Timothy P. Stanton

An Introduction to Rip Currents Based on Field Observations

Abstract Rip currents are fascinating, natural, surf zone phenomena that occur daily on many beaches throughout the world. My colleagues, students, advisors, and I have been studying rip currents for more than 10 years and have performed more than 10 comprehensive field experiments on various beaches throughout the world using different observational techniques and model simulations to improve our understanding and prediction of rip currents. We have written a series of scientific articles describing the intricacies and complexities of rip current behavior using statistical and mathematical equations. These manuscripts are typically published in professional journals, which often do not communicate our results to those who would benefit from the information—the beachgoing public and ocean swimmers. Herein, we summarize our findings to help people of all ages gain a better understanding of currents at the coast.

An Approach to Measuring Turbulent Stresses in the Nearshore Region

The velocity field is modeled for sea waves propagating over a sloping bottom covered by megaripples. The flow is assumed to be irrotational, and long waves of small amplitude are considered. The analysis has been carried out to investigate whether an appropriate rotation of the coordinate system exists such that the cross-correlation of the velocity components due to waves vanishes, allowing an estimate of turbulent Reynolds stresses from the correlation between tilt corrected vertical and horizontal velocity components. The results show that such a system exists, and that the tilting angle depends on the bottom profile and on the characteristics of the wave field. As suggested by Stanton and Thornton (1996), the angle can be computed by minimizing the measured cross-correlation. However the present analysis suggests that the procedure should consider different wave components separately and should be applied sequentially at different heights from the bottom. Introduction Many efforts are devoted to modeling nearshore circulation, because knowledge of the current field is necessary to understand phenomena taking place close to the shore, and in particular to quantify sediment transport and to predict erosion and deposition processes. The actual models include details of the turbulent stress distribution which have yet to be constrained by field observations. In an attempt to provide information to the modelers, field programs have been designed to collect data resolving the spatial and temporal distribution of turbulent stresses in Environmental Engineering Department University of Genova, Via Montallegro 1, 16145 Genova, Italy 2 Department of Oceanography Naval Postgraduate School Monterey, 93940 California, U.S.A; phone 831 656 3144;. 48 COASTAL ENGINEERING 2000 49 the nearshore region (Haines and Gelfenbaum, 1996; Stanton and Thornton, 1996). In literature it is known that eddy correlation measurements of the turbulent stresses in the field are quite sensitive to the orientation of the coordinate system relative to the vertical because of the presence of the wave motions which can provide large contributions to the cross-correlation of the velocity components. Linear wave theory over a horizontal bottom predicts that the horizontal and vertical wave velocities are in quadrature, and therefore do not contribute to the turbulent stresses. However for a sloping bottom, in a gravity oriented vertical reference system, a vertical velocity component is induced by the waves, which is in-phase with the horizontal one, and so the cross-corelation of the two velocity components does not vanish. Hence for a sloping bottom experimentalists usually employ a bed-normal coordinate system. Since the nearshore bed typically consists of both a mean slope and mulit-scaled ripples, instead of trying to measure the appropriate bottom slope, Stanton and Thornton (1996) used a hydrodynamic method to establish the coordinate system for turbulent stress calculations, and determined the correct orientation of the reference system by finding for the tilting angle which minimizes the value of the velocity cross-correlation. The approach suggested by Stanton and Thornton (1996) is supported by the analysis of the flow induced by an irrotational wave close to the sea bed, but it is not clear whether the same reference frame can be used also far from the bottom. The results described in Blondeaux et al. (1999) seem to indicate that the tilting angle depends on the distance from the bed, on the wave frequency, and on the reflection coefficient of the beach. Thus the procedure suggested by Stanton and Thornton (1996) should consider different wave components separately and should be applied sequentially at different heights from the bottom. These conclusions are based on the study of the propagation of small amplitude waves in shallow waters. Moreover the local depth is assumed to change on spatial scales comparable to the wavelength of the surface wave. However the sea bottom is often characterized by the presence of bedforms of different length scales, and in particular by bedforms characterized by lengths comparable with the local depth (megaripples). Hence the local slope may significantly differ from its average value. In the present paper we consider the propagation of a monochromatic surface wave which propagates in the nearshore region when megaripples are present. The irrotational flow is determined following Blondeaux et al. (2000) and the cross-correlation of the velocity components is evaluated. The results allow an estimate of the tilting angle 0 which should be used to make the cross-correlation of the wave velocity components to vanish. The tilting angle 0 is found to depend on the characteristics of the incoming wave and on the beach profile (in particular 0 depends on the amplitude and wavelength of megaripples). Moreover the tilting 50 COASTAL ENGINEERING 2000