Attachment C - Stormwater report

5.4.         Hydrologic Modelling
The XP-SWMM runoff-routing model been has used to estimate design flood discharges within the site. The
catchment node is defined by its pervious (undeveloped) and impervious (developed) area, fraction
impervious and average catchment slope. A fraction impervious of 0% has been applied to the
undeveloped portion and 100% to the developed portion. The net rainfall is routed through the network
after appropriate losses (initial and continuing) and roughness factors are applied, resulting in a surface
runoff hydrograph for each sub-catchment. Hydrologic modelling has been undertaken using the
Laurenson Runoff Routing Method.

5.4.1.       Rainfall Losses
Initial Loss (IL) and Continuing Losses (CL) have been applied to the modelling, and again these values have
been varied for the developed (impervious) and developed (pervious) portions of the catchment. The
following loss rates have been applied:

        Developed catchment                     IL = 1mm            CL = 0mm/hr; and
        Undeveloped catchment                   IL = 12mm           CL = 2mm/hr

5.4.2.       Roughness Coefficients
Manning’s roughness coefficients (n values) have been applied to represent the undeveloped and
developed portions of the catchment. XP-SWMM allows a range to be applied to represent the varied
degree of roughness that could be expected within the catchment. The following values have been applied:

        Developed catchment                     n = 0.014 and
        Undeveloped catchment                   n = 0.04.

5.5.         Peak Flow Rate
The unsteady state, 1D hydrodynamic model XP-SWMM was used to analyse the peak flow rate of the site.
Flood levels, discharge and velocity can be extracted from the model as functions of time at required
locations.

The key objective of the hydraulic modelling was to demonstrate that the post development peak discharge
from the site for 63% AEP and 1% AEP storm event is less than or equal to the pre development peak
discharge.




Stormwater Management Plan – Lot 1 on SP108094, 305 Lake Macdonald Drive                             Page 22

D’VINERIPE
R003370