Surf Observations and Longshore Current Prediction
Surf Observations and Longshore Current Prediction

Author: James H. Balsillie

Publisher:

Published: 1975

Total Pages: 0

ISBN-13:

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Simultaneous field observations of breaker and current behavior have been made using techniques of the Littoral Environment Observation (LEO) program. This program, developed at CERC, is used to collect data on beach and surf conditions. The data base for this report represents a concentrated data collection effort over a period of 1 year at three beach sites at Point Mugu, California. Longshore current behavior is investigated in two ways: (a) Observed; and (b) predicted, using LEO data. Observed longshore currents are represented by the behavior of dye injected into the surf between the shoreline and breakers. Predicted longshore currents, based on the observation that longshore currents are generated primarily by waves, are estimated using a relationship for the average longshore current across the surf. The prediction model relies upon consideration of the radiation stress or wave thrust on water inside the breakers, and is evaluated using LEO breaker heights, angles of wave approach at breaking, and estimated widths of the surf zone. The degree of similarity between observed and predicted longshore currents is significantly high. This similarity not only suggests that the relationship for prediction of longshore currents is reasonable in field situtations, but that there is a surprising degree of internal consistency within typical LEO data sets.

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 Field Investigation of the Spatial and Temporal Structure of Longshore Currents
A Field Investigation of the Spatial and Temporal Structure of Longshore Currents

Author: Guy Allen Meadows

Publisher:

Published: 1978

Total Pages: 200

ISBN-13:

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This field investigation was conducted to obtain simultaneous and continuous measurements of the horizontal, vertical and temporal variability of the longshore current flow field. The present study has resulted in a two-dimensional mapping, across the surf zone and with depth, of the longshore current field. The vertical structure of the mean longshore current flow field is nearly uniform with depth, with a narrow bottom boundary layer and sharp velocity gradients at the water-sediment interface. This investigation has also shown that the total longshore current velocity vector, at any point across the surf zone, is composed of three distinct velocity components. These components are: (1) a steady longshore current velocity component; (2) a long-period fluctuating velocity component which tends to be out-of-phase with the incident wave field and; (3) a short-period fluctuating longshore current velocity component which tends to be in-phase with the incident wave field. The results of this study have further indicated that neither the deterministic radiation stress approach to the prediction of longshore currents, nor a probabilistic formulation, provide adequate prediction of the magnitude or distribution of the longshore current velocity across the surf zone. In addition, the existence of a low velocity zone in the longshore current flow field has been isolated over the submarine bar. It appears that existing analytical formulations for longshore current flow prediction must be re-evaluated in light of the findings of this study. (Author).

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.