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Open-File Report 2008-1159

TRIGRS—A Fortran Program for Transient Rainfall Infiltration and Grid-Based Regional Slope-Stability Analysis, Version 2.0

By Rex L. Baum, William Z. Savage, and Jonathan W. Godt


Thumbnail of cover and link to report PDF (1.2 MB)

The Transient Rainfall Infiltration and Grid-Based Regional Slope-Stability Model (TRIGRS) is a Fortran program designed for modeling the timing and distribution of shallow, rainfall-induced landslides. The program computes transient pore-pressure changes, and attendant changes in the factor of safety, due to rainfall infiltration. The program models rainfall infiltration, resulting from storms that have durations ranging from hours to a few days, using analytical solutions for partial differential equations that represent one-dimensional, vertical flow in isotropic, homogeneous materials for either saturated or unsaturated conditions. Use of step-function series allows the program to represent variable rainfall input, and a simple runoff routing model allows the user to divert excess water from impervious areas onto more permeable downslope areas. The TRIGRS program uses a simple infinite-slope model to compute factor of safety on a cell-by-cell basis. An approximate formula for effective stress in unsaturated materials aids computation of the factor of safety in unsaturated soils. Horizontal heterogeneity is accounted for by allowing material properties, rainfall, and other input values to vary from cell to cell. This command-line program is used in conjunction with geographic information system (GIS) software to prepare input grids and visualize model results.

Revised October 20, 2009

First posted June 12, 2008

  • Downloads Directory
    Contains the TRIGRS program for either Windows PCs or Macintosh computers.

For additional information contact:

USGS Central Region Geologic Hazards Team
Box 25046, Denver Federal Center MS 966
Denver, CO 80225-0046

Part or all of this report is presented in Portable Document Format (PDF); the latest version of Adobe Reader or similar software is required to view it. Download the latest version of Adobe Reader, free of charge.

Suggested citation:

Baum, R.L., Savage, W.Z., and Godt, J.W., 2008, TRIGRS—A Fortran program for transient rainfall infiltration and grid-based regional slope-stability analysis, version 2.0: U.S. Geological Survey Open-File Report, 2008-1159, 75 p.





Theoretical Basis of the Program

Program Operation

Comparison of Computed Results with Published Analyses


References Cited

Appendix 1. Listing of Files Included in This Distribution

Appendix 2. Tutorial

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