The presence of a row of circular cylinders along the centerline in a flume gives a significant drag to the flow (J. JSCE B1 71(4) I_1057 2015). The row can be seen macroscopically as a porous flat plate having both the permeability and roughness effects, and the flow over semiannular stripe roughness elements is the system obtained by removing only the permeability from the flow past the cylinders. In the present study, these flows have been studied comparatively by a two-dimensional numerical experiment to elucidate the impacts of the permeability and the roughness on the flow resistance. The drag on the row of cylinders is found to be roughly one order of magnitude higher than that over the roughness elements. In other words, permeability causes much more resistance to the flow than roughness. The importance of the former tends to be decreased with decreasing the number of cylinders (i.e., increasing the plate porosity). Visualization of the instantaneous flow fields reveals the presence of large scale vortices in both types of flows, and the strength of the vortices is significantly enhanced by the boundary permeability. This then promotes fluid mixing and momentum exchange across the whole flume width, and increases the resistance to the flow.

View full abstract

 Presently, species diversity is widely used as evaluations of the effect of river works. Thus, basic species distribution data and research results focused on the species diversity are increasing. However, there are remarkably few studies on relationships between river development and biomass of the aquatic organisms. The aim of this study is to develop the spatial biomass prediction model of benthic invertebrate in the water system of Saba River.
 Field surveys were conducted to understand the relation between hydraulic characteristics and biomass of benthic invertebrate, and we established GLM to predict the biomass using the hydraulic characteristics. The change amount of biomass of benthic invertebrate in mainstream of Saba River is predicted by using GLM and calculated hydraulic characteristics by fluid models. The result revealed that the total biomass of benthic invertebrate was reduced by the negative impact of water intake.

View full abstract

 Localized torrential rains yield large amounts of driftwood in rivers aggravate the flood events. For example, large driftwood blocks the river, which lead to a decreased discharge capacity and increased water levels. However, there is almost no detailed information about the pitching and yawing moment stability of driftwood. In the present study, three kinds of flume experiments (1. Drag force and Lift force measurement, 2. Pitching moment and Yawing measurement, 3. Driftwood accumulation experiment) were performed. We also examined the effect of the gravity center position on the directional stability of driftwood. Particularly, small increase in angle of attack caused a significant increase of the projected area of driftwood. The results revealed that the blocking probability of driftwood at bridge decks and backwater rise due to driftwood accumulation depend on the projected area of driftwood.

View full abstract