Empirical Evaluation of Longshore-Current Models

Abstract

This article evaluates ability of three models (Longuet-Higgins, Komar and Inman, and a modified Longuet-Higgins approach) to replicate observed longshorecurrent velocities. These models were based on concept of local driving forces induced by radiation stress. Observation of longshore-current velocities was part of a field investigation of nearshore system at Sandy Hook, New Jersey. Correspondence between observed and predicted velocities was assessed statistically. The modified Longuet-Higgins approach provided best estimate of both site-specific and general velocities. LONGSHORE currents are generated by energy dissipation in breaking Largely confined to surf zone and adjacent nearshore, these currents may flow parallel to shore for relatively long distances or may occur as a part of a cellular circulation associated with rip currents. The flow is caused by an oblique angle of wave approach, an alongshore variation in wave height, or both. Despite these variations in their origins and form, longshore currents retain several basic attributes of importance to coastal geomorphologists. The role in sediment transport has particular interest, because advection of sediments by a longshore current is fundamental to development of different beach morphologies. Reliable models of behavior of longshore currents are central to an understanding of nearshore sediment transport. Much research has been devoted to description of these currents and to development of theoretical explanations of their generation. There have been four main approaches to modeling of longshore currents: energy and momentum equations (Putnam, Munk, and Traylor 1949), mass-continuity equations (Brunn 1963), empirical equations based on statistical approaches (Harrison and Krumbein 1964; Nummedal and Finley 1979), and radiation-stress equations (Komar and Inman 1970; Longuet-Higgins 1972; Sherman 1982). Although radiation-stress equations are also momentum based, theory differs radically from traditional momentum approach. At present, most attention remains focused on use of radiation-stress concept to model forcing of longshore currents. The purpose of this article is to discuss radiation-stress-based models of Longuet-Higgins (1972) and Komar and Inman (1970) as well as a modification of Longuet-Higgins equation (CERC 1984) and to assess their efficiency in replicating prototype measurements of longshore-current velocities. * Fieldwork for this study was funded by a grant (principal investigator, Karl Nordstrom) from National Park Service. Data analysis was supported by National Science Foundation grant SES8521344 to Mr. Sherman. * Department of Geography, University of Southern California, Los Angeles, California 90089-0663. This content downloaded from 207.46.13.115 on Sat, 08 Oct 2016 05:02:06 UTC All use subject to http://about.jstor.org/terms LONGSHORE-CURRENT MODELS SIMPLE LONGSHORE-CURRENT MODELS Bowen (1969) was first to describe manner in which radiation-stress gradients might force nearshore circulation. Longuet-Higgins and Stewart (1964) defined radiation stress as the excess flow of momentum due to presence of waves. They show that radiation stress for linear waves propagating normal to shoreline in shallow water is a proportion of wave energy,