Effects of sand addition on turbulent flow over an immobile gravel bed

[1] The factors controlling the complex interaction of a coarse streambed with flow and sediment are difficult to measure. However, planning for reservoir flushing or dam removal requires knowledge of these interactions. In both cases, impounded sediments are introduced to channel beds that have had fine sediment particles removed without replacement. The channel bed pore space interacts with the flow and provides storage for particles. In order to address the need for information on such systems, an adjustable-slope, recirculating laboratory flume was used to study the changes in flow and turbulence caused by sand added to an immobile gravel bed. Detailed measurements were made using an acoustic Doppler velocimeter that collected three velocity components at a rate of 200 Hz. Because of the rough nature of the bed, individual velocity profiles varied significantly; therefore, in order to determine general trends, the data were spatially averaged over six 10 × 20 cm planes parallel to the bed with the lowest plane about 2 cm below the maximum gravel elevation. The increasing elevation of sand relative to the gravel layer resulted in decreased bed shear stress, decreased Reynolds stress, increased relative turbulence intensity, and a near-bed shift toward sweep-dominated turbulence.