Figure 9. Spring tides occurred between 18 and 25 July while neap tides 

 occurred between 27 July and 2 August. Figure 9 shows the effect of 

 breaker height on the water table. Precipitation on 14 July raised the 

 water table nearly 30 centimeters. This, along with the large breakers 

 of 15 July, resulted in an even higher ground water level. Analogous 

 conditions occurred on 4 and 5 August, when precipitation and an increase 

 in breaker height resulted in a rise of the ground water surface level. 

 The rapid rise of the ground water level with the onset of storm condi- 

 tions would seem to increase the erosion associated with a storm. 



Duncan (1964) found that the position of the water table with respect 

 to the beach surface was important in controlling erosion and deposition: 



"Swash water, upon transgressing over and above the inter- 

 section of the water table with the foreshore surface, rapidly 

 percolates into the sand. This reduction of water volume is 

 accompanied by a decrease in velocity as well as deposition of 

 sediment transported in that portion of swash which vanished 

 into the interstices of the beach sands. Hence, a dry beach 

 (low water table) facilitates deposition in the upper reaches 

 of the foreshore until slope equilibrium is gradually reached 

 and backwash velocities prevent further net accretion." 



5. Longshore Current Measurements . 



One of the initial problems in measuring longshore drift is defining 

 longshore drift or littoral drift velocity. There are three basic modes 

 of littoral transport (U.S. Army, Corps of Engineers, Coastal Engineering 

 Research Center, 1966) (Fig. 10) . Beach drift is "material which is moved 



A 



berm crest 



Stillwater line 



path of sand grains 



/ : sand grain 

 / \ movement outside 

 xrfthe surf zone 



direction of wave-induced 



— — , — < 



currents 



breaker zone 



Figure 10. Longshore or lateral movement of littoral 

 drift (U.S. Army, Corps of Engineers, 

 Coastal Engineering Research Center, 1966) 



29 



