Valley Fire (CA-CNF 002833) Descanso Ranger District Cleveland National Forest September 20, 2020

Burned Area Emergency Response (BAER) Assessment Specialist Report – GEOLOGIC HAZARDS

Barton Wills – Geologist, Deschutes NF

Introduction: The Valley Fire started on September 8, 2020 and was 95% contained as of September 19, 2020. The fire burned on the Descanso Ranger District of the Cleveland National Forest, as well as across San Diego County and private property. Out of a total size of 16,769 (BAER) acres, 257 acres (2%) were high burn severity, 9,989 acres (60%) were moderate burn severity, 5,417 acres (32%) were low burn severity, and 1,106 acres (7%) were very low or unburned burn severity. The unburned acres refer to unburned areas within the fire perimeter (Figure 1). This report describes and assesses the increase in risk from geologic hazards within the Valley Fire burned area.

Objectives: When evaluating Geologic Hazards, the objectives of the “Geology” function on a BAER Team are identifying the geologic conditions and geomorphic processes that have helped shape and alter the watersheds and landscapes, and assessing the impacts from the fire on those conditions and processes that potentially could affect downstream Critical Values (CV). The fire removed vegetation that helps keep slopes and drainages intact, changed the structure and erosiveness of the soil, and altered the stability of the landscape. Using the understanding of rock types and characteristics, geomorphic processes, and distribution of geologic hazards helps predict how the watersheds will respond to and be impacted by upcoming storms.

Critical Values:

Human Life and Safety: • People living, working, traveling or recreating through and below burned areas – Loss of life or injury could take place as a result of debris slides, debris flows, rock-fall, or flooding in and downstream of the burn area.

Property:

• Forest Service structures, roads & trails, private access roads, and water systems – As a result of the fire, higher runoff and flows are expected, stability of slopes over trails is compromised. Debris slides, debris flows, rock-fall, and flooding could cause damage to these systems.

• Residential houses and other structures, powerlines, water systems, and other properties - As a result of post-fire conditions, higher runoff and sheet flows are expected, stability of slopes is compromised. Debris flows, rock-fall, and flooding could cause damage to this infrastructure.

Natural Resources:

• Critical habitat for Federally & State Listed Species: Suitable habitat is known to occur in Pine Creek for Least Bell’s Vireo. Run-off, excessive sedimentation and/or debris flows from the burned areas in these watersheds will adversely affect water quality, negatively impacting these suitable and critical habitats.

• Water quality - As a result of the fire, excessive sedimentation and debris will adversely affect water quality in creeks flowing through and below the burned area.

• Noxious and invasive weeds – As a result of the fire and associated suppression disturbance the probability of introduction of noxious and invasive weeds has increased. Equipment transported from out of the area may introduce new exotic species to the forest or disperse seeds from existing infestations to new areas. Small infestations of exotic species within the burned area may rapidly regenerate and expand post-fire where dense native vegetation previously inhibited their emergence or growth. The large area of wildland urban interface and private lands within and adjacent to the burn area create high probabilities of seed transport from off forest, particularly along roads and trails.

Cultural Resources:

• The increase in runoff and debris, resulting from a reduction in surface vegetation and compromised soil structure in addition to the downslope movement of materials, can also expose cultural resources, subjecting them to risk from vandalism and theft. Construction materials commonly used during the historic era may also be hazardous when burned or pose potential threat to other resources, habitat, or visitors to NFS lands. • No previously documented pre-contact sites were noted during the initial assessment of the NFS lands within the Valley Fire burn area vicinity. Cultural resource concerns off NFS lands are being considered under other emergency assessments.

Resource Condition Assessment: Resource Setting –

Geomorphology and Geology:

The Valley Fire area is located in the Santa Anna Block of the lower Peninsular Range region, a subset of the greater Peninsular Ranges Geomorphic Province of California. The Santa Anna Block is approximately bounded to the east by the Elsinore Fault Zone, bounded to the north by the Transverse Ranges, bounded to the south by Baja California, and bounded to the west by the Pacific Ocean (Baird and Miesch, 1984).

