, Volume 21, Issue 2, pp 246-254

Opening and closure of a marginal Southern California lagoon inlet

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Abstract

Over the past 50 yr, direct observations of the inlet status (open or closed) of San Dieguito Lagoon, a typical southern California lagoon located in Del Mar, California, have shown that river flooding is the major natural determinant of inlet conditions on time scales longer than a few years. River flooding is strongly dependent on rainfall in the San Dieguito River watershed and on the influences of two water storage reservoirs in the area. Rainfall fluctuates on yearly and longer time scales and undergoes cycles of wet and dry periods. Over short time periods, ranging from a few months to several years, inlet status is primarily determined by the available tidal prism and littoral sand transport. Recognition of these factors is crucial in order to correctly evaluate the probability that a small lagoon will remain open naturally. A probability approach is essential because the variables controlling inlet conditions are random in nature. The results of our study show that the inlet will remain open naturally 34% of the time. The tendency to remain open is vastly smaller during years of dry weather (12%) versus times of above-average rainfall (66%).