Coupling TRIGRS and TOPMODEL in shallow landslide prediction 1 Presenter: 王俊皓 Advisor: 李錫堤老師 Date: 016/10/13
Outline Introduction Literature review Methodology Pre-result Future work
Introduction 3 Motivation Shallow landslide is influenced by groundwater table depth of soil layer, predicting groundwater table depth can analysis slope stability effectively. Coupling hydrology theory to considerate lateral recharge of groundwater.
Introduction 4 Approaches to evaluate landslide Statistical approaches : Logistic Regression. Deterministic approaches : Physically-based models Steady state model : SHALSTAB MODEL(Montgomery et al., 1994) Transient model : TRIGRS (Baum et al., 00) Weight Slope angle Friction force
Introduction 5 Water behavior of a catchment Rainfall Infiltration Ground surface? Surface runoff Percolation River Groundwater flow Groundwater table Seepage
Introduction 6 Flow chart Geology and hydrology parameters Hydrological model(trigrs) Refine groundwater table depth with TOPMODEL Infinite slope model Prediction of landslide occurrence
Literature review 7 Literature review Combining an infinite-slope stability calculation with a transient, one-dimensional analytic solution for pore pressure response to transient rainfall infiltration. (Iverson, 000; Baum et al., 00; Savage et al., 003; Godt, 004) TRIGRS models were used for slope stability analysis in Taiwan. (C.C. Wu, 006; P.C. Wang, 007; S.H. Chung, 008) Concept of topographic index, ln(a/tanβ), and TOPMODEL. (Beven and Kirkby, 1979) A hydrological simulation based on a modified version of TOPMODEL was developed to estimate the temporal groundwater level for conducting the slope-instability analysis. (K.T. Lee, 009) Coupling TRIGRS and TOPMODEL. (H.W. Lee, 011)
Methodology 8 Diffusion equation(iverson, 000): φ t = ( D 0 cos δ ) φ z φ: Groundwater pressure head [L] t: Time [T] δ: Slope angle [ ] D 0 : Saturated hydraulic diffusivity [L /T] z: Depth in vertical direction [L] φ Z, t = Z d z cos δ I ZLT + N n=1 N n=1 InZ InZ TRIGRS(Transient Rainfall Infiltration and Grid-Based Regional Slope-Stability Analysis) (Baum et al., 00) A solution for pore pressure in the case of an impermeable basal boundary at a finite depth: K s D 0 H t t K n [ s cos δ (t t n)] D 0 Steady state H t t K n+1 [ s cos δ (t t n+1)] 1 m=1 1 m=1 ierfc ierfc Z: Vertical coordinate direction depth below the ground surface [L] t: Time [T] d z : Steady-state depth of the water table measured in the vertical direction [L] I ZLT : Steady (initial) surface flux [L/T] I nz : Surface flux of a given intensity for the n th time interval [L/T] d Lz : Depth of the impermeable basal boundary measured in the Z direction [L] N: Total number of time intervals H t t n : Heaviside step function and nt is the time at the th n time interval in the rainfall infiltration sequence ierfc x = 1 π e x x erfc(x) erfc x = x π 0 e t dt m 1 d Lz (d Lz Z) D 0 cos δ (t t n)] 1 m 1 d Lz (d Lz Z) D 0 cos δ (t t n+1)] (complementary error function) φ Z, t =0 Z = Groundwater table depth 1 + ierfc + ierfc m 1 d Lz + (d Lz Z) D 0 cos δ (t t n)] 1 m 1 d Lz + (d LZ Z) D 0 cos δ (t t n+1)] 1
Methodology 9 TOPMODEL(TOPgraphy based hydrological MODEL) (Beven. et al., 1979) TOPMODEL T is exponental decreasing with groundwater depth: T = T 0 e (z j m ) T: Lateral transmissivity of aquifer [L /T] T 0 : Lateral saturated transmissivity of ground surface [L /T] z j : Groundwater table depth in j-th grid [L] m: Coefficient of soil [L] S rz : Root zone S uz : Unsaturated zone D:Soil Depth Z w :Water table height q v :Vertical infiltration rate S rz S uz Groundwater table q v Z w
Methodology TRIGRS 10 Refine groundwater table depth a z j = z + m λ ln tan β j λ: Average topographic index m: Coefficient of soil [L] a: Specific catchment area [L] z: Mean groundwater depth of a catchment [m] TI(topographic index )= ln a tan β Unit contour length b Specific catchment Catchment area Area a=a/b a = A/b Contributing area A Stream line Contour line
11 Infinite Slope MODEL FS = resistance force driving force = τ r τ d Z = C + (γ sz γ w Z w )cos β tan ψ γ s Z sin β cos β C: Cohesion [M/LT ] γ s : Total unit weight of soil [M/L T ] γ w : Water unit weight [M/L T ] Z : Soil depth [L] Z w : Groundwater height [L] β: Slope angle [ ] ψ: Friction angle [ ] Groundwater table FS>1 Stable FS<1 Unstable β Weight Bedrock Friction force Z w
Pre-result 1 Sub-river basin of Tahan river basin: Piya (H.W. Lee, 011) Elevation (m)
13 C(cohesion) δ(slope angle) (N/m ) ( ) Pre-result Input parameter NDVI i, j + 1 C = C max Parameter Production Cohesion (Chung, 008) Slope angle Calculate from DTM. d Lz (depth of soil) Depth of soil (Chung, 008) (m)
14 Pre-result K s (hydraulic conductivity) (m/s) γ s (total unit weight of soil) Input parameter (N/m 3 ) Parameter Production Hydraulic conductivity (Chung, 008) Total unit weight of soil (Chung, 008) D 0 (diffusivity) Diffusivity (Chung, 008) (m /s)
Pre-result 15 Input parameter Parameter Production W i (initial groundwater depth) (m) Initial groundwater depth (Lee, 008) Topographic index(ti) (Lee, 008) Soil coefficient(m = 0.06) (Lee, 008) TI(topogratphic index) a z j = z + m λ ln tan β j λ: Average topographic index (5.384) m: Modulus parameter [L] (0.06) a: Specific catchment area [L] z: Mean groundwater depth of a catchment [m] TI(topographic index )= ln a tan β
Pre-result Pre-Result 16 Assume uniform rainfall (m) (m)
Future work 17 Future work Select new rainfall event and digit landslide inventory. Require more geology and hydrology parameters: Field sampling, Field test, Laboratory test.
18 Thanks for attention!