Modifications to the shoalhaven estuary and the coastal sediment budget over the past 40 years

© Australasian Coasts and Ports 2017 Conference. All rights reserved. The Shoalhaven River is one of the few rivers in NSW to supply significant quantities of sand to the coast at the present time. The bulk of sediment delivery does not occur continuously but rather in pulse events driven by storms and floods that breach the beach berm at Shoalhaven Heads, and modify the estuary and adjacent nearshore. Analysis of aerial photographs and Landsat imagery revealed that the river mouth at Shoalhaven Heads was open in 1961, 1974-1980, 1988-1994, 1998-1999, 2013-2014 and 2015-2016. This work presents a series of surveys that started in the 1980's and included repeated echo-sounding, sediment analyses, Light Detection and Ranging (LiDAR) and Real-Time Kinematic GPS (RTK-GPS) data to assess the volumetric change after the reduction of fluvial input imposed by the construction of Tallowa Dam in 1976. Results showed that the surficial sediments of the Shoalhaven estuary are composed of a complex general pattern characterized by a decrease in grain size from coarse sand in the upper estuary to medium sand at both Shoalhaven and Crookhaven Heads, and muddy areas where mean grain size reached 6 phi. Comparison of interpolated depths showed gradual deposition in the Shoalhaven Heads channel and erosion of the Crookhaven channel, with considerable changes in the estuarine and offshore volume over the years. Modification in the estuary between 1981 and 2006 accounted for a net deposition of ~400,000 m3 of sediment despite the loss of ~1,600,000 m3 at the lower end. During the same period, comparison of bathymetric data in the nearshore indicated a delivery of at least 1,065,000 m3 of sediment to the coast. These values seem to be in agreement with the average sediment yield of ~528,000 m3/y for the catchment upstream of Tallowa Dam and the delivery of ~90,000 m3/y to the estuary, downstream of the dam, over the past 40 years.