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- [Narrator] USGS scientists studying Mount Rainier,
have long recognized that the greatest hazard
posed by the volcano
is from landslides that begin high on its slopes
and transform into mud flows or lahars,
that travel great distances down river valleys.
Lahars are rapidly moving flows of water, mud, and rock.
They can move almost as fluidly as water,
yet be as dense as wet concrete.
Lahars can bury or carry away
nearly everything in their paths
and leave behind a layer of hard, rocky sediment.
Large lahars commonly accompany volcanic eruptions
but they can also develop from landslides
that occur without eruptive activity or any warning,
making it very important
that people understand lahar behavior and hazards.
USGS scientists and mathematicians teamed up
to developed state of the art computer simulations
that show what happens when large landslides
on Mount Rainier's western flank transform into lahars
and descend into the Puyallup and Nisqually River valleys.
The simulations show where and how far the lahars can go,
when they arrive, and how deep the lahar would be.
This simulation starts with a landslide
in the Sunset Amphitheater area.
The timer shows a lapse time after the landslide begins.
The colors represent depths.
Light yellow is less than 3 feet,
dark red is more than 150 feet.
In this scenario,
260 million cubic meters of rock and ice,
a volume similar in size to the Electron Mudflow
that occurred at Mount Rainier about 500 years ago,
slides off the volcano.
If the material is dense hard rock,
the landslide stabilizes before flowing very far.
This is the low mobility scenario
and it never reaches communities downstream.
But if the rock is weak and has a lot of clay,
the landslide will transform into a highly mobile,
swift-moving lahar.
Part of the lahar flows south
and enters the Nisqually River valley.
But most of it moves into the Puyallup River drainage.
The lahar reaches the town of Orting in about 50 minutes.
The lahar slows and spreads across the wide valley
between the Puyallup and Carbon Rivers.
The shades of orange represent depths of six to 16 feet.
Along the river corridors,
it is much deeper.
After passing through Orting,
the lahar continues to the Sumner and Puyallup lowlands,
arriving in three to four hours after the landslide begins.
The lahar will slow and come to rest in four to five hours.
But the deposit may remain liquified for many more hours,
and flooding may occur downstream.
This next simulation starts with the collapse
of the steep cliffs,
of the Tahoma Glacier Headwall.
The timer shows a lapse time after the landslide begins.
The colors represent depths.
Light yellow is less than three feet,
dark red is more than 150 feet.
In this scenario, 260 million cubic meters of rock and ice
slides off the volcano.
If it is mainly dense hard rock,
the landslide stabilizes before flowing very far.
This is the low mobility scenario
that never reaches communities downstream.
But if the rock is rich in clay,
the landslide will transform into a highly mobile
swift-moving lahar.
The lahar that travels down the Puyallup River drainage
will arrive in Orting in a little over 60 minutes.
There, it slows and spreads out across the wide valley.
After passing through Orting,
the lahar continues to the Sumner and Puyallup lowlands.
The lahar that travels down the Nisqually River drainage
will arrive at the national park boundary
in about five minutes,
and the community of Ashford in about 15 minutes.
The lahar reaches the Alder Lake reservoir
in about 50 minutes.
The lahar begins filling the upper end of Alder Lake.
The sediment-rich lahar, which is the brown color,
mixes with lake water, which is blue.
The lahar doesn't generate a substantial wave.
Instead, over the next 50 minutes,
the lake level slowly rises.
About an hour-and-a-half after the landslide begins,
water starts overtopping the Alder Lake dam.
The computer simulation ends two hours
after the landslide begins
and does not address what happens to the dam
when water begins flowing over it
or what might occur downstream of the dam.
We do not know when the next large lahar
at Mount Rainier will occur or how big it will be.
These computer simulations
provide estimated lahar travel times and depths,
so emergency officials and communities in lahar hazard zones
can continue to take the steps necessary
to prepare for a future lahar by developing alert systems
and evacuation routes that lead away from valley floors.
You can do more by following three steps.
Learn.
Inquire.
Plan.
First, learn about the volcanoes and hazards
that could affect you.
Second, talk to local officials
about their plans to handle a volcano emergency.
Third, develop a family preparedness and communications plan
and a kit with emergency provisions.
Know how to access information during an emergency
and be ready to act.
If you are at risk from lahars,
and are aware that one is in progress,
move rapidly off the valley floor.
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