Ch. 15] 



FACTORS INFLUENCING THE PROBLEM 



283 



pendent on adjacent areas. Thus, for example, we may consider an 

 island as a physiographic unit; it is probable that the entire Atlantic 

 coast of New Jersey from Sandy Hook to Cape May is a unit; and on 

 a smaller scale, Monterey and Emerald bays (Fig. 6) in California are 

 units. The physiographic unit defines the boundaries of the area to be 

 studied when considering shore problems. 



Fig. 6. Emerald Bay, California. A physiographic unit. 



The second basic concept is that of material-energy balance within 

 the physiographic unit. Material balance concedes equality between 

 the quantity of material supplied to and that lost from a shore area. 

 Obviously a time factor must be introduced which requires that the 

 volume of material considered be thought of as a rate of supply or loss. 

 We are concerned chiefly with material balances pertaining to days, 

 seasons, years, and storm durations. Since material is moved into and 

 away from any given beach or shore section only by the application of 

 forces, we must think also of rates of doing work, or energy. 



Let us examine briefly the factors involved in material balance. 

 If we assume some hypothetical shore stretch and consider the material 

 situation therein, we find that formulation of a complete balance re- 

 quires knowledge of (1) the rates of littoral drift to and from the 

 area; (2) the rates of supply and loss from and to the adjacent sea 

 bottom; (3) the rates of supply and loss from and to the adjacent 



