Storm Water - Lex Toth

                                                                            Callaghan & Toth


4.        STORMWATER DESIGN AND ASSESSMENT


4.1       Proposed On-Site Stormwater Treatment Process

The developed site was divided into Sub-Catchments as shown on Figure 1.1 and the
detention system is listed below:

1. Rainwater Detention Tanks will be required to collect water from all the roof areas of the
   new house. A total of two (2) tanks with a capacity of 3,000 litres per tank will be
   provided for the new house. The tanks are to an overflow pipe discharging to the
   underground drainage system. The total tanks capacity is to be 6,000 litres and is
   adequate for the approximate attenuation of runoff back to predevelopment
   concentrations for AEP1%, 10%, 50%.



4.1.1 Rainfall Data
The meteorological data used for the BOM IFD table for Cooroy was used for the runoff
calculations



4.2       HYDROLOGY

The site has been modelled using HydroCAD 10 to determine the detention capabilities of
the rainwater tanks.

The rainwater detention tanks adopted have been sized so that runoff concentrations are
reduced back to predevelopment levels as shown in our calculations in Appendix 1.

As the existing runoff and catchment extents to Opal Street is not to be affected by this
development, there will be no change to the capacity of drainage to Opal St..

As the new runoff from the new house in Opal Lane is to be attenuated to pre-development
levels, there will be no change to the capacity of existing drainage in Opal Lane.




Erthart                           1905 SWMP_Rev0.doc                           April 2019   8
1905