E3S Web of Conferences 415, 04006 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341504006

Snowmelt-triggered debris flows in seasonal snowpacks
Benjamin Hatchett1*, Steven Bacon2, W. Tyler Brandt3, Anne Heggli1, and Jeremy Lancaster4
1

Division of Atmospheric Sciences, Desert Research Institute, Reno, Nevada, USA
Division of Earth and Ecosystem Sciences, Desert Research Institute, Reno, Nevada, USA
3
Center for Western Weather and Water Extremes, Scripps Institution of Oceanography, La Jolla, California, USA
4
California Geological Survey, Sacramento, California, USA
2Abstract. Snowmelt-triggered debris flows commonly occur in mountains. On 14 June 2019, a debris flow
occurred on a steep, east-facing slope composed of unconsolidated glacial and periglacial sediments in
Yosemite National Park. Originating as a shallow landslide, ~1,300m3 of ripe snow was instantaneously
entrained into the debris flow carrying boulders, trees, and soil downslope. The forested area at the toe ofthe slope strained out debris leaving a muddy slurry to issue across Highway 120 during dewatering. We
document this mass movement and assesses its initiation using local snowpack and meteorological data as
well as a regional atmospheric reanalysis to examine synoptic conditions. A multiday warming trend andripening of the snowpack occurred prior to the event as a 500 hPa ridge amplified over western North
America leading to record warm 700 hPa temperatures. Anomalous temperatures and cloud cover prevented
refreezing of the snowpack and accelerated its ripening with meltwater contributing to soil saturation.
Similar conditions occurred during the catastrophic 1983 Slide Mountain debris flow, also hypothesized tobe snowmelt initiated. With projected increases in heat waves, our findings can support natural hazard early
warning systems in snow-dominated environments.

1 Introduction
Snowmelt-triggered debris flows occur in mountains
worldwide with a range of impacts depending on
downstream values at risk. Icy debris flows [1], poolingof snowmelt that ultimately induces failure [2],snowmelt-induced landslides [3], as well as rock and ice
fall [4] can trigger debris flows, damaging downstream
life and property. The typical initiation of debris flows
results from pooling of snowmelt in persistentsnowfields, leading to runoff-generated debris flows.
Ongoing warming of snow-dominated mountains
will increase likelihoods of natural hazard occurrences
relating to rapidly melting ice or snow [5]. Here, we aim
to (1) document a rarely observed mass movement in
California's seasonal snow zone during 2019, and (2)address the hypothesis that the 2019 event, and anotherin 1983, resulted from saturated soils following multiday heat waves that contributed to snowpack ripening
and melt with amplified melt rates resulting from highrelative humidity, strong solar radiation, anddownwelling longwave radiation all of which prevented
snowpack refreezing.
In contrast to debris flows resulting from poolingbeneath persistent snowfields, our study areas focus on
seasonal snowpacks that undergo an annual cycle ofaccumulation and melt. By applying a multidisciplinary
framework to analyse the geomorphic setting andoutcome of the 2019 event, the synoptic and local scaleatmospheric conditions preceding and during both 2019

and 1983 events, and the snowpack conditionsassociated with the 2019 event, our analysis can provide
guidance for the development of improved early
warning systems for natural hazards in mountainenvironments. As mountains become increasingly
exposed to events that were infrequent or rare undercooler climate conditions, such systems can reduce
impacts to life and property.

2 Geomorphic
Description

SettingandSite

The debris flow originated at approximately 3,148 m on
a 40° east aspect in a nivation hollow beneath protalus
ramparts along the Gaylor Peak Ridge at approximately
37.90180°N, -119.26402°W (Figure 1a). This slope isnear the Tioga Pass Entrance Station to Yosemite
National Park on Highway 120 (Figure 1b) and locatednear the middle of a 3.3 km long north-south trending
ridge culminating with Gaylor Peak to the north.
The protalus ramparts above the slope are
combinations of weathered and fractured Creteaceous
granodiorite bedrock, gendarmes, and boulders. In thenivation hollow below the ramparts, the slope iscomposed of a jumble of clast-supported, poorly sorted
glacial deposits with rounded talus and scree to finegrained silts. Along the upper slopes, soils are poorly
developed but thickened with the transition into forested
slopes halfway downslope. The slope transitions to an* Corresponding author: benjamin.hatchett@dri.edu© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0(https://creativecommons.org/licenses/by/4.0/).

E3S Web of Conferences 415, 04006 (2023)
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alluvial fan before terminating in a flat, seasonally wetmeadow (Figure 2a).
Sentinel satellite imagery from 2019 shows novegetation in the torrent channel, consistent with its June
activity, and indicates the small depositionalenvironment in the meadow (Figure 2b). Figure 2b also
demonstrates the likely scar (landslide source) from themost recent mass movement event and the three fingers
of water-rich debris issued beyond the debris fan into themeadow. These fingers are consistent with thephotograph taken on the day of the event (Figure 1d).
The presence of the depositional fingers indicates theimage was acquired after the event. Figures 1e-f showthe location of the origin of the shallow landslide andsubsequent snowpack collapse and entrainment.

