Bond, J., Green, A.N., Cooper, J.A.G. and Humphries, M.S., 2013. Seasonal and episodic variability in the morphodynamics of an ephemeral inlet, Zinkwazi Estuary, South Africa.
The inlet dynamics of estuary and lake systems with ephemeral inlets have been little studied compared to systems with semi-permanent tidal inlets. Here we document the meso-scale dynamics of a barrier and its associated ephemeral inlet. The inlet is typically open during the rainy season and experiences closure during winter low flow periods. It does not migrate. The inlet is formed by fluvial overtopping of the barrier and when active, forms a small delta seaward of the channel and a small flood tide delta in the back-barrier. The tidal prism is insufficient to maintain the inlet and it is quickly sealed through wave-reworking of sand from the ephemeral delta when fluvial discharge diminishes. During a large ocean swell event coupled with abnormally high tides in 2007, a departure from this seasonal behaviour occurred. The barrier migrated 100 m landwards and formed a gently (1.74°) seaward dipping sand sheet. Whereas the barrier had previously been narrow, it was widened by 80 m and rose 1.5 m in elevation. A breach later occurred and rapidly migrated northwards, establishing a significantly deeper inlet. This closed following wave reworking of a large ephemeral delta. Since then the barrier has maintained its landward position yet the inlet has continued to function as it did prior to the storm surge. This episodic barrier retreat appears to represent crossing of a morphodynamic threshold that triggered historically unprecedented rates of barrier rollover, creating a new set of inlet morphodynamic processes. These were short-lived and the system appears to have reverted to the typical seasonal morphodynamic processes despite such rapid rollover.