Abstract: Relative Stabilities of Tidal-Inlet Component Features--Computer- Aided, Photointerpretive Approach

John H. Barwis

Computer-aided aerial photointerpretation provides a quantitative historical review of the morphologic changes associated with tidal inlets. Additionally, it provides an approximate inlet-sediment budget by yielding information on size-location relations of intertidal geomorphic components.

A 20-year historical photoanalysis, encompassing the effects of several major storms, was performed for Fire Island Inlet, New York. The locations of shoals, channels, shorelines, berm crests, and dune scarps were digitized from 18 semicontrolled photomosaics (post-1962). These digitized data then were analyzed to rectify photos to identical scales, compute areas of inlet morphologic components, and determine the geographic frequency for both ebb-tidal deltas and flood-tidal deltas.

The growth or attrition of intertidal component features occurs by two modes of sediment influx. Mode 1 is evidenced by the area of the updrift spit being inversely proportional to the size of the main ebb-tidal delta. Sand deposited on the ebb-tidal delta is eroded from the updrift spit by waves and longshore currents; the spit elongates and becomes welded to the ebb-tidal delta until breaching causes the distal end of the spit to become part of the ebb-tidal delta. Mode 2 is related to a net influx of sand to the system as a whole, caused by an increase in gross longshore-transport rate. The influx is distributed among all inlet intertidal component features in amounts proportional to their areal extent. Sand exchange between components commonly occurs in this dynamic equilibrium mo e, but not at the net expense of any given component. The exposed and protected beaches respond in opposite ways to storm conditions; during storms, the ocean beaches are eroded, whereas beaches within and behind the inlet apparently store sand. The trend is reversed during periods of calm.

Comparison of these data to photointerpretive investigations of other inlets supports two conclusions about the gross stability and sediment budgets of tidal inlets: (1) tidal inlets are long-term sediment sinks only if the inlet migrates alongshore, providing new depocenters for flood-tidal deltas; (2) the shoreline and tidal-delta configurations of an inlet system display less variability when the inlet is subject to high ratios of net to gross longshore-transport rate, even though the inlet is more likely to migrate.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC