130 



Water 



Just as important as the diurnal and an- 

 nual cycles is the biweekly cycle of tides: 

 spring (when the earth, moon, and sun are 

 in line, moon either new or full) and neap 

 (when the moon is at first or last quarter). 

 One of the biological effects of the biweekly 

 cycle was pointed out in connection with 

 the egg-laying habits of grunion. A geo- 

 logical effect is that of erosion of the upper 

 part of the beaches during spring tides and 

 deposition during neap tides. During spring 

 tides the sea reaches higher on the beach 

 than at other times (Fig. 117), permitting 

 erosion of that zone; when the tide falls or 

 rises, sea level moves through the midtide 

 zone so quickly that little erosion is accom- 

 plished. In contrast, during neap tides the 

 tide range is small and sea level remains 

 most of the time near midtide. As a result, 

 during spring tides the wave energy is carried 

 at high tide across most of the beach, per- 



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PERCENTAGE 

 PACIFIC STANDARD TIME TIME 



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mitting concentrated erosion high on the 

 beach. Some of the eroded sand may be 

 deposited atop the berm, but most of it is 

 usually carried seaward to be deposited at 

 or below mean tide (LaFond and Rao, 1954). 

 During neap tides the situation is reversed 

 with most of the erosion near or below mid- 

 tide and most of the sand carried landward 

 to build up the upper foreshore. For a 

 whole year, of course, the histogram of sea 

 level position approaches a normal frequency 

 curve with the mode near mean sea level. 



Other effects on the beach are produced by 

 seepage of water into and out of the sand. 

 As the tide rises, water enters the sand and 

 flows landward; however, before it can sat- 

 urate the beach and form a horizontal water 

 table the tide falls and the water on the sea- 

 ward side of the beach escapes from the 

 sand, lowering the water table there. Thus 

 the rise and fall of the tide against the beach 

 produces a wave of tidal period that moves 

 landward through the sand (Fig. 118) but 

 lags in phase behind the tide and diminishes 

 in amplitude in a landward direction (Emery 

 and Foster, 1948). The escape of water 

 from the beach during low tide parallels the 

 lesser inward and outward movement pro- 

 duced as a continuation at depth of the 

 orbital movement of water particles under 

 passing waves (Putnam, 1949). Both kinds 

 of water movement lead to some elutriation 

 and removal of fine silt and clay from the 

 sand, thus tending to preserve the well- 

 sorted character of beach sands. In some 

 areas where heavy pumping of water wells 

 has greatly lowered the water table (Poland, 

 Piper, and others, 1956), some of the sea 

 water in the beach probably drains away 

 landward. In other areas a landward de- 

 crease in salinity of interstitial water indi- 

 cates a seaward loss of fresh water at low 

 tides. 



Figure 1 17. Tide curves for days of typical symmetrical 

 spring tide near equinox, of an asymmetrical neap tide, 

 and of an asymmetrical spring tide near solstice. Histo- 

 grams show percentage of day that sea level stayed within 

 each 1-foot (one-third meter) interval above or below 

 mean lower low water; the difference in modal positions 

 of sea level during spring and neap tides produces ero- 

 sion at different parts of the beach. 



Tidal Currents 



In the southern California region the tide 

 wave moves from southeast to northwest at 

 such a rate that high tide reaches Point 

 Conception about a half-hour after passing 

 San Diego. Associated with the tide are 



