| Since the landslide dam is formed by rapid accumulation,whose structure is generally loose,so it is prone to burst,forming the “Outburst Deposits”.As material records,the outburst deposits of landslide-dammed lakes are known to contain abundant geologic and geomorphologic information,which become an important way to understand the dam-break process,along with the mountain environment evolution law of landslide-dammed lakes.This information has an important guiding function for the effective prevention and control of future geohazards.The Diexi paleolandslide-dammed lake inferred from field investigations is located in the Diexi Reach of the Upper Minjiang River,from Diexi Acient Town to Taiping Villige,which is approximately 30 km in length.Recent field investigations have located the natural outburst deposits of a landslide-dammed lake in the Diexi segment of the Upper Minjiang River.As indicated by several lines of evidence,these outburst deposits are approximately 5 km long.This type of deposit has seldom been seen.Field geological investigation,sampling,laboratory tests,and quantitative calculation methods were comprehensively used in this study to analyze the sedimentary characteristics and grain-size characteristics of the outburst deposits induced by the Diexi paleolandslide-dammed lake.Then,the quantitative inversion of dam-break flood was studied.Finally,the cause of the Diexi ancient damming incident and occurrence time of the Diexi paleolandslide-dammed lake outburst event were analyzed.The following results are obtained in this study:(1)The outburst deposits induced by the Diexi paleolandslide-dammed lake were mainly formed by two types of sediment transport regimes: the hyperconcentrated flood flows and fluctuating and river flows.The imbricate,cavitation,matrix,massive,and gravel support-stacked structures formed in the outburst deposits.Moreover,A special rhythmite interbedded sedimentary structure had developed in the accumulation profiles.Two types of sediment facies developed in the outburst deposits: the boulder facies(Bcm)and Gravel and sand facies(Gm,Gfm,GRm,Sh,Sp and St).Each of the boulder facies presented a single event of high fluvial discharge,whereas the gravel and sand facies had been deposited by fluctuating and river flows.(2)The outburst deposits is mainly composed of boulders,gravel(fine to coarse),sand(fine to coarse),silty sand and silty,and a small amount of clay,in which,the coarse particles may have originated from the dam-body materials.Meanwhile the fine particles may have partly originated from the dam-body,and also from the lacustrine sediment in the upstream sections of the barrier lake,along with the river-bed material.(3)Obvious fractal features were observed in the outburst deposits,which displayed a self-similarity in geometrical shape,and a scale invariance in size magnitude.Furthermore,the particle-size fractal dimension value of the deposits presented a decreasing trend from the upper segment to the lower segment.(4)This study concluded that the large Diexi ancient landslides were most likely formed by ancient earthquake events.It could be deduced that the time limits of the preserved lacustrine sediments ranged between 10 and 40 ka BP according to the the results of the aforementioned research studies.This indicated that the occurrence time of the Diexi paleolandslide-dammed lake outburst event occurred during the early Holocene period.Estimated by the water flow energy method in water dynamics,the average velocity of outburst flood induced by the Diexi paleolandslide-dammed lake was 17.23 m/s,with a peak flow of 49821.28 m3/s,which was dozens of times the Minjiang River perennial flow. |
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