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Projected Effects of Climate Change on Post Wildfire DebrisFlow Hazards Applied to the 2017 Thomas Fire, California USA
Zane White1 and Paul Santi2
1

WW Wheeler & Associates, Inc., Englewood Colorado USA
Colorado School of Mines, Department of Geology and Geological Engineering, Golden Colorado USA

2

Abstract. Climate changes are expected to increase the likelihood, volume, and overall hazard of postwildfire debris flows, but it is a challenge to estimate specific changes and impacts from these hazards. In
this study we use climate change models to modify the parameters in predictive equations to demonstratechanges for the 2017 Thomas Fire in California USA if a similar event were to occur in the years 2050 or
2075. Our results show that, based on changes in fire size, the number of burned drainage basins in 2050
would increase by 105% and by 2075 they would increase by 147%. Based on changes in fire size andrainfall effects, the overall volume of debris produced by debris flows would increase by 96% in 2050 and
147% in 2075. Finally, there would be a notable shift in hazard level towards basins classified as highhazard. The hazard models for the 2017 fire classified 54% of the drainage basins (937 total) as high hazard,
but by 2050 there would be 60% (1869 total) classified as high hazard. By 2075, 67% of basins would behigh hazard (2385 total). This represents an increase in high hazard drainage basins of 99% by 2050 and
155% by 2075. The results of this exercise indicate a substantial future increase in area impacted by post
wildfire debris flows, the amount of debris produced, and the level of hazard posed.

1 Introduction
In recent history, the size, amount, and severity of
wildfires are increasing, and climate change is the main
driving component of these changes. Climate models
predict dryer conditions, an increase in temperatures,and changes in precipitation patterns. Dryer conditions
fuel fires to burn larger areas with higher severity. In
addition to increased variability in precipitation return
periods, rainfall intensities are expected to increase withchanges in the climate. These changes are expected toincrease the likelihood, volume, and overall hazard of
post-fire debris flows [1]. After a wildfire is
extinguished or even possibly still burning, post-firedebris flows are an immediate significant geohazard.
One of the challenges for debris-flow hazard
management is estimating how future climate conditions
would modify the impacts of post-fire debris flows.
For example, in December of 2017 the Thomas Firein the community of Montecito in Southern California
USA burned 114,078 hectares in 38 days [2]. Before thefire was 100% contained, heavy precipitation hit thearea causing several debris flows that killed 23 people
and caused extreme damage to infrastructure [2]. One ofthe debris flows, in San Ysidro Creek, was over 297,000
m3 in volume, so the hazard generated to the urban area
downstream was tremendous [3]. The Thomas Fire burnarea covered a wide range of drainage basin sizes, burnseverities, and created debris flows with varying
characteristics. This event is, therefore, an ideal study
area to demonstrate how the Thomas Fire would change

in size and severity under predicted future climate
conditions. Additionally, the study area is also suited fordemonstrating how post-fire debris-flows volume,
likelihood, and hazard would change under predicted
future climate conditions.
In this study we use climate change prediction
models to modify the parameters in the equations usedby the U.S Geological Survey (USGS) to predict debrisflow likelihood [4] and volume [5] In addition, we use
wildfire size prediction models to demonstrate changes
in affected area. By combining these, we show how thedebris-flow hazard would be expected to change for theThomas Fire area, if a similar event were to occur in theyears 2050 or 2075 (approximately 25 or 50 years in the
future).

2 Global Climate Models
As a first step, we use global climate models to predictchanges in temperature, which in turn allows us topredict changes in rainfall intensity. Temperaturechanges are predicted using different carbon emission
scenarios, referred to as Representative Concentration
Pathways (RCPs). We use RCP 8.5, which is a worstcase scenario model where there are no changes inemissions, as well as RCP 4.5, which is a scenario thatassumes moderate emissions mitigation (Table 1). Datafor these models is derived from [6, 7, 8].
Temperature changes are related to changes inrainfall intensity, with heavier rainfall events occurring
more often in a warmer climate [2, 7, 9, 10, 11]. We use
the Clausius-Clapeyron relation to calculate change,© 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/).

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which estimates the intensity of heavy rainstorm events
to increase by 6% to 7% for every degree Celsius
increase in global average temperature [11, 12, 13](Table 1).
The USGS model for the Thomas Fire area relies ona design rainstorm of 15-minute, 40 mm/hr intensity to
predict debris flow likelihood, volume, and hazard. This
design storm has a return period of approximately 1-2
years in the Montecito area [14]. Therefore, the values
in Table 1 use 40 mm/hr as a starting point and apply theClausius-Clapeyron relation to predict changes to thedesign rainstorm.

burn severities [19] On this basis, we assume theproportion of low, moderate, and high burn severity toremain comparable to the current proportions.
Other studies estimated the expected increase inareas burned under future climate change scenarios. The
total area burned in the western United States is
projected to increase by 47% to 86% relative to 20012010 for a typical fire year and 48% to 61% for extreme
fire years in 2041 to 2050 for the Mediterranean
California ecoregion [20]. The extreme fire years have alower increase in total area burned since they have alarger total area burned and are less likely to have thesame increase in total area burned than that of a normal
fire year. Spracklen et al. [18] projected that the total
burned area across the western U.S. to increase by 54%
for 2046-2055 relative to 1996-2005.
We elected to use the Spracklen et al. [18]projection of 54% increase to represent the change by
2050, as shown on Figure 1. This value is a conservative
estimate of the models shown on the figure.
Extrapolated to 2075, this would result in an increase inburned area of 81%.

