The geomorphology and evolution of intermittently open and closed estuaries in large embayments in Victoria, Australia

Intermittently open and closed estuaries (IOCE) are common landforms on open-ocean microtidal coasts. They are often viewed as evolutionary end members of open estuaries where all the accommodation space has been infilled by sediment. While IOCE are typically recognised to form in wave-dominated, microtidal environments, they can also form within embayments where the relative balance between waves and tides is highly spatially, and temporally, variable. This study examines the geomorphology and infill of two such embayed IOCE in Victoria, southern Australia, in order to assess how these systems compare to their open ocean counterparts in order to gain an understanding of the boundary conditions of such landforms. It is found that IOCE's can occur in embayments where there is sufficient energy to form a beach berm. The infill of these systems broadly follows the transgressive – stillstand pathways of open-coast systems, but the facies are modulated by the size of the catchment and open-ocean wave exposure. These boundary conditions also affect the evolution of the coastal plain in which the embayed IOCE may form and in turn the estuary planform. The entrance dynamics of these estuaries are highly variable, and it appears that they may switch between permanently open or closed states in relation to modification of boundary conditions such as roughness. It is inferred that the relative tidal range (RTR) of the marine environment at the estuary mouth is an important determination of entrance condition with the observed systems being close to the maximum RTR value on the open coast at which these landforms can exist