At the arc or grid prompt, type "&r model" SHALSTAB was written in Arc Macro Language (aml) before the GUI interface of arcview and arcmap were developed. It is still run at the command line, but this exercise uses the simple menu capabilities of command-line (workstation) ArcInfo.

Shalstab can be run with a constant rainfall and used to solve for critical cohesion or any other paramter. In this exercise, we are solving the amount of steady-state rainfall (in mm/day) necessary to cause instability. As steady-state rainfall is hard to come, the numbers should be taken as relative measures of instability. Cells that are "stable" should hold even when saturated. Cells that are "unstable" are predicted to fail even when dry. They indicate a problem with the input data, e.g. misrepresented slopes, incorrect cohesion (Retaining walls have very high cohesion.) or soil depth (The cell may have previously failed to rock, i.e. soil depth equals zero.) or they might indicate problems with shalstab itself, such as it's ignorance of stratigraphy.

The only spatial data that SHALSTAB requires is a DEM, but intermediate grids are created to speed this exercise. Transmissivity, bulk density, soil depth, phi, and cohesion can be either constants or spatially variable grids. Any spatially variable data is hard to come by, and we have been reasonably happy in using the same values around the state. A value of -1 is a flag to indicate a spatially variable grid, which, in our case, we have not got.

The menu is supposed to be self-explanatory, but isn't. Let me explain: Set the 5 paramters to your desired values, then hit the "RUN MODEL" button. The results of that model are now listed in the two scroll lists. You can click on any model to see it plotted on the screen, on either the left or right side. The model parameters are listed on the screen, along with the number of 10-meter cells in each critical rainfall category. As you complete a run, you can plot it on the screen next to a previous run. If you get confused by your model runs, click the "show runs" button and the parameter values for each run will appear in the text window from which you launched the menu. To zoom, hit the zoom button, drag the mouse in the most recently drawn side of the screen, and click on a run to redraw it. The zoom will apply to both sides of the screen.