Analysis Of Runoff/sediment Reduction Efficiency Of Check-dam In Its Lifespan

Check dam is an effective and widespread measure for soil and water conservation around the world.Check dam plays an important role in the Chinese Loess Plateau,which suffers from serious soil erosion.According to the government report,there will be 163 000 check dams in the Loess Plateau by the end of 2020 and consequently 50 000 hm2 sedimentary lands will be created in gullies,which will inevitably influence the flow and sediment processes in small catchments of Loess Plateau.However,there is still lacking comprehensive and quantitative understanding of how a heavy silting check dam impacts hydrologic responses and sediment transport processes.In this study,a check dam is treated as a dynamic system and its whole lifespan is separated into two stages:(1)before reaching its design storage capacity(DSC);(2)after reaching its design storage capacity.Based on sufficient sample data and observation data,this paper employs distributed hydrologic model(InHM)to quantitatively simulate and analyze the maximum runoff/sediment reduction efficiency of a check dam in Loess Plateau and comprehensively model the short-term and long-term impacts of check dams on surface hydrologic response and groundwater recharge.The main results and conclusions are as follows:(1)By designing different silting scenarios,flow movement and sediment transport on the sedimentary land are simulated,and the sediment trapping efficiencies of different scenarios are calculated and compared for a heavy silting check dam.The mechanism of check dam to intercept water and sediment is changed from "dam-dominated block" in light silting stage to"sedimentary-land-dominated slowing down" in heavy silting stage.The latter mechanism enables a heavy silting check to reduce and delay flood peak by 7.67%and 5-8 minutes.(2)A heavy silting check dam can directly trap sediment on the damland and indirectly attenuate erosion(or promote deposition)in tail-end area and gully-head areas.As the silting storage capacity reaching DSC,the indirect efficiency is increasing while the direct efficiency is decreasing but still non-negligible,making most of the sediment deposit in the tail-end area and gully-head areas.For a heavy silting check dam which reaches its DSC,its can still directly trap around 11700 tons sediment and indirectly reduce 760.70 tons sediment yield for a single sub-gully in a 90 mm hr-1 rainfall event.(3)By conducting continuous rainfall-runoff events,the maximum runoff/sediment trapping efficiencies of a check dam after reaching its DSC are simulated and compared among different rainfall intensities.The check dam has a large and stable extra storage capacity(ESC)which is around 24.30%of its DSC in low and moderate-intensity rainfall series.While the check dam has a small and unstable ESC which is only 4.68%of its DSC in high-intensity rainfall series.The check dam has nearly zero ESC in extreme-intensity rainfall series.(4)Deposition can occur in the tail-end area and erosion occurs in the middle area and near-dam area when check dam reaches its ESC,forming a "trapezoid-shape" silting profile and a "right-triangle-shape" silting profile in low and moderate intensity rainfall series,respectively.However,in high and extreme-intensity rainfall series,erosion process dominates in the whole dam-influenced area.(5)Check dams can redistribute the water-balance components by increasing subsurface outflow and groundwater recharge both in the short-term and in the long-term.Check dams can elevate water table in near-dam area shortly after its construction,and averagely rise the water table in the upstream and downstream channel by 3?5 m in the long-term.