Narellan Creek Overland Flood Study (Vol 1) - Flipbook - Page 53
Narellan Creek Catchment
Flood Study
5.2.2
Areal Reduction Factors
The design rainfall intensities presented in the preceding section are only applicable for
catchment areas of up to 1 km2. Therefore, ARR2019 includes areal reduction factors that
recognise that there is unlikely to be a uniformly high rainfall intensity across all sections of
large catchments.
The primary input variable to calculate the areal reduction factors is the contributing
catchment area. One of the main difficulties in applying the areal reduction factors for a
flood study such as this is the fact that the contributing catchment area varies considerably
across the study area. For example, the contributing catchment areas vary from less than
1 km2 at the upstream end of each major subcatchment (and smaller tributaries) to more
than 30 km2 at the downstream end of the catchment.
However, as discussed, the primary focus of the current study is to define overland flood
behaviour across the urbanised sections of the catchment, rather than the main
creeks/waterways that have been the subject of previous flood studies (refer Section 2.2).
As discussed in Section 3.2.1, overland flow paths can start to form across urban catchments
when the contributing catchment comprises as little as 1 hectare (i.e., 0.01 km2). Therefore,
the contributing catchments areas across urban overland flow areas are most commonly less
than 1 km2. Based on this, no areal reduction factors were applied to the point rainfall
intensities to ensure the flood risk was not understated across the overland flow areas.
A review of the areal reduction factors was completed to understand application of no
reduction factor may over-state design flow estimates in mainstream areas. As noted above,
the catchment area draining to the catchment outlet is more than 30 km2. However, this
area is dominated by the Nepean River backwater. Outside of the Nepean River backwater
area, the contributing catchment areas are typically less than 15 km2 and most commonly
less than 5 km2. The typical areal reduction factor for a 15 km2 is around 5% to 15% for
storm durations up to 6 hours, while a 5 km2 catchment would have an areal reduction
factor of between 2% and 10% applied. Therefore, adoption of no areal reduction factor
may over-state the design rainfall for areas with a contributing catchment area larger than
1 km2, but the differences would be typically less than 10%. This difference is considered
reasonable, noting that the ARR2019 rainfall estimates area lower than ARR1987.
5.2.3
Rainfall Losses
As discussed, the WBNM model was developed to include the
