Narellan Creek Overland Flood Study (Vol 1) - Flipbook - Page 44
Narellan Creek Catchment
Flood Study
Appendix E and indicates that the 2016 event produced rainfall that was around the same
severity as a 1% AEP design rainfall event for some periods. However, for the critical storm
duration for the catchment (i.e., 1 to 9 hours), it was closer to a 5% AEP design rainfall.
Rainfall Losses
As discussed in Section 4.2.3, the initial-continuing loss model was employed to represent
rainfall losses across the catchment. The following rainfall losses were adopted for pervious
sections of the catchment for the June 2016 simulation:
Initial loss = 10mm (based on adopted initial rainfall loss adopted in 8Update of Narellan
Creek Flood Study9 for June 2016 simulation)
Continuing loss rate = 1.48 mm/hour (based on continuing loss rate downloaded from
ARR2019 Data Hub with NSW jurisdictional advice adjustment applied i.e., 0.4
multiplier).
For effective impervious surfaces, an initial loss of 1mm was adopted along with a
continuing loss rate of 0 mm/hour (i.e., impervious surfaces fully contributed to runoff once
rainfall depths exceeded the initial loss vale).
Results
The WBNM model was used to simulate rainfall-runoff behaviour for the June 2016 flood
based upon the rainfall and rainfall loss information presented in the preceding sections.
This enabled discharge hydrographs to be generated for each subcatchment. A summary of
peak discharges for each WBNM model subcatchment for the 2016 event are included in
Appendix F.
The hydrographs generated by the WBNM model were subsequently routed through the
TUFLOW model. Further discussion on the TUFLOW model simulation results, are provided
below.
4.2.2
TUFLOW Modelling
Change to Reflect Historic Conditions
As noted earlier in this report, some sections of the Narellan Creek catchment have
undergone significant development over the past decade. As a result, the TUFLOW model,
which was developed to represent contemporary catchment conditions, was not necessarily
reflective of catchment conditions at the time of the 2016 flood. Therefore, the following
updates were completed to the TUFLOW model to provide an improved description of
conditions at the time of the flood:
Terrain representation was modified based upon 2019 LiDAR
Material/roughness was reverted back to grass/trees across new development areas
Stormwater pits and pipes were removed from new subdivisions
Boundary Conditions
Discharge hydrographs generated by the WBNM hydrologic model were used to define
inflows to the TUFLOW model.
Hydraulic computer models also require the adoption of a suitable downstream boundary
condition to reliably define flood behaviour throughout the area of interest. The
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