Post-Fire Debris Flows in : an Atmospheric Perspective

Nina Oakley, Ph.D., WRCC/CNAP/CW3E Jeremy Lancaster, CA Geological Survey ALERT User Group Conference, Ventura, CA April 2018

Atmospheric river influenced post-fire debris flow near Santa Barbara, CA, Jan 20 2017

Western Regional Climate Center What is a post-fire debris flow?

USGS baseline rate: 25 mm h-1 rate for 15 min

2 Factors • Soils/geology • Burn severity • Steepness of terrain • Rainfall intensity • Are there people there? Source: CA Alluvial Fan Task Force Community of Rancho Mirage CA, situated on an alluvial fan Max intensity may not be triggering intensity

Nov 2009, burn area (Los Angeles County) Staley et al. 2013, Landslides Outline: CA Post-Fire Debris Flows

1. Climatological study in Transverse Ranges 2. Observations in central/northern California 3. Post-fire debris flows in a future climate

Post-fire debris flow on burn area 12/12/2014, Camarillo, CA Outline: CA Post-Fire Debris Flows

1. Climatological study in Transverse Ranges 2. Observations in central/northern California 3. Post-fire debris flows in a future climate

Post-fire debris flow on Springs Fire burn area 12/12/2014, Camarillo, CA Study Area

You are here Cool Seasons 1980-2014: 19 storm events triggering debris flows on 12 burn areas

Oakley et al. 2017 Natural Hazards7 Atmospheric Rivers

13 of 19 (68%) of events associated with Atmospheric River

Image Source: CIMSS Thanks to Shawn Roj for assembling 8 Conceptual- Synoptic Conditions

Oakley et al. 2017 Natural Hazards9 Conceptual-Cross-Section

Oakley et al. 2017 Natural Hazards10 Old/GrandFire and Flood: Prix Debris Old/Grand Flows, Dec Prix 25 2003 2003

Debris flows from the Old/Grand Prix Fires killed 16 people

Oakley et al. 2017 Natural Hazards Old/Grand Prix Debris Flows, Dec 25 2003

San Gabriel San Bernardino

Radar shows high reflectivity mostly confined to mountains

Image: NCEI radar archive Other Features: Squall line/NCFR

Oakley et al. 2017 Natural Hazards13 Other Features: NCFR

Dec 12 2014: Camarillo Springs Debris Flow

14 Jan 9 2018 Montecito Debris Flow

Thanks to D. Staley for making images into video

Images: CNRFC Summary: Southern CA

• In study: built catalog of PFDF events • ARs present in 13/19 events • Southerly low-level flow • Strong upper level jet with exit over S. CA • Orography plays big role, several cases additionally have NCFR/squall line Damage in Montecito following Jan 9 2018 debris flow

16 Outline: CA Post-Fire Debris Flows

1. Climatological study in Transverse Ranges 2. Observations in central/northern California 3. Post-fire debris flows in a future climate

Post-fire debris flow on Springs Fire burn area 12/12/2014, Camarillo, CA Squall line over burn area March 6, 2016

Mono Lake Angels Camp Stockton

Courtesy Jeremy Lancaster, CGS 12/3/15 (Before) Butte Fire burn area after intense rain March 6, 2016

Rain rates ~1.5 in/h

3/9/16 (After) Debris flow at Hawver Rd, North Fork Calaveras River watershed

Courtesy Jeremy Lancaster, CGS 12/3/15Detwiler (Before) Burn Scar: March 22 2018

Band of intense t-storms

Modesto Mammoth 3/9/16 (After) Turlock

San Jose

Detwiler burn area Madera

Image: CNRFC 12/3/15Detwiler (Before) Burn Scar: March 22 2018

*Preliminary information*

Sediment on roadways, many road closures, damage to several homes Rain rates up to 0.92 inches in 30 minutes observed

Images: K. Mattarochia, NWS Hanford History of debris flows in Big Sur SP Following June 2008

Apr 7, 2009 Precipitation rate 0.84 in/hr (21.3 mm/hr)

Courtesy Jeremy Lancaster, CGS Monitoring Soberanes Burn Area (winter 2016/17)

Possibility of debris flow above 12/29/16 Pfeiffer Falls… 01/10/17

Small debris flows in Palo Colorado Cyn, rate of ~20 mm/h for 15 min Photo: CGS Why no debris flows in Soberanes?

