Field Experiments on Longshore Sand Transport in the Surf Zone

Eight fluorescent sand tracer experiments were performed in energetic surf zones on natural beaches and on beaches near structures to measure the short-term longshore sand transport rate. Tracer of up to four distinct colors was injected on a line crossing the surf zone to investigate the on-offshore distributions of the longshore sand adveetion velocity and transport rate. The tracer advection velocity, v , and the depth of mixing into the bed, b, were determined from large numbers of cores taken in situ throughout the sampling area. The sand advection velocity and mixing depth were not constant across the surf zone, but usually exhibited a maximum either toward the shoreline or toward the breaker line, or in both regions. The local breaking wave height, H_b, and horizontal current velocity in the surf zone (yielding an average longshore current velocity $\overline{\text{V}}$ ) were also measured. The data were interpreted with simple dimensional arguments to give the following results: b₂ = 0.027 H_b, v_a = 0.014 $\overline{\text{V}}$, and the volumetric transport rate Q = 0.024 ${\text{H}}_{\text{b}}^2\overline{\text{V}}$. Agreement was also found between the measured total longshore sand transport rate and a predictive expression due to Bagnold involving the breaking wave power and average longshore current velocity. Although the results appear reasonable and consistent, a problem remains concerning the apparent decrease in tracer advection speed alongshore recorded in most experiments at the longer sampling times.