effect will be a movement of the littoral material from the lips inward along each flank of 

 the channel, thus eroding the hps and shoaHng the inner channel. As the process continues, 

 the channel banks accrete toward tlie center of the channel, fed by material from the beach 

 on either side of the entrance. Unless tidal currents are strong enough to maintain an 

 opening against the forces tending to shoal the entrance, the channel will soon be closed. 

 Where the prevailing wave approach is oblique to the shoreline, sediment being transported 

 along the shore by littoral currents will be interrupted at the channel opening near the 

 updrift lip and that lip will soon begin to accrete. As the wave -induced longshore current 

 again begins to "feel" the shore downdrift of the channel mouth, it attempts to reacquire its 

 sediment load. As a result, tlie downdrift lip of the channel will erode at about the same rate 

 the updrift lip accretes, and the channel mouth will migrate in the downdrift direction. 



In each of these cases, the forces of nature are attempting to reestablish the littoral 

 balance that was present before the channel was excavated. The above account is an 

 oversimplified version of an extremely complex process and excludes consideration of the 

 effects of sandbar formation, eddy currents, and tidal channel meandering. 



The customary solution to entrance shoaHng is the construction of jetties along each 

 flank of the entrance channel from the hps of the mouth seaward beyond the breaker zone. 

 The structural features of the jetties must be such that the materials will not be washed 

 through or over the structure into the channel. A typical section of a sandtight 

 rubble-mound jetty is shown in Figure 14. If the littoral transport from one direction 

 predominates and the entrance is stabilized by jetties, accretion will occur along the updrift 

 shore and erosion along the downdrift shore. Mechanical bypassing of this httoral material 

 must be instituted to minimize the influence of the jetty structures on longshore transport. 

 Since littoral transport rates in excess of one million cubic yards annually have been 

 recorded along some reaches of the continental coastline, bypassing may be costly but 

 necessary for harbor maintenance. 



Longshore-type sediment movement can also occur inside a harbor where any segment of 

 the perimeter is left as a beach for wave absorption or recreational purposes. If such a beach 

 is near the entrance, residual wave action entering the harbor may cause longshore 

 movement of the beach material. The best means of avoiding this problem is to determine, 

 before harbor construction, the approach direction of the waves that will pass through the 

 entrance to the vicinity of the beach, and then to aline the beach normal to this direction. If 

 a revetted slope or bulkhead wall is to be located on either or both ends of the beach sector; 

 the beach must be recessed behind the general aHnement of the revetment or wall so that 

 the beach toe does not extend beyond the toe of the revetment or wall. Because the residual 

 wave action inside the harbor is usually diverging, the beach alinement should be made so 

 that the incoming wave will be normal to the beach at all points. 



Boat-wake generated waves can also cause sediment movement along beaches within a 

 harbor. This may be a minor problem that can be solved at low cost by using mechanical 

 equipment to periodically deepen the channel and restore the beach. If the beach existed 

 before the harbor was developed, and it was not intended for recreational use or esthetic 



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