淤地坝“淤满”后的水沙效应及防控对策

黄土高原大规模的淤地坝建设在减少黄河泥沙以及改善区域生态环境方面发挥了巨大作用.但是,在淤地坝“淤满”的极端条件下,关于其水沙效应变化及防治对策的研究还较少涉及.经分析,淤地坝“淤满”后:1)坝控范围内坡度降低,径流长度减少,沟道比降降低,而横断面由原来的“V”型沟道,演变为“U”型沟道;2)以关地沟4号坝为例,使用RUSLE计算,修建淤地坝前,坝控范围内平均每年土壤侵蚀模数为4 472 t/(km2 ·a),淤满后,土壤侵蚀模数下降至4 019 t/(km2·a),降幅约10％,“原地”减蚀作用显著,从修建至淤满阶段,拦沙作用巨大;3)淤地坝淤满后,坝地流速显著降低,从修建淤地坝前的0.83 m/s降至0.27 m/s,但坝体外坡的流速显著增加,特别是坡底,最大流速可达3.76 m/s;4)淤地坝淤满后,淤地坝“异地”减蚀作用会降低.基于上述变化,针对淤地坝淤满后的极端条件,本文提出如下防治对策:1)以小流域为单元,以溢洪道为主体,完善沟道排洪设施布设,提高支沟内以及支沟与主沟的连通度,提升排洪能力;2)遵循“因地制宜”原则,科学合理植树种草、修建梯田,加强坡面治理,减少坡面来水来沙,消耗和分散坡面来水侵蚀能量,降低坝地淤满后被损毁的风险;3)采取“截水沟和排水沟相结合,工程措施和植物措施相结合”的方法,做好坝体陡坡防治,提高坝体外边坡植被覆盖度.研究结果以期为黄土高原淤地坝建设提供理论支撑.

Runoff-sediment effect of "fully filled" check dam and related prevention and control measures

Background] Large-scale check dams in the Loess Plateau of China play a key role in sediment reduction and regional environmental improvement in the Yellow River basin.However,changes in the effects of runoff-sediment and related prevention and control measures on check dam under full capacity conditions have been rarely studied.Methods] Based on geospatial data,we employed the RULSE and GAST hydrodynamic model to calculate soil erosion modulus and flow velocity,respectively.Results] Here,we analyzed check dam once they reached full capacity,with the following changes found:1) within the check dam control area,the surface slope and gully channel gradient both decreased,flow length was shortened,and the cross-section evolved from a V-shaped gully to a U-shaped gully;2) using RUSLE computations and Guandigou No.4 check dam as an example,the mean soil erosion modulus within the check dam control area was 4 472 t/ (km2 · a) before the check dam was built,but this decreased by 10％ to 4 019 t/(km2 ·a) after the check dam was fully filled,thus in-situ erosion reduction was significant and sediment retention occurred from check dam construction to full capacity;3) runoff flow velocity fell significantly from 0.83 m/s before construction to 0.27 m/s after the check dam reached capacity,but runoff flow velocity on the outer slope of the check dam increased significantly,in particular,the maximum runoff flow velocity at the bottom reached 3.76 m/s;and 4) ”off-site“ erosion reduction of the check dam decreased once the check dam reached full capacity.Conclusions] Based on the above changes observed under the extreme siltation conditions,the following prevention and control measures are proposed:1) spillway-based flood discharge works should be established in the gully channels of all catchments to improve connectivity within a branch gully and between branch and main gullies,thereby increasing flood discharge capacity;2) the planting of trees and grasses and creating terraced fields built with local conditions considered should be undertaken to enhance slope erosion control,reduce flood and sediment supply to the slope surface,dissipate water erosion energy on the slope surface,and lower the risk to dam farmland after full capacity is reached;and 3) catchwater drains and drainage ditches,as well as engineering and vegetation measures,should be implemented to help prevent and control dam and steep slope erosion and increase vegetation cover of the outer slopes of a dam body.The results of this study are intended to provide scientific evidence to help improve check dam construction in the Loess Plateau of China.