Rain gauges needed at Somersby and Kariong

A study into overland flooding in the Somersby and Kariong Catchment has concluded more rain gauges are needed in the area, rather than having to rely on resident feedback.

It identified that of 36 rainwater gauges available in the study area owned and operated by the Bureau of Meteorology, Manly Hydraulics Laboratory and NSW Water, only 14 were in operation.

And there were no stream gauges in the study area to inform model calibration or validation.

“All local stream gauges incorporate significantly larger catchments which are not a part of this study,” the report said.

The study also concluded that the area is sensitive to stormwater infrastructure blockages.

Central Coast Council’s Environment and Planning Committee has recommended Council adopts the 352-page draft report to put it out on public exhibition.

Two years ago, 2,275 residents were given a questionnaire about flooding in the area.

A total of 133 questionnaires were returned; 5.8% of the number sent out.

Return rates of between 5% and 10% are typical for initial consultation on a flood study, the report said.

The model calibration and validation process relied upon observed peak flood level estimates and flood free property observations provided as part of the community consultation questionnaire responses. 

“In the absence of surveyed flood levels or suitable stream gauge within the catchment, they represent the best available information upon which to base the model calibration and validation,” the report said.

“Responses from community consultation often required interpretation due to the nature of the responses. 

“Flood level estimates are often constructed from the memories of residents, estimated from photographs taken following a flood event or from debris marks observed after a flood event.”

The report noted that due to some of the responses being anecdotal, such as stating the depths were “thereabouts” or “approximately”, location and reported depths may be inherently inaccurate or unreliable.

The Somersby and Kariong catchments cover an area of about 66.5sqkm and primarily drain west towards the Hawkesbury River with a small area draining east to Brisbane Water. 

A ridgeline traverses through the study area and a majority of residential and industrial development sits in close proximity. 

“As such, the catchments experience overland flooding caused by short and intense rainfall events,” the report said.

“Council has previously developed flood models and undertaken drainage assessments across the catchments, however there is a lack of flood information to provide an in-depth understanding of the existing flood behaviour across the study area. 

“In addition, a portion of the study area has been identified as a regionally significant growth area as part of the Central Coast Regional Plan 2041  highlighting the need to define the existing overland flooding behaviour to support planning procedures.”

Merilyn Vale