E3S Web of Conferences 415, 04003 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341504003

The spatial distribution of debris flows in relation to observed
rainfall anomalies: insights from the Dolan Fire, California
David B. Cavagnaro1*, Scott W. McCoy1, Matthew A. Thomas2, Jaime Kostelnik2, and Donald N. Lindsay3
1Department of Geological Sciences and Engineering, University of Nevada, Reno, Nevada, USA
2U.S. Geological Survey, Landslide Hazards Program, Golden, Colorado, USA
3California Geological Survey, Redding, California, USA

Abstract. A range of hydrologic responses can be observed in steep, recently burned terrain, which makes
predicting the spatial distribution of large debris flows challenging. Studies from rainfall-induced landslides
in unburned areas show evidence of hydroclimatic tuning of landslide triggering, such that the spatialdistribution of events is best predicted by the observed rainfall anomaly relative to climatic norms rather
than by absolute rainfall. In this paper, we test whether the spatial distribution of debris flows in responseto rainfall can be similarly predicted by rainfall anomaly. The 520 km2 Dolan Fire burn scar in Monterey
County, California, USA, spans a sharp hydroclimatic gradient and experienced a widespread storm inJanuary 2021 that triggered floods and debris flows, providing a natural experiment in which to test thishypothesis. In this study, we use remote and field methods to map debris-flow response and examine its
spatial heterogeneity. Together with rainfall data, our mapping reveals that the observed anomalies in peak
15-min rainfall intensity (I15) relative to the intensity of the 1-yr return interval storm predict debris-flow
occurrence better than the absolute peak I15. Our findings indicate that debris-flow processes and threshold
rainfall required for debris-flow initiation may be tuned to local hydroclimate.

1 Introduction
1.1 Post-wildfire debris flows
Anthropogenic climate change and human activity haveincreased the size and scope of severe wildfires in theWestern United States, increasing the amount of steep
burned area in proximity to population centers andcritical infrastructure [1]. These burned areas face a
range of post-fire hydrologic hazards, from clear waterfloods to catastrophic debris flows.
Although this range of flow types is often observedin burned landscapes, predicting the type of flow thatwill debouch from any particular basin still proveschallenging. This difficulty becomes more pronounced
as one moves into areas with limited observations of past
post-fire debris flows on which existing empirically
based predictive models can be trained [2, 3].
1.2 Hydroclimatic tuning of surface processes
Studies of rainfall-induced shallow landslides haveshown evidence that landslide initiation is moresensitive to rainfall anomaly (the observed rainfall
normalized by metrics of rainfall climatology) than it isto the absolute quantity of rainfall delivered, possibly
due to a coevolution of surface processes with long-term
local climate [4, 5, 6, 7].

In this study, we test whether the spatial distribution ofrunoff-generated debris flows across a hydroclimatic
gradient can be similarly explained through the use of
rainfall anomaly maps.
1.3 Study area and rainfall event
In August 2020, the Dolan Fire burned ~520 km2 ofsteep, remote, heavily vegetated terrain in Monterey
County, California. The resulting burn scar stretched up
to ~30 km inland from the Pacific coast, ~40 km northto south, and spanned a sharp east-west hydroclimate
gradient with mean annual rainfall ranging from ~50130 cm/yr [8] and the climatological 15-minute rainfall
intensity at 1-year return interval (1yr I15) ranging from~26-52 mm/hr [9].
On 27 January 2021, the area received widespreadheavy rainfall from an atmospheric river event,triggering numerous debris flows across the burn scar,as well as widespread rainfall runoff that exited basins
simply as flood flows. This triggering rain event,
together with the diverse hydroclimatology at the site,
makes the Dolan Fire a suitable natural experiment for
testing whether the observed heterogeneity inhydrologic response can be explained in part byobserved rainfall anomalies.

