| Check dam is most important channel governance measure for the prevention and control of soil erosion on the Loess Plateau,has significant effect on retaining sediment and reducing erosion,plays an important role in reducing the sediment flowed into the Yellow River.Due to the construction quality,operation management and extreme heavy rains,failure events of check dam system occurred frequently.Failure of check dam system has become a bottleneck restricting in the development of check dam.It is of great significance to accurately simulate the rainfall and runoff process of watershed constructed with check dam system and systematically and deeply analyze the failure process of check dam system,so as to reveal the disast-causing mechanism of the failure of check dam under different silting conditions and reduce the risk of check dam damage This study took a typical check dam system in the Loess Plateau as the research object.Through methods of field investigation,laboratory model test and numerical simulation,the discharge calculation methods of a perforated riser principle spillway in multiple operating conditions were put forward,and a rainfall and runoff model coupled with the drainage process of drainage structure for a watershed with check dam system was established,and the regulation effect of check dam system on flood process in watershed was expounded,and the law of failure of the check dam system on flood process under extreme rainstorm was revealed,and effects of overtopping of check dam under different silting conditions on flood sediment process was quantified.The main conclusions of this study are as follows:(1)Based on the physical model test of perforated riser principal spillway in laboratory,the calculation formula of discharge process of perforated riser principal spillway in check dam project under normal and unusual work conditions was put forward.The calculation formula of the discharge of the side inlet orifice which is suitable for the discharge calculation of the perforated riser principal spillway in check dam project under normal working conditions was selected,and the calculation of the discharge coefficient was optimized.The deviation between the discharge calculated with the optimized formula and the measured discharge was less than 15%,the determination coefficient was 0.95,and the Nash efficiency coefficient was 0.91.Referring to the discharge calculation formula of the pressurized flow state of a vertical pipe,the calculation formula of the discharge process of the perforated riser principal spillway in check dam project was derived,and based on variables of head and roughness,the approximate expression of Darcy-Weisbach friction factor is proposed.which solves the problem of iterative calculation when the discharge is unknown.The deviation between the calculated value of the the discharge calculated with the formula and the measured discharge was less than 5%.(2)The rainfall and runoff model for a watershed with check dam system,which considering the storage and drainage processes of the check dam system,was established,and the regulation effect of the check dam system on flood process of the watershed was clarified.Only two parameters needed to be calibrated,which were Manning coefficient and dimensionless parameter CN.Manning coefficient is more sensitive than CN.The simulation accuracy of the peak discharge can be improved by calculating the drainage process of the perforated riser principal spillway with the improved discharge calculation formula.Compared with the original formula in the code and without considering the drainage process of drainage structure,the simulation accuracy of the flood peak discharge is increased by 7.42%and 19.58%,respectively.The flood control and detention effect of the check dam system is obvious.The flood control effect of the check dam system decreases and the flood detention effect increases with the increase of the rainfall return period.Under the rainfall conditions of different return periods,the flood peak discharge and total flood amount can be reduced by 49.55%-55.62%and 15.91%-21.62%respectively,and the flood duration can be increased by 83.50%-168.16%.(3)The chain failure process of typical check dam system under extreme rainstorm condition was reconstructed and the law of failure of the check dam system on flood process of watershed was revealed.The collapse time of the burst check dam was more concentrated,about 70%of them occur before and after the heaviest rain.The peak of outburst flood of the series check dam on the branch does not superpose with the flood peak of the downstream main channel and has no obvious influence on the flood peak discharge of the downstream main channel.Compared with the condition without dam break,the failure in check dam system with over 55%silation induced an increase of flood peak flow at the outlet of the basin by 20%.Compared with the condition without check dam,the seriously silted check dam system induced a reduction the flood peak discharge at the outlet of the basin by 87%despite the failure occurred.Sediment deposition limits the expansion process of dam breach,retards the evolution process of dam-break flood of check dam system,and has obvious reduction effect on cascading burst flood.(4)Based on the failure model of check dam considering the erosion process of the siltation body in front of the dam,the overtopping failure process of check dam with different silting states was simulated,and the influences of silting on the flood and sediment process were clarified.Without considering the erosion process of the sediment body behind the dam would induce the underestimate of the size of the final dam breach.The burst flood process of the check dam is affected by both the inflow flood process and the siltation ratio.The peak discharge and its increase ratio of dam break flood firstly increase and then decrease with the progression of sediment deposition.With the increase of siltation ratio,the size of the final dam breach will gradually decrease,the length of erosion gully in the sediment deposited in the reservoir will gradually increase,and the plane shape of erosion gully changes from "wide and flat" type to"narrow and long" type,and the critical siltation ratio will be between 60%and 70%.The sediment erosion volume of the siltation body increases with the increase of the siltation ratio and rainfall return period,and the average proportion of sediment erosion volume increases linearly with silation ratio,and will keep stable when it exceeds a certain threshold.During the failure of check dam system in "July 15" rainstorm event in Wangmaogou basin,the sediment erosion volume was about 3,800-4,400 m3,accounting for about 2.7%to 3.2%of the total erosion of the whole basin. |
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