Incipient resuspension of silt-clay deposits in oscillatory and unidirectional flows Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/r781wk41f

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  • Incipient motion of cohesive sediment and the role of flow parameters were investigated for separate and combined unidirectional and oscillatory flows. Sediment from Sturgeon Lake, mainly silt and clay with organic matter, was used as bed material In recirculating and wave flumes. Bench-type experiments indicated that when the submerged sediment was under no active hydraulic disturbances, it consolidated substantially. The removal of fine sediment and organic matter from the sediment surface caused greater consolidation of the remaining coarse sediment. The mechanism for incipient motion of cohesive sediment under the action of flowing water was found to be complex. The top surface of the fine-grained sediment exhibited a skin layer which governed the the incipient motion process. Formation of this layer was evidently caused by physicochemical bonding of sediment particles by organic matter and iron in the sediment. When induced shear stress exceeded that required to initiate erosion, the bonded particles started to peel off, leaving pit marks and causing the development of streak lines. Suspension of sediment particles was a part of this process. Incipient motion for unidirectional flow could be expressed in terms of the mean velocity and shear stress necessary to initiate erosion of the cohesive sediment surface. Incipient motion for oscillatory flow could be expressed in terms of maximum orbital velocity and maximum shear stress. The results of the two sets of experiments could be used to speculate on the combined effects of oscillatory and unidirectional flows. It was found for fine cohesive sediment that: (1) the principles governing incipient motion under unidirectional flow are applicable for oscillatory flow; (2) the effects of combined unidirectional and oscillatory flows can be estimated from separate effects of each flow; (3) when induced shear stress exceeds skin bonding strength, peeling off of the skin layer begins; (4) sediment consolidation has little effect on incipient motion; (5) incipient motion of the sediment bed occurs at lower shear stress and velocity under oscillatory flow than under unidirectional flow; (6) a bed with a rough surface is more susceptible than a smooth bed to incipient motion; and (7) bioturbation affects incipient motion of fine-grained sediment.
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