© 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, 04003 (2023)
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https://doi.org/10.1051/e3sconf/202341504003

2 Methods

3 Results

2.1 Debris-flow mapping
We qualitatively mapped flow type at each segment of
the stream network using pre- and post-event
WorldView satellite imagery [10], according to thefollowing scale: 0- no erosion visible; 1- minor erosion
visible interpreted to be from standard flood flows; 2severe erosion visible with potential to be a debris flow;
3- clear evidence of debris flows. We performed field
investigations in June 2021 and January 2022 to refine
our remote mapping using direct observations of deposit
and channel morphology. The locations of fluvial
erosion and debris flows are summarized in Figure 1.
2.2 Rainfall data acquisition
Due to the sensitivity of runoff-generated debris flows
to short-period (~15 min) rainfall, we focus on shortperiod rainfall and associated climatological metrics.
Rainfall data for the triggering event were recorded by
12 nearby tipping-bucket gauges (time resolution 1-15min) and several satellite-based products (timeresolution 60 min). In this study, we omit radar products
due to a poor Radar Quality Index [11] in the remote,mountainous study area, choosing to focus on
instantaneous gauge-based products and 1-hour satellite
products from PERSIANN and IMERG for validation
purposes [12, 13].
Using QGIS 3.16.8, we created a spatiallycontinuous raster of gauge-based peak 15-minute
rainfall intensity (peak I15) through inverse distanceweighted spatial interpolation of the gauge peak values.
We calculated and inspected the spatial pattern of peak
60-minute rainfall intensity from the satellite estimates
to ensure that there were no major discrepancies
between the continuous satellite product and the gaugeinterpolation scheme. We acquired gridded data on
long-term rainfall-intensity climatology from theNational Oceanic and Atmospheric Administration(NOAA) Atlas14 dataset [9].

Fig. 1. Map of observed peak I15 (A) and I15 anomaly (B)across the study area, with debris flows and fluvially eroded
stream segments shown.

3.1 Remote and field-based mapping
Our field verification efforts indicate that close
assessment of pre- and post-event satellite imagery
generally produces an accurate characterization offluvially eroded and debris-flow-dominated stream
segments in this landscape, but field investigation may
be necessary in densely forested areas where the channel
is not visible on satellite imagery. The lack of intenserain in the months following the triggering event
allowed us to conduct field investigation six months toa year after the event, but more rapid assessment may benecessary in landscapes with frequent intense rain.

2.3 Data analysis
To compute the rainfall anomalies across the study area,
we normalized the measured peak I15 map by the I15 map
for a 1-year return interval storm [Equation 1].
𝐴𝑛𝑜𝑚𝑎𝑙𝑦 =𝑂𝑏𝑠𝑒𝑟𝑣𝑒𝑑 𝑝𝑒𝑎𝑘 𝑖𝑛𝑡𝑒𝑛𝑠𝑖𝑡𝑦
1𝑦𝑟 𝑟𝑒𝑡𝑢𝑟𝑛 𝑖𝑛𝑡𝑒𝑟𝑣𝑎𝑙 𝑖𝑛𝑡𝑒𝑛𝑠𝑖𝑡𝑦(1)

Hotspots of debris-flow activity were mainly foundin the northwestern, southern, and eastern portions of theburn scar. Evidence of fluvial erosion was morewidespread, except for a large region in the north-centralarea. This region of little to no erosion spans acrossmuch of the variation in peak I15 [Figure 1A], but lies
almost entirely below the 1.0 rainfall anomaly contour[Figure 1B].

We then associated each stream segment with acorresponding measured peak I15, 1-year climatologicalI15, and corresponding rainfall anomaly. To test whether
rainfall anomalies can explain the observed locations ofdebris flows better than peak I15, we constructed
histograms of total network length for each responsetype above and below 1.0 rainfall anomaly and aboveand below an I15 of 30 mm/h (the fire-averaged thresholdpredicted by the existing operational model) [14].

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4 Discussion

Fig. 2. Percent of network length (e.g., probability ofresponse given a particular rainfall) in the Dolan Fire and ineach response class, above and below 1.0 I15 anomaly (top)and 30 mm/h I15 (bottom). Compared with the overall stream
network (black horizontal dashed line), debris flows are~37% more likely to occur at sites with rainfall anomaly
greater than 1. Meanwhile, debris flows were only 1.6% more
likely to occur at sites with absolute rainfall above a 30 mm/hthreshold.

Fig. 3. Phase space depicting the 1-year return interval I15 (xaxis) and the peak observed I15 (y-axis) across the Dolan Fire.
The 1.0 rainfall anomaly contour from Figure 1B plots as adiagonal 1:1 line in this space, while any absolute rainfall
contour plots as a horizontal line (30 mm/h is shown). The
overall stream network (blue shading) occupies much of thisspace, while the vast majority of debris-flow basins (redcircles sized by basin area) are confined to the region above
the 1.0 rainfall anomaly line.