This portion of the Peninsular Ranges is underlain by Jurassic and Cretaceous plutonic rocks of the Peninsular Ranges Batholith. Geologically, the area is underlain by two principle rock types: Tonalite and Gabbro (Figure 2). Tonalite is a phaneritic, intermediate-felsic intrusive igneous rock and is composed of primarily feldspar with some quartz and other accessory amphiboles and pyroxenes minerals. Gabbro is a phaneritic, mafic intrusive igneous rock composed of calcium rich plagioclase and pyroxene with minor amounts of amphiboles and olivine (Baird and Miesch, 1984). There are no mapped metamorphic rocks, sedimentary rocks, or Quaternary surficial deposits other than valley alluvial deposits within the area of the Valley Fire (Quaternary Surficial Deposits, California Dept. of Conservation). Quaternary surficial deposits would include geologic mass wasting hazards such as debris flows, rock fall, and landslides.

The faults that bound the structural block post-date the emplacement of the plutonic rock, and movement on the faults have possibly continued to Holocene times. There is a buried fault trace on the southwest boundary of the fire perimeter that strikes to the west-northwest approximately underlying Hauser Canyon and Barrett Lake (Geologic Map of California, California Dept. of Conservation). However, there are no mapped Quaternary faults within the area of the Valley Fire (Earthquake Hazard, USGS Fault and Fold database of United States).

Physiographic setting of the Peninsular Ranges which are part of the North American Coast Range that run the length of the Pacific Coast from Alaska to Baja Mexico. The Valley Fire perimeter lies within the southern Laguna Mountains of the Peninsular Ranges, which is largely contained within the Cleveland National Forest. The highest point within the fire perimeter is Gaskill Peak, elevation of 3,836 feet, which is on the west side of the perimeter overlooking Lawson Valley. The lowest elevation is approximately 1600 feet elevation where Pine Creek enters Barrett Lake. Pine Creek is the only major drainage that cuts through the fire perimeter. The highest relief areas (slopes greater than 60 degrees) in the fire perimeter are the northwest slope of Gaskill Peak, the slopes north of Carveacre Road (FS Road 16S03), all slopes along FS Road 17S06 east of Pine Creek, slopes on both sides of Pine creek drainage, and north facing slopes on north end of fire perimeter near Hidden Glen. Most of the slopes within the fire perimeter are low to moderate steepness (Figure 4).

Burned Area Reconnaissance and Data Collection:

Reconnaissance of the burn area was a rapid assessment and included only field ground surveys that could be reached by vehicle. A flight recon was requested but denied. Ground surveys were conducted on September 16 & 17, 2020. Day one of survey included Japitul Station, Horse Thief trailhead, Lyons Valley Road (County), FS Road 17S06 (west of Pine Creek), Camp Barrett (County), Barrett Lake (County), Hidden Glen, and FS Road 16S02 (Glen Lonely Rd). Day two ground survey included Lower Pine Valley Watershed, Skye Valley Ranch, FS Road 17S06 (east of Pine Creek), and Lawson Valley area. Assessment of these areas included identification of Critical Values in and downstream of burned area, identification of pre-fire slope failures and pre-fire slope and channel failure deposits, measurements of slopes, identification of geological units, field verification, of soil burn severity, notes of observations and photography. In addition to ground reconnaissance, a review of published geologic maps and articles and study of aerial photography and LiDAR imagery was conducted.

Field Observations:

HUC 6 watersheds that were impacted by the Valley Fire include: portions of the Loveland Reservoir- Sweetwater River at the west end of the burned area, portions of the Taylor Creek watershed at the north end of burned area, portions of the Middle Pine Valley Creek watershed at the northeast end of burned area, Lower Pine Valley Creek watershed at the east end of the burned area, and Morena Reservoir-Cottonwood Creek watershed at the south end of the burned area (Figure 3).

During ground surveys, evidence of mass wasting as rock fall are widespread but that little evidence was found of landslides, debris slides or debris flows. In the steeper high relief areas, pre-fire rockfall was common. The burned area is principally underlain by very resistant, very strong competent igneous bedrock materials and is exposed over much (estimate 30-40%) of the burned area with pre-fire sparse vegetation. In the burned area, the bedrock is unchanged from the pre-fire condition, with the exception of some spalling due to very hot temperatures (Photo 6).

The native vegetation of chaparral and desert grasses of this fire adapted ecosystem does not appear to be holding the rock in position. Not observing large differences between pre-fire and post-fire in bedrock dominated areas. Only in high burn severity areas did heat penetrate into the soil and scorch roots. Roots in the low and moderate severity soil were still present.

Channels within the burned area were observed to mostly sand and small gravel sized sediment. Large boulders in the channels are from rockfall and not debris flow. During ground surveys did not observe any past debris flows type materials. In addition, most slopes and drainages in the burn area appeared to have low to moderate amounts of stored material, drainage areas were small, and channels were mostly low to moderately gradients (Figure 4), and generally has relatively low erosion rates despite its mountainous character. The Pine Creek drainage in the burned area is only a very small part of the overall drainage. It is very low gradient and very low probability of a channeling a debris flow to Barrett Lake. There is a slightly greater chance of small amounts of sediment by flooding.

