Extensive drainage works in the Geographe Catchment including over 400 kilometres of rural drains, river diversions, floodgates and culverts have enabled development of water inundated land and reduced the risk of flooding to the township of Busselton. The drainage network efficiently conveys not only water but also quickly transfers nutrients and sediment off the catchment into waterways. The action area – Rethink Drainage focusses on reviewing and optimising the current drainage network to both move water off the catchment and improve water quality.
The Vasse Diversion drain was built in 1927 to reduce the risk of flooding to Busselton and diverts around 90% of the upper Vasse and 60% of the Sabina Rivers directly to Geographe Bay. In response to community interest, the Reconnecting Rivers model was developed by the Department of Water and Environmental Regulation to look at options for redirecting flows back into the Lower Vasse River and Vasse Estuary to improve water quality. The model identified a number of options to reduce flow into the Lowe Vasse River and Vasse estuary without increasing flood risk, however the model also demonstrated that increasing flows also increases nutrient loads.
Informed by this study, the Water Corporation will be doubling the size of the culvert between the Vasse Diversion Drain and Lower Vasse River to increase winter flows in their 2020 upgrade of the Vasse Diversion Drain. The study also investigated a number of options of increasing flows off the catchment from purposely built storage dams and recycled water from the Busselton Waste Water treatment plant. Recycled water was the most cost effective option and will be further investigated as a potential water source to maintain summer flows into the Lower Vasse River.
The Toby Inlet is a small estuary to the east of Dunsborough. Artificial drainage has substantially reduced water flow into the Inlet and during the summer months the Inlet bar opening to the ocean is often closed resulting in poor water quality. To assess options to improve flushing of the Inlet a hydrology model was developed by the Department of Water and Environmental Regulation to investigate the feasibility of reconnecting Toby Inlet to the ocean and re-diverting flows off the catchment.
Outcomes of the model showed the most effective action to improve tidal flushing in the Toby Inlet was to maintain the opening of Toby Inlet sandbar. This action alone would flush an estimated 70% of the Inlet with seawater and is a management action that the City of Busselton will implement from 2017. Options for increasing flows off the catchment were limited due to the small size of the catchment and current storage options.
The Vasse Diversion drain is a major arterial drain built in 1927 to reduce the risk of flooding to the townsite of Busselton. The drain discharges nutrient rich water into Geographe Bay with its lower section running through urban areas and public open space regularly used for dog walking, bike riding and fishing. The vegetation growing on the drain is degraded and the banks are eroding in some locations.
The Vasse Diversion Drain enhancement project enhanced and revegetated the lower section of the drain (beachside of Bussell Hwy) to improve the quality of water entering Geographe Bay and the liveability and visual amenity of the drain. GeoCatch led this project in partnership with the Water Corporation and City of Busselton. The project has been informed by a community survey carried out in partnership with Busselton High School students. Weed and erosion control, revegetation with native plants, a viewing/fishing platform and the reduction of nutrients were the key issues identified by the community to be considered in the enhancement project. A successful trial opening of the ocean sand bar was undertaken in 2019 to improve flushing in the lower section of the drain.
The Geographe catchment is characterised by a network of over 400km of artificial drains that allowed agricultural development in seasonally inundated areas. Traditionally the rural network has been managed to quickly remove water off agricultural areas with little consideration to managing water quality. To explore options to optimise the rural drainage network to better manage water quality Busselton Water carried out a pilot study on the Buayanyup subcatchment to identify actions to undertake a more holistic management approach of the drainage network. A copy of the report can be found here.