Boundary layer development in roughness layer at modeled armored gravel bed stream and the sand movement in the region

Norio Tanaka, Yuta Yoshizawa, Saqib Habib

Thursday 2 july 2015

14:50 - 15:05h at Amazon (level 1)

Themes: (T) Hydro-environment, (ST) Ecohydraulics and ecohydrology

Parallel session: 12B. Engineering - River

Sediment supply to downstream of a dam by dredging sediment in a reservoir and placing sediment in a downstream flood basin of the river is recently tested in Japan for solving a riverbed environmental problem caused by armoring. Although the three-dimensional turbulence structures at the armored bed are assumed to greatly affect the invertebrate dynamics, not much studies have been conducted especially for armored bed and the relative water depth to gravel or stone size is small. Experiments were conducted in a flume with14 m in length and 0.5 m in width for clarifying the turbulence structure and shear stress characteristics around and inside the armored layer, sand motion in the armored layer, and effect of boundary layer development on the velocity distribution. For expressing the sand movement in armored layer, three size gravel were selected. Firstly, 1.6 cm average diameter gravel with a thickness of 3.5 cm (Group-2) was placed on bed, and secondly, 10 cm stone with a density of 68 stones/m2 (Case1) and 43 (Case2) (Group-1) was put on the Group-2. The relative water depth and stone height was set as 3 and 4 for Case1 and Case2, respectively. Particle image velocimetry was used for obtaining the turbulence structure in and around the bed layer. Velocity distribution and Reynolds stress were obtained by time and spatial-averaging the visualized velocity field in totally 5 sections and stream-wise spatial turbulence structures were obtained. With the small relative water depths condition, the form-induced sublayer is thick and log-law has been found not to be applied. The roughness layer width becomes thin with the development of boundary layer. Compared with the Case 1, the sand movement in the roughness layer is very rapid in Case2. This is due to the difference of the flow structure in the roughness layer.