Response Behavior Of Groundwater In Vegetated Slopes To Rainfall And Mechanism Operating

Landslide disasters which are spread in the mountainous area worldwide play an important role in the evolution of global geomorphology.Recently,due to the intensification of the greenhouse effect,extreme weather events occur frequently,affecting the stability of slopes at different time and spatial scales.The torrential floods,landslides,debris flows and other regional geological disasters caused by rainfall will have great destructive power in the narrow and steep valley environment,which cause serious threat to the lives and properties of mountain residents.Researches on the process chain of “rainfall?rain infiltration into slope groundwater?sloping groundwater and rock-soil interaction?landslide” mostly focuse on the starting point and the end point,as well as the water-rock interaction,but the conversion process of rainfall to slope groundwater still lags behind.Therefore,it is necessary to carry out the response behavior of groundwater to rainfall and its control mechanism in vegetated slopes.In this paper,the Touzhai watershed of Zhaotong,Yunnan,where a landslide event with a volume of about 9 × 106 m3 occurred and killed 216 lives,is taken as the research object.Through the methods of in-situ monitoring,testing and indoor theoretical analysis and numerical simulation,the distribution characteristics of rainfall in mountain areas and the process and mechanism of rainfall converting to groundwater in slopes are studied.The main achievements in this thesis are as follows.1.The field and laboratory test results of the physico-mechanical behavior of the basalt slope in the study area indicate that the grain size distribution of the slope soil within 1 m depth is homogeneous,the density and specific gravity increase with the increment of the soil depth,and the expansion degree has a trend of decreasing with depths;the annual moisture contents of the soil samples within 1 m depth in two test points are extremely high,with the average value of 144.3%,which is much higher than that of the general soil.2.Affected by topography and elevation,the annual rainfall amount in the upstream(MS.2)of the Touzhai valley is 1.8-fold greater than that of the downstream(MS.1);lowintensity precipitation appears more frequently at MS.1,and rainfall at MS.1 in 1 day is more dispersive than that at MS.2;the independent rainfall events at MS.2,whether rainfall or rainfall duration,are always greater than those at MS.1,and during heavy rainfall,the rainfall intensity level at MS.2 is always higher than that at MS.1;the characteristics of individual precipitation events indicate that,for both rainfall amount and duration,the respective magnitude is greater at MS.2 than at MS.1,and the rainfall grade at MS.2 shows a trend that is one grade or more greater than that at MS.1 during intense rainfall events;the rainfall at MS.2 always starts earlier and ends later than that at MS.1.The distribution of diurnal rainfall at both MSs is heterogeneous,showing a pattern of raining less during the daytime and more during the nighttime;the spatiotemporal distribution of rainfall in Touzhai valley suggests that the quality of rainfall data can be improved by positioning more rain gauges at/near the landslide detachment area for a single potential failure,rather than by increasing the rain gauge density at inhabited areas situated at a relatively low elevation.3.The monitoring results of the stream flow in the study area indicate that the annual runoff distribution in the study area is extremely uneven,and the flow rate of the stream shows a rapid “growth-peak-fall” situation.Each link of the process can respond quickly to the variation of rainfall within one day,even if the daily rainfall changes only a few millimeters.4.The similar magnitudes of Si O2 concentrations in the stream water and leachate suggest that the stream water is almost entirely recharged by the groundwater in the valley slopes.Thus,the rapid variation of stream flow rate with rainfall reflects the real-time fluctuation of groundwater table following rainfall.5.The in-situ experiments of double-ring infiltrometer and dye tracer infiltration show that the soil of the vegetated slope is excellent in permeability,which is attributed to the largescale macropore system developed in the soil.The average values of the hydraulic conductivity(KS)of the soil surface,10 cm and 60 cm depth at the two test points are 1.94×10-2?7.54×10-3? 1.13×10-2 cm/s,respectively.These macropores are mainly divided into “macropores related to biological activities” and “structural macropores”.The former mainly includes root-soil interstice,decay root channel and animal passage,the latter mainly includes inter-aggregate pore,soil-gravel interstice and soil crack.In addition,the mechanical barrier and reduction of evaporation of the litter layer on the surface of the slope soil promote the formation,development and maintenance of macropores.6.The CT scan of the large volume(28 × 28 × 50 cm)undisturbed soil sample and the three-dimensional reconstruction of the macropores based on it show that most of the macropores are round tubular with different sizes;the maximum effective diameter is 4 cm;the macroporosity of the sample surface(Z = 0)is 28.08%.As the depth increases,the macroporosity decreases linearly.At the depth of Z = 399 mm,the macroporosity decreases to 11.37%.7.The three-dimensional model of the soil macropores is compiled into a MODFLOW readable model by programming,and the steady and unsteady seepage simulations are carried out.Both results showed that the migration velocity of the water in macropores is much larger than that in the soil matrix,with the limit of 0.02 m/s;the maximum migration velocity of water in the some large pores of >1 cm in diameter is between 10 and 30 cm/s;the water moves faster in macropores with better connectivity and has the highest velocity at the “throat”.Taking the macropore centerline as the reference,the water flow velocity gradually decreases during the movement to the edge.In addition,the unsteady seepage simulation shows that the migration velocity of water gradually decreases with time elapsing,which is caused by the decrease of the initial head over time.8.The macropore system developed in the well vegetated slope soil provides a series of relatively unimpeded preferential flow channels for rainwater infiltration,enabling the water flow to reach the soil-bedrock interface in a short time and transforming into groundwater to recharge stream,due to which the stream flow could respond quickly to rainfall.This rapid response behavior indicates that only long-duration and high-intensity rainfall may cause the slope failure.In the face of the difficulty in monitoring the mountainous area prone to fail,the rapid response of stream to rainfall provides an effective and easy-to-promote method for understanding the dynamics of slope groundwater.The monitoring of stream flow may be one of the important means of warning and forecasting rainfall-induce slope disasters.In the actual implementation process,stream monitoring locations can be moved down to the residential area to avoid difficulties in power supply and communication in high-altitude mountainous areas.