Erosion,Deposition Of Sediment And Morphologic Response To The Removal Of The Small Dams

Dam plays important roles in flood control,power generation,irrigation,water supply,navigation and so on.At present,China has built about 98000 dams,most of which are mainly small in size.These dams have problems of plan,design,construction and management in some extent due to the limit of the economy and technology at that time.With the design working life approaching,the negative effects on economy,environment and security have become increasingly prominent,so it is necessary to perform demotion or retirement for some reservoir dams with little rehabilitation values.Instead of dam operation,dam removal will destroy the balance of ecological environment again,resulting in radical changes of hydraulic-dynamics and sediment,at the same time,influencing the biological habitat indirectly.Now,Sediment movement has become the biggest challenge in dam removal,it is particularly important to research the changes of hydrodynamic and sediment process and morphologic response after dam removal,which will provide some references for the theoretical research and technical support.This paper took the Xihe dam removal of Guizhou as an example,the rainfall-runoff process and sediment concentration in Xihe un-gauged basin have been derived.Through the establishment of two-dimensional mathematical model of flow and sediment,the long-term sedimentation in the reservoir has been calculated and analysed.Both the short and long term changes in hydraulic-dynamics and sediment have been simulated,as well as channel evolution.In addition,the influence of different removal approach on the sediment transportation and morphological evolution has been explored,we get the following main conclusions:(1)For ungauged small basins,through analysing long time serial datas of adjacent hydrological station,hydrological analogy method and rainfall-runoff frequency method were used to derive rainfall-runoff process,the result shows that there exists consistency between two methods,the annual average run-off fits regional distribution law and underlying surface condition,it can be used as model input to calculate.(2)By simulating process of water and sediment with dam,the findings show that dam's building cause the large quantity of sediment deposit in the resevior,sedimentation shape behaves “conical shape”,building dams also cause the fluid flow to be stead and velocity tends to zero in front of the dam.(3)By simulating the short and long term water and sediment processes,indicating that in a short period after dam removal,because of a huge drainage in reservoir,the high head difference generates a high velocity around dam site,resulting in erosion occurred in the reservoir area,sediment erosion is very serious in the main river.At the same time,the knickpoint in the reservoir migrates to upstream and erodes more sediment in the upper reaches;Deposition has been observed on both sides of the main river due to the rapid decline in water level;Sediment will not silt downstream of the dam because the velocity is too large.After dry and average seasons,there is little sediment scouring and silting in the channel,but after a flood season,a large number of sediment is transported to the downstream and silt on both sides of the river;The channel morphology changes dramatically and devolops to be narrow in the upstream,but changes may be less significant downstream.(4)In comparison with one-time removal,phased removal reduced the head difference and the positive and negative waves are smaller,the flow rate is relatively low,so erosion is not serious enough and erosion distance is shorter.But in the long run,the sediment scouring and silting distribution is similar to the one-time removal.