Empirical equation for longshore current velocity

Abstract

Ninety-eight observations of longshore current velocity, beach slope, and the breaker height, period, angle, crest length, and trough depth were made on an Atlantic ocean beach. Precision inshore surveys during the observation period revealed an essentially plane beach with an offshore bar parallel to the shoreline during two-thirds of the study period. Analysis of replicate measurements of current velocity revealed no topographically-induced non-“uniformity” in the current and suggested that an individual determination of velocity was within about ±10 percent of the true velocity value. A linear multi-regression analysis performed on the data resulted in the following empirical correlation, for the four strongest independent variables: V¯=-0.170455+0.037376(α¯b)+0.031801(T¯b)+0.241176(Hbs)+0.030923(m¯) where V¯ is in meters per second, α¯b is the mean acute angle between the breaker front and the shoreline in degrees, T¯b is in seconds, Hbs is the significant breaker height in meters, and m¯ is the mean beach slope in degrees. The total reduction of variance R2 for α¯b is 0.46 and for α¯b plus T¯b it is 0.53; Hbs and m¯ each add 0.02 to successive values of R2. The similarity between this equation and that obtained by Sonu et al. (1967), who used similar variables and analytical techniques, suggests that a general equation of this type may be valid for many Atlantic beaches. The present equation may be applied to values of Hbs, T¯b, and m¯ falling within certain specified ranges, when α¯b falls between about 2.0° and 15.0°.