Agusan River Basin Mangrove Wildlife Conservation Network

Title: Sediment budget approach to assess the impacts of harbour infrastructure on reef shoreline Description: Oral presentation by Stephanie Duce (James Cook University) during the 7th IOC WESTPAC International Conference in Kota Kinabalu Malaysia last May 21-25, 2008. Abstract: Reef islands are dynamic landforms that are vulnerable to natural and human-induced environmental changes. Reef islands in Torres Strait annually experience a seasonal reversal of prevailing winds, waves, and wave-generated currents (southeast for approximately 9 months of the year, to northwest during the summer monsoon). Distinct seasonal patterns of sediment transport direction on reef island beaches accompany these changes and drive seaonally consistent changes in beach morphology. On Masig Island, natural beach processes on the northwesterly shore have been disrupted by a harbour excavated in the narrow reef flat and associated marine facilities including a shore perpendicular rock barge ramp. Since the harbour was built, the beach east of the harbour has eroded and now threatens important infrastructure whereas the beach to the west has significantly accreted. Using an integrated approach including accurate topographic, hydrodynamic and sedimentological analyses, we have quantified the impact of the harbour on sediment transport and shoreline dynamics. The eroding shoreline east of the harbour has an annual deficit of approx 120 cubic meters while the accreting area west is in surplus by more than 700 cubic meters, largely because longshore sediment transport has been interrupted by the barge ramp. Loss of the beach through the harbour channel is minimal. Although the southeasterly wind season is longer, a greater volume of sediment is moved during the shorter northwesterly monsoon due to greater wave penetration to shore over a narrower, deeper reef flat. Hydrodynamic analyses suggest that if mitigation measures are not implemented, current shoreline trends will continue and possibly increase with sea level rise. Yearly renourishment of approx 500 cubic meters on the eastern side of the jetty, or assisted sediment bypassing would correct the sediment deficit and stabilize the eroding shoreline.

Posted by arbmwcnetwork on Jun 11, 2008

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