Development of a Landslide and Debris Flow Hazard Map for the Old and Grand Prix Fires: San Bernardino National Forest

1. Development of a Landslide and Debris Flow Hazard Map for the Old and Grand Prix Fires:San Bernardino National Forest Juan de la Fuente John Chatoian Allen P. King Christy B. Till Alisha R. Miller Robert G. Taylor USDA Forest Service

2. Outline I. Background • Chronology • Context of Project • Project Goals II. Methods • Remote Sensing Data • Air Photo & Field Assessments: 2003-2005 Storm Effects • Air Photo Assessment: 1969-1972 Storm Effects • Literature Review • Data Analysis • Hazard Map Development III. Summary of Key Slope Processes and Watershed Response • Key Slope Processes After the Fires: Painting a Picture • Observations of Watershed Response IV. Findings of This Study • USFS Mapping on 1972 and 2005 Air Photos • USGS mapping on 2004 Air Photos IV. Hazard Map • Elements • Uses V. Conclusions & Acknowledgements

3. I. Background Chronology: • Fall 2003- Old & Grand Prix Fires occurred; Assessed by Burned Area Emergency Rehabilitation (BAER) teams; • December 2003- Moderate storms, widespread debris flows, fatalities; BAER team re-convened in January 2004 and recommended Debris Flow Hazard Map • January 2005- BAER Team mobilized again to evaluate effects of 2005 storms • March 2005- USFS, NRCS, CGS, USGS personnel met in Davis, CA to coordinate work in post fire assessments and to discuss USFS hazard map for Old & Grand Prix Fires • April 2005-September 2005- New air photos acquired, field investigation, photo mapping, GIS coverages, analysis, consultation with researchers, development of Hazard map. Context: • USGS conducting Southern California Debris Flow Hazard assessment (Sue Cannon)- • USFS collaborating with agencies doing landslide work in California (USGS, CGS, NRCS. FEMA, Counties) • Will be linked to Southern California USFS geomorphic mapping Goals: • 1. Determine extent of post-fire landslides & Debris Flows for Old & Grand Prix Fires • 2. Investigate the response of watersheds in the Old & Grand Prix Fire area to previous storms; • 3. Summarize current knowledge of post-fire response in Old & Grand Prix Fire areas • 4. Explore possible links between bedrock, slope, vegetation, land management with fire and watershed response; • 5. Develop prototype hazard map for landslides and debris flows; Utilize sound, state of the art techniques, and results of recent studies; make the product simple and easy to use • Not to evaluate BAER post-fire mitigation measures (see USFS monitoring reports) • Not to provide site specific assessments or mitigations

4. LOCATION MAP Map 1

5. II. Methods (Goals 1-2) Air Photo & Field Assessments: 2003-2005 Storm Effects The Forest Service mapped altered channels and landslides on 1:40,00 Color Infrared air photos taken in April, 2005. The term “Altered Channel” is used here to identify those channels which experienced sufficient disruption of the channel bed (scour, deposition, or removal of riparian vegetation) to be visible on air photos, and distinguished from those in which the bed was not mobilized. Some altered channels experienced debris flows, some hyperconcentrated flows, and some simply mobilization of the bed due to high flows. Distinctions between the three were not discernible on the air photos. It identified features associated with the first two winters after the storm, and covered ~140,000 acres burned during the 2003 Old & Grand Prix Fires on the San Bernardino National Forest. The mapped area completely overlaps 1972 (USFS) and 2004 (USGS)mapping. • USFS 2005 Air Photo Mapping- New air photos acquired (April 2005); Landslides & altered channels mapped; Limited field verification; • USFS Field Work- Field investigations conducted in May, 2005 to calibrate air photo mapping and interpretations; • USGS 2004 Air Photo Mapping- USGS is mapping landslides and altered channels on 1:12,000 color air photos taken in February and March 2004. Will identify 2003-2004 features. Mapping criteria are similar to those being applied by the Forest Service, and this should facilitate comparison of findings. About 100,000 acres of the burn will be mapped, focusing on urban and urban interface lands. Scheduled completion in 2006. • USGS Field Work- Field investigations from 2003-2005 done as part of debris flow hazard assessments; Air Photo Assessment: Previous Storm Effects (1969) • USFS 1972 Air Photo Mapping- 1972 air photos similarly mapped in a sample area within the Old & Grand Prix Fire (about 21,000 acres Warm & East Twin Cr). Time period selected to show effects of 1969 storms. Part of mapped area burned in 1970.

