Continued Correa Bay siltation 'is likely'

Correa Bay is like to continue to suffer from sedimentation, with Bulls Hill quarry being "an easily-erodible sediment source", Gosford Council has been told by its environment and planning director Ms Colleen Worthy-Jennings.

She was responding to a question raised by Cr Peter Freewater.

Cr Freewater asked council staff to investigate the source of the sediment that fills Correa Bay from Woy Woy Creek during rainfall events and provide an accurate report on the source.

The report stated that as a result of the topography of Woy Woy Creek, any silt and sand sediments that are transported from the catchment will deposit near the creek entrance.

It found that Woy Woy Creek was the source of sand within Correa Bay and other finer sediments would come from other parts of the catchment.

The other creeks in the area, Tip Creek, Railway Tunnel Creek and Everglades Creek, were not considered to be sources of the sand.

Modelled average siltation rates in Correa Bay, which were investigated as part of the Brisbane Water Estuary Processes Study 2009, were in the order of 20mm per decade.

Near the outlet to Woy Woy Creek the siltation rate during average conditions was approximately 10mm per year.

Ms Worthy-Jennings said that, while detailed modelling suggested that siltation was concentrated near the entrance of Woy Woy Creek, these investigations were based on the current catchment land use and did not necessarily reflect siltation rates which may have historically occurred following major land use changes in the catchment.

The Draft Brisbane Water Estuary Management Plan included the goal "to achieve a standard of water and sediment quality that protects and promotes a healthy aquatic ecosystem, and allows aesthetic enjoyment and appropriate recreational use".

A specific management option was included in the plan to investigate appropriate stormwater control measures to reduce sedimentation in Correa Bay.

This action followed observations of large sediment loads in catchment flows draining from Woy Woy Creek into Correa Bay after wet weather, she said.

The plan provided for an investigation of potential stormwater control measures aimed at providing some pre-treatment of stormwater flows before they reached Correa Bay.

Ms Worthy-Jennings said it was possible that the system had the potential to retain a significant portion of total suspended solids currently deposited in the bay.

Action would be taken to achieve this as part of a larger Water Sensitive Urban Design strategy for the estuary catchment.

This would involve conducting a site inspection to identify sediment sources and assess site constraints and opportunities.

It may also require additional catchment modelling to assess options to reduce sediment loads in stormwater runoff, and to identify preferred stormwater control measures.

Priority would be given to tributaries contributing the highest sediment and pollutant loads, she said.

Council Agenda COR. 103, 6 Sep 2011