Understanding the Littoral Current Next Previousback

LONGSHORE CURRENT EXCERCISE

Understanding the littoral current Next PreviousBack

Introduction

The longshore current is the movement of water along the shoreline and is caused by (1) waves approaching at an angle to the shoreline, (2) tidal currents, and (3) wind pushing water along the beach. Sand moves to and from the beach in an onshore-offshore direction, but it also moves along the shoreline. This alongshore sand transport is referred to as littoral drift, and because of it, beaches have been called “rivers of sand”. The longshore current moves the sand along the beach. At a particular time, the longshore current may be moving sand in either direction along the beach. The total movement of sand along the beach is called the gross littoral drift. Over the course of a year and at most locations, the volume of sand moved in one direction will exceed the amount moved in the opposite direction. The difference in the amounts of littoral drift between the two directions over a period of one year is called the annual net littoral drift. Knowing the annual net littoral drift is extremely important because the major source of sand for Texas beaches is from updrift beaches, and it is the longshore current that drives the littoral drift. This exercise will use data you collected in the field to illustrate the relationships between the wind direction and speed, wave direction and height, and the longshore current.

Instructions

1)Compile your data for a location and time and list it below. Compute the averages of the three observations taken in the field for wave direction, breaker height, and longshore current. If one of the three measurements is very different from the other two, you might consider leaving it out of the average.
Shoreline orientation to the east _____ Shoreline orientation to the west _____
Wind direction _____ Sustained wind speed (km/hr) ______
Wave direction _____ Wave breaker height (cm) _____
Longshore current direction _____ Longshore current speed (cm/s) _____

2)Draw the shoreline. On the circular plot, draw a line from the center to the outer edge at the place corresponding to the shoreline orientation. Draw another line in the opposite direction.

3)Plot the wind direction and wind speed vector. Place a straight edge connecting the center of the plot and the place on the outer ring corresponding to the direction from which the wind was blowing. Now plot the wind vector along the straight edge by starting at the inner “0” magnitude circle and drawing a line the length of which corresponds to the wind speed as indicated by the scale on the plot. Draw an arrow on the vector and label it.

4)Plot the wave vector. Repeat the process used to plot the wind vector in step 3.

5)Plot the longshore current vector. The direction of the longshore vector is either to the west or the east parallel to the shoreline. Therefore, place a straight edge parallel to the shoreline and plot the length using the appropriate scale. Draw an arrow on the vector.

Inquiry

1)What is the relationship among the various parameters?

2)Is this an expected relationship, why/why not?

3)What are some reasons you may not have found what you expected to find?

4)Plot your data from other times and locations and see if you can develop a relationship among the various parameters and the longshore current.

Profile Name______Date (yr/mo/dy) ______Start Time ______

Recorder:______

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