Vesta-Little North Tributaries of the Chehalis

Landslide Map
Mapped Stability at phi=45°, Coh=0 kN/mē

Analysis with stability.aml created:
qust45: rainfall for instability in the old fashion, using a q and no cohesion,
qust15: rainfall for instability, with phi = 45°, choh = 15kN/mē,
qust8: rainfall for instability, with phi = 45°, choh = 8kN/mē,
qust2: rainfall for instability, with phi = 45°, choh = 2kN/mē, and
critcoh100: critical cohesion, assuming a steady-state rain of 100mm/day.

Comparison with Field Data

The landslide coverage has 262 slide polygons, 180 road-related, 75 not, 3 'i'. slides 379-382 have no data-base entries (but are als). The slide categories are
alsancient landslide,
ssdsmall sporadic deep (slumps),
dfdebris flow, and
srshallow rapid
The background polygon--and some slivers--have no code. Only debris flows and shallow rapid flows will be tallied. The 4 roadtype = "i" will be tallied with the non-road slides. When we rasterize the slides on a 30-meter grid with the standard ARC/INFO algorithm, 13 slides fall through the cracks. The best solution is to force rasterization to count a cell as a slide if it touches a slide at all. We can tally results by polygonal slide area or by slide count. Slide areas are problematic, because slides were digitized to include runout paths.

Some slides are represented as two or more polygons, we our clever compilation program can work around this problem.

NOTE: The shallow slumps were accidentally included in the road-related column. Tables were updated May 15. The difference is 2 slides.

NOTE: The Not-road-related column of the normalized table for each case were incorrect until corrected on May 31 but you must have seen that.

Here are results for