reference to beach erosion are generally agreed to be: (1) they shift the 

 zone of wave attack on the beach and (2) they set up currents that may 

 assist or oppose the movement of the sand particles which compose the beacho 



Currents resulting from tidal action are generally noticeable along 

 the coasts only in the vicinity of bays^ estuaries,, or inlets. The rise 

 and fall of the tide produces a fl.ood and ebb of the current filling and 

 emptying the estuary. This current is usually weak but can reach sufficient 

 magnitude in tte vicinity of the estuaery to cause erosion by its cairn action o 

 Where the currents are too small to cause movement of the beach sand alone 

 they can be effective by •working in conjunction mth the vra.ves breaking on 

 the beach. Waves approaching the shore at an angle will produce a littoral 

 current substantially parallel to tte shore o 



Sand Moveme nt 



The effect of ivaves on the sand beaches is especially pronounced at 

 the breaker line^ where comparatively large quantities of sand are tbj:-o"wn 

 into suspensions An angularity in the direction of wave approach frequently 

 results in a littoral current being set up substantial!;/ parallel to the 

 shore . This wave generated littoral current carries the suspended sand 

 alongshore. 



The net effect of the action of waves and currents is to produce a 

 beach which may be accreting or eroding, may show cyclic or seasonal changes,, 

 and is seldom if ever completely st-able. It m.ight be pictured as a river 

 cf sand whose direction and velocity are determined by the cha.racter of the 

 forces impressed upon it in the form of winds ^ -waves, and cixrrents. 



It is well to remember that once a sand grain is put into suspension^ 

 the slightest movement of the surrounding water, will produce a corresponding 

 movenent of the sand particle. As long as the particle remains in contact 

 with the bottom, it will take currents of appreciable strength to move even 

 the fine sand. However, the almost Tmceasing wave action on the beaches con- 

 tinually keeps a substantial amount of sand in suspension. Woan the waves 

 approach normal to tte beachj the sand movement would be essentially an 

 oscillation back and forth of the sand particles with little progressive 

 movement parallel to the shi^re. Howeverj, if even a slight littoral current 

 generated by tides or wj.nds is present^ the alongshore movement of material 

 can be relatively great. 



To avoid confusion we must remember that littoral current is the move- 

 ment of the water mass while littoral drift is the movement of the solid 

 particles. The coast in the vicinity of Santa Barbara, California,, is com.- 

 sidered to have a fairly large rate of littoral drift^ the direction of 

 drift being from northwest to southeast. Here the average rate of drift is 

 about 300,000 dibic yards of sa.nd per year. A rate of littoral drift rang- 

 ing from 20j000 to 50,000 cubic yards per year is consMered relatively low. 

 Wind blowing over the beach can also be an effective agent in mo\d.ng the 

 sand and should always be considered when ras,king a study of a specific 

 locality. 



17 



