Unintentional Tipping of Waterway Channels into Long-Term Scour Modes

An investigation into a proposal for a relatively minor expansion of a small fishing port on the mid New South Wales (NSW) coast in Australia uncovered unexpected channel scouring that was having an adverse impact on existing infrastructure. This included the undermining and destabilizing of the main road bridge that was the critical connection between two coastal towns. The purpose of the initial investigation was replaced by studies aimed at understanding why an estuary, previously thought to have a shoaling problem, was now experiencing significant ongoing scour. It became apparent that the evolution of a twin jettied entrance had tipped the estuary, which featured a lake connected to the ocean by a series of relatively long channels, into a long-term scour mode. The sand scoured from the estuary was being discharged offshore at the ocean end, and into the lake as a growing delta at the lake (“bay”) end. An initial conceptual model was developed that explained the reason for the change in hydraulic performance. Using the conceptual model, the analytical approach of Escoffier was combined with the empirical studies of O’Brien to produce a predictive model that enabled future trends to be projected. The approach was then applied to the three other similar NSW lakes connected to the ocean by relatively long estuary channels and where twin jetties had been constructed. All four systems were found to demonstrate the same tipping from shoaling mode into long-term scour as a result of jetty construction and it was possible to predict the development of the scour and changes in the estuary and lake hydraulic responses. Over the 40 years since the initial studies the estuary behavior at the original location has evolved along the predicted lines.