Sediment budget of a river-fed wave-dominated coastal compartment

Abstract

A sediment compartment approach is an appropriate framework within which to undertake planning and management of coastal environments, and a hierarchical scheme has been adopted by several state governments in Australia. This study applies a geospatial comparison of terrain modelling to estimate decadal-scale sediment transfer in a closed wave-dominated coastal compartment in southeastern Australia. The Shoalhaven River, one of the larger rivers in southern New South Wales, drains from a temperate catchment of 7151 km2 into Shoalhaven Bight, a secondary compartment. The river has infilled the barrier estuary at its mouth in recent millennia, and delivers sediment that nourishes a well-developed strandplain to its north. We estimate sediment yield from the heterogeneous catchment based on deposition between 2003 and 2014 bathymetric surveys in a reservoir formed as a result of dam construction. Delivery of ~86,000 m3/y to the estuarine channel was calculated, adopting a known trap efficiency at the dam, augmented by sediment from a further unmodified tributary. Volumetric change of the estuary and adjacent nearshore was determined by comparison of surveys in 1981 and 2006. A complex pattern of estuarine surficial sediments reflects modification of the river's natural course, which is now artificially diverted to exit at Crookhaven Heads. During major flood events, the river intermittently discharges material to the shoreface at Shoalhaven Heads, its former mouth, which is impounded again by deposition of a sandy berm in the months following such storms. Between 1981 and 2006, at least 1,020,000 m3 was added to the estuary, and 1,065,000 m3 of sand was discharged to the shoreface. Sand sourced from the Shoalhaven River is retained within the northernmost of three tertiary compartments, Seven Mile Beach, as cliffed headlands and rocky reefs restrict along-shore sediment contributions from the south and inhibit leakage to the north. Balance of the budget based on shoreline accretion during 41 years is consistent with cross-shore transport and an ongoing shoreface supply rate of 1–2 m3/m/y. Although a number of uncertainties constrain the final sediment budget, with complex patterns of erosion or accretion in response to natural events or engineering interventions, this preliminary study indicates the potential for deriving decadal-scale estimates of sediment pathways and transfer rates using geospatial analysis of terrain changes, which can provide a basis for planning and management actions.