Did not observe any strait channels with very steep chutes. Within the burned area there are abundant low gradient areas for deposition of sediment. No persistent rill networks were observed in the upper areas of the watershed. Based on ground surveys, past fires in the same area, such as 2006 Horse Fire and 2001 Viejas Fire, did not seem to produce any notable mass wasting besides increased sedimentation and rockfall as would be expected. Noted that check dams and culverts along Lyons Valley Road were full of fine sediment and need to be maintained and cleaned out in order to properly function.

One principal debris flow “watch out” slope is the northwest facing slope of Gaskill Peak. This slope has a moderate basin probability or likelihood (40-60%) of a design storm of a peak 15-minute rainfall intensity of 24 millimeters per hour (mm/h) rate with a moderate combined hazard. This channel funnels directly an existing alluvial fan and private lands of Lawson Valley (Photo 4). Due to the nature of a rapid assessment and on private land, this hazard was not investigated more thoroughly.

In addition to these Critical Values described above there are others located off NF Lands in and downstream of the burned area. These Critical Values will be analyzed carefully by CAL Fire Watershed Emergency Response Team (WERT Team). Just as the BAER Team evaluated changes to the watershed conditions on federal lands, the WERT Team will evaluate similar changes, primarily on private land in and downstream of the burn area.

1. Emergency Determination:

The emergency to Critical Values from geologic hazards caused by the fire includes adverse effects to the health and safety of people, FS, State, County, Tribal, City and private properties, and facilities, roads, trails, and natural and cultural resources. Risk of loss of life and limb is of particular concern.

2. Treatments to Mitigate the Emergency:

Treatments to mitigate the emergencies include warning notifications and signs, administration closures, and road stormproof treatments. The objective of these treatments is to save life, limb and property. Further detailed treatments can be found in Appendix B.

Conclusion/Discussion

In conclusion, as a result of the removal of vegetation by the fire, soils are exposed and have become weakened, hydrophobicity conditions have changed and rocks on slopes have lost their supporting vegetation. Due to these post-fire new conditions, roads, trails and water systems are at risk from numerous geological hazards as rolling rocks, potential debris flows and hyper-concentrated floods. Risks to human life, infrastructure, facilities, roads, trails, water systems and natural resources is elevated in most areas in and downstream of the Valley Fire.

Within the burned area of the Valley Fire, evidence of mass wasting as rock fall are widespread but that little evidence was found of landslides, debris slides or debris flows. In addition, most slopes and drainages in the burn area appeared to have low to moderate amounts of stored material, drainage areas were small, and channels were mostly moderately steep gradients (Figure 4).

USGS Debris Flow Assessment: A debris flow is a form of rapid mass movement in which a combination of loose soil, rock, organic matter, air, and water mobilize as a slurry that flows downslope. Factors that influence a debris flow are steepness of slope, soil type/geology, high intensity storm events, post-wildfire conditions (intensity of burn, hydrophobicity, and loss of vegetative cover). In order to assess the probability and potential volumes of debris flows in the burned area the assistance of the US Geological Survey (USGS) - Landslide Hazards Program was obtained. Their ongoing research has developed empirical models for forecasting the probability and the likely volume of such debris flow events. To run their models, the USGS uses geospatial data related to basin morphometry, burn severity, soil properties, and rainfall characteristics to estimate the probability and volume of debris flows that may occur in response to a design storm (Staley, 2016). Estimates of probability, volume, and combined hazard are based upon a design storm with a peak 15-minute rainfall intensity of 12 – 40 millimeters per hour (mm/h) rate. We selected a design storm of a peak 15-minute rainfall intensity of 24 millimeters per hour (mm/h) rate to evaluate debris flow potential and volumes. A 15-minute rainfall intensity of 24 mm/h is equivalent to the accumulation of 6 mm of rain in 15 minutes. In English units, this is equivalent to approximately 0.25 inches of rain in 15 minutes. This design storm is used for three reasons: 1. Post-fire debris flows are most often triggered by high-intensity, short-duration bursts of rain. 2. A 24 mm/h rain burst is likely to happen in most areas of the western U.S. (i.e. a 1-5 year recurrence interval). 3. A 24 mm/h rain burst is known to trigger debris flows at USGS monitoring sites in burn areas. (Landslide Hazards, USGS, https://www.usgs.gov/natural-hazards/landslide- hazards/science/emergency-assessment-post-fire-debris-flow-hazards)

Debris flow probability and volume were estimated for each basin in the burned area as well as along the upstream drainage networks, where the contributing area is greater than or equal to 0.02 km², with the maximum basin size of 8 km². The debris flow model only represents accumulations zones and does not represent deposition zones.

