Seasonal and episodic variability in the morphodynamics of an ephemeral inlet, Zinkwazi Estuary, South Africa

Abstract

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 o) 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.

Genetic diversity of inlet systems along non-tidal coasts: examples from the Black Sea and Sea of Azov (Ukraine)

“…11), as well as Bilosarayka, Kiltichiya, Lazovatka, and other streams characterized by low drainage conditions and distinct seasonal discharge. Similar features have been described in microtidal lagoons and estuaries of South Africa (Cooper 1990;Bond et al 2013).…”

Overwash‐dominated stratigraphy of barriers with intermittent inlets

“…Smaller inlets, such as those of the Zinkwazi and Umdloti barrier systems may close due to smaller, less intense storms that occur throughout the year (Figure ). This may be the result of the small tidal prisms and associated weak tidal currents that are readily overwhelmed by wave action (Cooper, ; Bond et al, ). Current outflow is also diminished when the impounded lagoon water has drained, and tidal flows are dominant.…”

“…The opening mechanisms for inlets can be diverse, and include overwashing and breaching during storms (e.g. Froneman, ; Bond et al, ); fluvial flooding (e.g. Green et al, ; Clarke et al, ); and the combination of the two.…”

“…The inlet forms in the wave shadow of a rocky headland (in the southern part of the barrier), with the estuary located in a headland‐bound embayment. The ~ 250‐m‐long sandy barrier has a swash‐aligned morphology and the volume of fluvial discharge controls the length of time the estuary remains open to the Indian Ocean (Bond et al, ).…”