510 / 2024-06-10 08:14:45

Experimental study on sediment deposition considering surface eddies and local separated flow structure around vegetation patches on sloping middle flood channel

suspended load, sedimentation, floodplain

Subtheme 3: Eco-hydraulics and Environment > Topic 3: Eco-environmental Effect of Water Engineering

Under the increase in flood intensity by climate change, overflowing from river levee frequently occurs in Japan. After a flood disaster, river channel and floodplain excavation including tree removal are being implemented to improve the flood flow capacity. However, the excavated channels are not always good landscape from the point of ecological biodiversity because of the lack of riparian vegetation at the initial condition after the excavation. Even in that case, re-sedimentation can occur after the next floodings because of the suspended load transfer from low channel and trapping by grasses on floodplain intruded after the excavation. In this study, a river channel model consisting of a low channel, a sloping floodplain (middle channel) with vegetation model, and a high-flat floodplain was set in a flume. Sloping of middle channel in cross-stream direction was considered for decreasing sedimentation at flood event. Three-type vegetation, a pioneer grasses (submergent grasses), emergent vegetation and trees, were considered for expressing the riparian vegetation. Plant cover rate was also considered as an important parameter. The deposition amount of suspended load on the sloping floodplain was investigated when the sand was supplied from the upstream side of the channel. Moreover, the flow condition and the horizontal eddy structure were visualized using sawdust. The velocity fluctuation by horizontal surface eddies were also checked using electro-magnetic flow meter. The experimental results showed that vegetation at the sloping floodplain leads to a significant sediment deposition on the floodplain, mainly on grasses and the separated flow region behind patchy-type grasses. The coherent horizontal eddies generated by the velocity difference between the sloping floodplain and low channel can contribute sediment fluxes on floodplain and the patch type grasses can trap the suspended load. The capacity of trapping amount is decided not only by the surface eddies but by plant cover and vegetation type. Vegetation type can greatly change the sedimentation amount due to the formation of separated region which traps suspended load and separated eddies which causes shifting sands. The experiment indicates the important point not to increase sedimentation even when riparian vegetation is provided at the excavated regions.

Under the increase in flood intensity by climate change, overflowing from river levee frequently occurs in Japan. After a flood disaster, river channel and floodplain excavation including tree removal are being implemented to improve the flood flow capacity. However, the excavated channels are not always good landscape from the point of ecological biodiversity because of the lack of riparian vegetation at the initial condition after the excavation. Even in that case, re-sedimentation can occur after the next floodings because of the suspended load transfer from low channel and trapping by grasses on floodplain intruded after the excavation. In this study, a river channel model consisting of a low channel, a sloping floodplain (middle channel) with vegetation model, and a high-flat floodplain was set in a flume. Sloping of middle channel in cross-stream direction was considered for decreasing sedimentation at flood event. Three-type vegetation, a pioneer grasses (submergent grasses), emergent vegetation and trees, were considered for expressing the riparian vegetation. Plant cover rate was also considered as an important parameter. The deposition amount of suspended load on the sloping floodplain was investigated when the sand was supplied from the upstream side of the channel. Moreover, the flow condition and the horizontal eddy structure were visualized using sawdust. The velocity fluctuation by horizontal surface eddies were also checked using electro-magnetic flow meter. The experimental results showed that vegetation at the sloping floodplain leads to a significant sediment deposition on the floodplain, mainly on grasses and the separated flow region behind patchy-type grasses. The coherent horizontal eddies generated by the velocity difference between the sloping floodplain and low channel can contribute sediment fluxes on floodplain and the patch type grasses can trap the suspended load. The capacity of trapping amount is decided not only by the surface eddies but by plant cover and vegetation type. Vegetation type can greatly change the sedimentation amount due to the formation of separated region which traps suspended load and separated eddies which causes shifting sands. The experiment indicates the important point not to increase sedimentation even when riparian vegetation is provided at the excavated regions.