Drawing a line in the sand: extrapolating the future from the past for the management of coastal developments
The Coast and Marine Conservation Branch of the South Australian Department for Environment and Heritage and PlanningSA (part of Department of Primary Industries and Resources SA) required a “line in the sand” where development (both current and future) could be at risk due to sand dune movement and increase. Sea-change coastal development is putting increasing pressure on dune environments, both due to increased temporary usage such as camping, and more permanently due to housing and other infrastructure developments.
The area under investigation is considerable: all 3,816km of coastline of mainland South Australia. Due to the length of coastline, traditional survey methods are logistically difficult. An alternative method was proposed that comprised processes of photogrammetry, remote sensing image classification, and GIS.
Orthorectified aerial photography (1979 and 2004) was used to map existing blowouts in representative dunes. The end result was not so much a map of historical change, though that was produced as a by-product of the analysis, but a vector showing direction and distance of worst-case sand boundary change. The change vector varied depending on the aspect of the dunes to prevailing winds, with southern aspect dunes showing a 4.8 metres/year rate of change over 25 years, while more sheltered western aspect dunes were around 2 metres/year.
This vector was then used to predict, not the future of existing blowouts, but the maximum area of risk of any possible blowout anywhere along the dune fields for a 100-year period (2004-2104). Buffering the back of the dune fields by 480 metres then identified areas to be excluded from development in the 21st century. The effects of climate change or sea level change may alter this figure, so the boundary line is only an indicative line subject to future research and modelling, but is sufficient for current planning purposes.