STABILITY OF COASTAL INLETS

Abstract

This paper is a continuation of papers of earlier date (4) and (5) and is an abstract of (6). Pertinent factors involved in inlet stability are discussed briefly. Results of analysis of existing data are mentioned and a future research program is outlined. In order to obtain a stable tidal inlet in alluvial material it appears to be an inevitable assumption that littoral drift material is being supplied continuously to the inlet. Part of this material is deposited on the inlet bottom where the tidal currents will move it forward and back as a kind of "rolling carpet." In order to obtain a relatively stable situation this carpet must not move back and forth too rapidly since it thus runs the risk of being lost at both ends (the ocean and the bay). Nor can it be allowed to move too irregularly, changing its velocity and travel time rapidly, since it may soon "get stuck" at one or at both ends in the form of excessive deposits. If - because of insufficient littoral drift supply -- inadequate amount of material is available for building up this carpet the inlet will be constantly "shaved" and will gradually develop non-scouring open bay or perhaps estuary characteristics. Fig. 1 shows longitudinal sections through inlets of different length. In the first case the (unstable) channel is so short that the rolling carpet extends outside the inlet floor, which in turn causes material to be deposited on shoals in the sea and in the bay by the material-loaded ebb and flood currents. In the second case the (also unstable) channel is so long that material is now deposited inside both ends of the inlet channel because it gradually became so long that currents were too slow to carry the material load out in the sea or in the bay for depositing. The third case demonstrates a stable "status quo" situation between inlet length, current velocities, and material load.