Table 1. Predicted temperature changes using Cal-adapt [15](upper table). Projected rain intensities for the years 2050 and
2075 under RCP4.5 and RCP8.5 scenarios (lower table).

Projected Temperature Change in Ventura County (C°)
Model
Obs. Hist.
RCP 4.5
RCP 8.5
Year
1976-2005 2050
2075
2050
2075
Max
20.3
22.6
23.1
22.7
24.3
Min
6.1
7.9
8.3
8.3
9.2
Average
13.2
15.3
15.7
15.5
16.8
Temp. Increase
0.0
2.0
2.4
2.3
3.5

Projected 15-min-40mm/hour Rainfall Intensity Change in Ventura County
Projection Scenario
RCP 4.5
RCP 8.5
Year2050 2075 2050 2075
Temperature Increase (C°)
2.0
2.4
2.3
3.5
Current 15 min-Intensity (mm/hr)
40
7% Rainfall Intensity Scaling (mm/hr) 46
47
46
50
Based on the range of resulting rainfall intensitychanges in Table 1, we project a design rainfall intensityof 46mm/hr for 2050 (both RCP 4.5 and 8.5 produce thesame results), and 50 mm/hr for 2075 (based on RCP
8.5).

Fig. 1. Projected increase in burned area using models by [18and 20]. We use Spracklen et al. [18] values (gray curve) for
our analysis herein.

3 Wildfire Change Models

4 Debris-Flow Likelihood, Volume, andHazard Models

Since the 1980s, large forest fire occurrence has
increased and is expected to continue to increase under
projected future climate conditions [2, 7, 9, 16, 17].
Burn severity, a measure of the heat produced fromthe burn and the duration of the burn, is also expected tochange with climate change. The area of high burnseverity from wildfires has increased in large parts of thewestern United States, but Southern California does notappear to have clear data trends for soil burn severities[13, 18]. The unclear trends in fire severity are thought
to be attributed to the burning of chaparral and replacing
it with grass which burns at lower severities [13]. In oneof the few studies evaluating changes in burn severity,
Parks et al. [19] reviewed previous soil burn severity
data for fires greater than 400 hectares in the years 19842012 and evaluated the potential response of burnseverity to climate change in the western United States(under the RCP 8.5 scenario). The produced models
show that fire burn severity in mid-century (2040-2069)
for the Montecito area will likely be low to moderate

The debris flow likelihood model we used was
generated by Staley et al. [4] and is used by the USGS
for their post-fire hazards analysis. It is a logistic
regression model that calculates the probability of adebris flow based on several input parameters:
P = ex / (1 + ex)(1)

where P is the probability of debris flow and x is
the exponential function based on equation 2:
x = -3.63 + (0.41 × X1R) + (0.67 × X2R) + (0.7 ×
X3R)(2)
where:

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X1R = proportion of upslope area classified as highor moderate soil burn severity with gradients ≥ 23°,
multiplied by the peak 15-minute rainfall
accumulation of the design storm (in millimeters[mm])
X2R = average differenced normalized burn ratio(dNBR) of the upslope area, multiplied by the peak
15-minute rainfall accumulation of the design storm(in millimeters [mm])
X3R = soil KF-Factor of the upslope area,multiplied by the peak 15-minute rainfall
accumulation of the design storm (in millimeters[mm]). KF factor is the average soil erodibility
index (Staley et al. [4])For our conservative estimated changes, only thepeak 15-minute rainfall is assumed to change, using thevalues in Table 1.
Post-fire debris-flow volumes are predicted for agiven basin at the basin outlet (pour point) using
Equation 3 [5]. Volume estimates are classified bymagnitude ranges of 0–1,000 m3; 1,000–10,000 m3;
10,000–100,000 m3; and greater than 100,000 m3.

Fig. 2. Debris-flow likelihood model for the Thomas fire.

ln(V) = 4.22 + (0.13 × sqrt (ElevRange)) + (0.36 ×ln(HMkm)) + (0.39 × sqrt(i15))(3)
where:•••

ElevRange = range (maximum elevation–minimum
elevation) of elevation values within the upstream
watershed (in meters)
HMkm = area upstream of the calculation point thatwas burned at high or moderate severity (in km2)
i15 = spatially averaged peak 15-min rainfall
intensity for the design storm in the upstream
watershed (in mm/h)

Fig. 3. Debris-flow volume model for the Thomas fire.

