Research On The Topography And Rainfall Actors For Debris Flow

Debris flow is a common geological hazards on slopes or valleys,It triggered by the excitation of external conditions such as rainstorm,snowmelt and lake burst,usually with fast speed of the movement,sudden occurrence,enormous energy.Shallow landslide-type debris flow belong to rainfall debris flow,It consists two stages,firstly soil on the slope is saturated after water uptake by rainfall,and then soil slide as internal mechanics properties changes,last it forms debris flow in the process of sliding,hence,this kind of debris flow has dual nature of the both landslide and debris flow,it has not only huge impact and destructive.Due to the change of forming conditions,conventional method is no longer applicable.Finally,It is meaningful to study the formation condition of shallow landslide-type debris flow with reasonable method and to provide scientific theoretical foundation for the disaster prevention and reduction.Any kinds of debris flow form without geology,topography and rainfall,rainfall is an element of triggering debris flow,and geological and topography condition is the potential term.The previous researches on debris flow occurrence focused on the analysis of single condition such as rainfall,geology,topography,without consideration of The comprehensive conditions.The article study the "6.6" debris flow within Wangmo,Guizhou,discusses that the one of comprehensive factor included rainfall and geology expresses the critical topography value in the shallow landslide-type debris flow's formation process under the premise condition of consistent geological condition,and then the model is applied to verify the shallow landslide-type debris flow in Chenyulan River watershed,Taiwan after The Midulle typhoon.The article studys under the premise condition of consistent geological condition,it means that formation lithology and geological structure in the study region is basically the same,we divide Chenyulan River watershed,Taiwan into four regions according to the outcrop,and analysis the comprehensive factor of topography and rainfall in each region.(1)In the "6.6" debris flow within Wangmo,Guizhou,there are thirty seven gullies belong to shallow landslide-type debris flow among one hundred gullies along the Wangmo river from Dayi town to Xintun town.The formation mechanism of this kind of debris flow is that heavy rainfall triggers shallow landslide and landslide form debris flow in the process of slide.As shallow landslide-type debris flow could form in the any area of the slope in both sides of channel,the research region is the whole basin.(2)It is the gully-shape debris flow if there are debris flow outbreak but no landslides distribute in the basin when confirm the type of debris flow;It is shallow landslide-type debris flow when the volume of landslides is equal to the value of the deposition area and fine grain washed away by flood;When debris flow in the some gullies washed to river and we can't calculate accurate volume,they may be both types of gully-shape and shallow landslide-type debris flow and be major with one type,Or may be break out shallow landslide-type debris flow and the gully-shape debris flow can't confirm,we define five hundreds as the critical volume of landslide,it is shallow landslide-type debris flow when the volume is greater than or equal to 500m3,it is gully-shape debris flow when the volume is less than 500m3.(3)The article chooses topography partments that include the basin A,the hillside degree S,the longitudinal shrinking slope J,it ascertain 25°?45° is the slope that most beneficial to shallow landslide-type debris flow development according to comparison,Between the A and S and J are by contrasted,It obtains the critical topography factor of T(dimensionless),that its expression is T=S(A/A0)0.2J0.3.Among the topography actors,the hillside degree is the most important,the hillside degree and the longitudinal shrinking slope are equally important although the weight of them are low.(4)On the base of formation mechanism of shallow landslide-type and in combination with the characteristics of the rainfall and time of debris flow outbreak,the article chooses the I-P critical rainfall mode included cumulative rainfall and hourly rainfall,that its expression isR=B+KI/R0,it confine K-value is 5.5 according to the formation mechanism and characteristics of the rainfall in the basin,average annual rainfall is to normalize the critical rainfall.(5)Between the topography parameter T and rainfall parameter R are by contrasted,it get a comprehensive factor P(dimensionless),that its expression is P=RT0.45=(B+55/R0)(SJ0.3A0.2)0.45,p can express the formation condition of shallow landslide-type debris flow in the base of same geological condition.(6)Using the parameter P to the 91 gullies in Chenyulan River watershed,Taiwan after The Midulle typhoon and no consideration of differences of soil in different regions,we get the same result as well as Wangmo,so it is universal that the study of formation condition included topography and rainfall about shallow landslide-type debris flow in this article.