Interaction between seagrass wrack and coastal structures: Lessons from Port Geographe, south-Western Australia

Charitha Pattiaratchi, Sarath Wijeratne, L. Roncevich, J. Holder

Research output: Chapter in Book/Report/Conference proceedingConference contributionResearchpeer-review


Coastal infrastructure projects such harbours and marinas require the construction of breakwaters to maintain a stable entrance channel but they also interrupt the alongshore transport of sand resulting in accumulation of sand on the upstream side of the breakwater. Some developments have installed sand bypass systems which artificially pump sand across the entrance. There are many successful projects which have been constructed and operational using this approach. In some cases they have been unsuccessful and remedial measures had to be undertaken and lessons learnt. This paper presents a case study of an unsuccessful system constructed on the above principles. Port Geographe, a marina and residential canal estate development was developed in the early nineties. The breakwaters were designed to prevent sand bar formation at the harbour entrance channel with a concentrated water flow through the channel to maintain navigable depth, and also to avoid high wave action within the channel itself. A sand bypass system was designed to pump sand across the harbour entrance from the west (upstream) to nourish the beaches in the east. However, the artificial bypassing has become a challenging issue due to the presence of sea grass wrack, which was more efficiently trapped by the sand trap than sand. Up to ∼100,000 m3 of wrack is trapped on the beach to depths of several meters and 1-2 km in length. Wrack accumulation along the breakwater and the adjacent beaches resulted in severe environmental problems (e.g. odour, beach use) on the western side of the development and erosion of beaches to the east at Wonnerup. Solutions to alleviate this problem were developed through the development of a seagrass wrack transport model linked to an existing morphological model (MIKE-21). An optimum configuration for the coastal structures, to promote natural bypassing of sand and sea grass wrack, included: an angled groyne on the western side of Port Geographe as a replacement to the existing breakwater, which is perpendicular to the shoreline and removal of groynes associated with the pocket beaches by a foreshore seawall. The model simulations guided a reconfiguration of the marina entrance which was implemented in 2014.
Original languageEnglish
Title of host publicationAustralasian Coasts and Ports Conference 2015
Place of PublicationAuckland, NZ
PublisherEngineers Australia
ISBN (Print)9781922107794
Publication statusPublished - 2015
EventAustralasian Coasts & Ports Conference 2015: 22nd Australasian Coastal and Ocean Engineering Conference and the 15th Australasian Port and Harbour Conference - Pullman Hotel, Auckland, New Zealand
Duration: 15 Sep 201518 Sep 2015


ConferenceAustralasian Coasts & Ports Conference 2015
CountryNew Zealand
Internet address

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