Shoal Bypassing in Mixed Energy Inlets: Geomorphic Variables and Empirical Predictions for Nine South Carolina Inlets

Daniel J. Gaudiano, Timothy W. Kana

Abstract


In mixed energy settings, tidal inlets undergo an episodic process known as shoal bypassing, whereby discrete bars are released from the ebb-tidal delta and migrate onshore. Such events can transport large volumes of sand, in the form of landward-migrating shoals, to adjacent beaches. Nine tidal inlets in South Carolina were analyzed to determine if there are predictable relationships among the volume of sand in the ebb-tidal deltas and in the individual bypassing shoals, the time interval between bypassing events, and the tidal prism. Historical photographs spanning up to 58 years yielded 221 discrete shoals at various stages of bypassing for the analysis. Building on earlier work by KANA (1995), mean shoal-bypassing event time intervals and the mean bypassing shoal volumes were found to be related to tidal prism. Larger inlets underwent shoal-bypassing events less frequently than smaller inlets, but produced larger bypassing shoal volumes. The relationship between average event interval and tidal prism was based on the equation, I = 0.046Tp + 4.56, where I is the average shoal-bypassing event interval (years) and Tp is the tidal prism (106 m3). The relationship between average shoal volume and tidal prism was based on the equation, S = 6.42Tp + 113.4, where S is the average bypassing shoal volume (103 m3). Both relationships are statistically significant. Bypassing shoals represented, on average, small volumetric percentages (0.6% to 6.6%) of their respective ebb-tidal delta volumes, yet in mesotidal, moderate wave energy settings such as South Carolina, shoal bypassing can be the single most important process contributing to a locally accreting beach.


Keywords


Beach accretion; inlet change; shoals; shoal bypassing; shoreline change; tidal inlets.

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