The lifecycle of seagrass wrack in Geographe Bay; produced in offshore meadows, the wrack accumulates over spring and summer before early winter storms move it onto the etches. From there, complex processes result in it moving on and off shore, while being transported along the shore. It accumulates around groynes where it decomposes, sometimes releasing large amounts of H2S.
Beach wrack dynamics
Detached macrophytes (seagrass and macroalgae) are transported from offshore areas and accumulate in substantial volumes on beaches, commonly called wrack. Wrack is an important feature of coastlines as it can play a major role in subsidising terrestrial production and supporting marine food webs. However, with human alteration of beaches such as groynes, the build-up of wrack can create management issues.
This project involves a multidisciplinary team from Edith Cowan University, The University of Western Australia and DHI to investigate seagrass wrack dynamics, especially transport and decomposition on beaches. There are three main phases:
- Building a hydrodynamic model of the study site, Geographe Bay in Australia’s southwest.
- Characterising when and what type of wrack moves on and off beaches.
- Identifying the key drivers of decomposition processes on the beach.
The information generated by the project has been used to help with the management of wrack build-up on beaches with altered structure. A 2D hydrodynamic numerical model of Geographe Bay using DHI’s MIKE21 has been built and calibrated. We have used our data on wrack particle characteristics and wrack dynamics in the water and on beaches to model the movement of wrack. This has allowed the team to examine different groyne configurations that minimize wrack buildup. We have also determined the conditions under which wrack decomposes, allowing us to make recommendations on managing the production of H2S on the beaches.
Dr Kathyn McMahon
Professor Paul Lavery
The University of Western Australia, Professor Carolyn Oldham
The University of Western Australia, Winthrop Professor Chari Pattiaratchi
The University of Western Australia, Brett Branco
DHI, Tony Chiffings
DHI, Morten Rugbjerg
The Department of Planning and Infrastructure
The Busselton Shire