Narellan Creek Overland Flood Study (Vol 1) - Flipbook - Page 45
Narellan Creek Catchment
Flood Study
downstream boundary condition is typically defined as a known water surface elevation
(i.e., stage). The downstream boundary of the TUFLOW model is located at the Nepean
River. Accordingly, the water level in this area will be governed by the prevailing water level
in the river at the time of the flood.
Although there is a water level gauge located at Camden Weir (near the Narellan
Creek/Nepean River confluence), no water level information was available for this gauge for
the June 2016 flood. However, a surveyed flood mark of 65.31 mAHD was collected
approximately 700m upstream of the Narellan Creek/Nepean River junction following the
2016 flood. Therefore, the downstream boundary was defined as a static water level of
65.30 mAHD within the Nepean River.
Blockage
As noted in Chapter 3, there is potential for drainage infrastructure (e.g., culvert, bridges,
stormwater pits) to become partly blocked by debris during a flood. Consequently, these
drainage structures will typically not operate at full efficiency during floods.
Blockage factors were calculated for all bridges and culverts based upon 8Australian Rainfall
and Runoff: A Guide to Flood Estimation9 (Ball et al, 2019). The blockage calculations are
summarised in Appendix D.
As shown in Appendix D, the blockage factors are assigned based on the severity of the
flood with larger floods being assigned a higher blockage factor due to the potential for
larger floods to increase the mobilisation of debris. As the June 2016 rainfall was generally
between a 5% AEP and 0.5% AEP event, the design blockage factors were assigned based on
this frequency (i.e., second last column in Appendix D).
For stormwater pits, a 20% blockage factor was applied to
