The Coorong SA01.02.01 Regional Setting This compartment extends from Cape Jaffa to Middleton. The dominant regional processes influencing coastal geomorphology in this region are the Mediterranean to humid cool-temperate climate, micro-tides, high energy south-westerly swells, westerly seas, carbonate sediments with interrupted swell- driven longshore transport, and the Southern Annular Mode (driving dominant south- westerly swells and storms). Regional hazards or processes driving large scale rapid coastal changes include: mid-latitude cyclones (depressions), storm surges and shelf waves. Justification of sensitivity The overall sensitivity rating is a 5. The transgressive dune barrier is eroding, biogenic sediment supply from shelf is no longer available, and overall sediment is predicted to decline. Younghusband Peninsula is a coastal sand barrier (~150 km in length) to the east of the Murray Mouth, formed during the last 7ka, and has Australia’s longest beach and a very high energy wave environment. The Younghusband barrier varies in width, from 3km near Murray Mouth, to a few hundred metres at Granites. The Sir Richard Peninsula is a shorter (~11km length and up to 500m in width) coastal sand barrier on the western side of the Murray Mouth stretching to Goolwa township. The sediment source for both barriers has come from high carbonate production on shelf which has been brought onshore to form dunes with 10-50% carbonate sand. Cape Jaffa to Kingston has lower energy at the southern end of the compartment. There is sediment accumulation against the marine breakwater and erosion on the down-drift side. Further erosion exists towards Kingston. Average historic long-shore rates of sand movement are high (260,000 m3/year), with potential for up to 1 million m3 in some years. The Murray Mouth migrates up to 1.5km, depending on the dominant direction of long-shore sediment drift. Historic river regulation has significantly reduced Murray River flow, contributing to silting up of the river mouth. From Goolwa to Middleton (the western end of the sediment compartment), there is a wide, sandy, fine sand beach exposed to waves >2m, breaking across a 500m double bar surf zone. Other comments Common features of the subtidal environment include: Bare Sand (88.79%), Dense Seagrass (9.9%), Heavy Limestone or Calcarenite Reef (1.31%). At the western end of the compartment, there has been significant cliff and foredune erosion (~200m) in historic times, possibly triggered by earth movement, but cliff slumping has slowed. On the coastal dune barrier, coast surveys by the Coast Protection Board show negligible sediment supply currently on the shelf, demonstrating a lack of replenishment source for Younghusband Peninsula. Murray Mouth dredging programs need to assess the implications of long-shore sand movement at sand dumping sites. Short & Cowell (2009, p.1) make the following conclusions in regard to the impact of sea level rise on the Younghusband Peninsula: Modelling of shoreline response to sea level rise in the three coastal Cells results in a remarkable uniform longshore response, with no significant difference between the three Cells. Shoreline recession would be driven by the sea level rise, together with sand loss to the dunes, the Murray Mouth flood tide delta and periodically to storm demand. Maximum recession for a +1.5 m sea level rise along the Coorong beach (1% probability) to 2030, 2050 and 2109 is on the order of 40 m, 95 m and 250 m respectively. Maximum recession for a +1.0 m and +1.5 m rise in sea level over the next 100 years (to 2109) with a probability of 1%, is on the order of 200m and 250m respectively, while minimum recession with a probability of 100% would be between 150m and 200m respectively. When the full range of sea level scenarios is modelled to 2109, the recession ranges from the maximum of 250m (1% probability) to a minimum of 40m (99% probability). As the narrowest section of barrier along the Sir Richard Peninsula is 350- 400m wide, even at the maximum rate of shoreline recession the barrier would not be breached by rising sea level by 2109. Confidence in sources High confidence in sources: The State has some excellent coastal data sources through various reports of the Coast Protection Board. In particular, there is a series of Coastal Action Plans covering the whole coast in a similar format, apart from Kangaroo Island (which was conducted in house by the Council). All other action plans clearly identify ‘coastal cells’ considered to be suitable management sizes. These cells are much smaller than the secondary compartments so that a number of cell descriptions need to be consulted for each sediment compartment. South Australia also has GIS-based mapping for the whole coast showing coastal types, coastal hazards and location of coastal profiles for monitoring. Unfortunately, data from each profile is not readily accessible for the map but can be obtained from DEWNR. Some of this profiling is extremely comprehensive and continuous back to 1975, in the case of the metropolitan coast. Additional information (links and references) Caton, B, Quinn, J., Detmar, S., Fotheringham, D., Rubbo, N., Royal, M., Sandercock, R. and Laurence, S. (2011) Limestone Coast and Coorong Coastal Action Plan and Conservation Priority Study, South East NRM Board and Department of Environment and Natural Resources, Adelaide. Caton, B, Fotheringham, D C Lock, M Royal, R Sandercock, R Taylor (2007) Southern Fleurieu Coastal Conservation Assessment and Action Plan, Prepared for: Adelaide & Mt Lofty NRM Board Alexandrina Council City of Victor Harbor Department of Environment & Heritage Goolwa to Wellington LAP, Yankalilla Council Harvey, N, (1996) The Significance of Coastal Processes for the Management of the River Murray Estuary, Australian Geographical Studies, Volume 34 (pp 45-57) Short, A. and Cowell, P (2009) Coorong Sea Level Rise Vulnerability Study, report CS 0902 by Coastal Studies for the Department of Environment and Heritage, 66pp. Short, A.D. and Hesp, P.A. (1980) Coastal Engineering and Morphodynamic Assessment of the coast within the South East Coast Protection District South Australia, Coast Protection Board, Adelaide, South Australia. Short, A.D. and Hesp, P.A. (1984) Beach and Dune Morphodynamics of the South East Coast of South Australia, Technical Report No. 84/1, Coastal Studies Unit, Department of Geography, University of Sydney. GIS-Based Coastal Mapping: South Australian Government Nature Maps for Coast and Marine and Coastal Management layers (accessed March 22, 2016) http://spatialwebapps.environment.sa.gov.au/naturemaps/?locale=en- us&viewer=naturemaps .