Thanos Papanicolaou
IIHR - Hydroscience & Engineering, The University of Iowa

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Application Note: Sediment Transport
 
Noncohesive Sediment: Understanding the mechanisms involved in the transport and fate of noncohesive soils in natural channel systems remains an open case in water-related engineering disciplines.  The main challenge in studying noncohesive sediments is the complex character of the bed geometry which governs the velocity, as well as the turbulence structure of the flow which in turn, control the sediment-carrying capacity of the flow.  Bed geometry is controlled by stochastic processes and subject to drastic changes due to changes in the flow condition.  Bedforms, clusters, step and pools are few examples of such complicated bed geometry.  In addition, sediment may consist of a wide range of grain sizes.
   
Incipient of motion: An incipient motion criterion for movement of sediment as bedload in open channels is introduced based upon an operational definition of the probability of entrainment.  The definition accounts for the stochastic nature of sediment movement in turbulent flow and admits a straightforward physical interpretation. The balance of moments of the instantaneous hydrodynamic forces acting on a sphere positioned in a 3-D particle arrangement was considered around the sphere's pivoting point.  The magnitude of the minimum moment required for the commencement of the sediment motion was calculated by assuming that the probability of sediment entrainment is equal to the probability of the occurrence of these flow events that are capable of dislodging the sphere.  This probability concept leads to an objective, quantitative and consistent approach to the incipient condition for sediment motion.  The approach was explored experimentally in a laboratory investigation of the effect of sediment availability on bed behavior and flow conditions at the incipient condition.  It was found that an incipient criterion based upon the probability concept can yield relatively active sediment beds where the level of activity is strongly dependent upon particles availability.