FORMATION CONDITIONS OF SLOPE TYPE MUDFLOW IN NINGSHAAN COUNTY, SOUTHERN SHAANXI,AND ITS INDUCING MECHANISM

The occurrence of slope mudflow is a process in which a landslide first occurs and then rapidly changes into mudflow.It has some properties of landslide and mudflow concurrently.The slope mudflow around the seat of Ningshaan County occurs on a steep slope of Middle Devonian strata.The eluvial soils on slopes where surface flow occurs are mainly clayey soils with some blocks or debris.They have good grading distribution and a thin soil horizon.The clay minerals in the eluvial soils consist of illite/montmorillonite,illite and kaolinite with low and intermediate dilatability,so their permeability is low or very low.When rainstorms persist,such soils will start to slide down the slope and be eroded,and furthermore,they rush down and form slurry,thus giving rise to slope mudflow.The canyons of slope mudflow are shallow and wide and mostly occur downslope,and sometimes those that obliquely intersect the slope direction but are parallel to the strike of the strata are also seen.They are embryos of canyon type mudflow.Therefore,it is necessary to study the dynamic features of slope type mudflow to provide a scientific basis for early-warning of mudflow disasters.