A criterion for sustainably stable tidal inlets

Abstract

A criterion for tidal inlets to achieve sustainable stability was derived from theoretical considerations and validated with historical data. The criterion states, in part, that an inlet will achieve sustainable stability if its Keulegan’s repletion coefficient K exceeds 1.86. Based on the fundamental hydraulics of tidal inlets that holds regardless of timescale, the criterion relates the stability of an inlet to a suite of throat parameters explicitly. The criterion distinguishes between general stability and sustainable stability, thereby permitting a statement on the inlet resiliency in the context of climate change. The criterion was evaluated for 44 global inlets with known stability conditions at the time of data. The results were found to validate the criterion. The criterion suggests that inlet stability exists across a semi-infinite space of K rather than at (or around) a certain throat area Ac as implied by the O’Brien-Jarrett criteria and expounded by Escoffier. For a stable inlet, the effective tidal prism is in a dynamic equilibrium with a suite of throat parameters rather than with the throat area Ac alone. The tidal prism as normalized will remain unchanged as long as the sustainable criterion is met even as the throat parameters modulate individually or collectively in a changing physical environment on any timescales. Since all of the throat parameters involved are relatively easy to obtain for a given inlet, the criterion can be conveniently applied in coastal planning and conceptual design where initial scoping, problem identification, and evaluation of frequently numerous alternatives may require a simpler, less costly tool than numerical modeling.