TWEED RIVER ENTRANCE AND BYPASS SEDIMENT DYNAMICS

Abstract

A sediment budget analysis model was applied to the Tweed River entrance, and was used to evaluate different coastal management scenarios. Construction of training walls at the Tweed River entrance resulted in the accretion prior to 1994 of an estimated 7 million m3 along Letitia Spit, New South Wales (to the south), and erosion of beaches at the Gold Coast, Queensland (to the north). The Tweed River Entrance Sand Bypassing Project (TRESBP) was established in 1994 and is responsible for bypassing sand from south to north, through dredging campaigns and a permanent bypass jetty. The Inlet Reservoir Model was developed by Kraus (2000) as a tool to analyse morphology changes at inlets. The inputs for the model were the estimated monthly longshore sediment transport rate, and the monthly pumping and dredging volumes from the TRESBP. Side-scan sonar surveys of the entrance bathymetry were used to validate the model over the period from 2000 to 2009. The validated model was used to examine bypass pumping strategies to manage the Letitia and Gold coast beaches, maintain navigability and minimise dredging costs. According to the model results, annual bypass pumping needs to be less than 325,000 m3 to manage the recovery of the Letitia Spit shoreline, and annual dredging of approximately 125,000 m3 is required to maintain full navigability in the Tweed River entrance.