New Report Released
PGLOA has 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 reconfiguration. It is devoid of anecdotes and emotion. All Port Geographe land owners have a potential financial liability in wrack issues because under existing agreements, our SAR money can be deployed to operational management of wrack in the vicinity of the port development.
Principle conclusions from this study are:
- The build-up of seagrass against the western groyne of Port Geographe (both original and re-configured) has been, and continues be, the cause of abnormal build-up of wrack on the Western Beach.
- The Western Beach lies within a segment of coastline that displays the highest sand accreting rates in Geographe Bay.
- Based on quantitative volumetric estimates of sand accreting rates on the Western Beach, and known sand flux rates, the beach should reach an equilibrium shape by the summer of 2020.
- The ideal beach shape will be a catenary-style gentle embayment between the tip of the Western Groyne and the Russell Street Node, and its shape will mimic the shape of previous beach lines going back to 1941. This will enable natural by-passing of both sand and wrack.
- The equilibrium profile will still be subject to normal wrack accumulation and dispersal processes as are found elsewhere around Geographe Bay.
- The accumulation of wrack plays an important role in assisting the build-up of sand into the elbow of the beach-groyne interface.
- The mechanical intervention in October 2018, intended as a wrack management trial, has neither enhanced nor impeded the build-up of sand in the Western Beach; however it is acknowledged it has improved the beach amenity of local residents.
- As a general principle mechanical intervention that removes wrack from the Western Beach should not be contemplated.
- There is no necessity for any artificial sand nourishment program to accelerate the attainment of equilibrium of the Western Beach.
The full report can be viewed here.
Central to the study is Figure 3 of the report, a larger version of which can be viewed here.
Figure 3 Historic shorelines of Western Beach
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.