The effect of bed porosity on turbulent flow in gravel-bed rivers

This thesis presents the main findings from laboratory experiments that were specially designed to study the effect of gravel bed porosity on the surface flow characteristics. The thesis also contains five primary and two secondary research papers in which the results are discussed in detail. In order to study the effect of porosity on flow resistance and near-bed turbulence, a non-porous counterpart of a water-worked gravel-bed surface was produced using a casting technique. The casting technique was also used to study the influence of gravel grain orientation on bulk flow resistance. The accuracy of the casting technique was evaluated, and the results indicated that the cast replicated the original surface accurately. The vertical distribution of porosity in the water-worked gravel-bed was also investigated. Hydraulic measurements were performed over three different bed types: water-worked gravel-bed, cast-bed and rotated cast-bed (cast tiles were rotated 180 degrees). The rotated cast-bed imposed larger flow resistance on the flow, thus the grain orientation played an important role determining the flow resistance. Moreover, differences in flow resistance and turbulence characteristics between porous water-worked gravel-bed and non-porous cast-bed are discussed. The porous water-worked gravel-bed imposed slightly larger resistance on the flow than the non-porous cast-bed. The analysis of turbulence flow characteristics between water-worked gravel-bed and cast-bed revealed that a higher momentum flux occurred over the water-worked gravel-bed. Moreover, the form-induced stresses were higher over the cast-bed than over the gravel-bed. This was associated with a strong recirculation of the fluid over the cast-bed as fluid cannot infiltrate. In contrast, fluid can infiltrate into the subsurface of the water-worked gravel-bed, causing less recirculation in the water-sediment interface