Terrain Stability Mapping Using the ArcView SINMAP extension David G Tarboton, Robert T Pack, Craig N Goodwin Acknowledgements n The SINMAP software was developed through a collaboration of Terratech Consulting Ltd., Utah State University, and Canadian Forest Products. n Funding was provided by Forest Renewal B.C., Canada.

Key points n An example of the combination of GIS and engineering analysis. n Hydrology, Geology, Geomorphology and Geotechnical Engineering. n Extendibility of ArcView. n Programming to provide integration of modeling into ArcView.

Key points continued n Synthesis of GIS based digital elevation data with field observations. n Interactive calibration to combine fitting to data with engineering judgement. n Recognition of parameter uncertainty through probability distributions. n Free – In the public domain at

Outline n The Problem n Theory n Inputs and Outputs n Implementation n Demonstration

The Problem

Norrish Watershed, BC.

SINMAP Does: Geographic Information System Based Mapping of Slope Stability with Uncertain Parameters n Is based on an objective physical model n Can be quantitative or qualitative n Is expressed in terms of stability indices n Can be combined with geomorphic and statistical analysis

n Forest and watershed management, forestry and forest engineering. n Determine volume of harvestable timber in annual allowable cut calculations. n Better plan timber development to minimize occurrence of landslides and resulting impacts. Where is it Used

Lardeau Creek, BC

Contour Map

Slope Map ° 22.5°- 33° 33°- 45°

Specific Catchment Area Map

The first SINDEX map

The SINMAP Formulation n Based on the Infinite Plane Slope Stability model with wetness (pore pressure) obtained from a topographically based steady state hydrology model (e.g. Montgomery and Dietrich, 1994, WRR p1153) n Uses Grid DEM data format n Spatial distributions rely on shallow subsurface groundwater flow convergence and topographic slope n Parameter uncertainty incorporated through ranges of soil and hydrology parameters

Limitations n Applies only to shallow transitional landsliding controlled by shallow groundwater flow convergence. n Not applicable to deep seated instability such as deep earthflows or rotational slumps. n Maps potential landslide initiation zones - not runout or hazard areas. n Constrained by DEM resolution.

Theoretical Basis Infinite Plane Slope Stability Model 1. Relative Wetness Density Ratio 2. Dimensionless Cohesion (assumed a constant of 0.5)

1. Relative Wetness Defined n Assumes lateral depth integrated flux q is proportional to specific catchment area a q = R·a [L 2 /T] n Soil transmission capacity = T·sin  where T is the soil transmissivity and  is the topographic slope n “Relative Wetness” is defined as W = Min{(R·a / T·sin  ),1}. Specific catchment area a [m 2 /m  m] (per unit coutour length) DwDw D Slope angle  q

Specific Catchment Area “a” Upslope catchment area divided by unit contour width n Uses GRID structure n Calculated for each grid cell n Grid cell size depends on original DEM grid cell size n Expressed as the variable “a” with units of m 2 /m  m AREA 1 AREA

The D  Algorithm Tarboton, D. G., (1997), "A New Method for the Determination of Flow Directions and Contributing Areas in Grid Digital Elevation Models," Water Resources Research, 33(2): ) (

Specific catchment area a is the upslope area per unit contour length [m 2 /m  m]

2. Dimensionless Cohesion Defined (includes both soil and root effects)

Topographic Slope “  ” Defined n Uses GRID structure n Calculated for each grid cell n Expressed as the variable  with angle units in radians or degrees SLOPE 1 SLOPE 2

Example slope map

FS > 1 w = 1 Stable, Saturated FS < 1 Unstable FS > 1 w < 1 Stable, Unsaturated Area-Slope Plot

But Soil parameters, C, , T are uncertain and spatially variable. n Recharge parameter R is spatially and temporally variable. P C2C2 C1C1 range

Definition of Stability Index SI n SI = Prob(FS > 1) n If FS > 1 using most conservative end of each parameter range SI = FS

Area-Slope plot with uncertainty FS* > 1 SI = FS* *(using most conservative end of each parameter range) FS < 1 for all possible parameters SI = 0 Most conservative parameter limits Most destabilizing parameter limits SI = Prob(FS >1)

SI derived analytically for each region in Slope vs Contributing Area plot e.g. for region 2

Stability Class Definitions

Summary of SINMAP Inputs Topography (dictated by DEM) a - specific catchment area θ - slope angle Soil Parameters (given as a range) C - dimensionless cohesion tan Φ- tan of soil internal friction angle R/T - soil hydraulic parameter

Summary of SINMAP Outputs 1. Specific Catchment Area Map 2. Stability Index Map 3. Soil Wetness Map 4. Calibration Plot 5. Statistical Tables

Stability Index Map

Soil Wetness Map

SI = minimum safety factors SI indicates lower than average potential instability SI indicates higher than average potential instability SI < 0 indicates instability Possibly Saturated Calibration Plot

Slope-Area plot interactive calibration

Percentage of Average Landslide Density

Customization of the Graphical User Interface Avenue Script Programming Language Core computations in C++ DLL for speed and supportability Access ESRI grid data using grid application programmers interface (GRIDIO) Result: Tight integration between hydrologic modeling and GIS Implementation Exploiting ArcView 3.2 Extendability

Implementation Spatial Analyst includes a C programming API (Application Programming Interface) that allows you to read and write ESRI grid data sets directly. Excerpt from gioapi.h / * GetWindowCell - Get a cell within the window for a layer, * Client must interpret the type of the output 32 Bit Ptr * to be the type of the layer being read from. * * GetWindowCellFloat - Get a cell within the window for a layer as a 32 Bit Float * * GetWindowCellInt - Get a cell within the window for a layer as a 32 Bit Integer * * PutWindowCell - Put a cell within the window for a layer. * Client must ensure that the type of the input 32 Bit Ptr * is the type of the layer being read from. * * PutWindowCellFloat - Put a cell within the window for a layer as a 32 Bit Float * * PutWindowCellInt - Put a cell within the window for a layer as a 32 Bit Integer */ int GetWindowCell(int channel, int rescol, int resrow, CELLTYPE *cell); int GetWindowCellFloat(int channel, int rescol, int resrow, float *cell); int GetWindowCellInt(int channel, int rescol, int resrow, int *cell); int PutWindowCellFloat(int channel, int col, int row, double fcell); int PutWindowCellInt(int channel, int col, int row, int icell); int PutWindowCell(int channel, int col, int row, CELLTYPE cell);

Avenue Script ' SINMAP.c.area ' BaseName is the full directory path name of the input DEM grid BaseName = Self.Get(0) 'get the procedure from the dll library area = DLLProc.Make(_avlsmDLL,"area",#DLLPROC_TYPE_INT16, {#DLLPROC_TYPE_STR,#DLLPROC_TYPE_STR,#DLLPROC_TYPE_INT32, #DLLPROC_TYPE_INT32}) if (area = NIL) then av.Run("SINMAP.Error.OpFail",{2}) end error = area.Call({BaseName+"ang",BaseName+"sca", 0, 0}) if (error > 0) then av.Run("SINMAP.Error.OpFail",{"Grid and/or memory error reading/writing in area.c"}) end External C/C++ function compiled as a DLL

Customizing the GUI

Conclusions n Extended the infinite plane slope stability model with topographically based wetness to account for parameter uncertainty. n Successfully mapped potentially unstable terrain. 56% of landslides in 8.5% of area classified as “upper threshold”. n Implementation is an interactive ArcView © extension, named SINMAP. n Available from

Are there any questions ? AREA 1 AREA Demo of the Software