downwelling (three-layer flow) but is not returned onshore. Niedoroda, 

 Swift, and Hopkins (1985) also supported the loss of sediment from the 

 beach-inner shelf system only during storms. However, they state that 

 some of the sand transferred from beach to inner shelf during storms will 

 return. 



Luternauer and Pilkey (1967) employ the use of minerals (i.e. phos- 

 phorite) at the North Carolina coast at Onslow Bay to document the inter- 

 change of sediment between the beach and the inner shelf. They found 

 that the shelf is an important source of beach sediments. This suggests 

 that the shelf is a major contributor of phosphorite to landward beaches. 

 Another interesting finding of this study was that a small amount of long- 

 shore transport occurs on the shelf as phosphorite content is limited to 

 Onslow Bay and does not spill over to other embayments. This indicates 

 that phosphorite is a useful tool for determining sediment provenance and 

 transportation. 



Thus, several studies support the interchange of sediment between the 

 beach and the inner shelf. However, there are examples in the literature 

 where no sediment interchange occurs. For instance, Meisburger (1989) 

 investigated the interchange of sediment between the beach and Gilbert 

 Shoal, a nearshore linear shoal off Florida. He determined that the major 

 sediment source to the beach is from littoral processes, while a lesser 

 amount of sediment comes from the shoal. However, the shoal and sur- 

 rounding seafloor receive little, if any, sediment from the beach or nearby 

 St. Lucie Inlet. The shoal obtains sediment from the nearby shelf floor 

 and from in situ shell production. 



Depth of inner shelf sediment transport 



When considering sediment interchange between the shelf and the 

 beach, the next logical question is to what depth is sediment transported 

 and/or affected on the continental shelf. This topic was previously consid- 

 ered in the "Depth of Closure" section of Chapter 2 as discussed by 

 Draper (1967); Harlett (1972); Komar, Neudeck, and Kulm (1972); Smith 

 and Hopkins (1972); Sternberg and Larsen (1976); Channon and Hamilton 

 (1976); Sternberg and McManus (1972); Gadd, LaVelle, and Swift (1978); 

 Vincent, Swift, and Hillard (1981); Larsen et al. (1981); and Wright et al. 

 (1986). In addition, Grant and Madsen (1979a,b, 1986), Madsen and 

 Grant (1976), Larsen et al. (1981), and Niedoroda et al. (1982) compute 

 bed load transport at depths. Evidence of sediment transport at consider- 

 able depths (greater than -40 m) follows. 



Direct current measurements on the central and outer continental shelf 

 of Washington and Oregon by Smith and Hopkins (1972) at the -50-m and 

 -80-m water depths showed that significant sediment transport in an off- 

 shore direction, most importantly by suspended load, occurs only during 

 storms. A storm with current speeds of up to 60 cm/sec transports on the 

 order of 6 m^/hr/m of sediment of shelf length, while a 70-cm/sec storm 



Chapter 3 Evidence of Cross-Shore Sediment Transport 



33 



