Evaluating the influence of boulder arrangement on flow resistance in gravel-bed channels

Gravel bed flow resistance is affected by the shape and size of the roughness elements and their arrangement on the channel bed surface (spacing between elements, direction with respect to flow streamlines, and protrusion of the elements from the channel bed). Previous studies demonstrated that the flow resistance of open channel flows can be obtained by integrating the power velocity profile. This paper aims to study flow resistance in gravel-bed channels with different concentrations of boulders having staggered arrangements. At first, the equation relating Γ coefficient of the power velocity profile, and the Froude number was calibrated using measurements performed in a flume covered by hemispheric roughness elements for partially submerged and completely submerged hydraulic conditions. The roughness elements were evenly spaced (staggered) and arranged using three different concentrations of 9, 25, and 49%. Moreover, the relationship between Γ, slope, and the Froude number, calibrated using literature measurements performed with the same experimental setup but with a square arrangement, was tested for the measurements obtained with the staggered arrangement. The results showed that i) the Darcy-Weisbach friction factor can be accurately estimated by the proposed flow resistance equation, ii) the differences in flow resistance behavior between the two different investigated arrangements (staggered, square) occur only for the partially submerged hydraulic condition, and iii) for the staggered arrangement, skimming flow is reached for lower element concentrations as compared to the square one.