1967; Keulegan, 1951; and van Dom, 1953.) These surface currents drift 

 in the direction of the wind at a speed equal to 2 to 3 percent of the 

 wind speed. In hurricanes, winds generate surface currents of 2 to 8 

 feet per second. Such wind-induced surface currents toward the shore 

 cause significant bottom return flows which may transport sediment sea- 

 ward; similarly, strong offshore winds can result in an offshore surface 

 current, and an onshore bottom current which can aid in transporting 

 sediment landward. 



4.125 Geologic Factors . The geology of a coastal region affects the 

 supply of sediment on the beaches and the total coastal morphology. Thus, 

 geology determines the initial conditions for littoral processes, but geo- 

 logic factors are not usually active processes affecting coastal engineer- 

 ing. 



One exception is the rate of change of sea level with respect to 

 land which may be great enough to influence design, and should be exam- 

 ined if project life is 50 years or more. On U.S. coasts, typical rates 

 of sea level rise average about 1 to 2 millimeters per year, but changes 

 range from -13 to +9 millimeters per year. (Hicks, 1972.) (Plus means 

 a rise in sea level with respect to local land level.) 



4.126 Other Factors . Other principal factors affecting littoral processes 

 are the works of man and activities of organisms native to the particular 

 littoral zone. In engineering design, the effects on littoral processes 



of construction activities, the resulting structures, and structure main- 

 tenance must be considered. This consideration is particularly necessary 

 for a project that may alter the sand budget of the area, such as jetty 

 or groin construction. In addition biological activity may be important 

 in producing carbonate sands, in reef development, or (through vegetation) 

 in trapping sand on dunes. 



4.13 CHANGES IN THE LITTORAL ZONE 



Because most of the wave energy is dissipated in the littoral zone, 

 this zone is where beach changes are most rapid. These changes may be 

 short-term due to seasonal changes in wave conditions and to occurrence 

 of intermittent storms separated by intervals of low waves, or long-term 

 due to an overall imbalance between the added and eroded sand. Short-term 

 changes are apparent in the temporary redistribution of sand across the 

 profile (Fig. 4-1); long-term changes are apparent in the more rtearly per- 

 manent shift of the shoreline. (See Figures 4-3, 4-4, and 4-5.) 



Maximum seasonal or storm- induced changes across a profile, such as 

 those shown in Figure 4-1, are typically on the order of a few feet verti- 

 cally and from 10 to 100 feet horizontally. (See Table 4-1.) Only during 

 extreme storms, or where the available sand supply is restricted, do un- 

 usual changes occur over a short period. 



Typical seasonal changes on southern California beaches are shown in 

 Table 4-1. (Shepard, 1950.) These data show greater changes on the beach 



4-6 



