The design of check dam openings for debris flow control has been identified as a longstanding challenge, with no definitive solution yet identified. In this study, a quantitative analysis of the control efficacy of check dams with varying opening rates is presented. Field investigation data of 67 check dams located in Wenxian County, Gansu Province, were utilized to gain a preliminary understanding of their running state and damage situation. Building upon this, five check dams with opening rates ranging from 2.1% to 10.4% were designed and subjected to testing. Parameters including volumetric water content, pore water pressure, deposit morphology, and particle size distribution were analyzed to investigate the effect of varying opening rates on debris flow control. The results showed that: 1) As the opening rate of the check dams increased, the peaks of volumetric water content and pore water pressure behind the dam first decreased and then stabilized. When the opening rate was increased to 6.3% or higher, these parameters reached stable values. 2) Check dams with different opening rates all demonstrated good effects in retaining the coarse and sluicing the fine, resulting in the average particle size behind dams was coarsened by 2.65 times. This coarsening was primarily attributed to an increase in the proportion of retained particles within the 2-5 mm size range. 3) An optimal opening range of 4.2%-6.3% was identified for effective debris flow control. Compared with other dams, Dam II with opening rate 4.2% exhibited superior performance in mitigating flow energy and intercepting coarse particles, but it imposed stringent strength-related requirements.

Understanding the changes of sediment concentration in rivers, especially under heavy rains, is of great significance to accurately evaluate the effectiveness of soil and water conservation measures in the Loess Hilly Area. On July 26, 2017, an extremely heavy rain (maximum rain intensity of 66.6 mm/h, the highest rainfall of 256.8 mm in 10 h, and a return period of 100 years) occurred over the 821 km2 Xiaoli River watershed in the Wuding River watershed. In order to understand the sediment concentration during the heavy rain event, 10 dam-controlled catchments in the center of the rainstorm (3 in the Xiaoli River watershed and 7 in its adjacent watersheds) without drainage and damage were selected. All the retained runoff and sediment in the check dams were measured by the cross-sectional survey according to the flood marks left by the heavy rain. Combined with the observed runoff and sediment transport data by the hydrological station at the outlet of the Xiaoli River watershed as well as the deposited sediment investigated in the watershed during the heavy rain event, the change of sediment concentration and the role of check dam were analyzed. Results showed that under heavy rain, all the sediment concentrations of runoff in the dam-controlled catchments ranged from 344 to 552 kg/m3, suggesting that the sediment retaining effect of slope measures (woodland, grassland, and terrace) is limited and cannot prevent the occurrence of hyperconcentrated flows. The gully measures (check dams) in the Xiaoli River watershed can hold a large amount of sediment under heavy rain and make the runoff no longer a hyperconcentrated flow at the watershed outlet. In order to control the transport of hyperconcentrated flows to the downstream, more attention should be paid to the construction of check dams while carrying out vegetation ecological construction in the Loess Hilly Area.

Among the most widespread structures for successfully retaining water and checking erosion on the semi-arid portions of China's Loess Plateau, check dams retain silt at slower than projected rates, leading to flood control issues. Meanwhile, the shortage and the uneven distribution of time and space of water resources in semi-arid areas can easily cause droughts and floods, which seriously restricted the rapid development of the socioeconomic. However, some of the high-quality rain and flood resources accumulated in the check dams can be used to alleviate part of the water resources crisis instead of causing flood. With the goal of holistically maximizing a projected check dam array's water resource, ecosystem and socioeconomic benefits, a Check Dam Benefit Maximization Model (CDBMM) was first developed. The CDBMM was first applied to the Si Jiagou Basin, and the model showed the total costs represent 7.07% of the total and rather significant benefits. Water resources benefits accounting for 45.40% of the total benefits, indicating that the water resources benefits were substantial and should be considered as the main influencing factors in the basin's ecosystem-friendly design and construction. Use of the CDBMM in watershed planning will allow a more efficient use of water and soil resources and greatly alleviate water crises in the semi-arid area. It can further provide a reference for both check dam system planning and the system benefits analysis.

Check dams and afforestation are widely used to control debris flows; however, the combined effects of mitigation changes caused by sedimentation behind the check dams as well as the growth of vegetation are unclear. This paper reports long-term measurements of the erosion base level behind check dams along the Shengou gully, which is an active debris flow gully with 19 shallow soil landslides along its banks. The loose sediment on the shallow soil landslides, which is affected by afforestation, is the main source of material and energy for debris flows in this gully. The change in the mechanical properties of the loose sediment on the shallow soil landslides was determined. Based on the mechanical properties of the shallow soil landslides and the erosion base level behind the check dams, the volume and potential energy of the shallow soil landslide sediment were calculated. A model of the vegetation coverage, shallow soil landslide energy, and debris flow volume was established and applied to the Shengou gully. The results show that the model can evaluate the different benefits of check dams and afforestation on debris flow mitigation given the sedimentation behind the check dams and the vegetation growth.