10. Long Term Impacts of Jetties and Training Walls on Estuarine Hydraulics and Ecologies

Data and theory show that the inlets of several large estuaries on Australia’s eastern seaboard that appeared to be stable within a range of entrance conditions are demonstrating unstable scouring modes and have been doing so for decades, if not centuries, since entrance jetties had been constructed. Jetties have increased the hydraulic conveyance of the entrance channels by removing sand bars and extraneous littoral currents that impeded ebb tide discharges. Field data comprising comprehensive water level monitoring in the bays, enabling the definition of tidal planes to a high resolution, have shown that the spring tidal ranges of these bays has been increasing steadily for decades with high tide planes rising and low tide planes falling. The field data have indicated that these changes show no signs of stabilizing and Escoffier analyses have indicated that it could take centuries for these inlets to reach new stable hydraulic regimes. Implications have included extensive scour in the entrance channels requiring channel erosion protection works, subsidence of road bridges, collapse of foreshores including buildings, sedimentation in the bays and on adjacent beaches and permanent changes to fringing marine ecologies and fisheries. Changes to the distribution of seagrass, saltmarsh and mangrove forests have been observed to coincide with and confirm the expectations of impacts on marine ecology that could derive from jetty construction. While jetties have improved flood conveyance significantly the increases in ebb tide velocities have resulted in navigational hazards for recreational boating.