Shoreline erosional-depositional processes associated with tidal inlets

Abstract

Tidal inlets strongly influence the overall dynamics of barrier island shorelines. The average barrier length along a coast is controlled by the size and number of tidal inlets, which in turn, are primarily a function of a region's tidal range and bay area. The greatest magnitude erosional-depositional changes along a barrier chain occur next to tidal inlets, and these losses and gains of sand are a direct consequence of tidal inlet processes. During a transgression, as at present, ephemeral, migrating and stable inlets provide a means for transferring a large proportion of the coastal sand reserves landward past the eroding shoreline. This is accomplished through the transport of sand to flood-tidal deltas, tidal creeks and the marsh surface, and through the building of recurved spits. Ebb-tidal deltas represent a large sand reservoir which may be comparable in volume to that of the adjacent barrier islands. Slight changes in the size of an ebb delta, due to changes in the inlet tidal prism, can gready affect the sand supply to nearby beaches. The sand shoals associated with ebb-tidal deltas may act as natural offshore breakwaters, reducing wave energy on landward beaches. Their removal may cause or accelerate shoreline erosion. Along mixed energy shorelines inlet sediment bypassing occurs by three major mechanisms: 1. inlet migration and spit breaching, 2. stable inlet processes, and 3. ebbtidal delta breaching. Migrating tidal inlets tend to have relatively shallow inlet channels that erode through non-resistant sediments. Spit breaching is an uncommon event, but its occurrence results in the bypassing of large amounts of sand. In the other two methods of bypassing, large bar complexes are formed on the downdrift side of the ebb-tidal delta. These bars, which may be over 1 km long and may contain more than 500,000 m3 of sand, migrate onshore welding to the landward beach. The process of bar attachment dictates the shape of many barriers.