8 August 2019
The South Tahoma Glacier/Tahoma Creek Outburst Flood & Debris Flow on 5th August 2019
Posted by Dave Petley
The South Tahoma Glacier/Tahoma Creek Outburst Flood & Debris Flow on 5th August 2019
With thanks to Scott Beason, Park Geologist, Mount Rainier National Park.
On Monday evening, 5th August 2019, an outburst flood and debris flow occurred at Mount Rainier National Park. Between approximately 6:48 – 7:58 PM PDT (8/6/2019 01:48 – 2:58 UTC), the park’s seismographs picked up four separate debris flow sequences in the Tahoma Creek area. Scott notes that:
“On arrival in the park on Tuesday morning, I observed extreme sediment transport/hyperconcentrated flows at the road bridge, approximately 13 km downstream of the glacier. Once I got in the office, I checked the seismic records and saw a clear debris flow signature (see below). After checking with the USGS/Cascades Volcano Observatory, who concurred on the debris flow signature, we closed the park’s West Side Road (which parallels Tahoma Creek and is downstream of the South Tahoma Glacier) until we could undertake a ground and aerial reconnaissance to determine the source and additional hazards”
The seismogram record that captures the Mount Rainier Debris Flow. Provided by Scott Beason. Data from Pacific Northwest Seismic Network (PNSN) and USGS/Cascade Volcano Observatory.
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Seismic data shows that the debris flows had four discrete pulses (see the earthquake spectra image and seismic records below) and Scott has found numerous debris flow levees and boulder fields in the field.
The earthquake spectra for the Mount Rainier debris flow event. Image provided by Scott Beason. Data from Pacific Northwest Seismic Network (PNSN) and USGS/Cascade Volcano Observatory.
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Detailed seismic record for the Mount Rainier debris flow event. Image provided by Scott Beason. Data from Pacific Northwest Seismic Network (PNSN) and USGS/Cascade Volcano Observatory.
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Scott and his colleagues were able to collect images of the aftermath of the event via a helicopter flight and fieldwork. These are available via a Flickr page – they are fabulous:-
The bulking zone of the Mount Rainer debris flow, Image by Scott Beason via Flickr.
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Initial satellite image analysis by Scott suggests that the outlet stream from the South Tahoma changed locations:-
An initial analysis, by Scott Beason, of the source of the Mount Rainier debris flow.
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Scott’s initial hypothesis that this was a catastrophic change that then rapidly incised into debris-covered stagnant ice and ground-cored moraine just downstream and between bedrock “steps” below the glacier. Once it had enough momentum, it then rapidly bulked up into little ice age ground moraine several km’s below the terminus. The run out ended about 6.5 – 7 km’s downstream where the boulder fields were deposited.
Interestingly, this event is remarkably similar to the 2015 debris flow sequence.
There is much more to write about this event, and I will return to it in the next few days. In the meantime, many thanks to Scott for highlighting it to me, and for the amazing data.
Wow!
Dave Petley is the Vice-Chancellor of the University of Hull in the United Kingdom. His blog provides commentary and analysis of landslide events occurring worldwide, including the landslides themselves, latest research, and conferences and meetings.
Two days ago @ckchizzle on instagram posted two very close commercial airplane fly by photos on his feed. I think the flow route can be seen on the second one.
Thank you for your detailed report. Living in Tacoma near Stadium High School between 1958 through 1971 I recall the effects of the Great Alaskan Earthquake of 1964 like it was yesterday! I look forward to your next report.
Kate/ living in northeastern Skagit county now.
The western flank of Mt. Rainier is the most deeply scalloped by glacial erosion, forming the aptly named “Sunset Amphitheater”. At its base, the retreating Tahoma and South Tahoma Glacier are still powerful conveyors of mass plucked from the massif and rockfall collected from the chemically weakened head-walls towering above them. Counter-intuitively, the relatively small South Tahoma glacier has produced more substantial debris flows, especially in the last 30 years, than any other drainage emanating from the stratovolcano. Most were much larger than this recent Lahar. The accumulations down-stream are raising the valley floor adjacent to the frequently-closed West Side road of Mount Rainier National Park. All along it’s route, multiple-acre swaths of living forest have been inundated, with successive events distinguished by each tree-stands varying stages of decay since death. As alluded to above, this side of Rainier is also the most rotten. When conditions permit, Sulfur-steam fumaroles can be seen all over the vast cliff faces and cleavers within the Amphitheater, from a concentration of chemically altering, hot outgassing . What would be an exposure of fairly solid andesite (with even a little high-silica basalt) on any other flank of the mountain, here it’s a deteriorating, crumbling mess…. looking for the slightest excuse to let gravity have it’s way.