Establishing uniform longshore currents in a large-scale sediment transport facility

A large-scale laboratory facility for conducting research on surf-zone sedi ment transport processes has been constructed at the U.S. Army Engineer Res earch and Development Center. Successful execution of sediment transport ex periments, which attempt to replicate some of the important coastal process es found on long straight beaches, requires a method for establishing the p roper longshore current. An active pumping and recirculation system compris ed of 20 independent pumps and pipelines is used to control the cross-shore distribution of the mean longshore current. Pumping rates are adjusted in an iterative manner to converge toward the proper settings, based on measur ements along the beach. Two recirculation criteria proposed by Visser [Coas tal Eng. 15 (1991) 563] were also used, and they provided additional eviden ce that the proper total longshore flow rate in the surf zone was obtained. The success of the external recirculation system and its operational proce dure, and the degree of longshore uniformity achieved along the beach, are the subjects of this paper. To evaluate the performance of the recirculatio n system, and as a precursor to sediment transport experiments, two compreh ensive test series were conducted on a concrete beach with straight and par allel contours (1:30 slope), one using regular waves and the other using ir regular waves. In the regular wave case, the wave period was 2.5 s and the average wave height at breaking was approximately 0.25 m. In the irregular wave case, the peak wave period was 2.5 s and the significant breaking wave height was approximately 0.21 m. The longshore current recirculation syste m proved to be very effective in establishing uniform mean longshore curren ts along the beach in both cases. This facility and the data presented here are unique for the following reasons: (1) the high cross-shore resolution of the recirculation system and the ease with which changes can be made to the longshore current distribution, (2) the degree of longshore uniformity achieved as a percentage of the length of the basin (even near the downdrif t boundary), (3) the scale of the wave conditions generated, and (3) the re latively gentle beach slope used in the experiments (compared to previous l aboratory studies of the longshore current). Measured data are provided in an appendix for use in theoretical studies and numerical model development and validation. (C) 2001 Elsevier Science B.V. All rights reserved.