Storm/Fair- Weather Sediment Transport 



Several researchers (Hayes 1967a,c; Murray 1970; Morton 1981; Green 

 at al. 1988; Wright et al. 1991) have documented the differences in cross- 

 shore inner shelf mechanisms and resulting sediment transport during fair- 

 weather and storm conditions (refer to "Significant (Storm) Event 

 References" in Appendix B for additional references concerning this 

 topic). 



Green at al. (1988) and Wright et al. (1991) in Mid-Atlantic Bight ex- 

 periments measured suspended sediment movement, wave heights, and 

 mean current flows between the -7-m and -17-m depth contours at Duck, 

 NC, in 1985 and 1987 and at Sandbridge,VA, in 1988. The purpose of 

 this work was to identify modes, directions, rates, and causes of shore- 

 normal sediment flux over the inner shelf in response to different energy 

 conditions (Table 3). Field measurements were compared to energetics 

 mathematical models of sand transport (Bowen 1980; Bailard 1981; Guza 

 and Thornton 1985 a,b; Roelvink and Stive 1989) who compared the con- 

 tributions of mean and oscillatory flows, and separated cross-shore compo- 

 nents of immersed weight sediment transport into bed load and suspended 

 load. 



Table 3 



Summary of Environmental Conditions at Duck, North Carolina, for 



Different Events (after Wright et al. (1991)) 



Parameter 



Summer 

 Fair Weather 

 July, 1987 



Post-Hurrlcane 

 Fair Weather 

 August, 1991 



Winter 



Swell-Dominated 

 January, 1988 



Extra-Tropical 

 Storm 

 October, 1991 



Bed roughness 



Large ripples, 

 biogenic activity 



Ripples on 

 mounds and holes 



Small ripples, 

 irregular 



Highly mobile 

 plane bed 



Depth of 

 instrumentation 



8m 



8m 



7m 



13m 



Current speed 



9.0-16.5 cm/sec 



10.6-13.0 cm/sec 



4.0-13.6 cm/sec 



2.0-49.5 cm/sec 



Wave height 



0.35-0.40 m 



0.29-0.40 m 



0.9-1 .4 m 



1 .0 - > 4.0 m 



Wave period 



8.7-9.0 sec 



7.0 sec 



9.7-12.9 sec 



9.0-14.0 sec 



Wave/current 

 angle 



45°-75° 



26°-34° 



29°-66° 



36°-85° 



Fair-weather sediment transport 



In documenting fair-weather processes, Green et al. (1988) and Wright 

 et al. (1991) examined data collected at the -8-m and -17-m depths during 

 two data collection periods at Duck (1985 and 1987). Green et al. (1988) 

 and Wright et al. (1991) found that although tides and oscillatory wave 

 motion strongly influence both onshore and offshore sediment transport 



Chapter 3 Evidence of Cross-Shore Sediment Transport 



35 



