Abstract
A drop structure installed in a river channel alters the riverbed configuration around it and brings both advantage and disadvantage to the aquatic life. This study investigated the effect of a drop structure on river environment through field measurements of topography and flow structure at several study sites. At first, we clarified that a drop structure create a riffle, a pool, and a run around it, but the pool is not created in the area where the riverbed degradation occurs. Additionally, the riffle bed in such degraded area lacks pebbles and small cobbles. The run upstream of a drop structure has two kinds of riverbed configuration in it. In a convex side, very coarse pebbles or small cobbles and sand covers the riverbed, while the uniform pebbles covers the concave side. So loose gravel bed is created in the concave side of a run. In a pool, vertical inflow from a drop structure causes the turbulent flow. When the discharge is small, fast flow appears near the surface and the water in the middle and bottom layer is stagnated. When the discharge is large, fast flow intrudes into the bottom of a pool and circulating flow is created. The topographic characteristics mentioned above are often changed by size distribution of riverbed material, alignment of a river channel and riverbed profile, as well as the specification of a drop structure.
