Narellan Creek Overland Flood Study (Vol 1) - Flipbook - Page 65
Narellan Creek Catchment
Flood Study
Indirect damage costs which are costs incurred outside of the specific inundation event,
such as clean-up costs and loss of trade (for commercial and industrial properties).
Costs that cannot be readily accounted for in monetary terms (e.g., emotional stress) were
not included in the damage calculations.
Building Database
A building database was developed to enable damage calculations to be prepared for
residential, commercial and public buildings. The database was developed in GIS and
included floor levels for all habitable buildings located within the PMF extent (3,673
buildings). For residential dwellings, the lowest habitable floor level was estimated, while
the lowest operational floor level was estimated for commercial and public buildings.
For buildings located within the 5% AEP flood extent, the floor levels were estimated using a
8drive by9 survey technique. This was completed using Google Street View. The floor level
was most commonly estimated by counting the number of steps or bricks between the
ground level of the property and the front door. The number of steps/bricks were then
multiplied by the step riser/brick height (180mm height for steps and 90mm height for
bricks) which was then combined with the LiDAR DEM to provide the floor level estimate.
For buildings located outside of the 5% AEP extent, but within the PMF extent, the floor
levels were assumed to be located 150 mm above the ground elevation. Although this
approach provides a less precise floor level estimate, the rarer floods have a much smaller
contribution to the average annual flood damage calculations relative to the more frequent
events. Therefore, a lower level of precision will not have a significant impact on the overall
damage calculations.
Damage Calculations
The flood damage estimates were prepared using the NSW Government9s Flood Damage
CBA Tool. The Tool requires a number of inputs to provide a reliable flood damage estimate
for the study area. A copy of the adopted inputs is provided in Appendix M.
The inputs within the Flood Damage CBA Tool are used to derive curves describing the total
flood damage costs relative to the depth of above floodings for different property types
(e.g., residential, commercial). A copy of the flood damage curves adopted as part of the
current study are also provided in Appendix M.
Each building point from the property database was intersected with the flood level results
to provide an above floor flooding depth for each building and each design flood. The above
floor flooding depths were then combined with flood damages curves to provide a flood
damage estimate for each property for each design flood.
As part of the damage cost calculations, the number of buildings subject to above floor
inundation during each design flood was calculated. This information is summarised in Table
16 and Table 17 for residential and commercial buildings respectively. The number of
properties subject to above ground inundation (even if above floor flooding is not predicted)
are also provided in Table 16 and Table 17. This includes damage to external items such as
fences, sheds and garages.
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