Coastal management at Port Geographe includes four main issues:

• Seagrass wrack accumulation on Western Beach.

• Maintaining navigation depth in the Entrance Channel.

• Water quality in the canals and Lagoon.

• Erosion at Wonnerup Beach.

Geographe Bay contains some of the largest seagrass meadows in Western Australia. Just like a forest sheds leaves, seagrass sheds leaf material. If you live next to a forest you must expect to get some leaves in your yard. Living next to seagrass meadows, some seagrass wrack on the beach is a normal part of the environment.

In the absence of manmade structures like groynes, breakwaters and jetties, the seagrass wrack in Geographe Bay accumulates on the beach in May through to July and then starts to disperse. Some remains and is an essential part of the coastal environment as it assists in binding the sand.

Accumulation of seagrass wrack up to several metres thick, on what is now referred to as Western Beach, occurred prior to the construction of Port Geographe. The wrack dispersed naturally and was not regarded as a nuisance. With construction of the breakwaters at Port Geographe, the natural dispersal process was impeded.

The original breakwaters at Port Geographe were design to form a sand trap with sand to be bypassed to Wonnerup Beach by dredge, pump or trucks. An unintended effect was trapping seagrass wrack which no longer dispersed.

In 2011 a study was undertaken and a new alignment for the breakwaters selected to better allow for natural bypassing of seagrass and sand. The realignment was constructed in 2013/14.

For the new alignment to allow for the natural dispersion of seagrass wrack, the bay west of the Western Breakwater needs to be filled with sand to form an arc from about Morgan Street to about 80m from the tip of the Western Breakwater. During the period from 2014 until 2022 the arc of sand was still developing, and had not formed sufficiently for natural dispersion of seagrass to occur. The formation of a sandbar near the tip of the Western breakwater is evidence that the final arc is close to being achieved.

In 2021 extensive mechanical intervention was undertaken to accelerate seagrass wrak dispersion and the summer of 2021/22 had a beach with good amenity.

When seagrass suspended in turbulent (high energy) water reaches placid water (low energy) the seagrass settles. Water, with suspended seagrass, is carried into the entrance channel of Port Geographe by an incoming tide and settles partly blocking the channel. This must be removed by dredging. In 2021 the arrival of the dredge was delayed, and the entrance channel became partly blocked, causing a problem for keel boats trying to exit the marina. A similar problem of settling seaweed also partly blocked the entrance to the Lagoon.

These seagrass banks in the entrance channel and Lagoon are not just a navigation problem. They inhibit water exchange from the canals to the ocean. Key to maintaining good water quality in the canals is dredging of the main channel in early September.   The entrance to the Lagoon is too narrow for the large dredge and a mini-dredge or long reach excavator will be required for that area.

Erosion at Wonnerup Beach is a more difficult problem to solve, but it must be recognised it is very localised. So far it has been tackled by construction of a seawall, construction of groynes, dredging sand from an offshore spoil bank, dredging sand from the Port Entrance and hauling in sand by truck. The results have not always resulted in good beach amenity. PGLOA has encouraged further studies to find a long-term solution.

Submission to Port Geographe Technical Working Group

Following public meetings expressing concern about the performance up until 2018 of the Port Geographe breakwater reconfiguration, it was agreed that a technical working group be convened by Department of Transport and Busselton City, to examine the issue.

PGLOA made a submission which is largely based on its own technical review of Western Beach in 2018. In essence we believe the realigned Western Breakwater gives all indications of future good performance and the beach will reach dynamic equilibrium within a few years and facilitate natural bypassing of seagrass wrack.

2018 Report on Western Beach

PGLOA released its own review of the trial sea-grass management program conducted jointly by the BCC and DoT in October 2018. The report draws on the scientific and engineering expertise within the PGLOA and is technically based on observational data and quantified sediment dynamics deduced from time-series imagery of historic shorelines, before and after breakwater reconfiguration

As part of the 2018 review by PGLOA, historic shorelines for Western Beach were mapped, see Figure 3 of the report, a larger version of which can be viewed here.