Distance Seaward (km) 



Figure 8. Cross-shelf profile of the inner shelf off Duck, North Carolina (after Wright 

 et al. (in press)) 



Smith and Hopkins (1972) determined in their study of Columbia River 

 sediments that an average particle on the shelf moves about 40 km/year in 

 a longshore direction and 7 km/year in an offshore direction. The major- 

 ity of this transport occurs only during a few storms each winter. Esti- 

 mates of sediment transport indicate that the sand fraction moves much 

 more slowly as bed load than the silt fraction as suspended load. 



Seasonal effects on inner shelf cross-shore sediment transport 



Seasonal cross-shore transport of sediment along the southern Califor- 

 nia coast has been documented by Shepard (1950), Shepard and Inman 

 (1950), Inman (1953), Inman and Rusnak (1956), and Aubrey (1979). Dur- 

 ing summer, the subaerial beach accretes, while the offshore loses sedi- 

 ment. In winter, the subaerial beach erodes, while the offshore accretes. 

 These changes are a result of variation in wave frequency and directional 

 properties (e.g. Pawka et al. (1976)). Small-amplitude, long-period waves 

 dominate in summer, while higher-energy, high-frequency storm waves 

 dominate in winter. 



Aubrey (1979) examines temporal changes in beach/inner shelf profile 

 configuration using eigenfunction analysis of profile data for southern 

 California profiles for a 5-year period. Two seasonal pivotal points sepa- 

 rating eroding and accreting regions are documented at -2 m to -3 m, and 

 at -6 m. A simple model of depth-dependent seasonal sand movement 



Chapter 3 Evidence of Cross-Shore Sediment Transport 



31 



