storms. Dune deposits require longer to form - months or years - because 

 wind transport usually moves material at a lesser rate than wave transport. 

 I£ the immediate beach area is the control volume of interest, and budget 

 calculations are made based on data taken just after a severe storm, allow- 

 ance should be made in budget calculations for sand that will be stored in 

 berms through natural wave action. (See Table 4-5.) 



4.734 Offshore Slopes . The offshore area is potentially an important 

 sink for littoral material. Transport to the offshore is favored by: 

 storm waves which stir up sand, particularly when onshore winds create 



a seaward return flow; turbulent mixing along the sediment concentration 

 gradient which exists between the sediment-water mixture of the surf zone 

 and the clear water offshore; and the slight offshore component of gravity 

 which acts on both the individual sediment particles and on the sediment- 

 water mixture. 



It is often assumed that the sediment sorting toss that commonly 

 reduces the volume of newly placed beach fill is lost to the offshore 

 slopes. (Corps of Engineers, Wilmington District, 1970; and Watts, 

 1956.) A major loss to the offshore zone occurs where spits build into 

 deep water in the longshore direction. Sandy Hook, New Jersey, is an 

 example. (See Figure 4-47.) It has been suggested (Bruun and Gerritsen, 

 1959) that ebb flows from inlets may sometimes cause a loss of sand by 

 jetting sediment seaward into the offshore zone. 



The calculation of quantities lost to the offshore zone is difficult, 

 since it requires extensive, accurate, and costly surveys. Some data on 

 offshore changes can be obtained by studies of sand level changes on rods 

 imbedded in the sea floor (Inman and Rusnak, 1956), but without extending 

 the survey beyond the boundary of the moving sand bed, it is difficult 

 to determine net changes. 



4.735 Submarine Canyons . Probably the most frequently mentioned sinks 

 for littoral materials are submarine canyons. Shepard (1963) and Shepard 

 and Dill (1966) provide extensive description and discussion of the origin 

 of submarine canyons. The relative importance of submarine canyons in 

 sediment budgets is still largely unknown. 



Of 93 canyons tabulated by Shepard and Dill (1966), 34 appear to be 

 receiving sediment from the coast, either by longshore transport or by 

 transport from river mouths. Submarine canyons are thought to be espe- 

 cially important as sinks off southern California. Herron and Harris 

 (1966, p. 654) suggest that Mugu Canyon, California, traps about 1 mil- 

 lion cubic yards per year of the local littoral drift. 



The exact mechanism of transport into these canyons is not clear, 

 even for the La Jolla Canyon (California) which is stated to be the most 

 extensively studied submarine feature in the world. (Shepard and Buff- 

 ington, 1968.) Once inside the canyons, the sediment travels down the 

 floors of the heads of the canyons, and is permanently lost to the litto- 

 ral zone. 



4-127 



