Bathymetric rejuvenation strategies for morphologically degraded estuaries

Bathymetric adjustment of estuaries created by anthropogenic stressors is a common global issue. Typical stressors are tidal levees, jetties, infilling, barrages and flow redirection. Removal or alleviation of stressors should in part, reinstate previous conditions. Generic and site specific rehabilitation strategies were assessed using a simple regime model (FORM) for the excessively silted Tamar River estuary in Tasmania, Australia. The model calculated the net sediment adjustment resulting from each strategy and was applied to evaluate projects designed to mitigate previously identified stressors and two separate barrages. Results show that a combination of projects involving removal or mitigation of stressors potentially eroded >7 × 106 m³ of silt over the study area whereas both barrages caused silt accretion, one potentially doubling the volume of silt accumulated since the early 1800s. It was concluded that a substantial rejuvenation of the estuary was possible utilising various strategies, including creating a tidal lake, removing tidal levees, reconstituting an old meander system, and creating an additional waterway, whilst mainly negative trade-offs would result from installation of a barrage. In a general sense, the recommended strategies would apply to similarly degraded estuaries elsewhere.