Flood study tells of 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 outlines of buildings based on LiDAR.
"This change can allow more infiltration at parts of allotments and less ponding to be simulated at allotments.
"Changes to the design rainfall is also substantial.
"Rainfall intensities are smaller in this study than the 2010 study where rainfall intensities in 1987 ARR guidelines were applied.
"This is particularly evident in short duration events.
"While only one temporal pattern was available for each duration of design event, 10 temporal patterns were examined and the one producing a mean peak flow was adopted in 2D modelling.
"The adopted antecedent groundwater conditions for design runs also differ significantly.
"Both the 2010 study and current study adopted the 80th percentile groundwater level as the antecedent groundwater condition using the long-term groundwater simulations.
"However, the current study's antecedent condition is based on the updated groundwater model which was well calibrated against much longer records of bore data in the recent Woy Woy Integrated Water Management and Case Study Everglades Catchment Study (DHI, 2021).
"The antecedent groundwater levels adopted in the 2010 study are approximately one to two metres higher than the one in the current study.
"This is much higher than the 90th percentile groundwater level derived in the current study, which is only 0.1 to 0.2 metres higher than the 80th percentile groundwater level.
"A higher antecedent groundwater level means more wet soil prior to a rain event and thus less infiltration loss.
"This could have resulted in overestimation of flooding, especially groundwater driven flooding at some locations and have had some consequences on the selection of critical durations in the previous study.
"Given the improved groundwater model, the prediction of the 80th percentile groundwater level and the antecedent catchment condition for design events in this study has a greater confidence."
SOURCE:
Website, 18 Jan 2023
Floodplain Manaagement Study, Central Coast Council