Rapid erosion of sand dunes in response to seasonal changes in hydrodynamics - A potential hazard for seagrass communities in the Torres Strait 2006

Presentation Abstract: Submerged sand banks in the northwest of Torres Strait typically have smaller dunes superimposed upon them. Survey work undertaken as part of the Torres Strait CRC measured the rates of sand dune migration on these banks in an effort to gauge their potential impact on local seagrass communities. Marine surveys at the end of the monsoon and trade wind seasons measured very similar hydrodynamic conditions but substantially different patterns of dune migration. At the end of the monsoon season migration rates of up to 17 meters were measured over a 14 day period, at the end of the trade wind season migration rates of up to 4 meters were measured over a similar time period. Wind data acquired for the two weeks before and during the monsoon season survey indicated the onset of the trade wind season during this time. As a result, the elevated levels of dune migration observed at the end monsoon season are probably the result of wind driven-currents, changing from predominantly eastwards during the monsoon season to predominantly westwards with the onset of the trade wind season. Dunes that were influenced by wind-driven currents during the monsoon season were east facing at the start of the trade wind season but these dunes then became hydrodynamically unstable under the influence of the westward-directed wind-driven currents and experienced accelerated levels of migration to the west. By comparison, the lower rates of sand dune migration observed during the trade wind season survey are considered to be representative of dunes that are hydrodynamically stable. The main conclusions from this research are that the dunes in the study area have been observed to move rapidly in response to seasonal changes in hydrodynamics however the typical rates of dune migration suggest that only seagrass communities in close proximity to the sandbanks are likely to be threatened by dune migration.