For a conservative prediction of debris-flow volume
changes for this study, we assume only the 15-minute
rainfall will change.
The debris-flow hazard model is a combination ofprobability and volume maps. The probability estimates
are divided into bins of 1-5 with 1 representing low
probability and 5 representing high probability (1 = 020%, 5 = 80-100%). The volume estimates are divided
into bins of 1-4 with 1 representing small volumes and4 representing very large volumes (1 = 0–1,000 m3, 4 =>100,000 m3). The ranks are added together, with 9
representing the highest hazard. The hazard is thenclassified as low (1 = sum of ranks is 2-3), moderate (2= sum of ranks is 4-6), and high (3 = sum of ranks is 79).
The resulting likelihood, volume, and hazard
models for the 2017 Thomas Fire are shown in Figures
2, 3, and 4, respectively. These are maps produced fromour modeling using the USGS equations, and not theactual maps produced by the USGS, so there are slightdifferences.

Fig. 4. Debris-flow hazard model for the Thomas fire.

5 Results
5.1 Projected change in fire size and severity
In order to develop polygon outlines of drainage basins
and pour points, we use the ArcGIS Watershed tool.
Any basins less than two hectares in size were removed.
The fire area that burned in 2017 covered 1736 distinct

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drainage basins. Our projections for 2050 (a 54%
increase in fire boundary expanded evenly around theentire 2017 perimeter) resulted in 3115 burned basins.
The projection of 2075 resulted in 3559 burned basins.
The resulting change in shown in Figure 5.

Fig. 7. Projected burn severity map for 2075 for the Thomas
fire (81% increase in fire boundary).

5.2 Projected change in debris-flow likelihood,
volume, and hazard
The models for the 2017 likelihood, volume, and hazard
shown in Figures 2, 3, and 4 were modified for projectedchanges in rainfall intensity (Table 1), burned area
(Figure 5), and burn severity (Figures 6 and 7). The
resulting hazard models incorporating climate change
effects are shown in Figures 8 (2050 projection) and 9(2075 projection). Compared to Figure 4, one can notethe shift towards higher hazard classification in addition
to the significant increase in the total number of drainage
basins affected (because of the significant increase infire size).

Fig. 5. Projected increase in burned area for the Thomas fire.

Assuming the same proportion of burn severity,
estimated maps of severity for 2050 and 2075 are shownin Figures 6 and 7, respectively.

Fig. 6. Projected burn severity map for 2050 for the Thomas
fire (54% increase in fire boundary).
Fig. 8. Projected debris flow hazard map for 2050 for theThomas fire (81% increase in fire boundary).

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In 2017, 54% of basins were classified as highhazard, which means that out of 1736 total basins,
937 would be high hazard.
In 2050, 60% of basins were classified as highhazard, but with a larger projected fire size, there
would be 3115 total basins, so 1869 basins would
be high hazard. This is an increase of 99% of thenumber of high hazard basins compared to 2017.
In 2075, 67% of basins were classified as highhazard, and with a larger projected fire size, there
would be 3559 total basins, so 2385 basins would
be high hazard. This is an increase of 155% of thenumber of high hazard basins compared to 2017.

Although previous studies have indicated theincreased likelihood (frequency), volume, and impactsof debris flows under future climate change scenarios,
this study shows the specific changes expected using
well-established models applied to an important recent
debris flow event. We conclude that even with moderate
carbon emissions mitigation, post wildfire debris-flowhazards and impacts will increase substantially.

Fig. 9. Projected debris flow hazard map for 2075 for theThomas fire (81% increase in fire boundary).

Table 2 summarizes the changes in total volume ofdebris-flow material expected under future climatechange scenarios, as well as the shift towards higher
hazard basins.

References

Table 2. Comparison of 2017 debris-flow hazards to future
projected scenarios. Note the significant change in overall
volume and the shift toward high hazard basins

1.

Year

Rainfall Intensity(mm/hr)Fire Size %RCP scenario

Number ofdrainage basins

Total volume ofdebris flows m3% High hazard
basins

2.

2017

40

100

--

1736

11,354,351

54

2050

46

154

4.5/8.53115

22,292,364

60

2075

50

181

8.5

3559

28,003,404

67

3.

4.
5.
6.

6 Discussion and Conclusions
7.

We applied established climate change models toestimate future changes in debris flow likelihood, size,
affected area, and overall hazards when applied to the2017 Thomas Fire in California USA.
Using
conservative assumptions, we demonstrate that the
number of drainage basins burned by a similar fire, wereit to occur in the year 2050, would increase by 105%
(3115 basins in 2050 compared to 1736 basins in 2017).
Were the fire to occur in the year 2075, the number ofburned basins would increase by 147% (3559 compared
to 1736). Looking at the overall volume of debris
emanating from these basins, we show 96% morevolume is possible in 2050 and 147% more volume in
2075.
The overall debris-flow hazard, measured as a
combination of debris-flow likelihood and debris-flow
volume shows a distinct shift towards the high hazard
category:

8.

9.

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