Study on Local Scour and Sediment Exchange around Spur Dikes
|Keywords:||丁壩;局部沖刷;掛淤機制;河岸保護;壩高-水深比;spur dike;local scour;sediment exchange;riverbank protection;overtopping rate|
Spur dikes are hydraulic structures that extend from a river bank into the river channel, and in on-site applications, they are present in groups. In river engineering, spur dikes have diverse functions. When applied to erodible banks, spur dikes can deflect near-bank currents to the river center. In addition, the recirculation zones behind spur dikes have a slow flow velocity, which increases sedimentation at the riverbank footing and reduces riverbank erosion. Although spur dikes serve multiple river management functions, their inherent structure hampers water flow from upstream to downstream. Thus, when water flows approach a group of spur dikes, the potential energy of the flows is increased to overcome the resistance engendered by the spur dikes. This increase in potential energy indicates that water flows passing through spur dikes demonstrate increasing turbulence intensity of flow, resulting in complex bed change near spur dikes. The local scour issue of a single spur dike has been studied by many researches, but fewer studies analyzed local scour of spur dikes. In addition, less study discussed the relationship between local scour mechanism and sediment exchange around spur dike field. Based on the field investigation, numerical simulation and flume experiment, this study explained the mechanism of spur dike scour and sediment loss in the spur dike fields. In the submerged case, the sediment loss in the spur dike fields caused by upward vortex and the scour zone development of first spur dike. The thickness of sediment loss in the spur dike fields can be calculated by local scour depth of first spur dike, the distances from upstream and overtopping rate. In the emerged case, the sediment loss in spur dike field is less than the submerged case. Moreover, an empirical formula was developed to estimate the maximum scour depth around the on-site spur dike group.
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