Kean et al. (2013) and Staley et al. (2016) have identified that rainfall intensities measured over durations of 60 minutes or less are best correlated with debris-flow initiation. It is important to emphasize that local data (such as debris supply) influence both the probability and volume of debris flows. Unfortunately, local specific data are not presently available at the spatial scale of the post-fire debris-flow hazard assessment done by the USGS. As such, local conditions that are not constrained by the model may serve to dramatically increase or decrease the probability and (or) volume of a debris flow at a basin outlet.

Based on USGS debris flow modeling it appears that under conditions of a peak 15-minute rainfall intensity storm of 24 millimeters per hour (0.25 inch/hr.), the probability of debris flows occurring is very low to low (0-40%) in a majority of the channels/creeks in the Valley fire burn area to moderate (40-60%) in a minority of the channels/creeks. Under these same conditions, predicted volumes in these channels are expected to less than 1K cubic meters in channels. Based on the low probabilities of debris flow initiation and low predicted volumes of debris flows, a majority of creeks in the burn area appear to present a low to moderate combine hazard. Field observations match the low to moderate probability, low (<1K cubic meters) volume, and low to moderate combined hazard of the debris flow model.

The conclusion of our field observations is that whether the primary post-fire process is rock-fall, debris slides, debris flows or sediment laden flooding, the cumulative risk of various types of slope instability, sediment bulking, and channel flushing is high along a majority of slopes and drainages in and below the burn area following the Apple Fire. Based on the above, special attention and caution is recommended in areas where people are living or traveling through, working or recreating in or below the burned areas during and after storm events.

In order to reduce risk to life, it is our recommendation to coordinate warning notifications with the National Weather Service, post warning signs and enforce administration closures.

Beyond threats to life and property, as a result of the fire, excessive sedimentation and debris could adversely affect the quality and capacity of streams and other critical habitats for Federally & State Listed Species in and below the burn area, in addition to adversely impacting cultural resources.

References:

Baird, A.K. and Miesch, A.T., 1984, Batholithic Rocks of Southern California – A model for Petrochemical Nature of their Source Materials, U.S. Geological Survey Professional Paper 1284, Dept of Interior.

Compilation of Quaternary Surficial Deposits, California Geological Survey, California Department of Conservation, https://maps.conservation.ca.gov/cgs/qsd/app/

Debris Flow Program, National Weather Service, https://www.weather.gov/lox/debrisflow

Geologic Map of California, California Geological Survey, California Department of Conservation https://maps.conservation.ca.gov/cgs/gmc/App/

Kean, J.W., McCoy, S.W., Tucker, G.E., Staley, D.M., Coe, J.A., 2013, Runoff‐generated debris flows: Observations and modeling of surge initiation, magnitude, and frequency, Journal of Geophysical Research Earth Surface, 118(4), p. 2190-2207.

Landslide Hazards Program, U.S. Geologic Survey, https://www.usgs.gov/natural-hazards/landslide- hazards/science/emergency-assessment-post-fire-debris-flow-hazards

Maps and Data, California Geological Survey, California Department of Conservation, https://www.conservation.ca.gov/cgs/maps-data

Staley, D.M., Negri, J.A., Kean, J.W., Tillery, A.C., Youberg, A.M., 2016, Updated logistic regression equations for the calculation of post-fire debris-flow likelihood in the western

Staley, D.M., Negri, J.A., Kean, J.W., Tillery, A.C., Youberg, A.M., 2016, Updated logistic regression equations for the calculation of post-fire debris-flow likelihood in the western United States: U.S. Geological Survey Open-File Report 2016-XXXX, 20 p., available at http:// pubs.usgs.gov/of/2016/XXXX/ Staley, D.M., Kean, J.W., Rengers, F.K., 2020, The recurrence interval of post-fire debris-flow generating rainfall in the southwestern United States

Staley DM, Kean JW, Cannon SH, Schmidt KM, Laber JL, 2013, Objective definition of Rain-fall intensity– duration thresholds for the initiation of post-fire debris flows in Southern California. Landslides 10:547– 562

