often carry this information.) Tidal currents are particularly important in 

 transporting sand at entrances to harbors, bays, and estuaries. 



Currents induced by storm surges (Murray, 1970) are less well known 

 because of the difficulty in measuring them, but their effects are undoubtedly 

 significant. 



The change in vater level caused by tides and surges is a significant 

 factor in sediment transport since, with a higher water level, waves can then 

 attack a greater range of elevations on the beach profile (see Fig. 1-8.). 



d. Winds . Winds act directly by blowing sand off the beaches {deflation) 

 and by depositing sand in dunes (Savage and Woodhouse, 1968). Deflation 

 usually removes the finer material, leaving behind coarser sediment and shell 

 fragments. Sand blown seaward from the beach usually falls into the surf 

 zone; thus it is not lost, but is introduced into the littoral transport 

 system. Sand blown landward from the beach may form dunes, add to existing 

 dunes, or be deposited in lagoons behind barrier islands. 



For dunes to form, a significant quantity of sand must be available for 

 transport by wind, as must features that act to trap the moving sand. 

 Topographic irregularities, the dunes themselves, and vegetation are the 

 principal features that trap sand. 



The most important dunes in littoral processes are fovedunes, the line of 

 dunes immediately landward of the beach. They usually form because beach 

 grasses growing just landward of the beach will trap sand blown landward off 

 the beach. Foredunes act as a barrier to prevent waves and high water from 

 moving inland and provide a reservoir of sand to replenish the nearshore 

 regime during severe shore erosion. 



The effect of winds in producing currents on the \«ater surface is vvell 

 documented, both in the laboratory and in the field (van Dorn, 1953, Keulegan, 

 1951; and Bretschneider, 1967). These surface currents drift in the direction 

 of the wind at a speed equal to 2 to 3 percent of the windspeed. In hurri- 

 canes, winds generate surface currents of 0.6 to 2.4 meters (2 to 8 feet) per 

 second. Such wind-induced surface currents tovard the shore cause significant 

 bottom return flows vdiich may transport sediment seaward; similarly, strong 

 offshore winds can result in an offshore surface current and an onshore bottom 

 current which can aid in transporting sediment landward. 



e. 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 geologic factors 

 are not usually active processes affecting coastal engineering. 



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 examined 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 relative rise in sea 

 level with respect to local land level.) 



4-5 



