Heuristic Models of Sand Transport in the Surf Zone

TLDR

Physical mechanisms are described for both the onshore/offshore and longshore modes of sand transport within the surf zone, based on wave steepness and the ratio of fall velocity to wave period, and on the advection of sand by longshore currents. These models should be useful in beach restoration projects and other coastal engineering projects concerned with sand transport within the surf zone. Sand suspended during wave breaking is assumed to consume a fixed portion of the wave energy flux, and the average longshore current is modeled using the Longuet‑Higgins representation. Using data from three earlier investigators, the model predicts the onshore or offshore direction of sand motion correctly in 161 of 184 runs, and the derived expression for littoral drift closely matches empirical equations while incorporating beach slope, grain diameter, and roughness.

Abstract

Physical mechanisms are described for both the onshore/offshore and longshore modes of sand transport within the surf zone. The onshore/offshore mode of motion is based on consideration of whether the initial sand suspended during the wave breaking undergoes a net shoreward or seaward displacement as it falls to the bottom. The two important parameters are the wave steepness and ratio of fall velocity to wave period. Using data from three earlier investigators, and choosing one empirical constant, it is found that these two parameters can predict the correct onshore or offshore direction of sand motion in 161 of 184 individual experimental runs. The longshore mode of motion (littoral drift) is considered to be the advection, by the longshore current, of the sand suspended by the breaking waves. The quantity of sand suspended is taken to be an amount which, during its fall to the bottom, consumes a fixed portion of the wave energy flux into the surf zone. The average longshore current is based on the simplest representation by Longuet - Higgins. The resulting expression for littoral drift is quite similar to earlier empirically determined equations; however, the relationship here obtained includes beach slope, sand grain diameter, beach roughness, etc. These models should be useful in beach restoration projects and other coastal engineering projects concerned with sand transport within the surf zone.