Overall, we are in a fortunate position in WA, with relatively low levels of coastal development, a history of well-established coastal setbacks and a substantial proportion of the coast where the coast is resilient to changing coastal pressures. Much of our coast is determined by an erosion-resistant geological (rock) framework, which helps provide stability. There are large areas of sand storage, especially in coastal dunes, which act as a buffer to storm erosion and often provide protection from coastal flooding. However, there are locations where erosion and coastal flooding may occur.
Many of our present day erosion “problems” are merely cyclic behaviour that is related to sediment moving back and forth between different positions (such as Fremantle South Beach or Rockingham-Kwinana shores).
The vast majority of historic erosion problems are due to unintended consequences of coastal works (with Mandurah Northern Beaches and Wonnerup providing poignant examples), or deliberately sacrificing one section of the coast in preference to another (Harvey coast).
There are a few areas where the coast is a relic from older conditions, and not supported by present day conditions, so the coast gradually gets whittled away. This has mainly occurred on cuspate forelands, including those at Quinns Rocks, Lancelin and Seabird.
Within the last decade, there has been general erosion pressure on much of the Western Australian coast due to high mean sea levels during the extreme 2011-2013 La Niña conditions. This made some parts of the coast more sensitive to storm attack, with short-term erosion affecting several coastal facilities, such as Gnarabup café and Whitehill Rd at Drummond Cove.
Sea Level Rise and Variability
Projected sea level rise in the order of 0.5-1.0m over the next 100 years, or possibly higher, represents a substantial potential threat to our coastal lifestyle and highly valued coastal habitats. Much of this impact is projected to occur through the latter half of the 21st Century. However, we have been given a taste of things to come through sea level variability. A transition from low mean sea levels in the 1990s during the El Niño climate phase to high mean sea levels in 2011-2013 during La Niña conditions saw unheralded rates of sea level rise along the Western Australian coast, in places averaging more than 10mm/year for 20 years. Thankfully, this trend has reversed over the last five years, and we are presently (right now) at an ‘average’ state, after a very sustained high period from 2011-2013. However, coastal dynamics occurring over these decades provided an insight into how change may occur under sustained sea level rise.
In addition to the mean sea level process, there are two factors which affect high water levels, which are the inter-annual cycle of tides (for the southwest, this is due to peak around 2025) and the effects of storminess. The latter effect is largely a 'random' process, although it occurs in a cyclic fashion. The effect of the tidal modulation can add almost 0.2m to the typical daily maxima, so it is not insignificant.
Coastal Erosion
In all cases, there is an amount of erosion that can occur 'any time' due to storms - State Government recommend an approximate allowance of 40m for this, but it varies a lot between locations.
There is no direct calculation of how the coast will recede due to climate change and sea level rise. For the most part, much of the Western Australian coast is controlled by the presence of rock formations, and the modern coast was largely 'prestressed' by a previous sea level high stand of approximately 1m higher than present day levels, about 6,000 years ago. The means that many coastal features are comparatively insensitive to present day conditions, with the geomorphology showing that they have been far more active in the past.
This means is we presently have quite discrete areas, particularly those low lying areas that formed 'recently' in a geological sense (<2,000 years), which are quite susceptible to erosion threat. Some of these have been eroding for much longer, such as the 'peninsula' at Leschenault Estuary. Within the Swan River estuary, the locations of stress could be matched to an evaluation of wave conditions - in theory the same thing occurs on the coast, with a mixture of 'old' erosion sites and 'new' ones that are generated by present day conditions.
There are other locations, including the Perth Metropolitan coast, and much of the Swan River estuary, that have been highly modified by human activity. These features have a higher degree of instability than 'natural' systems and we would expect to see continued additional response.
For the southern part of the Swan Coastal Plain, one of the important mechanisms is the slowing or cessation of sediment supply along the coast, which runs broadly from south to north. We have already seen that this is sensitive to interannual variability, with a lack of supply affecting Mandurah from 2011-2013. Although this highlights that climate change / sea level rise is a threat, the greater potential problem is impoundment and disruption of this supply by people attempting to protect their section of the coast. This problem is already being faced (and strategies are being developed) for the Busselton area.
