Post-Fire Debris Flows in California: an Atmospheric Perspective

Transcription

1 Post-Fire Debris Flows in California: 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 Western Regional Climate Center

2 What is a post-fire debris flow? USGS baseline rate: 25 mm h -1 rate for 15 min 2

3 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

4 Max intensity may not be triggering intensity Nov 2009, Station Fire burn area (Los Angeles County) Staley et al. 2013, Landslides

5 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

6 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

7 Study Area You are here Cool Seasons : 19 storm events triggering debris flows on 12 burn areas Oakley et al Natural Hazards 7

8 Atmospheric Rivers 13 of 19 (68%) of events associated with Atmospheric River Image Source: CIMSS Thanks to Shawn Roj for assembling 8

9 Conceptual- Synoptic Conditions Oakley et al Natural Hazards 9

10 Conceptual-Cross-Section Oakley et al Natural Hazards 10

11 Old/Grand Fire Flood: Prix Debris Old/Grand Flows, Dec Prix Debris flows from the Old/Grand Prix Fires killed 16 people Oakley et al Natural Hazards

12 Old/Grand Prix Debris Flows, Dec San Gabriel San Bernardino Radar shows high reflectivity mostly confined to mountains Image: NCEI radar archive

13 Other Features: Squall line/ncfr Oakley et al Natural Hazards 13

14 Other Features: NCFR Dec : Camarillo Springs Debris Flow 14

15 Jan Montecito Debris Flow Thanks to D. Staley for making images into video Images: CNRFC

16 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 debris flow 16

17 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

18 Squall line over Butte Fire burn area March 6, 2016 Stockton Angels Camp Mono Lake Courtesy Jeremy Lancaster, CGS

19 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

20 12/3/15 (Before) Detwiler Burn Scar: March Band of intense t-storms 3/9/16 (After) San Jose Modesto Turlock Mammoth Detwiler burn area Madera Image: CNRFC

21 12/3/15 (Before) Detwiler Burn Scar: March *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

22 History of debris flows in Big Sur SP Following Basin Complex Fire June 2008 Apr 7, 2009 Precipitation rate 0.84 in/hr (21.3 mm/hr) Courtesy Jeremy Lancaster, CGS

23 Monitoring Soberanes Burn Area (winter 2016/17) Possibility of debris flow above Pfeiffer Falls 12/29/16 01/10/17 Small debris flows in Palo Colorado Cyn, rate of ~20 mm/h for 15 min Photo: CGS

24 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 Photo: CGS

25 North Bay: No significant debris flows this winter. Why not? Nuns Fire Atlas Fire landslides.usgs.gov/hazards/postfire_debrisflow/ Hypotheses: Low to moderate severity burn Rainfall intensities not high enough? (little known about area)

26 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 than S. CA Atlas Fire burns hills east of Napa on 9 Oct Photo: M. Short, SF Chronicle 26

27 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

28 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 No consensus on lightning activity Thomas Fire smoke over 101 Freeway near Carpinteria. Photo: S. Palley/Washington Post 28

29 Future Hourly Precipitation Extremes For Dec-Jan-Feb 2-3x more likely to exceed top 0.05% hourly rainfall in warmer climate in CA in DJF Change in exceedance probability of the top 0.05% highest hourly precipitation rates observed in control period (Jan 2001-Sept 2013) Based on RCP 8.5 (high emissions) scenario Prein et al. 2017, Nature Climate Change

30 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

31 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 Assess changes (historic, future) in high intensity, short duration precipitation Thomas Fire Photo: Ventura OES 31

32 Western Regional Climate Center Thank you! Research supported by: @CW3E_Scripps WRCC NASA astronaut Randy Bresnik photographed the Southern California plumes of smoke on 5 Dec 2017 aboard the ISS

33 EXTRA SLIDES Post-fire debris flow on Springs Fire burn area 12/12/2014, Camarillo, CA Western Regional Climate Center Providing climate services since

34 Atmospheric Rivers Source: AMS Glossary 34

35 Event Case Studies Data Sources: NARR (32 km) SSM/I radar wind profilers precipitation gauges Sample case study: 18 Jan

36 Determining Event Timing pressure transducer Laser stage gauge + rain triggered camera wildlife camera

37 Precipitation Observations ALERT research gauges RAWS gauges radar 15 min or better real-time observations most valuable!

38 CW3E Probability of AR Conditions Western Regional Climate Center Providing climate services since

39 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/

40 CNRFC Radar Archive Western Regional Climate Center Providing climate services since

41 Jan Montecito: Low-level winds Source: CANSAC WRF

42 Future Hourly Precipitation Extremes Prein et al. 2017, Nature Climate Change

43 Decrease in Number of Precipitation Days Future period: Baseline period: Pierce et al. 2013, J. Climate

44 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 ( minus ) Pierce et al. 2013, J. Climate