Simulation Research On Effects Of Check Dams For Debris Flow Mitigation In Bailong River Basin

Debris flows represent major hazards in Bailong River Basin of Gansu Province where they repeatedly result in disasters.Debris flows pose a serious threat to life,properties,the ecological environment and the advancement of local economic development.In the context of global climate change,extreme rainfall events and earthquakes occur frequently,debris flows become serious,the governments and people have paid more attention to debris flow mitigation.Since 1960 s especially after the ‘5.12' Wenchuan earthquake and “8.8” Zhouqu debris flow,the Chinese government had invested lots of funds to carry out reconstruction projects as means of mitigating the debris flow hazards.Building check dams are the basic methods in debris flow mitigation in Southern Gansu,however,check dams cannot stop debris flow,many of check dams are fully filled up only a few years later such as the dams in Dadigou valley and Macaogou valley,when a debris flow or a highly sediment concentrated flood comes down the dams often fail to check them.Once the check dams are fully filled up,a cascading failure of check dams further downstream will likely take place,which might lead to disasters,such as the ‘8.8' Zhouqu debris flow.They called for urgency in studying on effects of check dams for debris flow mitigation in Bailong River Basin of Gansu Province.This paper through field investigation and monitoring,literature review,data analysis,remote sensing interpretation and other methods,in-situ physical model and the field test model were established,and the Kanako numerical model was used to simulate the effects of check dams on debris flow mitigation.Furthermore,Sanyanyu debris flow valley and Goulinping debris flow valley were selected to study the effect of the check dams.The main study conclusions are as follows:(1)The characteristics,causes and susceptibility of debris flow hazards in Bailong River Basin were analyzed.Debris flows in the study area mainly rainstorm-induced debris flow,especially 10 min short rainfall-induced debris flow;Based on the monitoring of the typical gully,the rainstormtype debris flow warning equation is established by statistical fitting: I = 112.2t-0.82;The ZhouquLinjiang section along Bailong River,the Minjiang River and the Beiyuhe River are the debris flow prone areas.(2)Through the in-situ simulation test,the process and phenomenon of the debris flow are simulated in the situations including original channel,dam fulfilled and dam failure.The results show that when a dam is damaged,the cutting depth of the ditch bed is increased by 367%,the handling distance of the stone increases by 41.7%,37.5% and 23.1%,respectively,and the stress peak of the sensor increases by 15.4%,which indicates that the dam failure increase the erosion capacity and destructive power of the debris flow.The stress of the debris flow on the sensor in the situation of dam fulfilled decrease 78.46%,which indicates that when a dam is fulfilled,the dam still has a positive effect on the debris flow,because the dam has improved the erosion datum of the upper reaches of the dam,resulting in reducing of the upstream erosion.(3)The Kanako numerical model was used to simulate the change of total sediment discharges and channel under different scenarios including dam with different heights and different slopes of channel.The reduction rate of runout of sediment(k)is established to show the effects of check dam,the results show that: the effects of check dam are closely related to the height of the dam and the slope of the channel.The general rules are as flows: When the height of the dam is the same,the slope of the channel is in the range of 6 ° ~ 14 °,the k value decreases first and then increases,the slope of the channel is in the range of 14 ~ 20 °,the k value increases first and then decreases;When the slope of channel is the same,k is linearly positively correlated with the dam height,however when the slope is 6 °,the rate of increase is the biggest,when the slope is 14 °,and the rate of increase is the smallest;When a dam is fulfilled,it is still possible to reduce the erosion of the upstream of the project until the blockage of the upstream channel bed is reduced to the maximum siltation margin;The destructive power is amplified when the dam is damaged.(4)Sanyanyu debris flow Valley(with civil engineering works)and Goulinping debris flow Valley(without civil engineering works)were selected,this paper analyzed the effects of check dam on debris flow mitigation.It is believed that the dam failure in Sanyanyu Valley increased the destructive power of debris flow in the “8.8” debris flow hazards;Unwise city planning and disorderly occupation of the alluvial fan by dense structures were important factors which caused Zhouqu "8.8" debris flow disaster;In the context of global climate change,the extreme rainfall events increase,the failure of check dams has the potential to trigger even greater hazards,channel works that can direct debris flow to travel along desired paths in densely populated areas.It is concluded that channel works should be focused on,combined with ecological methods and check dams are probably preferred approaches to minimize the debris flow damage in debris flow catchments characterized with high mountains,concentrated rainfalls and active nontectonic movement.In this paper,the characteristics,causes and susceptibility of debris flow hazards in Bailong River Basin were analyzed comprehensively.The critical induced rainfall of small and medium debris flow was obtained through field monitoring.The effects of check dams were simulated by physical model and numerical model.Furthermore,this paper selected Sanyanyu and Goulinping debris flow valley to study the effects of the check dams,which provides the theoretical basis for the disaster prevention and mitigation of the debris flow in the Bailong River Basin,and provides new ideas for the mitigation of similar debris flow in other regions of the country.