Fig. 2. (a) Oblique Google Earth image of the shallow
landslide area on Gaylor Ridge with estimated watershedaccumulation area, scar, torrent track, and debris fan. (b)Sentinel imagery provided by CalTopo showing the scarfrom the landslide's zone of depletion, the torrent channel,
and the deposition into the debris fan above Highway 120
and into Dana Meadows (note depositional fingers
photographed in Figure 1d). (c) Shaded relief image showing
the torrent channel and debris fan.

3 Station and Meteorological Data
3.1 Station Data
Sub-hourly observations of temperature, precipitation,snow water equivalent, relative humidity, snow depth,and solar radiation were acquired from the Dana
Meadows and Tuolumne Meadows California
Cooperative Snow Survey Sensors via the California
Data
ExchangeCenter(CDEC;http://www.cdec.water.ca.gov) for the period spanning
12 am local time on 30 May 2019–12 pm 15 June 2019.

Fig. 1. (a) Initial view of the debris flow event. (b) Study
location in the Tioga Pass, California region with red dots
showing the locations of surface meteorology and snowpack
observations. (c) Several cyclists investigating the mud anddebris as the road became impassable to vehicle and bicycle
traffic. Active dewatering was occurring at this time. (d)Hyperconcentrated flow issuing out into Dana Meadows andincising into the snowpack. Note the deposition fingers in
the center of the image. (e) and (f) Main scarp showing thecharacteristic glacially deposited, clast-supported, poorly
sorted boulder- to silt-sized clasts, the flanks and crown ofthe snowpack collapse, head scarps of the shallow landslide,and rills incising the landslide rupture surface.

3.2 Meteorological Data Using Atmospheric
Reanalysis Products
Daily averages (00Z) of 36 km horizontal resolution 700hPa air temperature and 500 hPa geopotential height
were acquired from the North American Regional
Reanalysis (NARR) spanning the period 11-14 June
2019. The characteristic terrain height is near 3,200 m,
thus the 700 hPa pressure surface in the NARR is anideal model pressure level to assess conditions. 700 hPa
is also a very commonly output pressure level fromnumerical weather prediction models, thus can be used
to forecast free atmospheric conditions in the northernand central Sierra Nevada near and just above its crestelevation.
Anomalies of geopotential height and temperature
were calculated by differencing the daily event values

A digital elevation map provides additional evidence
for mass movement and fluvial activity. The hillshadefurther reveals the presence of an approximately 2 mwide torrent channel terminating in a classic triangulardebris fan (Figure 2c). The watershed contributing area,or accumulation area (Figure 2a), was calculated in
CalTopo to be approximately 28,000 m . Above theroad, the debris fan is estimated at 4,000 m . Including
the area inundated across Dana Meadows (mapped via
satellite imagery; Figure 2b), the total depositional area
is estimated to be on the order of 4,000 m .
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E3S Web of Conferences 415, 04006 (2023)
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4.2 Heat wave results from amplified 500 hPa
ridge and record 700 hPa temperaturesfrom long term averages of the corresponding calendar
days spanning 1981-2010. Daily rankings oftemperatures were calculated by sorting each grid pointin the NARR period of record (1979-2021) andidentifying the rank of the date of interest.

Midtropospheric (500hPa) height anomalies indicate thepresence of an amplified ridge over western North
America during 11-14 June (Figure 4). Amplified
ridging favors downward vertical motions andanomalously positive temperatures in the lower
troposphere and at the Earth’s surface. Mountain-top
temperature anomalies in the free atmosphere (700 hPa)
over the region during 11-14 June ranged from +3 to+8°C, with the hottest anomalies occurring on 11 June
(Figure 4a). Above-freezing 700 hPa temperatures,cloud cover, and high relative humidities prevented
nocturnal snowpack refreezing–indicated by theincrease in snowpack density (Figure 3e)–leading tocontinuous liquid water input to the soil surface.

4 Results
4.1 Heatwave precedes debris flow
Maximum temperatures at Dana Meadows exceeding
10°C started on 9 June and warmed through 11 June
(18.3°C) before slowly cooling until the time of theevent on the afternoon of 14 June (15°C; Figure 3a). The
remaining analysis focuses on 11-14 June that covers thepeak of the heatwave until the debris flow occurred.

Fig. 4. 700 hPa temperature anomalies (filled contours) and500 hPa geopotential heights (black contours) for a) 11 June
2019, a) 12 June 2019, a) 13 June 2019, and d) 14 June 2019.

Fig. 3. (a) Timeseries of meteorological and snowpack
conditions. a) Near-surface (2 m) temperatures at Dana
Meadows spanning 1200 hrs 7 June-00 hrs 15 June (localtime). (b) Near-surface (2 m) temperatures at Dana Meadows(red line) and Tuolumne Meadows (blue line), (c) solar
radiation (filled gold areas) and relative humidity (green line)
at Dana Meadows, (d) snow water equivalent (black line) andsnow depth (blue line) at Dana Meadows, and (e) snow density(filled dots) spanning 1200 hrs 7 June-00 hrs 15 June (localtime). The light brown vertical bar indicates the approximate
timing of the debris flow, constrained by human observations.

