Far-field impact and coastal sedimentation associated with the 2006 Java tsunami in West Australia

Abstract

A detailed assessment of the impact of a far-field tsunami on the Australian coastline was carried out in the Steep Point region of Western Australia following the July 17 2006 Java tsunami. Tsunami inundation and run-up were mapped on the basis of eyewitness accounts, debris lines, vegetation damage and the occurrence of recently deposited fish, starfish, corals and sea urchins well above high-tide mark. A topographic survey using kinematic GPS with accuracies of 0.02 m in the horizontal and 0.04 m in the vertical recorded flow depths of between 1 and 2 m, inundation of up to 200 m inland, and a maximum recorded run-up of 7.9 m AHD (Australian Height Datum). The tsunami impacted the sparsely populated Steep Point coastline close to low tide. It caused widespread erosion in the littoral zone, extensive vegetation damage and destroyed several campsites. Eyewitnesses reported three waves in the tsunami wave train, the second being the largest. A sand sheet, up to 14 cm thick and tapering landwards over 200 m, was deposited over coastal dunes. The deposits are predominantly composed of moderately well-sorted, medium-grained carbonate sand with some gravel and organic debris. A basal unconformity defines the boundary between tsunami sediments and underlying aeolian dune sand. Evidence for up to three individual waves is preserved as normally graded sequences mantled by layers of dark grey, organic-rich fine silty sand. Given the strong wind regimes in the area and the similarity of the underlying dune deposits to the tsunami sediments, it is likely that seasonal erosion will remove all traces of these sediment sheets within years to decades.