Topics in Longshore Currents
Topics in Longshore Currents

Author: John Casey Church

Publisher:

Published: 1993

Total Pages: 140

ISBN-13:

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The momentum equation governing mean longshore currents on straight beaches is a balance of forcing from the momentum transfer of the oscillatory wave motion, turbulent momentum transfer (mixing), and bottom stress. Of these, the wave's contribution is well understood, but the remaining two are not, principally due to the complicated hydrodynamics of the surf-zone. Addressing the bottom stress term, a longshore current model is developed which includes a modification of the bottom stress due to the effects of breaking-wave induced turbulence. A one-dimensional turbulent kinetic energy equation is used to model this breaking-wave induced turbulence, producing a spatially varying bottom friction coefficient. The modeled longshore current cross-shore profiles show improved agreement with field observations. In a second bottom stress study, vertical profiles of mean longshore currents are examined using field data obtained with vertically stacked electromagnetic current meters with the goal of measuring the bottom stress and its associated drag coefficient. The profiles are observed to become vertically uniform whenever the ratio of wave height to depth exceeds 0.3, indicating that nearly all of the waves passing a given location are breaking. Finally, horizontal turbulent momentum transfer (mixing) is examined for the case of shear instabilities of the longshore current.

A Simple Quasi-three Dimensional Model of Longshore Currents Over Arbitrary Profile
A Simple Quasi-three Dimensional Model of Longshore Currents Over Arbitrary Profile

Author: Antonio Fernando Garcez Faria

Publisher:

Published: 1995

Total Pages: 52

ISBN-13:

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The long shore current maximum observed in the trough of a barred beach during the nearshore dynamics experiment DELILAH at Duck, North Carolina, is not predicted by present theory. The simplest longshore curren models balance cross-shore changes in the alongshore wave momentum (radiation stress) with the alongshore bottom shear stress. Waves break over the bar, reform in the trough and again break on the foreshore resulting in changes in the radiation stress, which predicts two jets, one over the bar and the other at the foreshore, which does not agree with the observed current maximum in the trough. The advection of the momentum of the longshore current by mean cross-shore currents as a source of momentum mixing is investigated. The longshore current is strongest toward the surface and decreasing to zero at the bottom. The cross-shore mean current has an onshore transport in the wave crest/trough region and an offshore transport beneath (undertow). The net interaction can induce significant lateral mixing of the alongshore momentum of the mean currents, which is shown using a simplified three- dimension model of nearshore currents to explain much of the differences with observations.

Wind and Wave Forcing of Longshore Currents Across a Barred Beach
Wind and Wave Forcing of Longshore Currents Across a Barred Beach

Author: Dennis James Whitford

Publisher:

Published: 1988

Total Pages: 238

ISBN-13:

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Previous investigations of longshore currents have included simplifying assumptions and restriction (such as a planar beach, a steady and depth uniform flow, spatially-variant bed shear stress and turbulent momentum exchange, and the exclusion of surface wind stress. These assumptions are quantitatively investigated by calculating the relative importance of each term in the longshore momentum balance with an emphasis on the relative importance of wind forcing across the barred nearshore. Wind and wave forcing of longshore currents across a barred beach are examined using both a numerical model and field measurements. A local momentum balance was measured at various locations across the surf zone during the SUPERDUCK experiment held at the USACE CERC Field Research Facility, Duck, N.C. in October 1986. A moveable sled was instrumented with pressure, current, and wind sensors to measure the various terms in the longshore momentum equation. Stability-dependent atmospheric drag coefficients for the surf zone are determined from wind stress measurements acquired just beyond the surf zone and wind speed measurements acquired from an anemometer atop the 9 m sled mast. Breaking waves were visually identified and electronically marked on the data tapes. Keywords: Ocean currents; Air water interactions; Nearshore surf zone; Wind stress; Theses. (EDC).

Application of Short-crested Wave Theory in the Design of Three Dimensional Coastal Hydrodynamic Models
Application of Short-crested Wave Theory in the Design of Three Dimensional Coastal Hydrodynamic Models

Author: William Leo Wood

Publisher:

Published: 1980

Total Pages: 72

ISBN-13:

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In spring 1973 a series of field experiments was initiated to investigate the three dimensional structure of a coastal hydrodynamic system. These field investigations supported the concept that short-crested wave theory is applicable to modeling of incident wind-wave transformations from offshore to the outer surf zone. Application of short-crested wave theory to the design of coastal hydrodynamic models was also considered appropriate because of the theories inherent three dimensional structure. Concurrent with this work two field experiments were conducted in 1974 and 1976 to measure vertical and horizontal distribution of longshore current velocity and to monitor temporal variations in current velocity at a point. In fall 1978 a series of experimental laboratory investigations was initiated to make precise measurements, at close spatial intervals, of wave height decay after breaking. These experiments were carried out to determine a wave height decay expression based upon the assumption that an appropriate physical conceptualization of wave energy dissipation after breaking must consider turbulence dominant to bottom friction. This report presents a detailed summary of these investigations and their results. (Author).