| Check dams have played a huge role in soil and water conservation on the Loess Plateau,with their main functions being flood detention,sediment retention and land reclamation.However,large-scale engineering construction has changed the underlying surface conditions of the basin,and the water cycle process and rainstorm flood situation in the Loess Plateau region have undergone significant changes.The early-built Check dams generally have low flood control standards,insufficient flood discharge capacity,poor construction quality,lack of protection measures and other problems,which weaken their flood control and disaster reduction capacity,and water damage occurs from time to time.At the same time,due to the special geographical and climatic influences of the Loess Plateau region,summer precipitation accounts for about 2/3 of the total annual precipitation,and local short-duration heavy precipitation occurs frequently.In addition,small basins in loess hilly areas have significant nonlinear characteristics of runoff generation and confluence.Floods have short duration,high peak flow rate and small volume.They rise rapidly and fall rapidly to a certain extent.They pose a risk of dam failure to Check dams in basins.They seriously threaten people’s lives downstream of dam sites.At present,research on rainstorm floods for Check dams generally stays at investigating current engineering projects for Check dams.It concludes that each Check dam has flood control capacity in Check dam systems based on historical rainfall data for simulating flood processes in Check dam systems.It cannot achieve early warning on a certain time scale for Check dams based on meteorological forecast rainfall data;nor can it obtain real-time inflow flood processes for each Check dam based on measured rainfall data And inflow total amount of floods;carry out pre-warning risk size under current rainfall when affected by previous rainfall.The purpose of this study is to construct a multi-stage progressive risk warning system for flash floods caused by rainstorms at Check dams by using meteorological forecast rainfall data for 24 hours ahead 6 hours ahead And real-time rainfall data;provide early warning of flash flood risk for target small watershed Check dams;meanwhile by field investigation of dam-break data using HEC-RAS to simulate dam-break floods for target Check dams improve small watershed Check Dam Warning System.Through research,the following results are obtained:(1)Construction of storm flood simulation model for Check damIn this study,based on the rainfall model and the production and confluence model in the China Hill Flood Hydrology Model(CNFF),the hyperinfiltration production model and the Check dam reservoir model are introduced,and the Check dam storm flood simulation model is constructed after correlating the upstream and downstream Check dams to simulate the incoming flood and reservoir storage of the Check dam in the basin in turn.The model generalizes the basin into different elements such as sub-basin,river section,node,water source,diversion,depression,etc.according to the basin sub-basin conditions and production and confluence characteristics.Based on the topological relationships and hydraulic connections of these elements,the basin water cycle is calculated;flood simulation is performed according to rainfall-evaporation-production-sink-flow-reservoir storage,etc.(2)Check dam storm flood risk warning level classification and warning index determinationThe remaining reservoir capacity of the Check dam is the main part of flood storage,based on the remaining reservoir capacity,this study determines the characteristic reservoir capacity index of the Check dam according to the different proportion of reservoir storage,and divides the warning level;proposes the calculation method of rainfall warning index considering the risk factors such as flood holding capacity,rise rate and rainfall variability.The critical rainfall is the basis for determining the rainfall warning index,this paper uses the model trial algorithm to determine the critical rainfall;based on the Checky dam storm flood simulation model,considering the changes of soil water content and reservoir storage in the upstream watershed,the static critical rainfall and dynamic critical rainfall under different characteristic reservoir capacity index of the Checky dam are determined by the model trial calculation,and their static rainfall warning index and dynamic rainfall warning index are obtained by risk factor correction The static rainfall warning index and dynamic rainfall warning index were obtained by risk factor correction.(3)Multi-stage early warning model construction and applicationIn this study,a multi-stage early warning model for Checky dams with online operation is constructed by improving the original single warning model based on storm flood warnings of different foreseeable periods.The multi-stage and progressive flood risk warning model for Check dams was established based on a 24-h future storm flood risk warning model,a 6-h future storm flood risk warning model for Check dams,and a real-time storm flood warning model for Check dams.Based on this,a multi-stage flash flood risk warning was conducted for 204 pilot dams in the Yan’an area during the flood season,and the results were released to the dam management units for flood control reference.(4)Analysis and simulation of dam-break floodAccording to Tangjiahe River Basin’s breach of a Check dam,the impact of rainfall on the breach of a Check dam was analyzed.According to flood trace investigation data etc.,peak flow rate at control section was calculated.Based on investigation data,HEC-RAS was used to simulate breach floods for 5 Check dams respectively to obtain simulated flow rate at control section;results showed that in breach simulation of a Check dam using HEC-RAS software obtained results that were basically consistent with actual situation verifying feasibility of HEC-RAS software’s establishment of breach models for simulating breach floods. |
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