Change to alongshore sediment transport patterns affects various coastal morphology in different ways. Consequently, areas most sensitive to changes in sediment supply may provide useful indicators of erosion pressure. Areas most likely to be affected are those where the coastal structure is maintained by a balance of supply and loss, such as coastal spits, or on the updrift side of restrictions to littoral sediment transport. Many potential areas of sensitivity are therefore related to the concept of sediment cells, which have been mapped for much of Western Australia and provides a basis for regional monitoring being undertaken through the Peron-Naturaliste Partnership.
In theory, at some stage projected sea level rise will reach above the 'prestressed' conditions, and we would expect to see widespread coastal response. This is the 'big ticket' item associated with sea level rise, and is the problem being faced by other parts of the world, particularly across the northern hemisphere (and SE Australia) where they did not have previous highstand of sea level.
A key indicator of coastal dynamics for the southern half of the Western Australian coast is the mobility of coastal dunes. Under modern conditions, where the coast has achieved a degree of stability, the mobility of dunes, including blowouts and sand sheet formation, is substantially less than that occurring under higher sea levels. Coastal recession due to projected sea level rise, including consequent changes to sediment supply, are expected to significantly increase the occurrence of dune mobility, which is therefore available as a key indicator. However, care and attention must be paid to distinguishing oceanographic pressures from other sources, particularly uncontrolled access and vehicle use.
Origin of Statement
This statement was prepared by Matt Eliot (Damara WA & Seashore Engineering) for the South West Catchments Council in April 2018. The content represents a synthesis of findings from several regional to state scale studies and investigations, including:
Damara WA. (2011) Coastal Erosion Study: Assessment of Climate Change Impacts. Prepared for the Shire of Busselton, Final Report 96-00-01.
Eliot I, Nutt C, Gozzard B, Higgins M, Buckley E & Bowyer J. (2011) Coastal Compartments of Western Australia: A Physical Framework for Marine & Coastal Planning. Report to the Departments of Environment & Conservation, Planning and Transport. Damara WA Pty Ltd, Geological Survey of Western Australia and Department of Environment & Conservation, Western Australia.
Eliot M. (2011) Influence of Inter-annual Tidal Modulation on Coastal Flooding Along the Western Australian Coast. Journal of Geophysical Research, 115, C11013, doi:10.1029/2010JC006306.
Haigh ID, Eliot M, Pattiaratchi C & Wahl T. (2011) Regional changes in mean sea level around Western Australia between 1897 and 2008. Coast & Ports 2011. Proceedings : 20th Australasian Coastal and Ocean Engineering Conference and the 13th Australasian Port and Harbour Conference : diverse and developing held 28-30 September 2011, Perth Convention Exhibition Centre / ISBN: 9780858258860 (CD-ROM).
Eliot I, Gozzard B, Eliot M, Stul T and McCormack G. (2012) The Coast of the Shires of Gingin and Dandaragan, Western Australia: Geology, Geomorphology & Vulnerability. Report prepared by Damara WA Pty Ltd and Geological Survey of Western Australia for Department of Transport and Department of Planning. Report 113-01.
Eliot M. (2013) Application of Geomorphic Frameworks to Sea-level Rise Impact Assessment. Prepared for Geoscience Australia. Damara WA Report 193-01-Rev 0.
Stul T, Gozzard JR, Eliot IG & Eliot MJ. (2015) Coastal sediment cells for the Vlamingh Region between Cape Naturaliste and Moore River, Western Australia. Report prepared by Seashore Engineering Pty Ltd and Geological Survey of Western Australia for the Western Australian Department of Transport.
Damara WA (2016) Peron-Naturaliste Regional Coastal Monitoring Program. Report 245 Rev 0.
Eliot (2016) Coastal sediments, beaches and other soft shores. Coast Adapt Information Manual 8, National Climate Change Adaptation Research Facility.
Eliot M. (2018) Western Australian Sea Level Variability. PhD Thesis, University of Western Australia, The Oceans Institute.
Seashore Engineering Pty Ltd. (2019) Assessment of Coastal Erosion Hotspots in Western Australia. For the Department of Transport and Department of Planning, Lands and Heritage. Report SE052-01-Rev1.
Comments