Delineating sediment compartment boundaries in an urbanised embayment for geomorphic management of decadal-scale coastal dynamics

Coastal sediment compartments are used for quantifying landform dynamics for the purposes of coastal management. They are defined as primary, secondary and tertiary compartments. The smallest unit, the tertiary compartment, encompasses sediment movement over a decade. This scale is viewed as most relevant for managers, however defining them has proven difficult due to complexity of local geomorphology. In this paper we analyse the delineation of tertiary compartments and their boundaries in Australia and test their applicability in Port Phillip Bay. Sediment movement in a compartment is assessed based on geomorphic features related to wind and hydrodynamic processes, such as surf zone bars and aeolian dunes. Classifying tertiary compartments can be difficult for long (kilometre-scale) continuous beaches where sediment transfer from opposite ends does not occur in a decade. Management activities especially rock walls, groynes and training walls in enclosed bays also complicates the delineation of compartments. A five-point scale is developed for compartment boundaries from closed, leaky (i), leaky (ii), leaky (iii), to open. In the case of shorelines in urbanised areas, engineered structures most commonly define compartments. It is also found that tertiary compartment extent cannot be considered static over decadal scales.