Impacts Of Flood-relief Works On Local Scour And Deposition In Tidal Reach

The construction of flood-relief works is one of the water conservancy project measures for urban flood control.Introducing the mountain torrents directly into the river downstream bypass the city,then can resolve or minimize urban flood disaster,thus decreases the damage caused to the people and promotes social economy development.However,the original state of dynamic balance of river reach near the outlet will be broken.It needs some time for the riverbed to adjust to a new balance.Analysis of the impacts of flood-relief works on local scour and deposition in tidal reach,which is available for promoting the benefit of projects and maintaining stabilization of the river regime,Taking Kuiqi reach of Min River North Bank as the study region,in this paper,the variations of the hydraulic field of Kuiqi reach under the impact of flood discharge by a flood control project of Fuzhou is explored.How riverbed is influenced by the different flood discharge conditions is analyzed simultaneously.It has practical significance for developing energy dissipation and protection measures in the river at the tunnel exit to forecast hydraulic field and sediment erosion and deposition.A 2-D flow and sediment mathematical model which is based on the theory of the shallow water equations and finite volume method is used for simulation in this paper.Main research contents and results are as follows:(1)The flow mathematical model is applied to simulate the natural hydraulic field in the lower tidal reach of Min River.According to the validation and analysis of simulation results based on the measured data,the results can reflect the hydraulic parameters(such as water level and flow velocity)of this river section and can provide accurate boundary conditions for Kuiqi reach model.From the calculation of natural flow field of Kuiqi,the velocity of the ebb current is larger than that of the flood current and the ebb tide duration is longer than that of the flood.(2)According to the research on the influence of flood discharge on flow field in Kuiqi reach,it shows that a main flow region with large flow motion is formed at this reach near the tunnel outlet.It swings with the flood and ebb of river.And its influence area affected by the actions of ebb current is broader than that of the flood current.(3)Considering the factors of discharge,times and duration of flood discharge,four different conditions are taken to simulate the variations of local scour and deposition of riverbed in Kuiqi reach.The following conclusions have been come to:Due to the actions of tidal current,the range of bed deformation is characterized by a fan-shaped distribution centered at tunnel outlet.A scour pit forms rapidly at the location of the outlet,meanwhile,the sediment carried by flood discharge deposits on the bilateralis of the scour.During flood tide the siltation area of upstream grows more rapidly,but it grows relatively slow at ebb tide.While the deposition part of downstream forms mainly during ebb tide.The impact(including erosion range,scour depth and the height of deposition)on the local scour and deposition of riverbed increases with the increase of discharge,times and duration of flood discharge.The recovery rate of river bed change(the maximum scour depth and the height of deposition)can reach over 50%after the whole tidal cycle.This research results not only provide scientific evidence for taking energy dissipation and protection measures of Kuiqi,but also can be a reference for the similar engineering.