Table 2
Design or planning examples of lahars at Mount Rainier, an example of a probabilistic hazard assessment. NA, not applicable (Modified from Scott and others, 1992, p. 79)

Maximum Lahar

Case I

Case II

Case III

Debris flow type  muddy (cohesive)  muddy (cohesive) granular (noncohesive) probably muddy but may be partly or entirely granular, depending on source area
Recurrence interval ~10,000 years 500-1,000 years 100-500 years <100 years
Volume at lowland boundary >3 km3 230x105m3 60x106m3 (Puyallup River) 65x106m3 (Carbon River) NA
Mean Flow Velocity
Base of volcano  >40* m/s >30* m/s 10 m/s >30* m/s
 Lowland boundary >20 m/s ~20 m/s ~7 m/s NA
1 km on lowland ~10 m/s  ~8 m/s ~3-4 m/s NA
Cross-sectional area of flow at lowland boundary ~90,000 m2 ~16,000 m2 1,000 m2 (Puyallup River)
1,200 m2 (Carbon River)
Peak discharge at lowland boundary >1,800,000 m3/s ~320,000m3/s 7,700 m3/s (Puyallup River)
8,400 m3/s (Carbon River)
Depth of Moving Flow
Base of Volcano ~200 m ~50 m 15 m 55m
Lowland boundary ~100 m 22m 8 m NA
1 km on lowland up to 30 m ~10 m 1-3 m NA
Sediment concentration at lowland boundary** (percent by volume) >60% >60% ~40% (Puyallup River)
~45% (Carbon River)
Extent (or inundation area) To Puget Sound or Columbia River (in Cowlitz River drainage) Inundation of 36 km2 (Electron) to ~50 km2 (modern recurrence of same flow) All active flood plains (except Cowlitz River) above reservoirs, if present; otherwise upstream of Puyallup Runout phases of noncohesive lahar could extend an additional 10 km

* Estimated by comparison with similar flows at other volcanoes.
Estimated on assumption of linear relation of sorting and concentration observed at Mount St. Helens.

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