Previous preliminary advice on the suitability of the site chosen for the location of the intake and outlet diffuser for the Tugun Desalination Plant (Hughes and Tomlinson, 2006) suggested that the seabed was stable at water depths around 20m. Since then there has been a change in the design of the outlet and studies of seabed profiles (Patterson, 2006) along Main Beach and the Spit coastline clearly demonstrate that a net onshore movement of sand is occurring at depths of the order of 15 below MSL. In the light of these recent findings and changes a supplementary assessment of the seabed stability has been undertaken. Beach profiles have been examined for Tugun, Kirra and Coolangatta. The latter two are in the area deprived of sand over the last 40 years due to the construction of the extension to the Tweed Training Walls in 1962-64, and should represent the extreme movement of the seabed possible in the region. Survey transects from 1966 to 2000 were provided by the Gold Coast City Council (GCCC) for the Tugun area. These survey transects were then analysed with a program specifically developed for the purpose in Matlab.
The main continuous dataset are the ETA lines set up for the DELFT Report in the 1960s. These lines extended from Fingal Head to the top of South Stradbroke Island and generally were spaced at 400m. ETA 24 and ETA 25 are near the intake/diffuser site and the full available dataset of profiles are presented in a way to see the overall extent of seabed movement, including the storm bar from the 1974 storms. The 1974 storms are considered by many to have a return period of around 100years, although there is insufficient information available to do a proper statistical analysis.