| The check dam system is a major engineering measure to control gully soil erosion on the Loess Plateau.It plays an important role in intercepting rainwater runoff and sediment,peak shaving and energy dissipation,and the development and utilization of water and sediment resources.However,check dams still face a number of key problems,such as the risk of water damage in the event of storm flooding,and insufficient exploitation of water and sand resources within the dam.In this study,the damage characteristics of typical storm check dams were analyzed from the regulation of runoff and sediment under flood events and the effect of water and sediment control.The safety operation guarantee system of check dam system was constructed,and the temporal and spatial distribution characteristics of water and sediment culti vation in dams have been clarified.In addition,water and sedimen resource utilization was further analyzed.Which provides a scientific basis for the safe operation and the playing of ecological benefits of check dams on the Loess Plateau.The results of this study are as follows:(1)The influence of check dam on water and sediment process under rainstorm condition is analyzed.Factors such as runoff and sediment transport in the Wangmaogou watershed from 1980 to 2013 changed abruptly in 1987.In the rainfall-runoff depth-sediment modulus transport relation,which showed little change from 1980 to 1990,weakened from 1990 to 2000,and has been improving after 2000,the runoff depth-sediment modulus transport relation is stronger than the rainfall-runoff depth relation.During the ’7.15’ rainstorm flood in Suide in 2012,dam No.2 in Wangmaogou watershed intercepted 80%of the upstream eroded sediment.Sediment intercepts from the Wangmaogou watershed dam system are greatest in the Jiuyuangou watershed,and the total flood volume is the largest with the exception of the main channel dam system unit.During this storm,the amount of water stored and sediment retained at Jiuyuangou is far less than the remaining storage capacity of the entire dam system.During the ’7.26’ rainstorm flood in 2017,the deposition thickness showed a downward trend from the upstream to the downstream of the watershed,reflecting that the construction of the check dam system helped to improve the ability of individual dams to cope with extreme rainstorm flood.The deposition of large dam,medium dam and small dam accounted for 27.60%,35.60%and 36.80%of the total sediment,respectively.The check dam has a certain effect of silting up and discharging fine sediment on the sediment above the channel.The effect of silting up and discharging fine sediment of check dam with large control area is better than that of check dam with small control area.The sediment movement with runoff from coarse to fine presents the horizontal displacement trajectory law,and the sediment deposition law is particularly obvious.Channel sediment is the main source of check dam sediment,with a contribution rate of more than 60%,while slope sediment contributes less than 40%.As the deposition depth increases,the channel contribution rate to the check dam sediment increases,while the slope contribution rate decreases,and the check dam sediment still mostly comes from the channel.With the increase of deposition time,the trend of different deposition layers from bottom to top is that the contribution of channel sediment production gradually decreases,while the contribution of slope sediment production gradually increases,and mostly from the lower part of the slope.(2)The regulation effect of check dams on runoff and sediment transport under flood events was analyzed.There is a significant difference in peak flow between dam and no dam in the watershed.The peak flow in the basin without check dams is 14 times higher than in the watershed with check dams,and the runoff lag time is the main direct factor leading to the large difference in runoff.The check dam can increase the surface water storage of the watershed by more than 3 times.The construction of the check dam changes the spatial distribution of the maximum flow position,moves the maximum flow position upstream,and extends the time for runoff to reach the outlet of the basin by 3.6 times.Under the condition of no check dam,the maximum flow and average maximum flow rate of the watershed are 2.23 times and 1.52 times that of the state with check dams,respectively.Due to the gradual increase in sediment concentration in upstream water flow,the sediment carrying capacity of upstream to downstream water flow in the damless watershed section shows a decreasing trend.The maximum sediment carrying capacity of a section of the dam watershed occurs at 9#and the minimum occurs at 7#.The sediment carrying capacity of the section before and after the dam decreases from upstream to downstream.As the duration of the rainfall increases,the sediment carrying capacity of the non-dam section at each moment is greater than that of the dam section.For both the dammed and non-dammed watersheds,the sediment carrying capacity of the flow with the runoff process shows a form of double peak with dams and single peak without dams.(3)The causes of water damage of earth dam under typical rainstorm conditions are revealed,and the risk early warning system of the check dam system was constructed.During the rainstorm flood period,the damaged check dams are mostly small and medium-sized dams.During the ’7.26’rainstorm in Wuding River in 2017,the type of damage to the check dams in Zizhou County was mainly dam-body and spillway damage at the same time,while in Suide County the type was mainly dam-body damage.The main reasons for the flood damage of the check dam are as follows:the super-standard flood caused by heavy rainstorm exceeds the defense standard of the check dam,the supporting facilities of the dam system are not perfect,the check dam and supporting buildings are out of repair for a long time,and the management and maintenance are not in place.The flood control risk level of the check dam system increases with the frequency of rainfall,and the operational management risk,structural safety risk and leak assessment risk do not change with the increase.The flood-control safety assessment level for the Jiuyuangou check dam system is 3,which is at a high level.As the frequency of rainfall increases,the risk of flooding disasters increases from mild to moderate.There is no change in the evaluation grade of structural safety and seepage flow.Rainfall is a sensitive factor for the assessment of the flood control risk level of the silt-dam.As the rainfall increases,the flood-control risk of the check dams in the Jiuyangou basin is gradually increasing,and the large and medium-sized dams in the basic safety level are decreasing,especially the medium-sized dams are more prominent.When the frequency of rainfall changes from one in 10 years to one in 300 years,the basic level of safety in medium-sized dams is halved and the number of highly hazardous check dams in large dams is more than doubled.(4)The spatial and temporal distribution characteristics of water and sediment conservation in dam land were clarified,and the potential of water and sediment resource utilization was explored.The soil water content and soil water storage of Wangmaogou check dam system follow the order of large dam>medium dam>small dam,and the water storage of large dam accounts for 45.12%of the total water storage of dam land.From upstream to downstream,the soil water content of the series dam system show a gradual increaseing trend.The seasonal performance is autumn>winter>summer>spring.The soil water content is highest in each layer of large dams and lowest in small dams.From the front of the dam to the back of the dam,the soil water content decreases gradually,the soil water in the dam land have lateral flow.The STP,SOC,STN and SAP of dam land accounts for 12.67%,10.94%,11.33%and 11.34%of the total reserves of the basin,respectively,and gradually accumulated from upstream to downstream.The total reserves of STP,SOC,STN and SAP in large dam land accounts for 51.98%,43.50%,51.31%and 50.77%of the total nutrient reserves,respectively.Nutrients in the watershed are mostly stored in large dams,but small dams have higher soil quality than large and medium dams.The main sources of STP and SOC in dam land are sloping farmland,accounting for 50%and 65%of the total STP and SOC,respectively.STN and SAP are mainly derived from grassland.In the nutrient index,the interpretation of soil environmental factors as soil nutrients is about 1.70 times higher than that of surface environmental factors.STP,SOC,STN and SAP are significantly correlated with soil water content,and the soil nutrient content increasing significantly with the increase of water content. |
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