Draft flood plan proposes new minimum floor levels
A new floodplain management plan would raise minimum floor level requirements over most of the Peninsula.
Central Coast Council's draft Woy Woy Floodplain Risk Management Plan is currently on exhibition for public comment.
New floor levels will be almost 1.3 metres above the one per cent annual probability flood level, commonly referred to as the one-in-100-year flood.
They are based on a 500mm "freeboard" above the flood level, plus another 0.74 metres to accommodate sea level rise.
It also recommends that the ground level (or "landform") of low-lying land subject to tidal inundation be raised in response to sea level rise.
Without this, it says: "By 2100, there will be extensive daily flooding in the Woy Woy CBD and Booker Bay, while Ettalong will see significant flooding in the lowest roads and private properties".
Among a number of recommended changes to the council's planning provisions, the plan recommends provisions to ensure "significant increases in perviousness and rainfall infiltration across the Peninsula" and "allowing more flood compatible development (by) requiring open spaces which also convey flood water in rare storm events".
The plan also recommends a shift of focus to providing a "sustainable level of drainage service", away from prohibitively expensive drainage projects.
"Requiring all major stormwater systems to convey peak one per cent flows is likely to not be achieved given the initial costs associated with construction.
"A financially-sustainable approach to stormwater asset management will be required into the future.
"Even with existing infrastructure, the consequences of one per cent flooding are generally not significant on the sand flats of the Peninsula.
"However, ongoing maintenance will need to be undertaken to ensure services are not reduced back to unacceptable levels (eg blockage of pipes and culverts).
"Ideally, a future level of major stormwater system service will enable the entire system to be upgraded in a financially-sustainable manner."
It recommended that "within the sand flats only, the requirements of the Central Coast Council Civil Works Specification - Design Guideline be relaxed to provide an alternative level of service".
This is accompanied by recommendations for six infiltration pits in key locations and for measures which aim to ensure overland flood flow paths are kept clear.
"Property owners and residents living adjacent to the main drain (Teatree Creek) and Kahibah Creek can significantly affect flood behaviour with the types of structures constructed within the floodplain."
The plan said that "landscape works, sheds, stockpiling and other small works ... can either remove flood storage or deflect flood waters and increase surrounding flood levels" or "be a
source of debris causing blockage downstream".
It recommended an "encroachment policy to manage illegal structures in the floodplain" and an education program for property owners with obstructions that were not illegal.
The new plan has nine broad recommendations, which also cover setting up evacuation centres, establishing a flood warning system, providing flood signs and flood education programs.
The plan acknowledges that it is based on modelling that is "calibrated" against just one flood 35 years ago, using flood levels at just 20 locations, and without detailed rainfall data or drainage flow measurements.
However, the modelling shows "a smaller flood extent and shallower flood depths in general" than the previous flood study in 2010.
The plan also states that two associated studies were undertaken to prepare for this plan.
One was the Woy Woy Climate Change Adaptation Study (Rhelm, 2021b) "to identify feasible strategies to adapt the low-lying areas of Woy Woy to the impacts of sea level rise".
The other was the Woy Woy Integrated Water Management Cycle and Case Study Everglades Catchment (DHI, 2021) "to define flooding in this catchment utilising an integrated surface water and ground water model, and identify potential solutions to mitigate flooding".
Neither study has been circulated with other documents supporting the Plan.
Public submissions will be received until Wednesday, February 15.
Website, 18 Jan 2023
Your Voice Our Coast, Central Coast Council