Impact Mechanism Of Ecological Construction On Flood Dynamics In Typical Watershed Of Loess Plateau

The severe soil erosion on the Loess Plateau has led to the continuous deterioration of the local ecological environment and restricted agricultural production and socio-economic development.Monitoring and research indicate that the past 30 years have been the period of greatest changes in the underlying surface of the Loess Plateau over the past century.The implementation of comprehensive management measures such as returning farmland to forest(grassland),slope to terrace,and silt dam has caused significant changes in the soil erosion environment of the Loess Plateau,and the regional ecological environment has been significantly improved.Deeply studying the evolution law of rainstorm and flood in the basin under ecological construction and control is an important point of high-quality development of the Yellow River basin in the new era.Clarifying the regulation effect of ecological construction and control measures such as silt trap dam and land use change on the flood process in the basin is of great significance to deeply reveal the impact mechanism of ecological construction and control on the flood dynamic process.This study takes Jiuyuangou,a typical ecological construction watershed in the Loess Plateau,as the research object,through field research and field observation,combined with numerical model simulation and other methods,analyzes the evolution characteristics of rainstorm flood in the ecological construction watershed,studies the impact of the layout of warping dams on the flood process in the watershed,and quantifies the regulation law of the number and location of warping dams on the dynamic process of flood in the watershed,Clarified the mechanism of peak regulation and flood discharge,as well as sediment retention and erosion reduction in the silt dam system,and determined the response relationship based on extreme land use changes and flood processes.The main conclusions of this study are as follows:(1)The evolution law of rainstorm and flood in the basin under the ecological construction and control measures is studied,and it is determined that climate change and underlying surface change are the main driving factors of rainstorm and flood.Mann Kendall test and other methods are used to analyze the trend of the annual maximum rainstorm flood elements in Jiuyuangou watershed.The results show that the peak discharge,secondary flood runoff depth and sediment discharge series all show a decreasing trend,of which the sediment discharge trend is the most significant,from the average 530kg/m~3 in the 1960s to the average 129kg/m~3 in the 2000s,down 75.5%,and the runoff depth trend is the least significant.There are two factors that affect the changes in water and sediment in a watershed,namely climate factors and underlying surface factors.Among them,the underlying surface factor plays a leading role,and the weight of soil and water conservation measures in the peak flow is significantly greater than that of land use.(2)The impact of the layout of silt dams on the flood process in the watershed was revealed,and the regulatory effect of the number and location of silt dams on the flood dynamic process in the watershed was quantified.The construction of silt dam system significantly reduces the peak flow and total flood volume of the basin,increases the duration of the flood,and has a greater impact on peak shaving and flood detention than on retention.The peak reduction rate,flood volume reduction rate,and flood duration increase rate all show a linear increasing trend with the number of silt dams,and the silt dam system has the best linear response to the peak reduction rate;The construction of warping dam system reduced the peak flood flow by 67.78%,the total flood volume by 21.15%,and the flood duration by 172.97%under the rainstorm conditions of different return periods on average;As the number of silt dams increases,the variation coefficients of peak flow,average flow,and flow all show a decreasing trend.Silt dams not only weaken floods,but also make the flood process more stable.The silt dam system reduced the runoff velocity in the watershed,resulting in a 54%decrease in the average flow velocity and a 58.56%decrease in the peak value after the dam was built.It also had a greater impact on the runoff velocity in smaller return periods.As the return period increases,the shear force and runoff erosion power also increase accordingly.The construction of silt dams significantly reduces the erosion power intensity in the middle and lower reaches of the basin,effectively suppressing the loss of sediment in the channel.(3)Clarified the mechanism of peak regulation,flood discharge,and erosion reduction of the silt dam system,and determined the relationship between extreme land use change patterns and basin flood response.The terrain changes caused by the sedimentation of silt behind the warping dam system and its dam have significantly weakened the dynamic process of channel floods,increased the basin’s ability to regulate and store runoff,and changed the catchment process of the basin.The flow velocity and water level in front and behind the dam show a sudden decrease trend,significantly weakening the intensity of floods and having a high sediment interception effect.Through setting extreme land use scenarios,research has found that changes in land use have a significant impact on flood processes:the increase in arable land and residential land will enhance the intensity of flood peaks,with an average increase rate of 34.5%for residential land flood peaks;On the contrary,the increase of forest land and grassland can weaken the flood peak,with the weakening degree of forest land and grassland being 13.46%and 8.26%,respectively.In addition,forest land and grassland also have a good effect in reducing flood volume,with an average decrease of 8.18%and 11.27%,respectively.