Sediment Interaction at Modified Coastal Inlets: Processes and Policies

Abstract

A substantial number of the existing inlets in the United States has been modified (or constructed) to serve navigation, recreation and water quality. The hydraulics and sedimentary processes associated with inlets are complex and our ability to understand and predict the detailed behavior of inlets must be the product of considerable future field, laboratory and analytical research. Yet in the absence of this capability, assessments and interpretations will be required leading to design decisions.

Addressed in this paper is the sediment interaction at modified inlets with particular reference to effects on adjacent beaches and the most appropriate remedial measures in those cases where the effects are adverse. Important interaction factors include the net and gross longshore sediment transport, geometric and permeability characteristics of the jetties and sand disposal practices associated with maintenance dredging. Sediment losses to adjacent beaches can be the result of: (1) blocking of the net longshore sediment transport by the updrift jetty, (2) flow of sand over and through low and permeable jetties, (3) jetting of sand farther seaward to the ebb tidal shoals, and (4) removal of sand to maintain channel depth with disposal in deep water.

The only responsible policy of inlet management must be one that reinstates or improves upon the natural transport processes around the inlet. This can be accomplished best by a system which captures the net longshore sand transport in the active surf zone and places it in a downdrift location such that transport continuity will be maintained. The efficient design of such systems to mitigate against potential adverse effects requires considerable insight into inlet processes and presents a challenge for the