Author(s): Pietro Beretta Piccoli; Youichi Yasuda; Robert Boes
Linked Author(s): Youichi Yasuda, Robert Boes
Keywords: Gravel dikes; Meandering shape; Flow refuge; Habitat suitability modelling; Flood control
Abstract: The installation of alternative gravel dikes could represent valuable substitute to classic river restoration projects in channelized rivers, since no widening is necessary. This study assesses the changes in flow heterogeneity, ecological suitability and flood protection caused by this new strategy. The experiments are conducted in rectangular channel with slope 0.01, where gravel dikes are inserted on alternated shores during normal stage. The flow velocity and water surface are measured using electrical-magnetic current meter and point gauge respectively. The installation area is studied under different discharges and further data are collected once at flat gravel bed morphology. With small discharges, the water surface is flat, and the flow meanders and pockets with almost still water are found downstream of each gravel dike. Those characteristics gradually disappear with larger floods, where waves on the water surface and straightening of the flow are observed in the channel’s center, coupled with significant speed reduction closing to the lateral walls. Above flat gravel bed, homogeneous normal flow is recorded across the whole section. Further, experiments in prototype yield that the space flows in boulders are successfully used by living fishes to refuge from the flood flows. None of studied discharges reaches the critical Shields stress for erosion, while gravel dikes produce significant habitat improvement for all the studied species, especially where the pockets with almost still water are found. On the other hand, this alternative bed morphology brings unbalance to the hydraulic conditions of the channel and causes the water surface to rise, shrinking the cross-sectional capacity and increasing the flood risk. Future research must be conducted for the installation shapes of gravel dikes to become a valuable restoration strategy.
Year: 2020