Adelaide Coastal Waters Study

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PPM 1—Coastal sediment budget

Issue: What are the sources, sinks and fate of sediments in the coastal system?

In recent years, the natural seasonal changes in the Adelaide beach system have been adversely affected by increased sand transport, both onshore and offshore. Sand from areas previously protected by healthy seagrass beds has moved into the active beach zone. At the same time, changes to the seafloor topography have altered the size and direction of waves that reach the shore and therefore the rates of coastal erosion.

The Coast Protection Board (CPB) and the Coastal Protection Branch have investigated coastal erosion, which affects beaches and adjacent property.

Response: Coastal Sediment Budget

The Adelaide Coastal Waters Study’s Coastal Sediment Budget complements the CPB and Branch programs. The task is intended to produce a balance sheet, showing how much sand is entering the beach zone from offshore areas, its particle sizes and where it has come from, and where it is settling out. With this information, the CPB and Branch will be able to prepare more accurate sand budgets for the metropolitan coast, assisting in the review and development of appropriate coast protection strategies.

Looking to the future, the task teams will also determine how the sand is being mobilised, and their paths from seafloor to beach. This part of the task will be based on concurrent sampling of sediments and water from 0 to 5 km from shore, measurement of water conditions at each sampling spot, and video and still photography. The plan is to collect samples that best portray the likely minimum and maximum variations (that is, mid-summer and mid-winter), along with opportunistic catastrophic events such as major storms or a spill at sea, during 2004 –2005.

The results should allow predictive modelling that can deal with new situations developing after 2005 — for example, changes in nutrient cycles, loss or gain of seagrass meadows, increased boating activities or unusual storm activity.