NEW APPROACH FOR MODELING RIVER BED SCOUR

Mohammed Gamal Abd - Alla

Abstract


The discharge increase in rivers during floods induces short term bed erosion, thiserosionis an important constrain in river structuresdesign. Different methodologies are used to estimate the maximum scour associated to a flood.The simplest ones are based on empiricalrelations.On the otherhandmore complex computations include complete sediment transport simulations. The numerical models used inriver morphodynamics usually implement the Saint Venant-Exner equations system, different techniques exists for the coupling betweensediments and hydrodynamics, but most of the models consider thepassive tracer approach. This means that the sediment is considered as anon inertial tracer, neglecting the momentum transported by sediment and assuming that the transported solids velocity is thewatervelocity. Under these conditions, the sediment wave travels at the water wave velocity, as its known, the water wave velocity includesconvective velocity and gravity wave celerity. The real sediment acceleration is carried out by the momentum transfer betweenfluid andsolids through the drag forces, andthe water velocity represents an asymptotic limit for the sediment. Biphasic models are extensively usedto model hyper concentrated flowssuchas Fraccarollo,Capart 2002, Cao et al. 2004, Zechand Spinewine 2002models, they consider thesediment velocity as an independent variable and corresponding momentum transfer mechanisms are defined. In this work anewmixedapproach is used, the momentum transfer is neglected from the water point of view but considered in the sediment velocity. Finally, thesediment wave travels with its own velocity creating a delay between water discharge peak and sediment discharge peak. This non-equilibrium sediment transport results in erosion in rising part of the hydrograph and deposition in the diminishing part, asis usual influvial processes.

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