Coastal and Estuarine Environment Department

Longshore current on a barred beach

[Longshore current on a barred beach]

Field measurements on longshore current were carried out during four years commencing on January 5, 1987 at HORS. A spherical float having a diameter of 0.2m shown in Figure 1 was used in the measurements. Nearshore current velocities at 1m below the water surface were measured to avoid the effects of winds and waves. Thus the density of the float was adjusted to be slightly larger than that of seawater. The float was connected with an identification buoy by 1-m-long rope as shown in Figure 1.

In the measurements, the float attached to a 30m line was released from the pier, and the time for full extension of the line was measured with a stopwatch. The current velocity was calculated with the time and the length of the line. The transport direction of the float due to nearshore current was observed with a protractor. At each point, nearshore currents were measured three times. It takes about 90 minutes to measure the nearshore currents along the HORS. The mean breaking position and the breaking wave direction were visually observed.

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Longshore current velocities measured with the float were compared with those measured with an electromagnetic current meter to check the accuracy of the measurement method with the float. When the distance between the sensor of the current meter and the mean water level was about 1 m, longshore current velocities were measured with the float and with the current meter simultaneously.

Figure 2 shows the relationship between the time-averaged longshore current velocities measured with the float UFLOAT and those measured with the current meter UEMC; triangles show the data obtained when the longshore components of the wind velocities were over +8m/s. Although the velocities measured with the float are larger than those measured with the current meter, there is a very strong correlation between them. Furthermore, the effects of winds on the longshore current velocities measured with the float were small. Through this calibration, this simple method with the float was confirmed to be useful for the measurement of longshore current velocity.


The relationship between UEMC and UFLOAT is expressed as

 (1)

In the following analysis, the values of UEMC are used.

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The cross-shore distributions of the longshore current velocity measured over longshore bars and troughs when waves broke over the bars were analyzed. An example of measured longshore current is shown in Figure 3.

Figure 4 shows the frequency distribution of the seaward distance from the location of the bar crest ybar to the location of the peak longshore current velocity ypeak. Although cross-shore distributions of the longshore current velocity calculated by one-dimensional models of longshore current velocity have peak velocities seaward of the bar crests, 85 percent of the distributions measured at the HORS had peaks shoreward of the bar crests.

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The  cross-shore distribution of the longshore current velocity with a peak shoreward of the bar crest is reproduced by a numerical simulation model that includes mass and momentum fluxes due to bore, which are several times of that calculated with the formula for non-breaking waves (Figure 5).

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