Wrack line signatures of high-magnitude water-level events on the northwest Australian coast

• The NW Australian coast is a land of extremes.
• Wrack lines provide evidence of 12 high magnitude wave events in the last 2500 yr.
• The highest events have low frequency and have not been seen in historical times.
• Modern day infrastructure has not allowed for the risk associated with large events.

The Pilbara coast is affected by both intense tropical cyclones directing storm surges and high waves from the N and NW, and tsunamis generated off Indonesia. Wrack lines of shell and other skeletal material commonly occur not only 2–5 m above Australian Height Datum (AHD) but also up to 10 m AHD. They occur in distinct bands and may be continuous for several kilometres but are usually disjunct for tens of kilometres.This study assesses the potential use of wrack lines to identify extreme high water levels preserved on the coast. Elevated wrack lines occur on open coast dunes, storm bars, cheniers and spits and along lagoonal shores. Twelve wrack line assemblages were identified for the last 2500 years, with eight reported at more than one site and ≥ 5 m AHD. Six sites contain wrack lines ≥ 8 m AHD, and three reach 10 m AHD, higher than any historical tsunami inundation in the region. Ten wrack lines are found at 5–8 m AHD, and extend beyond the upper end of any reported storm and cyclone wave heights. Prehistoric wrack lines were also identified at 2–5 m AHD and are within the range of marine inundation by surges associated with present day tropical cyclones.