Effects of levee realignment on flood control in urban area

As an example,the impact of the levee realignment project on the flood control of Wenzhou city is studied by a 2-D tidal flow mathematical model.The verifications of the model show that the tidal levels computed by the model are in good agreement with the field data,and the tidal velocity and rate of discharge are also in accordance with the data recorded.The variance in computation versus actual results is less than 10%.Therefore,the model can be used to accurately predict the changes of tidal level,flow velocity and discharge to forecast the effects of the project.As the majority of the riverbank moved outward is in the concave area of the river,the project will create a smoother outward arc for the riverbank.It is shown from computed results that the maximum high tidal level increases 0.03-0.04 m and 0.04-0.08 m for project scheme 1 and scheme 2,respectively,in the flood frequency of 1%in the reach from the Yutoushan to the Mei'ao.With an annual mean discharge of 450 m³/s,the vertical average velocity in the reach from the Tashan section to Yuqishan section increases 6% to 22% after the completion of project scheme 2.The change in velocity in the reach outside of the project is small.The impact of the project on the bifurcation ratio of the Jiangxinsi branch is very small.The river blocked by project scheme 1 is much smaller than that by project scheme 2,and the change in velocity created by project scheme 1 is also smaller than that by project scheme 2.The results computed by the model can be used as a reference for decision making.