|
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) |
NA |
| 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) |
NA |
| 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) |
NA |
| 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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