Coastal Environment [i]
The Peron Naturaliste region is located in the southwest of Western Australia, which is a largely sandy coastline for approximately 200 kilometres. The area is bounded by rocky coastal areas of the Capes Coast to the south and the Garden Island ridge to the north. The coastline is varying with a north-northeast facing shoreline near Cape Naturaliste to a west-southwest facing shoreline near Cape Bouvard. Further north, the shore orientation is more irregular with embayments of various size located on the lee side of gaps in the large offshore limestone ridges. The largest of these embayments are Comet Bay, adjacent to Mandurah and Warnbro Sound, on the southern side of the Rockingham Peninsula.
Offshore, the margin of the continental shelf is much straighter than the shoreline, aligned north-northeast. This creates a 100 kilometre wide continental margin over Geographe Bay, at the southern end of the Peron-Naturaliste coast, narrowing to approximately 30 kilometres at the northern end near Point Peron. Sediments across the inner shelf are mainly calcareous, of various depths overlying limestone pavement, but typically shallow.
The Peron Naturaliste coast is predominantly sand overlying and abutting various relict geological features, principally Tamala limestone formations that mark coastal positions from eras of different mean sea level [ii]. These include the massive limestone ridges that define the Sepia Depression, which runs more than 50 kilometre from Mandurah to Rottnest. The geological framework supports a series of sand formations, including parallel dune fields, parabolic dunes and linear barrier dune systems. Sediment sequences from these dunes and adjacent landforms also indicate coastal response to varying sea level, albeit over a much shorter time frame than the geological history. The modern coast is largely a result of a rapid sea level rise over the late Holocene, which peaked 1-2m above present day levels approximately 6,000 years before present [iii]. This process swept huge volumes of sand from the inner shelf plain landward, forming coastal barrier dunes between Wonnerup and Rockingham, along much of the Peron Naturaliste coast [iv]. Subsequent coastal evolution has occurred through aeolian and marine processes as sea levels gradually fell to present day.
The change from rapidly rising to gradually falling sea levels produced a significant change in the southern part of Geographe Bay, by supporting a sand feed from the Capes coast, presently evident as the Dunn Bay Bar near Quindalup, and other smaller onshore feeds. The sand feeds have historically provided net accretion along the Busselton coast [v], supporting the use of groynes in local erosion management.
There is limited influence of rivers on the coastal sediments, with the largest rivers debouching into coastal lagoons and estuaries, rather than forming coastal deltas. Systems include the Vasse Wonnerup and Broadwater wetlands, coastal wetlands near Peppermint Grove Beach, the Leschenault Estuary and the Peel Harvey Estuarine System. Several of these waterbodies have been principally subject to incursion by marine sand over recent millennia rather than infilling by fluvial sediments. Historically, there have been extensive interventions on most rivers and streams through the Peron Naturaliste region, including weirs or floodgates on many natural channels, and new channels excavated to drain land subject to water-logging. The near-surface groundwater aquifers are also heavily used, mainly for agriculture.
Coastal dynamics along the Peron Naturaliste coast reflect both episodic and cyclic variations of coastal forcing. Although behaviour is largely characterised as wave-driven, with net northward sediment transport, interpretations of observed coastal change requires consideration of short-term reversals, cross-shore sediment transfer or local features which respond to other types of forcing, including currents or water levels.
Coastal interaction with estuarine systems occurs along the Peron Naturaliste coast, including the large estuarine systems of Peel Harvey, Leschenault Estuary, Vasse Wonnerup and Broadwater. The relative importance of estuarine foreshore management is increased due to the relative intensity of development in some areas, the small development setbacks and the extensive levels of intervention in natural processes. Estuarine foreshore dynamics are typically more complex than those of the coast due to more mobile sediments, the increased role of currents, significant local-scale variations in wave climate, exaggerated scale of cross-shore features and greater interactions with benthic or riparian vegetation.
[i] Damara WA. (2012) Peron Naturaliste Partnership Region Coastal Monitoring Program Coastal Monitoring Action Plan. Prepared for Peron-Naturaliste Partnership, Report 245-03
[ii] Hearty PJ & O’Leary MJ. (2008) Carbonate eolianites, quartz sands, and Quaternary sea-level cycles, Western Australia: A chronostratigraphic approach. Quaternary Geochronology, 3: 26-55
[iii] Wyrwoll K-H, Zhu ZR, Kendrick GA, Collins LB & Eisenhauser A. (1995) Holocene sea-level events in Western Australia: revisiting old questions. In: CW Finkl (ed.) Holocene cycles: climate, sea level, and coastal sedimentation. Journal of Coastal Research, special issue no. 17: 321–326. Coastal Education and Research Foundation
[iv] Short AD. (2010) Sediment transport around Australia-Sources, mechanisms, rates, and barrier forms. Journal of Coastal Research, 395-402.
[v] Searle DJ & Logan BW. (1978) A Report on Sedimentation in Geographe Bay. Sedimentology and Marine Geology Group, Department of Geology, University of Western Australia
The coastline includes 105 kilometres of urban coast (where the adjacent uses are predominately residential and commercial and there is a high demand for recreational activity), 19 kilometres of natural coast (with less intensive hinterland uses and concentrations of tourism and associated recreational and cultural activities) and 88 kilometres of remote coast (with limited opportunity for low key tourism and associated recreational and cultural activities). The area includes the large estuarine systems of Peel Harvey, Leschenault, Vasse Wonnerup and Broadwater [i].
There is a mixture of residential areas with four major centres; Rockingham, Mandurah, Bunbury and Busselton and a number of smaller coastal communities along the coast. The estimated population at 2015 for the region is in the vicinity of 360,000. Significant economies in the coastal areas in the region include; tourism, agriculture, mining related activities, port facility and fishing.
There are significant coastal values and assets in the region. Environmental values include protected areas and endangered species, EPBC Threatened Communities and Endangered Species. For example; Sandfire Meadows, Carnaby Cockatoo’s and Western Ringtail Possum and two internationally protected Ramsar listed wetlands – Peel Harvey and Vasse Wonnerup. These have intrinsic ecological values and are highly valued by the community and visitors.
The region has cultural connections, with a rich traditional ecological knowledge and significant known and unknown cultural heritage values. There are areas and specific sites of both historic and current cultural value that need to be conserved and respected.
The beaches, foreshores, natural and built environments in the region also provide significant recreational, tourism and social amenity. They provide a meeting place and area for mental and physical health, socializing and recreational activities. The coast and sea also form a part of the areas visual amenity and these ascetic, scenic and visual qualities of the landscape constitute a highly valuable resource in their own right.
[i] Acil-Tasman Pty Ltd. (2012) Climate Change Adaptation Options Assessment – Developing Flexible Adaptation Pathways for the Peron-Naturaliste Coastal Region of Western Australia. Prepared for Peron-Naturaliste Partnership