| Debris flow is a common natural disaster phenomenon occured in the mountains,which is a special flood triggered by rainstorm or other natural disasters included alarge amount of sand and stones. Debris flow is also the solid-liquid two-phase flowbetween the water flow and landslides.According to the formation of dynamicconditions Debris flow can be divided into two kinds, the soiled debris flow and thehydraulic-Droved debris flow.The Karst landscape is development in GuizhouProvince,where the geological structure is strong and gully debris flow isdevelopment.And the debris flow has resulted in more casualties and property losses.Therefore, The study of typical debris flows initiation mechanism by channel runoffof the Karst region in Guizhou province is very necessary,which can provide atheoretical basis and methods for the establishment of the prediction model of debrisflow in this area.Through field survey and basic data collection of the9debris flow gully inHouchang town of Weining County of Guizhou province,the initiation mechanism ofdebris flow in this area is studied,and the critical model of debris flow initiated isestablished combined with the laboratory experiments, the conclusions is followed:(1) The study area is located in the two-Tong River area of the origin of Sanchariver at Wujiang upstream.In the study area, the limestone is widely distributed andthe dissolution landform is development featured the typical Karst landscape. Affectedby the Karst landscape, the surface of debris flow gully is relatively dry, non-perennialwater flow.And the rain is usually infiltrated into the underground through the Karstcave and river underground. The longitudinal slope of channel is relatively gentle.Theloose material accumulated on the bed can be divided into two layers: on the upper isthe loose material which’s particulate is large and the lower is the smaller particulatematerial which is relatively unstability,and the two layers has high permeability. Thesefactors all affect the initiation of the debris flow in study area.(2) Through the field investigation and the mechanics analysis of debris flow instudy area,the initiation of debris flow in Karst area is mainly affected by the slope ofthe bed,material particle accumulated on the bed and the rainfall intensity,where therainfall plays a dominant role. The surface runoff generated by rainfall is the triggering factor to initiate the debris flow.According to the three stages of the formation of surface runoff,the initiation process of debris flow can be divided into three steps,such as, the saturated step of low-intensity rainfall conditions,the seepage occurring step of moderate-intensity rainfall conditions,the formation of overland runoff and the initiation of debris flow of high-intensity rainfall conditions.(3) According to the Mohr-Coulomb failure criterion,the method of static equilibrium is used to analyze the critical initiation state of the debris flow,and then to establish the critical depth about the surface runoff of the debris flow initiation in the study area.Through the formula, it is known that the critical depth (hw) of runoff is affected by the bed slope (θ) and the thickness (Z) of coarse layer on the bed.When the thickness (Z) is invariant,the critical depth (hw) required by the initiation of debris flow may decrease with the increase of the bed slope (θ).And when the bed slope (θ) is invariant, the critical depth (hw) may increase with the increase of the thickness (Z).(4) The analysis of experiment shows that the data from the experiment and theoretical analysis is uniform in the analysis of the effect of the coarse layer thickness Z and the bed slope θ to the initiation of debris flow,which proves that theoretically critical initiation formula is reasonable feasible,but the difference in value of the two methods shows that theoretical formula is imperfect and limited. The main reason causing this difference is the neglection of the effect of the cohesion among the particles of the coarse layer and the seepage underground on the initiation of debris flow. In addition, the density of the bed material is another reason for this difference.Therefore,it is necessary to amend the theoretically critical initiation formula according to the experiment results,and the following formula is obtained:(5)The comparison between the amended formula and experiment shows that,the amended critical initiation formula can be applied to the situation that the bed slope of the initiation area is not greater than21°,such as,θ≤21°.Therefore,it also can be used to predict the initiation of debris flow in the study area.(6) According to the amended initiation formula of debris flow in the study area,7debris flow gully is calculated to get ratio hw/Z from0.22to0.35and ho/Z from0.08to0.13.Based on these ranges, three early forecasting models are established,such as, when h/Z is from0.08to0.16, it’s blue alarm, when h/Z is from0.16to0.22, it’s orange alarmt, and when h/Z is over0.22,it’s red alert, as shown below. According to this prediction model,the hazard warning is announced timely to prevent the disaster.0.08<h/Z≤0.16Blue warning0.16<h/Z<0.22Orange warningh/Z≥0.22Red warning... |
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