An experimental study on the bedrock re-exposure by sandbar formation and channel curvature.

Takuya Inoue, Tomoo Ushiyama, Kazutake Asahi, Mitsuaki Yonemoto

Tuesday 30 june 2015

17:30 - 17:33h at Oceania (level 0)

Themes: (T) Sediment management and morphodynamics, (ST) River morphodynamics, Poster pitches

Parallel session: Poster pitches: 7B. Sediment - River

A weakly bedrock layer is partially exposed in the middle reaches of the Ishikari River (Hokkaido, Japan). The site had been a fully alluvial channel until the 1980s. The thickness of alluvial layer was gradually decreasing by the outflow of gravel. After the bedrock exposure, the bedrock began to be eroded by saltating bedload. Local bedrock erosion has led to insufficient embedded depth of bridge piers and damage to revetments. As those problems are becoming more serious, we are planning to be covered the bedrock by gravel in order to suppress the bedrock local erosion. However, the formation of gravel bars often cause partial bedrock exposure. The purpose of this work is to understand the mechanisms of the bedrock re-exposure using a large-scale flume experiment. We conducted an experiment using 1/50-scale model of the Ishikari River that contains both straight channel zones and meandering channel zones. The length and average width of the flume were 200m and 2m, respectively. The flume bed was non-erodible mortar bed of average slope 0.0034. The mortar was covered with sand of average diameter 1.1mm. The thickness of sand layer was 20mm. The experiment was bank-full flow with discharge of 35.5L/s. Sediment feed rate was equal to the sediment transport capacity. The experimental results showed that; 1) the bedrock was re-exposed by the bar formation near the flume banks; 2) in the straight channel zone, the throughput bedload moves over the exposed bedrock patches; 3) in the meandering zone, the secondary flow prevents the intrusion of bedload into the exposed bedrock patches.