Flood study explains smaller extent and shallower depths

The “technical volume” of the Woy Woy Floodplain Management Study provides an explanation for its map showing a smaller flood extent and shallower flood depths compared to the 2010 study, and claims a higher confidence level.

“There are various changes to modelling of design runs since the previous study,” it stated.

It listed sevenchanges.

The first was that “design rainfalls” were updated in the Australian Rainfall and Runoff guidelines.

“Both the rainfall intensities and temporal patterns were updated.”

It said it used an updated groundwater model.

“The antecedent groundwater condition for design runs have been updated using the updated groundwater model.

“The groundwater model for the Woy Woy Peninsula has been updated with the new LiDAR and recalibrated for the longer records of the bore data.”

Also, “the model topography was updated with the LiDAR 2013 data and refined from 10 metres to five metres”.

The study stated: “Refinement of the model allowed a finer representation of landuse.”

The stormwater drainage network had also been updated with the new database, “although this should not differ much from the previous study”.

“The model domain was extended to cover the entire escarpment and the Kahibah Creek catchment,” it said.

And “the updated model was recalibrated”.

“All these changes contribute to the different flood extents and depths,” it stated.

“Refinement of the model spatial resolution represents streets which works as surface drainage paths and the impervious areas in lots more accurately based on LiDAR.

“For example, where the 2010 study applied low permeability to all residential blocks, the current study represents approximate