AN ELEMENTARY DISCUSSION OF TIDES, CURRENTS, 

 AND WAVE ACTION IN BEACH EROSION 



Tides and Currents 



The subject of the tides has engaged the attention of mathematicians 

 and engineers for several centuries « From the viewpoint of beach erosion 

 our principal interest in tides is due to the fact that they shift the 

 zone of wave attack on the beach and may set up currents that affect the 

 movement of the sand particles which compose the beacho 



A connection between the moon and the tide was first recognized 

 about 350 Bo Co However j the explanation of the cause of this relation 

 was not forthcoming until Sir Isaac Newton published his "Principia" in 

 1687, wherein was set forth a statement of the law of gravitational pullo 



It is now generally recognized that the tide around the globe re- 

 sults from the gravitational pulls of the moon and the sun, with the 

 moon being the dominant factor due to its closer proximity to the eartho 

 In factj its tide producing force is some 2-1/4 times that of the suno 



On first consideration it might seem that we could expect only one 

 high and one low water each lnonar day of 24, hours and 50 minutes o The 

 mechanics of the gravitational system^ however, is such that in most 

 areas two highs and two lows are obtained each dayo This is due to the 

 fact that the effect of the gravitational prills of the moon and sun are 

 felt Qtt both sides of the eartho It is readily aaen that the water 

 mass on the side of the earth facing the moon will tend to rise toward 

 the moono On the other side, the moon tends to draw the earth mass 

 from the water mass due to the fact that the center of gravity of the 

 earth mass is in this case closer to the moon than the center of the 

 water masso The effect of tending to produce a high tide is the same 

 however Ihus we find that, generally speaking^ the tides around our 

 coast for the most part show two highs and two lows each lunar dayo 



Other factors at times enter the pictirre and serve to bring 

 variations in the tidal picture from place to place and from day to 

 day. Thus we find that the movement of the moon from the northern 

 hemisphere to the southern hemisphere and back again each 27|- days gives 

 rise to a diurnal inequality in the tide, that is, the range of the two 

 tides each day is appreciably differento This diurnal inequality is 

 particularly noticeable on our Pacific Coasto It is of little signifi- 

 cance on the Atlantic Coasto 



Another variable feature is the phase of tte moono At times of 

 full moon and new moon the moon and sun work together and give us high 

 tides known as spring tides » At times of the 1st and 3d quarter of the 

 moon, the moon and sun are working agains<t each other and we find that 

 we have low or neap tides » 



