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Debris Flow Assessment from Rainfall Infiltration Induced Landslide

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Debris Flow Assessment from Rainfall Infiltration Induced Landslide 7th International Conference on Debris-Flow Hazards Mitigation Debris flow assessment from rainfall infiltration induced landslide Yu-Charn Hsu a,*, Ko-Fei Liu a, Hung-Ming Shu b aDepartment of Civil Engineering National Taiwan University, No.1, Sec.4, Roosevelt Rd., Taipei 10617, Taiwan ( R.O.C.) b Taitung Branch of Taiwan Soil and Water Conservation Bureau, No.665, Sec. 1, Zhonghua Rd., Taitung City 950, Taiwan (R.O.C.) Abstract In the study, debris flows induced by landslides are studied through physical models. TRIGRS and DEBRIS-2D models are integrated for simulation of rainfall infiltration induced shallow landslide and the subsequent debris flows. TRIGRS is used to estimate unstable mass on the hillslope and provide the initial volume for debris flow simulation, and DEBRIS-2D is applied to simulate mass motion and assess the hazard zone mapping. The method is applied to Daniao tribe’s sediment disaster during Typhoon Morakot in Taiwan. The simulated final deposition zone and the disaster area in the real event are almost identical. All the geophysical parameters are obtained through official values and rheological parameters are obtained by in situ measurements. Keywords: TRIGRS, DEBRIS-2D, estimate unstable mass, the hazard zone; 1. Introduction Rainfall infiltration will increase soil moisture. As a result, shear strength is reduced and pore pressures and seepage forces are increased. Enough rainfall causes hillside failure, and the failure mass will slide down or turn into debris flows with enough water. Many studies used empirical or statistical method to obtain landslide potential analysis and realize the hazard zone mapping for debris flow. But physical process combining landslide prediction and debris flow simulation is considered more precise in smaller scale. Many researches have used coupled methodology to simulate a debris flow mobilization from a shallow landslide. Chiang et al. (2012) have combined a landslide susceptibility model in landslide prediction, an empirical model to select debris flow initiation points among predicted landslide area and a debris flow model to simulate the spread and inundated region of failed materials from the identified source areas. Gomes et al. (2013) have combined two physical models of SHALSTAB and FLO-2-D to model debris flow spreading area. Wang et al. (2013) have combined limit equilibrium theorem and 2-D depth-integral flow model to assess landslide and a debris flow processes. In this study, TRIGRS (Baum et al., 2010) and DEBRIS-2D (Liu and Huang, 2006) models are coupled in an assessment with rainfall infiltration amount. TRIGRS is a well-known model used in estimating collapse region from rainfall infiltration. DEBRIS-2D has been successfully applied to a hazard zone simulation of debris flow, but DEBRIS-2D needs input for failure volume and location. Therefore, TRIGRS is used to estimate unstable mass on the hillslope and provide the initial volume for debris flow simulation, and DEBRIS-2D is applied to simulate mass motion and assess the hazard zone mapping. This way, TRIGRS and DEBRIS-2D models are integrated for simulation of rainfall infiltration induced shallow landslide and the subsequent debris flows. _________ * Corresponding author e-mail address: [email protected] Hsu / 7th International Conference on Debris-Flow Hazards Mitigation (2019) 2. Fundamentals 2.1. Rainfall Infiltration Rainfall infiltration causes soil water content to increase until saturation and then raise the water table. Therefore, the pore pressure in the saturated state needs to be calculated first. Consider a rectangular Cartesian coordinate system with its origin at an arbitrary point on the ground (see Fig.1), the x axis points to down slope, the y axis points to tangents the topographic contour, and the z axis is normal to x - y plane and points into the slope. The fundamental of rainfall infiltration simulation is based on Iverson’s (2000) linearized solution of Richard’s equation. A generalized solution with an infinite basal boundary is expressed in equation (1), and an impermeable basal boundary at a finite depth is given by equation (2). The first term on the right hand side in equations (1) and (2) represents the steady solution and remaining terms on the right hand side represent the transient solution. (,)[Z tZd ] , (1) N I Z 2()[()][nz Ht t Dt t1/2 ierfc ] nn1 1/2 n1 KDttzn2[1 ( ) ] N I Z 2()[()][nz Ht t Dt t1/2 ierfc nn+1 1 1 1/2 n1 KDttzn2[11 ( ) ] (,)[Zt Z d ] . (2) (2md 1) ( d Z ) ierfc[]LZ LZ N 1/2 Inz 1/2 2[Dt1 ( tn ) ] 2()[()]Ht tnn Dt1 t K (2md 1) ( d Z ) nm11z ierfc[]LZ LZ 1/2 2[Dt1 ( tn ) ] (2md 1) ( d Z ) ierfc[ LZ LZ ] N 2[Dt ( t )1/2 ] Inz 1/2 1+1n 2()[()]Ht tnn+1 Dt 1 t +1 K (2md 1) ( d Z ) n1 z m1 ierfc[]LZ LZ 1/2 2[Dt1+1 ( tn ) ] Fig.1 Coordinate system diagram of TRIGRS model. Hsu / 7th International Conference on Debris-Flow Hazards Mitigation (2019) Equation (1) applies where hydraulic properties are uniform and equation (2) applies where a well-defined decrease in hydraulic conductivity exists at a finite depth. In the equations φ is the groundwater pressure head, t is time, θ is the slope angle of x axis, Z = z / cosθis the failure depth, d is the initial depth of the water table measured in Z direction in steady state, dLz is a depth of impermeable basal boundary measured in Z direction, β = λcosθ[λ = cos θ- (Iz - Kz)LT, with Kz the hydraulic conductivity in Z and Iz initial surface flux], Inz means a surface flux of a given th 2 intensity in n time interval, D1 = D0 cos θ with D0 the saturated hydraulic diffusivity), H(t - tn) is Heaviside function. The function ierfc is defined as 1 ierfc() exp(2 )erfc () (3) 2.2. Slope Stability Analysis Iverson (2000) used an infinite-slope stability analysis to model a hillslope stability. The ratio Fs called the factor of safety is calculated at Z depth by (4). tan CZt(,)w tan (4) FS tans Z sin cos where is the friction angle, C is the cohesion of soil, both for effective stress, w is specific gravity of water and s is specific gravity of soil. Equation (4) expresses the failure of the infinite slope by the ratio between resisting from basal Coulomb friction to gravitationally induced downslope basal driving stress. The hillslope fails for Fs < 1. Therefore, the depth H = z and area A of the hillside in unstable (Fs < 1) condition, the product H and A will provide volume for the mass motion simulation. 2.3. Debris flow A hillside fails when Fs < 1, and this mass will mix with water and become debris flow as it moves down slope. A physical model, DEBRIS-2D (Liu and Hung, 2006) adopted depth integrated form of conservation law under long wave approximation in the plug flow region and has been successfully applied in debris flow simulation, which is original developed by Liu and Huang (2006). DEBRIS-2D with inclined coordinate system (see Fig. 2), x coincides with flow direction, y tangent to topographical contour direction and z normal to x - y plane and points to depth direction. The velocity components in the x, y directions are u and v respectively, θ is the inclined angle, τ0 is the yield stress, H = h - B is the flow depth (where h is the free surface and B is the natural bottom of the debris flow). The momentum equations in conservative form are shown in equation (5) and (6), the continuity equation is shown in equation (7). uH u2 H uvH ()BH 1 u gHcos gHsin 0 , (5) txy x uv22 vH uvH v2 H ()1 B H v gHcos 0 , (6) txy y uv22 HuHvH 0 , (7) txy The initial velocities when the hillslope just fails are u = 0 and v = 0, and the initial depth H is obtained from the slope stability analysis results under the instability condition Fs < 1 in equation (4). Three unknowns H, u and v could be solved from three independent equations (5), (6), and (7). Hsu / 7th International Conference on Debris-Flow Hazards Mitigation (2019) Fig.2 Coordinate system diagram of DEBRIS-2D model. 3. Descriptions of Environment and Modelling 3.1. Surface Survey The landslide is located on the upper hillside of the Daniao tribe as in Fig. 3. The failure source on the ground was composed of slate, mudstone, sandstone and weathered gravel, which are all easily movable under external forces. The sieve analysis gives D10 = 0.98 mm, Dm = 55.94 mm and maximum is Dmax = 420 mm. (a) Photographs of Daniao tribe’s landslide after Typhoon Morakot (b) Particle distribution of Daniao tribe’s landslide Fig. 3 Surface survey results. 3.2. Topographical Analysis The 2 m × 2 m digital terrain model is used for the topographic analysis of the Daniao tribe’s sediment disaster. The watershed area is approximately 52.38 ha, and the elevation changes from 60 m to 480 m, and the slope distribution is from 0° to 70°, the major stratigraphic trend is from the east to the west. The landslide occurred mostly within the steeper area (slope greater than 15°). The distributions of elevation, slope and flow direction of the hazard zone are shown in Fig. 4. Hsu / 7th International Conference on Debris-Flow Hazards Mitigation (2019) Fig. 4 Topographic analysis results. 3.3. Rainfall Event During 2009 August, Typhoon Morakot struck Taiwan and induced sediment disasters throughout Taiwan. Daniao tribe watershed landslide is one of sediment disasters which is occurred in Eastern Taiwan. Typhoon Morakot produced heavy rainfall to Daniao tribe watershed from 2009/08/07 09:00 to 2009/08/10 03:00.
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