EMPIRICAL EQUATIONS FOR FORESHORE CHANGES OVER A TIDAL CYCLE

The following geometrical changes were investigated for a gently-sloping sandy beach, at Virginia Beach, Va., over the interval from one low water to the next: 1. (1) Net change in quantity of foreshore sand ΔQf (cross-sectional area of change in a plane perpendicular to shore, times a unit distance parallel to shore). 2. (2) Advance or retreat of the shoreline (mean high water line) ΔS. 3. (3) Mean slope of the foreshore, m ̄. The data for ΔQf, ΔS, and m ̄ were derived from fortyeight consecutive low water profiles made along a single transect. Fifteen environmental variables were also measured, concurrent with the monitoring of foreshore changes. Linear multiregression of the time-series data resulted in useful prediction equations of moderate strength. The importance of the interaction of swash and groundwater characteristics is brought out clearly by the regression analyses. Design of field or laboratory studies of foreshore changes must provide for adequate monitoring of groundwater-table and runup characteristics. Most of the variability in erosion or deposition on the foreshore is explained by: (a) a breaker-steepness index acting on the time of final low water; (b) the ratio of hydraulic head (h) in the groundwater table to the swash runup (b), acting at time of falling half tide; and (c) the angle of breaker approach, acting at time of high water. Advance or retreat of the mean high water line along this beach depends most closely on a ratio of mean grain diameter on the mid-foreshore to the trough-to-bottom distance in front of a breaking wave, acting at time of final low water. The next most important predictor of ΔS is the breaker-steepness index, acting at time of falling half tide. © 1969.