| Check dam is one of the most important engineering measures to control soil and water loss in the gully area on the Loess Plateau.According to the first national water conservancy survey in 2011,there are 58,446 check dams on the Loess Plateau.During the 14th Five-Year Plan period,the Ministry of Water Resources of P.R.C.plans to build 1461 new check dams to form arable land and 2559 new check dams to trap sediment.Such a large-scale construction of check dams would have a profound impact on the hydrological and sediment processes of the watershed.The systematic and in-depth analysis of the regulation effect of the check dam system on the hydrological and sediment processes of the catchment is of great significance for the comprehensive understanding the mechanism of the check dam system on sediment transport control and erosion reduction.And it would benefit for clarify the contribution of check dam system to the changes of runoff and sediment load in the hilly and gully region of the Loess Plateau.In the present study,the Wangmaogou catchment,a typical small catchment in the hilly and gully region of the Loess Plateau,was selected as research object.Based on field observation,laboratory testing and numerical simulation,this study analyzed the effects of check dam system on runoff generation and concentration processes in the catchment,quantified the regulation effects of check dam body and topographic changes caused by sediment deposition on flood process of catchment,revealed the regulation mechanism of check dam system on hydrodynamic process and sediment load process in the channel,and clarified the synergistic sediment reduction mechanism between check dam system and land use change.The main conclusions of this study were as follows:(1)The regulating effects of check dam system on runoff generation and concentration processes of the catchment were quantified.Compared with the catchment without check dam,the construction of check dam system reduced the flood peak discharge and total flood amount,extended the flood duration,and significantly reduced the flood peak.Under the rainstorms with different return periods,the construction of check dam system in Wangmaogou catchment decreased the flood peak discharge,total flood amount and flood duration by 68.06%-73.33%,14.96%-24.13%,and 125.45%-194.77%,respectively.With the number of check dams increased,the flood peak discharge,average flood discharge and variation coefficient of flood discharge were all decreased,and the flood process became more smoothly.The construction of check dam system obviously reduced the runoff coefficient,increased the regulation and storage capacity,extended the average runoff concentration time of the catchment.In rainstorms with a return period of 30 years,the runoff coefficient of the catchment reduced 20.72%and the average runoff concentration time increased 361.86%.(2)The regulating effects of check dam body and topographic changes caused by sediment deposition behind the dam on flood process of the catchment were expounded.The mechanisms of flood peak reducing effect and flooding process mitigating effect of check dam fully filled with sediment were revealed.With the continuous loss of the sediment storage capacity of check dam,the effect of check dam on floods changed from mainly interception to reduction and mitigation,and the check dam fully filled with sediment still had the reduction and mitigation effects on flood.When the designed sediment storage of Wangmaogou 1#check dam was fully filled by sediment,it still had effect on dispersing and mitigating floods;the maximum peak discharge reduction rate,maximum flood volume reduction rate and maximum flood lag time were 73.09%,5.12%and 19 min respectively.The sediment deposition in the channel altered the local topography,and the topographical changes contributed to the retard of the flood propagation by reducing the flow velocity,which could reduce the flood peak to some extent.When the designed sediment storage of Wangmaogou 1#check dam was fully filled by sediment,the maximum flood peak reduction rate and maximum flood volume reduction rate of topographic changes caused by sediment deposition were 4.03%and 0.90%,respectively.In addition,compared with a single dam,a check dam system containing multiple,cascading check dams had a more significant regulation effect on the hydrological and hydrodynamic processes of floods in the catchment.In the Wangmaogou check dam system,where the sediment deposition storage accounts for nearly 60%of the total reservoir capacity,only the topographic changes caused by the sediment behind the dam could reduce the peak discharge and the total flood volume by 9.61%,and 7.36%,respectively,and increase the flood duration by 71.76%.(3)Effects of check dam system on the hydrodynamic process in channel were expounded and effects of check dam system on sediment reduction in flood events were clarified.Compared with the catchment without check dam,the hydrodynamic process of the channel was significantly mitigated by the check dam system and the topographic variation caused by the sediment deposition behind the dam,and the reduction of hydrodynamics was obviously greater than that of discharge.Under the rainstorms with different return periods,the Wangmaogou check dam system,in which nearly 60%of reservoir storage capacities were lost to siltation,could decrease the average discharge,average velocity,average shear stress and average flow power by 75.71%,66.13%,78.69%,and 89.93%,respectively.The topographic variation caused by sediment deposition decreased the average discharge,average velocity,average shear stress and average flow power by 7.46%,44.09%,56.82%and 69.23%,respectively.The contribution rate of the topographic changes to the reduction of average discharge,average velocity,average shear stress and average flow power were 9.88%,66.67%,72.21%and 76.98%,respectively.The impact of topography variation on flow discharge was obviously less than that on shear stress and flow power.The Wangmaogou check dam system,in which most of reservoir storage capacities were lost to siltation,could still decrease the sediment load by 50.37%on average under the rainstorms of different return periods,while the sediment load deposited in channels reduced 9.42%due to topographic variation caused by sediment deposition only.The sediment reduction benefit of the check dam system and the topographic variation decreased with the increasing of the return period of rainstorm.(4)Effects of check dam system on erosion and sediment yield under different land use were analyzed,and the synergistic effects of check dam system and land use change on sediment reduction were clarified.The changes of land use during the period of 1970-2000 significantly reduced the soil erosion intensity and its distribution in the Wangmaogou catchment,and the area of severe erosion significantly decreased,while the area of slight and mild erosion significantly increased.The check dam system significantly reduced the sediment transport ratio.Under the same land use conditions,the average sediment transport ratio of the catchment before and after check dam construction were 53.69%and 5.40%,respectively,and the sediment transport ratio decreased 89.94%.Taking the land use scenario without dam construction in 1970 as the base period,the land use in 2000,2012 and 2017 reduced the annual sediment yield by 67.89%,71.52%and 70.00%,respectively.The sediment reduction rate of check dam system decreased from 90.93%in 1970 to 29.40%in 2000,further decreased to 25.27%in 2012,and slightly increased to 26.79%in 2017.With the increase of forestland and grassland area,the sediment reduction effect of land use change increased gradually,the sediment reduction effect of check dam system gradually decreased,but it still played a great role in extreme rainstorm conditions. |
