Summer seagrass wrack accumulations at Two Rocks, Western Australia – investigation and management pathways

Mr Timothy Stead1, Dr Fangjun Li1

1Department Of Transport, Fremantle, AU

Abstract:

This presentation will summarise an investigation into excess wrack accumulation during summer at Two Rocks southern beaches. Excess wrack has concerned local residents due to odours during decomposition, public safety risks associated with the depth and unstable nature of wrack piles, and loss of sheltered beach as a recreational asset.

WA Department of Transport undertook coastal monitoring to assist City of Wanneroo with this contentious issue. This included a two-year photomonitoring record at the beach immediately south of the marina, plus concurrent collection of nearshore water level, wave, and current data. Collected data were analysed to better understand dynamic wrack accumulation patterns. Seasonal water level variation and storm activity were observed to play critical roles in controlling wrack accumulations on-and-off the beach.

Further investigation focussed on occasions where wrack intermittently cleared from the beaches during summer conditions. Characterising these ‘clearing events’ informed forecasting windows for future trials of wrack-pushing back into the nearshore zone that emulate natural bypassing. Three types of wrack clearing event were found: a ‘Seabreeze’ event, ‘Storm’ event, or ‘Offshore’ event. All event types required rising tides, high wave heights, and wind-driven currents. However, it was found that wrack accumulations could return rapidly following any clearing event.

Three different mechanisms were hypothesised to explain how wrack rapidly returns to the beach after a clearing event. It remains unclear which mechanism(s) is correct, adding significant uncertainty to wrack management.  Notwithstanding, short- and long-term management pathways were proposed. The preferred immediate pathway is to trial wrack-pushing during clearing events.


Biography:

Tim Stead is a coastal scientist for the Western Australian Department of Transport. Tim has close to ten years experience working with WA’s diverse range of coastal projects and coastal management challenges. His experience in this state covers some of the largest tidal ranges in the world in the cyclone-prone northwest, which juxtaposes his work among some of the world’s smallest tidal ranges in the wave-dominated coasts of the south.