Coastal Management

Overview

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

The full report can be viewed here.

Figure 2 Western Beach February 2022

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.

Figure 3 Historic shorelines of Western Beach

Notwithstanding our view that the coastal process will equilibrate over a reasonable period of time, it is becoming increasingly evident that the channel will require annual or even bi-annual dredging.

It can reasonably be inferred that the Busselton City Council intends that the circa $380k annual contribution from the WMRF that holds our SAR money will be destined to channel dredging in perpetuity. The question why SAR ratepayers are the only local sector paying for dredging of a public port remains unanswered.

Coastal Management

PGLOA has an interest in coastal management, not only because it is the key signature of our precinct, but more importantly, because Port Geographe landowners are in part financing these activities through the SAR  contribution to the Waterways Management Reserve Fund. Interest is heightened by the fact that the extent of the designated Port Geographe Coastal Management Area (PGCMA), to which we are contributing, extends well beyond the SAR catchment zone.

Following the completion of the Groyne Reconfiguration Project in May 2015, Department of Transport (DoT) has the ongoing responsibility for the performance of the coastal features. DoT provides periodic updates on the monitoring and maintenance works.

The initial update of 2015 by DoT gave the first indication of a much improved natural by-passing in the first winter. Notwithstanding this, in late 2015 DoT undertook a substantial sand-nourishment program.

Quantitative measurements of post-winter wrack accumulations by DoT in the previous three years gave volumes in the range of 115,000m3 to 150,000m3.

The update for 2016 – a year of moderately strong storms – showed a residual wrack of 15,000m3. Significantly it also showed an encouraging accumulation of sand on the Western Beach, together with pockets of depletion at Wonnerup Beach, and accumulations of wrack and sand in the port channel. No sand trucking was undertaken that year, but dredging was required in the port channel.

The 2017 update noted a post-winter wrack build of only 12,000m3 and further accretion of sand at the Western Beach. There was further build-up in the port channel, again requiring dredging, and pockets of sand depletion at Wonnerup. The latter influenced DoT to undertake a major importation of sand all along Wonnerup Beach in the Autumn of 2018.

Images of Port Geographe courtesy Department of Transport

The 2018 update noted a large (unquantified) accumulation of wrack after a winter of strong storms. The wrack had compacted, and a steep face had developed on the seaward side. It was deemed to be hindering the natural movement of sand along the shore line and slowing the build-up of sand in the elbow of the western groyne. Consequently the City and DoT jointly committed to mechanically move some of the wrack in an effort to enhance the sand movement.

The 2018 update also found an immediate requirement to again dredge the channel. Dredged material of high wrack content will be pumped offshore, whereas material of high sand content will be pumped off the eastern revetment to assists the down-drift nourishment of Wonnerup Beach.

All the updates can be viewed at: www.transport.wa.gov.au/portgeographe

Based on these observations it can be concluded:

  • Post-winter wrack accumulations after groyne re-configuration are much less than prior to groyne re-configuration
  • More sand is arriving at the Western Beach than is naturally by-passing
  • Some of the sand that is naturally by-passing is accumulating in the port channel
  • Channel dredging will likely be at least an annual requirement, and possibly bi-annual.
  • Deposition of dredged sand may be enough to nourish Wonnerup Beach.
  • Overall the re-configured groynes are performing well
  • The coast-line in the immediate vicinity of Port Geographe is progressively equilibrating by natural long-shore littoral-drift processes.

It is noticeable that the DoT updates are not as yet estimating natural sand-volume movements. The necessity for large-scale “nourishment” at Wonnerup remains un-quantified. There is some evidence that pockets of so-called “erosion” on Wonnerup Beach are due to overfill, as shown in this image. But overall this part of the Geographe Bay coastline is accreting at a fast rate.

On the basis of the DoT reports, the PGLOA committee has adopted a view that no further works of a by-passing nature should be undertaken along the coastal strip for which SAR money is used, until natural processes have come to a new equilibrium, as identified by the annual reviews by DoT. We consider this will take about four years after groyne re-configuration. However we also advocate the merit of artificially assisting the natural build-up of Western Beach against the western groyne.

We all must remember that, because of its proximity to one of the largest seagrass meadows in WA, the entire Geographe Bay coastline has been plastered with seagrass wrack for at least five thousand years. The location of wrack can, depending on weather conditions, be both static and dynamic, with material arriving and passing in just a short time. This is strikingly evident with the inconvenient accumulations of wrack in the vicinity of the iconic Busselton Jetty after the storm of mid-May 2018.

Notwithstanding our view to let coastal process equilibrate over a reasonable period of time, it is becoming increasingly evident that the channel will require annual or even bi-annual dredging. We understand the same dredging contractor is engaged for Mandurah, Halls Head and Port Geographe, and this is an annual cycle of works.

It can reasonably be inferred that the Busselton City Council intends that the circa $300k annual contribution from the WMRF that holds our SAR money will be destined to channel dredging in perpetuity. The question why SAR ratepayers are the only local sector paying for dredging of a public port remains unanswered.