According to Longuet-Higgins (1970a, p. 6788), v^ is the longshore current 

 speed at the breaker position, T is a mixing coefficient which ranges between 

 0.17 (little mixing) and 0.5 (complete mixing) but is commonly about 0.2; 

 is the depth-to-height ratio of breaking waves in shallow water taken to be 

 1.2; and f^ is the friction coefficient, taken to be 0.01. Using these 

 values, Mj^ = 9.0 . 



Applying equation (4-20) to the two sets of data yields predictions that 

 average about 0.43 of the measured values. In part, these predicted speeds 

 are lower because v-u as given in equation (4-20) is for the speed at the 

 breaker line, whereas the measured velocities are mostly from the faster zone 

 of flow shoreward of the breaker line (Galvin and Eagleson, 1965). Therefore, 

 equation (4-20) multiplied by 2.3 leads to the modified Longuet-Higgins 

 equation for longshore current velocity: 



V = 20.7 m (gH^)^^^ sin 2ct^ (4-22) 



used in Figure 4-24. Further developments in the Longuet-Higgins' (1970b, 

 1971) theory permit calculation of velocity distribution, but there is no 

 experience with these predictions for longshore currents flowing on erodible 

 sand beds . 



5. Summary . 



The major currents in the littoral zone are wave-induced motions super- 

 imposed on the wave-induced oscillatory motion of the water. The net motions 

 generally have low velocities, but because they transport whatever sand is set 

 in motion by the wave-induced water motions, they are important in determining 

 littoral transport. 



Evidence indicates that there is only a slight exchange of fluid between 

 the offshore and the surf zone. 



Longshore current velocities are most sensitive to changes in breaker 

 angle and, to a lesser degree, to changes in breaker height. However, the 

 volume rate of flow of the longshore current is most sensitive to breaker 

 height, probably proportional to H . The modified Longuet-Higgins equation 

 (4-22) is recommended for predicting mean longshore current velocity of fully 

 developed flows. 



V. LITTORAL TRANSPORT 



1. Introduction . 



a. Importance of Littoral Transport . If the coast is examined on 

 satellite imagery as shown in Figure 4-25, only its general characteristics 

 are visible. At this elevation, the shore consists of bright segments that 

 are straight or slightly curved. The brightness is evidence of sand, the most 

 common material along the shore. The straightness often is evidence of the 

 effects of sediment transport. 



4-55 