Hypotheses: • Rainfall not intense enough • All sediment flushed out by 2009 event • Soil burn severity may not have been high enough (low-moderate) Ash movement on Palo Colorado Rd late Oct 2016. Photo: CGS North Bay: No significant debris flows this winter. Why not?

Atlas Fire Nuns Fire

landslides.usgs.gov/hazards/postfire_debrisflow/ Hypotheses: • Low to moderate severity burn • Rainfall intensities not high enough? (little known about area) Summary: Central/Northern CA

• Much less known about triggering rates • Generally higher than southern CA • Isolated thunderstorms, squall lines appear to dominate • Limited populations on alluvial fans; smaller fans burns hills east of Napa on 9 Oct than S. CA 2017. Photo: M. Short, SF Chronicle

26 Outline: CA Post-Fire Debris Flows

1. Climatological study in Transverse Ranges 2. Observations in central/northern California 3. Post-fire debris flows in a future climate

Post-fire debris flow on Springs Fire burn area 12/12/2014, Camarillo, CA California Fire in the Future

• Length of fire season, size of fires likely to increase • Forested areas prone to more intense fires • Southern CA remains “ignition limited” • Population growth a big factor in fire frequency/impacts smoke over 101 Freeway near • No consensus on Carpinteria. Photo: S. Palley/Washington Post lightning activity

28 Future Hourly Precipitation Extremes

For Dec-Jan-Feb

Change in exceedance probability of the top 0.05% highest hourly precipitation rates observed in control period (Jan 2001-Sept 2013) 2-3x more likely to exceed top 0.05% hourly rainfall in Based on RCP 8.5 (high warmer climate in emissions) scenario CA in DJF

Prein et al. 2017, Nature Climate Change Take-home messages: Post-fire debris flows in California

• Antecedent rainfall not needed for debris flow in burn area • Do not need “big storm” • High intensity rainfall key • Atmospheric rivers present, mesoscale features play key role • Extended fire season likely in future • Hourly precipitation extremes more frequent in future climate • ALERT observations critical to post-fire debris flow research! Looking down on Thomas burn area, Feb 2018 Future Work

• Establish sub-daily records for CA! • Process data, increase monitoring in northern/central CA • Improve understanding of NCFR/squall lines in CA • Evaluate weather model performance Thomas Fire Photo: Ventura OES • Assess changes (historic, future) in high intensity, short duration precipitation

31 Research supported by: Thank you!

Nina Oakley [email protected] @WRCCclimate @CnapRisa @CW3E_Scripps WRCC

NASA astronaut Randy Bresnik photographed the Southern California plumes of smoke on 5 Dec 2017 aboard the ISS

Western Regional Climate Center EXTRA SLIDES

Post-fire debris flow on Springs Fire burn area 12/12/2014, Camarillo, CA

Western Regional Climate Center Providing climate services since 1986 Atmospheric Rivers

Source: AMS Glossary 34 Event Case Studies

Data Sources:

• NARR (32 km) • SSM/I • radar • wind profilers • precipitation gauges

35 Sample case study: 18 Jan 2010 Determining Event Timing

wildlife camera Laser stage gauge pressure + rain triggered transducer camera Precipitation Observations

ALERT research gauges

RAWS gauges radar

15 min or better real-time observations most valuable! CW3E Probability of AR Conditions

Western Regional Climate Center Providing climate services since 1986 USGS Debris Flow Hazard Assessment

The area above Montecito was evaluated as having a high likelihood of debris flow with the design storm (peak 15 min intensity of 24 mm/h rate)

Western Regional Climate Center Providing climate services since landslides.usgs.gov/hazards/postfire_debrisflow/ 1986 CNRFC Radar Archive http://www.cnrfc.noaa.gov/radarArchive.php

Western Regional Climate Center Providing climate services since 1986 Jan 9 2018 Montecito: Low-level winds

Source: CANSAC WRF Future Hourly Precipitation Extremes

Prein et al. 2017, Nature Climate Change Decrease in Number of Precipitation Days

Future period: 2060-2069 Baseline period: 1985-1994

Pierce et al. 2013, J. Climate Increase in daily precipitation totals

Discrepancies on wetter/drier trend in future depends largely on changes in frequency of largest (>60 mm/day) events

Change in days per year exceeding thresholds (2060-2069 minus 1985-1994) Pierce et al. 2013, J. Climate