Staley DM, Negri JA, Kean JW, Laber JL, Tillery AC, Youberg AM (2017) Prediction of spatially explicit rainfall intensity–duration thresholds for post-fire debris-flow generation in the Western United States. Geomorphology 278:149–162

Quaternary Fault and Fold Database of the United States, Earthquake Hazards, USGS, https://usgs.maps.arcgis.com/apps/webappviewer/index.html?id=5a6038b3a1684561a9b0aadf88412fc f

Appendix A: – Figures & Photos

Figure 1 – Soil Burn Severity map of the Valley Fire area

Figure 2 – Geology map of the Valley Fire area

Figure 3 – Watersheds in the Valley Fire area

Figure 4 – Slope in percentage (%) map of the Valley Fire area

Figure 5 - Debris Flow Probability based on a peak 15-minute rainfall intensity storm of 24 mm/h rate storm in the Valley Fire area

Figure 6 - Debris Flow Estimated Volume based on a peak 15-minute rainfall intensity storm of 24 mm/h rate storm in the Valley Fire area

Figure 7 - Debris Flow Combined Hazard based on a peak 15-minute rainfall intensity storm of 24 mm/h rate storm in the Valley Fire area

Photo 1 – Steep slopes above Japitul Station and along Lyons Valley Road. Rockfall hazard along Lyons Valley Road. Low Probability (20-40%) of Debris Flow from peak 15-minute rainfall intensity storm of 24 mm/h rate storm. The volume from such an event is anticipated to be minor (less than 1000 cubic meters of material).

Photo 2 – Boulders in Pine Creek from Bridge (looking south towards Barrett Lake from bridge) are from rockfall, not debris flow.

Photo 3 – Channel along FS Road 17S06 leading to Camp Barrett and Barrett Lake.

Photo 4 – Gasskill Peak from Lawson Valley. One of the highest relief points in the burned are of the Valley Fire. Note unburned strip of vegetation in the middle of mountain and along sides compared to the burned. Also note abundance of very strong, resistant bedrock with very few channels on its steep flank.

Photo 5 – Rock spalling on boulder in drainage from very high severe burn. Northwest above Japitul Station and just below Lyons Valley Road near the pass.

Photo 6 – From west end of Barrett Lake looking east at the southern Laguna Mountains which were burned by the Valley Fire.

Appendix B: - Geology Inputs to 2500-8

Soil Burn Severity

Figure 1 – Soil Burn Severity map of the Valley Fire area

Geologic Units

Figure 2 – Geology map of the Valley Fire area

Hydrologic Units

Figure 3 – Watersheds in the Valley Fire area

Slope Percentage

Figure 4 – Slope in percentage (%) map of the Valley Fire area

Debris Flow Probability Peak 15 Minute Rainfall Intensity Rate of 24mm/hour

Figure 5 - Debris Flow Probability based on a peak 15-minute rainfall intensity storm of 24 mm/h rate storm in the Valley Fire area

Debris Flow Estimated Volume Peak 15 Minute Rainfall Intensity Rate of 24mm/hour

Figure 6 - Debris Flow Estimated Volume based on a peak 15-minute rainfall intensity storm of 24 mm/h rate storm in the Valley Fire area

Debris Flow Combined Hazard Peak 15 Minute Rainfall Intensity Rate of 24mm/hour

Figure 7 - Debris Flow Combined Hazard based on a peak 15-minute rainfall intensity storm of 24 mm/h rate storm in the Valley Fire area

Photo 2 – Boulders in Pine Creek from Bridge (looking south towards Barrett Lake from bridge) are from rockfall, not debris flow.

Photo 3 – Channel along FS Road 17S06 leading to Camp Barrett and Barrett Lake.

Photo 5 – Rock spalling on boulder in drainage from very high severe burn. Northwest above Japitul Station and just below Lyons Valley Road near the pass.

Photo 6 – From west end of Barrett Lake looking east at the southern Laguna Mountains which were burned by the Valley Fire. Appendix B: Geology Inputs to 2500-8

Part II – Burned Area Description:

Geomorphology and Geology: The Valley Fire area is located in the Santa Anna Block of the lower Peninsular Range region, a subset of the greater Peninsular Ranges Geomorphic Province of California. The Santa Anna Block is approximately bounded to the east by the Elsinore Fault Zone, bounded to the north by the Transverse Ranges, bounded to the south by Baja California, and bounded to the west by the Pacific Ocean (Baird and Miesch, 1984).