4.1 Absolute rainfall vs. rainfall anomaly
Compared to the overall stream network within theDolan burn scar, debris flows were 1.6% more likely tooccur above a fixed I15 threshold of 30 mm/h, but ~37%
more likely to occur above an I15 anomaly threshold of
1.0 [Figure 2]. This pattern indicates a higher sensitivityof debris-flow processes to climatologically extreme
rainfall than to rainfall above a constant,climatologically insensitive threshold.

4.2 Possible coevolution of geomorphic
processes to local climate
A correlation between geomorphic process thresholds
and local hydroclimate has been observed in priorstudies of landslide and debris-flow events in unburned
areas [15, 4, 5, 6, 7], and in a regional-scale analysis of
post-fire debris-flow initiation [16]. Such a correlation
may indicate a coevolution of processes to long-term
local climate, part of a long-studied link between
climate and geomorphology.

We also plotted the full stream network and debrisflow locations on a two-dimensional phase space, withthe x-axis representing the climatological 1-year I15 andthe y-axis representing the observed peak I15 at a given
point [Figure 3]. This visualization scheme allows us toreduce the geographic contours from Figure 1 to simple
lines which highlight the predictive power of rainfall
anomaly versus peak rainfall. Any point on the spacethat lies above the 1:1 line represents a point thatreceived a rainfall anomaly greater than 1, while any
point on the space that lies above a horizontal line
represents a point that received a peak I15 above a
particular constant value.

Our study did not seek to identify the precise
mechanisms driving the association between debrisflow initiation and local climate, and future studies
could further investigate this link. Following findings
from related aspects of geomorphic and landscapeevolution research, mechanisms may include theadjustment of soil hydraulic properties [17, 18],
mechanical properties [19], and/or drainage network
geometry [20], among others.

The full stream network occupies a large portion ofthis space, but the debris-flow locations are almost
exclusively limited to the area above the 1:1 line. This
indicates that a threshold equal to the local 1-yearrecurrence interval storm explains the prevalence ofdebris flows more effectively than any climatologicallyinsensitive threshold that would plot as a horizontal linein the space.

4.3 Implications for debris-flow hazard
prediction
The sensitivity of post-fire debris flows to local rainfall
anomaly has implications for debris-flow hazard
prediction in hydroclimatically diverse regions. Further
focus and understanding of this pattern may be criticalfor predicting post-fire debris-flow processes in areasthat are facing unprecedented levels of severe wildfire,
have sparse observational history, and/or are far fromareas in which existing predictive models were trained.

3E3S Web of Conferences 415, 04003 (2023)
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Additionally, many areas are facing hydroclimatic
shifts associated with climate change, such as increased
rainfall intensity [21]. The adjustment of geomorphic
process to long-term local climatology may havesubstantial implications for debris-flow frequency andseverity in areas that are facing hydroclimatic conditions
far more intense than those to which the landscape hashistorically adjusted.

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5 Conclusions

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The sharp hydroclimate gradient and widespread rainfall
event at the Dolan Fire in California provide a naturalexperiment to demonstrate whether debris-flow
processes and rainfall-intensity thresholds for debrisflow initiation are sensitive to local climatology.

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By mapping flow type across the burn scar throughremote and field methods and comparing with observed
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more effectively than any single fixed rainfall intensity
threshold.

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Similar to past studies of landslide and debris-flow
activity in unburned areas, this indicates that the factors
governing debris-flow generation may have co-evolved
with the local climatology of intense rainfall. Thisfinding has implications for post-fire debris-flow hazardprediction that will prove important as the size and scopeof severe wildfire expands globally, as well as forgeneral debris-flow hazard prediction as globalhydroclimatic conditions shift with climate change.

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17. N. Delgado, Quantification of wildfire-inducedThis research was supported by the U.S. Geological
Survey Landslide Hazards Program. We would like to thankAmy East, Don Lindsay, Nathan Delgado, Ingrid Suter,
Amelia Winner, Noah Finnegan, and Yonni Schwartz for their
help in the field, and Nina Oakley for meteorological andclimatological advice. Special thanks go to Mark Readdie andGage Dayton for providing accommodation at the Universityof California Landels-Hill Big Creek Reserve.

perturbations to soil-water retention curves and theirimpact on rainfall runoff, Univ. of Nevada Reno
Dissertations & Theses, (2022).

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Any use of trade, firm, or product names is fordescriptive purposes only and does not imply endorsement by
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21. N. Oakley, Earth’s Future, 9(7), (2021).

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