Narellan Creek Overland Flood Study (Vol 1) - Flipbook - Page 69
Detailed Overland Flow Study for the
Narellan Creek Catchment
Table 16 shows that above floor inundation is predicted to start during the 20% AEP flood,
although external damage is predicted in floods as frequent as the 50% AEP event. During
the 1% AEP event, 132 residential properties are predicted to experience above floor
inundation (and more than 400 are predicted to experience external property damaged).
During the PMF, more than 1,300 properties are predicted to be inundated above floor level
and a more than 2,600 are predicted to be damaged externally. Therefore, the property
impacts increase significantly during the PMF.
The breakdown of damage costs for each design flood are also summarised in Table 16 and
Table 17 for residential and commercial properties. The total flood damage costs for each
event (i.e., residential, commercial, public and infrastructure costs) are provided in Table 18.
Table 18 indicates that if a 1% AEP flood was to occur, more than $15 million worth of
damage could be expected. More than $10 million worth of the damage would be incurred
across residential properties. If a PMF was to occur, the economic impacts would be severe
with more than $280 million in damages anticipated.
The damage estimates were also used to prepare an Average Annual Damage (AAD)
estimate for each property. The AAD provides an estimate of the average annual cost of
inundation across the study area over an extended timeframe (in effect, how much money
would be need to set aside each year in order to pay for flood damage costs). The AAD for
the catchment was calculated as $3,082,616. Therefore, if the 8status quo9 was maintained,
residents and business owners within the catchment as well as infrastructure providers,
such as Council, would likely be subject to flood damage costs of around $3 million per
annum (on average).
5.6
Future Catchment Development
5.6.1
Turner Road Growth Area
The flood study was prepared based on topographic and development conditions from
February 2023. However, it was noted the Turner Road Growth Area was continuing to
expand/development while the current study was being prepared. Therefore, additional
simulations were completed to understand how further development across the Turner
Road area might impact on the existing flood information presented earlier in this chapter.
In this regard, design plans for the following properties were provided by Council to assist in
updating the hydrologic and hydraulic models to reflect development across the area:
135 and 165 Turner Road
8 Flaneur Avenue
The hydrologic model was updated to reflect the increased impervious survey associated
with the future land use. The following impervious proportions were adopted for the future
land uses and were informed by impervious proportions across neighbouring subdivisions:
Parks/open space: 5% impervious
Residential: 82% impervious
Roads: 100% impervious
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