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

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Application Note: Sediment Transport
 
Cohesive Sediment: Understanding the mechanisms involved in the transport and fate of cohesive sediment in natural channel systems along with effects on aquatic organisms remains an open case in water-related engineering disciplines.  The main challenge is that cohesive sediment dynamics are controlled not only by physical forces (e.g., inertia, buoyancy, drag, lift, friction) but also by electrochemical forces.  A complete identification of the properties of cohesive sediments typically involves twenty-four parameters (Commission of the European Community) and explains why the few studies referring to the transport and fate of cohesive sediment are site-specific rather than have a more fundamental nature. 
   
Bank erosion: Fluvial erosion of cohesive banks commences when the side-wall shear stress applied by the flow to the bank surface exceeds a surrogate measure of cohesion, known as the critical erosional strength.  Union Flat Creek, a gravel bed stream with cohesive banks in the Palouse region of WA, is characterized by a sequence of cross-sectional irregularities such as channel expansions and constrictions.  These features are believed to create an additional turbulent stress toward the banks of the stream caused by secondary currents, thus leading to excessive banks erosion.  In order to quantitatively understand and predict the banks shear stress forces and erosion rates, a laboratory study was conducted to determine the critical stress for fluvial erosion of cohesive bank sediments.  The method accounts for determining the fluid stresses present on banks by turbulence and secondary currents due to the cross-sectional irregularities.  This improved our understanding of the turbulence characteristics found in a gradual expansion streams.  The detailed measurements of the mean and turbulent flow quantities reveal the presence of some very interesting flow features that, to the best of our knowledge, have not been reported before in natural flows.  It is found that at the channel expansion, turbulence intensity increases with depth away from the channel bed, which suggests that turbulence momentum is not transferred from the core of the flow to the bed, but momentum is transferred from the bed to the free surface.  Furthermore, the analysis of these measurements shows that the presence of the secondary currents increases the magnitude of the side wall shear stress.  The side wall shear stress values are many times higher in magnitude than the corresponding stress for uniform flow conditions.  The results also show that the local side wall shear stress values are almost 2-3 times greater than the depth-averaged value.  It is suggested that use of the average values of the fluid shear stress may be a good approximation for simple channel geometries but not for natural channels characterized by width expansions and constrictions.