Longshore current on a barred beach: Field measurements and calculation

Abstract

Measurements of the longshore current on a barred beach made during the 1990 Duck Experiment on Low-Frequency and Incident-Band Longshore and Across-Shore Hydrodynamics (DELILAH) field data collection project conducted at Duck, North Carolina, revealed an unexpected and persistent broad peak in the current velocity in the trough between the nearshore bar and the shore. This paper introduces longshore current and associated wave measurements from DELILAH together with a numerical model capable of describing the field observations. An existing numerical model of the mean current is modified to include a general transport equation for the mean turbulent kinetic energy created by wave breaking, and Reynolds stress components needed to calculate the longshore current and mean water level are reexpressed to include the turbulent momentum transport. In comparison to predictions from the original model, the modified model produces much improved agreement with the measured current velocity on the barred profiles in the field measurements and with the velocity profile and mean water level generated on a uniformly sloping laboratory beach by monochromatic waves. Three forms of a bottom friction coefficient are examined, and the open-channel-flow Manning friction coefficient is selected because of best agreement and consistency. Values of the Manning friction coefficient required to calibrate the model agree with values normally assigned to the related bottom roughness in open-channel flow. 22,717 22,718 SMITH ET AL.' LONGSHORE CURRENT ON A BARRED BEACH

The Structure of Nearshore Currents Driven by Changes in Meteo-Marine Forcings

“…It is stronger on the bar slope than close to the shoreline. This result differs from the observations of Smith et al (1993a); Thornton et al (1996); the comparison should be made carefully as our inner measurement point is not in the trough but closer to the shore with respect to the points compared in those studies. Cross-shore component is also uniform, but less intense (about 0.2 m/s) and negative (seaward).…”

“…The analysis of the vertical profiles reveals that longshore component of the current is generally more vertically uniform than cross-shore component, which is consistent with respect to the rare existing field observations (Smith et al, 1993a;Thornton et al, 1996;Reniers et al, 2004). To document the response of the vertical flow structure to the change in meteo-marine conditions, four typical configurations are analysed below; results are illustrated with day-averaged profiles ( evolve permanently, vertical elevations are depicted with respect to the bottom.…”

Temporal and spatial variations of surf-zone currents and suspended sediment concentration

“…14) are considerably different from the classic shape, in which the peak current is located a short distance landward of the breaker line, as predicted by the analytical model of LonguetHiggins (1970) for regular waves and also from measurements made at the LSTF for regular and irregular waves impinging on a planar concrete beach . Relatively uniform longshore current or patterns with a low, broad peak have also been reported from field studies (e.g., Kraus and Sasaki, 1979;Smith et al, 1993). Field data collected by Thornton and Guza (1986) over a dissipative nearly planar beach showed a longshore current peak well within the surf zone.…”

Bar‐trapped edge waves and longshore currents

“…The trapping criterion from (9) is C-'(ghbaO u2 + V(XbaO; a Taylor expansion of (3) should have two maxima, one at or slightly shoreward of the bar and one near the shoreline, caused by incident wave breaking patterns [Larson and Kraus, 1991]. Field data suggest that the longshore current maximum is often more evenly distributed across the trough [Smith et al, 1993]. Though these studies give a general indication of the longshore current profiles one might expect, only detailed examination of the location and strength of naturally occuring current shear relative to real depth profiles will show whether these current-modified effective depth profiles occur and, consequently, whether longshore currents do play a significant role in determining edge wave shape on barred beaches as well as on plane beaches.…”

“…The cross-shore shape of the mean longshore current during DELILAH was characterized by a broad peak over the trough [Smith et al, 1993], which modulated both in size and location with the tide [Thornton and Kim, 1993].…”