B07056.W-01A Complete - Stormwater [As Published] · 132007.2398

Concept Stormwater Management Plan
RSL Care – Cooroy Retirement Village



5.0        DETENTION BASIN MODELLING
As identified in Section 3.0, measures are required to mitigate the increase in peak flows resulting from the
development.

The measures to mitigate the increase in runoff from North and South developed catchments is to enlarge the
two existing dams to provide adequate detention volume. Outlet structures will also be installed to control the
discharge from each basin.

This section discusses the concept designs of the two detention basins proposed, the modelling methodology
and results confirming their effectiveness to mitigate the increase in peak flows.

5.1                Concept Detention Basin Design
The concept design of the two detention basins has been based on the following:
      •   Mitigate the increase in peak flows resulting from the development;
      •   To incorporate the existing northern and southern farm dams located on the site.
      •   Allow for access by maintenance vehicles,
      •   Provide significant visual amenity.

Also incorporated into each basin are Water Quality Control Ponds (WQCP’s) that will form part of the water
quality treatment train for the site. While runoff from each catchment discharges to the detention basins only low
flows (up to 3 month ARI) will be treated by the WQCP’s. Refer to Section 6.0 for information on WQCP’s.

In the event of storm flows (2 to 100 year ARI events) the detention volume, outlet structures and overflow weirs
will reduce flows leaving each basin to below existing levels. Flow leaving the basins will discharge offsite to the
downstream drainage system.

After calculating preliminary detention basin volumes each basin was designed accommodating the above
constraints. The minimum level of detention storage was based on the top of the storage required for the
WCQP’s. Effectively this means that storage for stormwater quality treatment is not included in the detention
basin volumes.

Different outlet configurations were tested until configurations were achieved that mitigated developed catchment
peak discharges to existing levels. The developed site hydrological model set up using the DRAINS software
package and described in Section 3.0 was modified to include the Stage-Discharge relationship (Mitigated
Scenario).

Details of the proposed Stormwater Detention Basins are presented in Table 5.1.




B07056.W-01A.doc                                                                                             Page 9