Limiting slopes and depths at ebb-tidal shoals

Dense bathymetry surveys obtained by LIDAR at 13 small to medium coastal inlets of the continental United States were analyzed to quantify limiting (maximum) bottom slopes of ebb shoals and entrance channels. The LIDAR data were supplemented with conventional bathyrnetry measurements from five large inlets to obtain predictive relationships for the limiting (minimum) depth over crest of the ebb shoal. The sites, all located on sandy coasts, were chosen to cover a range in tidal amplitude, tidal prism, and average annual wave height. Wave-dominated inlets exhibited steeper slopes on their seaward margins than tide-dominated inlets. Slopes on ebb shoals typically do not exceed 4-6degrees, with seaward slopes being 1-2degrees steeper than landward slopes. Dredged entrance channels have steeper slopes than natural channels, with maximum slopes immediately after dredging reaching 6-8degrees. At one inlet having a series of LIDAR surveys, entrance channel maintenance dredging created 3-5degrees side slopes that decreased 0.5-1/year for the next 2 years to achieve a typical slope of 3degrees along much of the channel. Greatest bottom slopes are found in scour holes near jetties (10-12degrees) and at the entrance bars (8-10degrees) of (tideless) Great Lakes harbors. Limiting depth over crest of the ebb shoals is predicted well by the parameter (HsP)(1/4), where H-S is the average annual significant wave height, and P is the spring tidal prism. High correlation was also found between limiting depth and prism, and with limiting depth and wave height. (C) 2002 Elsevier Science B.V. All rights reserved.