The Dynamic Evolution Mechanism Of Sediment Source Of Debris Flow In Seismic Area And The Effect Of Check Dams

The 2008 Wenchuan earthquake triggered a significant number of co-seismic landslides and rock avalanches,resulting in sediment instability within the region.Consequently,the seismic area provided abundant loose deposits and potential sediment sources for subsequent debris flow events,thereby promoting their occurrence.The formation of debris flows is influenced by geomorphology,rainfall patterns,and the availability of loose sediments as sources.In the Wenchuan seismic area,characterized by deep-cut alpine canyons with concentrated annual rainfall,favorable conditions for debris flow formation are easily met.Under this geological setting,the frequency and magnitude of debris flows are primarily determined by sediment evolution within gullies.Additionally,sediment migration within basins accumulates in check dam regions,which also impacts their effectiveness.Given these circumstances,studying sediment evolution and assessing check dam efficacy in seismic areas holds great practical significance for preventing and managing subsequent debris flow events while ensuring effective maintenance of protective engineering measures.The evolution of sediment in different spatial and temporal scales was identified in this study by integrating multiple detection methods with typical debris flow events in Cutou Gully.Downscale physical models were employed to invert the evolution mechanism of sediment before and during debris flow events.By combining disaster phenomena with physical characteristics,a quantitative evaluation system for assessing the effectiveness of check dams in controlling sediment was proposed.Finally,the control effect of check dams on sediments was revealed through numerical simulation.The significant research findings and specific conclusions are described as follows:1.Considering the challenges associated with delayed field investigations on sediment source in debris flow,a historical tracing scheme of sediment evolution before the debris flow event is constructed by integrating satellite remote sensing images and In SAR technology,which carry valuable historical information.By focusing on the spatial distribution and magnitude variation of active sediment as well as the cumulative surface deformation in the catchment area,this study successfully identifies the macro-evolution and migration trends of sediment prior to the"8.20"debris flow event in Cutou Gully in 2019.The obtained results demonstrate:（1）significant responses of sediment to key disaster events such as the Wenchuan earthquake and the"7.10"debris flow event in 2013,while also revealing（1）pre-debris flow sediment distribution patterns under low rainfall thresholds,thereby providing essential data for future research endeavors.Specifically,seismic activity induces sediment instability,leading to an increase in sediment sources,while debris flow releases sediment sources and promotes sediment stability.Additionally,over time,there is a decrease in slope sediment and an increase in channel sediment within Cutou gully.2.According to the dynamic evolution of sediment at different spatial scales,such as inaccessible high elevations,along the main channel,and typical landslide or avalanche deposits,we monitored and quantitatively analyzed the macro-evolution of sediment during debris flow events using satellite remote sensing images,UAVs,terrestrial laser scanning,field investigation,and testing.Specifically,this analysis can be divided into two scales:（1）catchment scale and（2）deposits scale.At the catchment scale（1）,we reveal the processes and causes of the"8.20"and"8.17"debris flow events in 2019 and 2020 respectively.Furthermore,we explain the nature of catastrophic debris flows in 2020 under a low rainfall threshold by proposing a new perspective that highlights how continuous evolution of sediment within a catchment area coupled with reduced efficiency of check dams can lower the disaster threshold for debris flows or even induce repeated events.At the deposits scale（2）,The distinct evolutionary patterns of sediment and their implications for debris flow disasters are unveiled.As sediment continues to evolve,the heterogeneity and permeability of sediment gradually intensify.3.Taking the typical channel sediment in Cutou Gully as an example,a downscaled physical model was constructed based on the similarity principle.The different hydrodynamic conditions were established to simulate the evolution process of sediment before and during debris flow disasters.During the simulation,we focused on elucidating the macroscopic development process of sediment evolution,examining how the final form responds to hydrodynamic conditions,and analyzing variations in water content and pore pressure.Consequently,we revealed the underlying mechanism governing sediment evolution.The maximum water content of sediment is approximately 39%,and the evolutionary mechanism of sediment as well as its response to hydrological conditions exhibit significant disparities.When prior to debris flows,sediment evolution is primarily influenced by infiltration of fine particles and localized surges in pore pressure,exhibiting a certain degree of randomness in instability magnitude.Conversely,during debris flows,runoff erosion governs sediment dynamics with instability magnitude being directly proportional to runoff intensity.4.According to the field characteristics of debris flow and the physical model,we propose an evaluation system for assessing the sediment control efficiency of check dams.This system consists of three indexes(DEI,DEI_total,and DEI_d),and we have improved the analysis method for evaluating sediment control efficiency and studying the effectiveness of check dams.In conjunction with the evaluation system and quantitative results of sediment evolution,this study analyzes the sediment containment efficiency of check dams during recent debris flow events,which is influenced by the remaining reservoir capacity.The findings indicate that as the remaining reservoir capacity decreases（from 91.8%to 31.2%）,the check dam event promotes sediment release（DEI=42.7%to-75.5%）.Furthermore,using a simulation method that considers sediment erosion and accumulation,we develop a generalized numerical model for check dams to investigate how their size,spacing,channel gradient,and sediment particle size affect their sediment control efficiency,which is not burdened by excessive preconditions and restrictions and considering the physical characteristics of the dam.Based on the above results,the sediment control mechanism of check dam is revealed.The sediment control mechanism is not limited to the disaster protection of debris flow in the seismic area,and can be applied to the arrangement of different types of debris flow measures,which has remarkable universality and extensibility.