are normally planned for inclusion in urban stormwater management systems for one of the following purposes:

  • to remedy a situation where some part of the downstream drainage system is undersized, and cannot be enlarged conveniently or inexpensively;

  • to reduce flows from a developing area, so that the flows from the fully urbanised catchment are no greater than those which would occur under present conditions, or for the catchment in its natural state; or

  • to develop the most cost-effective drainage system possible, by reducing the sizes and cost of downstream pipes and channels, or as long as this reduces the overall net cost of the total drainage works

From the Catchment Data window, users can select or add an appropriate Detention/Retention procedure by clicking the button in the Onsite Detention/Retention area. 

Once a Detention/Retention procedure has been selected, the parameters can be further modified by clicking the Edit button. 

Data for rainwater tanks when used as an OSD device are required on a per lot basis. The accumulative water tank affect on the total sub-catchment discharge is derived by multiplying the single allotment device/s by the total sub-catchment area × the dev Area/Total Area ratio × the Average Allotment Density.

Further details on the field contents can be found here:

General OSD Options

Onsite Detention

Rainwater Tanks