Date Awarded

1993

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Virginia Institute of Marine Science

Advisor

L. D. Wright

Abstract

Near-bottom suspended sediment concentrations and velocities were measured on the inner shelf off Duck, N.C. from late October to early November, 1991. This period embraced both fair and storm conditions. Four bottom roughness models are tested using field data together with a wave-current boundary layer model. Bottom roughness plays a significant role in calculations of sediment concentration profiles and current velocity profiles. The importance of each of the three parts in the roughness models (grain roughness, ripple roughness, and sediment motion roughness) vary depending on forcing conditions. A new bottom roughness model is established and tested. The calculated concentration and velocity profiles using the new roughness model compare well to the measured concentration and velocity profiles. The effects of stratification and sediment composition on vertical profiles of current velocity and mean sediment concentration were also investigated. Stratification and sediment composition can have opposing effects. Since natural sediments always consists of multiple grain size components, the equivalent settling velocity is not a constant in the water column. The effects of multiple grain sizes on sediment concentration are more important in fair weather than in storms. Conversely, stratification is most effective during storms. Stratification damps the vertical turbulent transport of mass and momentum (reduces the turbulent eddy viscosity) and causes an increased shear in the current velocity profile. The limit above which stratification must be considered is represented by the stratification stability parameter (z/L = 0.03, where L is the Monin-Obukhov length). The resuspension coefficient &\gamma\sb0& was calculated from these data using a wave-current boundary layer model in association with two roughness models. The relation between &\gamma\sb0& and excess shear stress reported by Drake and Cacchione (1989), Vincent et al (1991) (i.e. resuspension coefficient decreases when excess shear stress increases) was reproduced from using both the Grant and Madsen (1982) and the new roughness models. The decrease of &\gamma\sb0& with increasing excess shear stress in that relation appears to be partially caused by the over-estimate of the sediment motion (movable bed) roughness and under-estimate of the resuspension coefficient when using the Grant and Madsen (1982) roughness model. The neglect of stratification and multiple grain size effects in the calculation of &\gamma\sb0& may also be responsible for the decline in resuspension coefficient with increasing excess shear stress. When the fraction of silt and clay is used in calculating the &\gamma\sb0& values, the &\gamma\sb0& values show no trend of being a function of the shear stress.

DOI

https://dx.doi.org/doi:10.25773/v5-55tq-zj21

Rights

© The Author

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