OFFSHORE SAND SOURCES

In 2009, Angus Gordon wrote a very important paper that appeared in the proceedings of the Coast and Ports Conference, Engineers Australia, Wellington NZ. The title was “The potential for offshore sand sources to offset climate change impacts on Sydney’s beaches”. The paper has much broader implications than just for Sydney. It highlighted the need to understand offshore sand  resources in the context of their future use for beach nourishment as a potential adaptive option in relation to adverse climate change impacts on beaches. His work stemmed from his engagement in offshore studies along the NSW coast from Tathra in the south to Byron in the north in association with colleagues from the former Public Works Department and Peter Roy, State Marine Geologist.

Angus pointed out that during the 1970’s and 1980’s there were several studies undertaken on the Sydney continental shelf including those associated with potential commercial sand extraction, the impact of 1974-78 storms on beaches, and  the plan to move sewer outfalls offshore away from the base of cliffs at North Head, Bondi and Malabar. Outfall design and performance considerations required both seabed mapping of sediment and rock reef distribution, and an understanding of seabed currents and sediment movements. The studies covered 62km of the Sydney coast out to 7km for the coast to 60m water depth in the north to 4km and 85km depth in the south. A series of detailed maps were produced  along with reports on shelf currents and sediment dynamics. The discovery of shelf sand bodies has been a major contribution to our knowledge of inner continental shelf geologic history in the Late Quaternary as documented by Roy and his PhD student Marie Ferland.

The NSW Government in the 1980s including Sydney Water, invested heavily in seabed mapping, sampling and coring of sand deposits. It was a tremendous effort but showed what is need if we are to fully appreciate the nature of sand resources near highly prized beaches that may be at risk from sand depletion under continued sea level rise and changes in storm type, magnitude and frequency.

Beaches today may be resilient to storm impacts within the closed sediment compartments of the Sydney coast; but there may come a tipping point when post-storm recovery is insufficient to maintain beach width against promenades and seawalls. This is when the sand resources from offshore as delineated by the outfall mapping and drilling will become a potential source for beach nourishment.

Recent announcements by the Minister for Planning on a funding package to support the development and implementation of Coastal Management Programs (CMPs) in NSW opens up the need for further discussion on what is known and not known about sand sources and movements  on the inner continental shelf. In the 1980s, Angus and colleagues used state-of-art technologies to explore the seabed. These are outlined in his 2009 paper and in an earlier paper by Gordon and Hoffman (1986) published by the Geological Society of Australia, NSW Division. There have been considerable improvements in recent years in seabed mapping using high resolution multibeam echosounder systems to collect a swath of georeferenced bathymetric soundings; these products can be now combined with topographic laser scan and aerial photography to produce more detailed seabed information to define seabed features. Seabed LiDAR images on the shelf off the coast in SW Western Australia quite dramatically show not just remnant submerged landforms but also streamers of sand indicating possible movements between cemented carbonate reefs. Substrate mapping has also become more efficient with the use of backscatter data sets, a technique increasingly in use around the UK coast by folk at the Channel Coastal Observatory, Southampton.

Through recent work for NCCARF and the NSW Government, I am of the view that more information on seabed sediment features and topography will be needed to improve our capacity to manage the Australian coast. We are able to build on some excellent work in places such as the Gold Coast, Byron Bay, Sydney and south west WA. Such work is needed to enhance sediment budget analysis to inform the design, operational performance and review of coastal management plans. They will also provide a better basis for decisions to nourish beaches to reduce risk of beach loss and property/infrastructure damage. In my view, the time is ripe for more consideration of offshore sand resources following the points made by Angus in the 2009 paper.

---

Words by Prof Bruce Thom. Please respect Bruce Thom’s thoughts and reference where appropriately: (c) ACS, 2016, posted 25th May 2016, for correspondence about this blog post please email admin@australiancoastalsociety.org.