Rankings of 700 hPa temperatures indicate the peak
day of the heatwave (11 June 2019) was the hottest in
the NARR period of record (Figure 5). Widespread top(highest since 1979) rankings occur throughout eastern
North Pacific and western North America encompassing
an area from 140°W to 110°W and from (20°N) to 56°N.

Temperatures at Dana Meadows (2987 m) versus
Tuolumne Meadows (2621 m) were consistently 3-5°C
hotter during the peak hours of the day (Figure 3b). Bothstations reached similar minimum temperatures withadditional cooling prevented after atmosphericsaturation achieved and latent heat released (interpretedfrom Dana Meadows relative humidity reaching 100%;
Figure 3c). Minimum temperatures at both stations
remained above 0°C throughout the period, with the last
below-freezing minimum temperature on 9 June.
Generally clear skies are suggested by solar
radiation, however greater cloud cover on 13 June andlight high-level cloud cover on 14 June (Figure 3c)
likely enhanced downwelling longwave radiation to thesnow surface, potentially further enhancing melt rates assnow effectively absorbs longwave radiation.

Fig. 5. Rankings of 11 June 2019 (hottest date of June 2019heatwave) 700 hPa temperatures with respect to the NARR
period of record (1979-2021). Gold dot indicates Tioga Pass.

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E3S Web of Conferences 415, 04006 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341504006

5 Application to 30 May 1983 Slide
Mountain Debris Flow

multiday temperatures and high solar radiation
following anomalously snowy winters. Anomalously
warm temperatures and solar radiation led to snowmeltthat likely increased soil moistures and soil porepressures and induced shallow landslide failure.
Because remnant seasonal snow remained, ripe (near0°C) snow was entrained and melted, creating a slurrythat flowed downslope and issued upon infrastructure
built along an alluvial fan, leading to road closures andminor damage. A similar sequence occurred during themuch larger and more damaging 1983 Slide Mountaindebris flow, providing confidence in our identified
sequence of events and supporting our hypothesis.
While snowmelt-triggered debris flows are
relatively rare in non-glaciated mountains, we expect
warming temperatures (higher maximum and fewer
minimum temperatures below freezing), increased
precipitation variability, and more frequent heat waves
to produce an increase in the likelihood of these events.
As in situ networks providing observations of relevant
parameters such as snowpack temperature and density,
air temperature, soil moisture, and solar radiation areuncommon in many mountain areas worldwide, wesuggest applying atmospheric reanalysis products aswell as satellite-based products of solar radiation andsnow albedo to develop predictive tools. These tools canthen incorporate weather forecasts as input to earlywarning systems.
Solar radiation is a key driver of snowmelt [6] andmay provide explanatory power for events that did notexperience anomalous high temperatures. Reductions insnow albedo from snow grain growth and deposition oflight absorbing aerosols (e.g., dust and black carbon)
multiply the effects of anomalous solar radiation inelevating snowmelt rates [7]. Forensic case studies of
unique events, such as presented here, can be used todevelop snowmelt-induced hazard early warning tools
for application in mountains with otherwise limited
observational networks.

The 30 May 1983 Slide Mountain landslide andsubsequent debris flow followed a near-record snowfall
year [3]. The slope failure initiated near the summit ofSlide Mountain, Nevada (eastern Sierra Nevada) andentrained snow on the southeastern aspect of the upperslopes. Water was also entrained as material and debris
flowed downslope into Upper Price Lake. The resulting
slurry (1.07x10 m ) flowed 4 kilometers down Ophir
Creek and issued onto an alluvial fan proximal toWashoe Lake, inundating Highway 395 and multiple
homes constructed on the alluvial fan, killing one.
Applying a similar analysis to the 2019 event, butusing mean temperatures spanning the 11-14 Juneperiod (Figure 6a), we find anomalous positive 700 hPa
temperatures throughout the 27-30 May 1983 period(Figure 6b). Temperature anomalies during the May
1983 event were of similar magnitudes compared toJune 2019 (exceeding +8°C above the mean), but weremore widespread across western North America. 700hPa temperature rankings during the May 1983 Slide
Mountain debris flow were also among the hottest in theNARR period of record (Figure 7).
63Fig. 6. Maximum mean 700 hPa temperature anomaly
rankings during (a) 11-14 June 2019 and (b) 27-30 May 1983.

References
1.
2.
3.

4.
Fig. 7. As in Figure 5 but for 30 May 1983 Slide Mountaindebris flow (hottest date of May 1983 heatwave shown).

5.

6 Discussion
We documented a unique snowmelt-triggered mass
movement during June 2019 in the Sierra Nevada (USA)that resulted from a sequence of extreme weather eventsin a high mountain environment. This sequence included
late spring heat waves with record-to-near record

6.

4

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