Physiographic setting of the Peninsular Ranges which are part of the North American Coast Range that run the length of the Pacific Coast from Alaska to Baja Mexico. The Valley Fire perimeter lies within the southern Laguna Mountains of the Peninsular Ranges, which is largely contained within the Cleveland National Forest. The highest point within the fire perimeter is Gaskill Peak, elevation of 3,836 feet, which is on the west side of the perimeter overlooking Lawson Valley. The lowest elevation is approximately 1600 feet elevation where Pine Creek enters Barrett Lake. Pine Creek is the only major drainage that cuts through the fire perimeter. The highest relief areas (slopes greater than 60 degrees) in the fire perimeter are the northwest slope of Gaskill Peak, the slopes north of Carveacre Road (FS Road 16S03), all slopes along FS Road 17S06, slopes on both sides of Pine creek drainage, and north facing slopes on north end of fire perimeter near Hidden Glen. Most of the slopes within the fire perimeter are low to moderate steepness (Figure 4). One principal debris flow watch out slope is the northwest facing slope of Gaskill Peak. This slope has a moderate basin probability or likelihood (40- 60%) of a design storm of a peak 15-minute rainfall intensity of 24 millimeters per hour (mm/h) rate with a moderate combined hazard. This channel funnels directly an existing alluvial fan and private lands of Lawson Valley.

Part III – Watershed Conditions Within the burned area of the Valley Fire, evidence of mass wasting as rock fall are widespread but that little evidence was found of landslides, debris slides or debris flows. In addition, most slopes and drainages in the burn area appeared to have low to moderate amounts of stored material, drainage areas were small, and channels were mostly moderately steep gradients (Figure 4).

It is estimated that in case of high intensity storms (>20 mm/hr.) that tend to initiate/trigger debris flows, including summer thunder-storms the probabilities of debris flows are low to moderate. In addition, based on ground surveys, rock-fall are very likely along numerous steep burned slopes within the burn area of the Valley Fire. However, no evidence was found during ground surveys of landslides or debris flows within the burned area of the Valley Fire. Also, no evidence was found while reviewing state and national geologic references of landslides or debris flows within the burned area of the Valley Fire. Based on ground surveys, past fires in the same area, such as 2006 Horse Fire and 2001 Viejas Fire, did not seem to produce any notable mass wasting besides increased sedimentation and rockfall as would be expected. In conclusion, as a result of the removal of vegetation by the fire, soils are exposed and have become weakened, hydrophobicity conditions have changed and rocks on slopes have lost their supporting vegetation. Due to these post-fire new conditions, roads, trails and water systems are at risk from numerous geological hazards as rolling rocks, potential debris flows and hyper-concentrated floods. Risks to human life, infrastructure, facilities, roads, trails, water systems and natural resources is elevated in most areas in and downstream of the Valley Fire.

Debris Flow Potential: A debris flow is a form of rapid mass movement in which a combination of loose soil, rock, organic matter, air, and water mobilize as a slurry that flows downslope. Factors that influence a debris flow are steepness of slope, soil type/geology, high intensity storm events, post-wildfire conditions (intensity of burn, hydrophobicity, and loss of vegetative cover). The US Geological Survey (USGS) - Landslide Hazards Program, has developed empirical models for forecasting the probability and the likely volume of post-fire debris flow events. To run their models, the USGS uses geospatial data related to basin morphometry, burn severity, soil properties, and rainfall characteristics to estimate the probability and volume of debris flows that may occur in response to a design storm (Staley, 2016). Estimates of probability, volume, and combined hazard are based upon a design storm with a peak 15-minute rainfall intensity of 12 – 40 millimeters per hour (mm/h) rate. We selected a design storm of a peak 15-minute rainfall intensity of 24 millimeters per hour (mm/h) rate to evaluate debris flow potential and volumes. A 15-minute rainfall intensity of 24 mm/h is equivalent to the accumulation of 6 mm of rain in 15 minutes. In English units, this is equivalent to approximately 0.25 inches of rain in 15 minutes. This design storm is used for three reasons: 4. Post-fire debris flows are most often triggered by high-intensity, short-duration bursts of rain. 5. A 24 mm/h rain burst is likely to happen in most areas of the western U.S. (i.e. a 1-5 year recurrence interval). 6. A 24 mm/h rain burst is known to trigger debris flows at USGS monitoring sites in burn areas. (Landslide Hazards, USGS, https://www.usgs.gov/natural-hazards/landslide- hazards/science/emergency-assessment-post-fire-debris-flow-hazards)