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Hydraulic roughness of armoured gravel beds : the role of grain protrusion Martín, Violeta


A new theoretical analysis was developed that accounts for variable grain protrusion within self-formed, stable gravel armour layers. A key feature of the analysis is accounting for the variation in drag coefficient, drag force and critical dimensionless shear stress with grain protrusion above the virtual bed, which is defined here as the elevation at which the extrapolated logarithmic velocity profile becomes equal to zero. The central hypothesis is that self-formed stable armour layers develop through adjustment of grain protrusion such that all grains are at the threshold of motion, at least in a statistical sense. This represents the limiting case of equal mobility. Testing of the analysis using published flume data shows good agreement between observed and predicted roughness height, mean velocity and flow depth. Experimental work on simulating gravel-bed armouring was carried out to obtain more data and test the assumptions underlying the numerical model. Velocity profiles across and along the flume were measured with an acoustic Doppler velocimeter (ADV). To determine the reliability of ADV measurements in turbulent flows over rough boundaries, a thorough data analysis was undertaken. Shear stresses obtained from the force balance (pgYSj), from the velocity profiles, or from the Reynolds stress measurements were compared and showed a reasonable agreement. A unique study on individual grain protrusion was carried out, in which the armoured beds were scanned, digital elevation models (DEM) were developed, and then combined with photographs to obtain the information on protrusions. These measured protrusions are in good agreement with those calculated in the numerical mod

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