32 



shows that during initial winter storms, sand is eroded from both the fore- 

 shore and from depths of -6 m to -10 m and is deposited at the -2-m to 

 -6-m water depth. During less energetic periods, sediment migrates both 

 onshore (to the beachface) as well as offshore (to a depth of -10 m) from 

 its winter site of deposition (-2 m to -6 m). This depth-dependent motion 

 contradicts the single pivotal-point model previously suggested for near- 

 shore seasonal cross-shore sediment motion and emphasizes the complex- 

 ity of nearshore sediment transport. A sediment budget for seasonal 

 cross-shore transport, based on the dual pivotal point model, consists of 

 exchanges of 85 m^/m at the -3-m pivotal point, and 15 m^/m at the -6-m 

 pivotal point. On a longer (5-year) time scale, beaches showed no erosion 

 or accretion, suggesting that the limited coastal region is stable over this 

 time period. 



Beach-Inner Shelf Sediment Exchange/Losses 



Now that evidence has been presented concerning the onshore and off- 

 shore components of cross-shore sediment transport, the actual exchange 

 of sediment between the inner shelf and the beach is considered. Boyd 

 (1981) emphasized that cross-shore sediment exchange represents a major 

 contribution to the inner shelf sediment budget. 



Studies by Pearson and Riggs (1981) extensively documented the ex- 

 change of sediment between the beach and the inner shelf at Wrightsville 

 Beach, North Carolina. It is this study which has accentuated the impor- 

 tance of the permanent loss of sediment from the beach-inner shelf sys- 

 tem. Two findings associated with this study are important. First, 

 Pearson and Riggs (1981) observed the offshore transport of replenish- 

 ment sand from Wrightsville Beach to a depth of -16.6 m. This is based 

 on the presence of beach nourishment sand (fine to coarse-grained gray to 

 black sand with oyster shells) which is easily distinguishable from North 

 Carolina continental shelf sands, which are brown in color. This suggests 

 that the depth of closure at Wrightsville Beach is at least -16.6 m. 



Secondly, Pearson and Riggs (1981) state that periodic renourishment 

 totalling 7,300,000 cu m of material placed since 1939 (which would 

 cover a 23.3-km'^ area with a 14.6-cm layer of sediment) is being effec- 

 tively and permanently removed from the nearshore system. This renour- 

 ishment sand requirement has not decreased over time, indicating that the 

 profile is not establishing an equilibrium profile. Pilkey et al. (1993) con- 

 tend that if the concept of the equilibrium profile were valid, then the vol- 

 ume of sand needed to nourish the profile should decrease over the years 

 as it accumulates above closure depth on the inner shelf. 



In studies of Hurricanes Carla and Allen, and tropical storm Delia on 

 the Texas shelf, Nummedal and Snedden (1987) document the cross-shore 

 exchange of sediment as a great loss of sediment from the beach-inner 

 shelf. They found that sand is moved offshore during storms due to 



Chapter 3 Evidence of Cross-Shore Sediment Transport 