6. II. Methods (Goals 3-4) Review of Past & Current Work • Previous Work- A literature review was conducted, and relevant articles summarized; • Current Work- Consultation With USGS, CGS, County, and USFS geologists Data Compilation & Analysis- • Air photo Interpretation: Done on overlays, ocular transfer to 7 ½ minute topo maps, digitized, GIS coverage developed & entered into geodatabase; • GIS Layer Compilation- Bedrock, 2 meter DEM (Slope Gradient, Elevation, Hillshade, 3-D Images), Burn Severity, Soil Slip Susceptibility, Watershed Response, Vegetation, Landslides & Altered Channels; • Data Analysis- GIS overlays & data base queries done to determine density of 2005 landslides and altered channels for different slope classes, burn intensities, bedrock types, etc. Statistical analysis was done on 1972 air photo data exploring these same relationships. 3-D images examined to evaluate utility for terrain stratification; Remote Sensing Tools Used in Analysis: • 1972 Air Photos- 1:16,000 Color (mapped Landslides and altered channels) • 2003 Post Fire Imagery and Digital Orthophoto- Identified pre-fire landslides & altered channels • 2004 Air Photos- (USGS) 1:12,000 Color (mapped Landslides and altered channels) • 2005 Air Photos- (USFS) 1:40,000 Color Infrared Plus 2005 Digital Orthophoto (landslides and altered channels) • Digital Elevation Models- (DEMs)- 10 meter, 3 meter, 2 meter (slope and elevation, stream network, 3-Dimensional Imaging Program);

7. II. Methods (Goal 5) Hazard Map Development: Hazard maps were developed to address the following Processes: • Debris Flow- • Debris Slide and Rock Fall- • Deep Seated Landslide- The Following Questions Guided Development of the Hazard Maps: • What happened after 2003 fires, where, how, what were the effects? • What happened with first storms after the fires; how did burned versus unburned land behave? How did burned areas respond to subsequent storms? • What happened in this area during earlier storms and fires over the past 50 years? • What and where are sensitive lands where landslides, rock falls, a of debris flows occurred? • What tools are best suited to efficiently and systematically delineate these sensitive and hazardous lands?

8. III. Summary of Previous Work Key Post-Fire Slope Processes:Review of BAER Team findings, published studies, consultation with others, and air photo & field work yielded the following summary of dominant slope processes in the Old & Grand Prix Fire areas. The fire was followed by the following sequence of events and processes: • Removal of Organic Material- The fires removed organic ground cover exposed soils to rain drop impact; loss of organic litter, plants, trees, and shrubs removed mechanical support • Dry Ravel- Loss of mechanical support provided by ogranic material on steep slopes facilitated dry ravel following fire; this process was documented in BAER team reports. • Soil Water Repellancy- Repellancey developed on about half of the grass/chaparral lands (BAER Report 2003), and probably accentuated runoff; • Root Support- Root support was lost from fire-killed vegetation; effects were likely not felt until the second winter after the fires (time lag for delay). • Channel Material- Channels were filled with dry ravel immediately after the fires; • Storms of December 2003- The storm around Christmas of 2003 acted on water repellant slopes, channels filled with dry ravel, and barren soil, producing widespread debris flows; • Source of Debris Flow Material- Channels were charged with dry ravel, barren steep hillslopes effeciently delivered material by surface erosion, rilling and gullying. Some studies (Moody 2001) indicate that for non debris slide debris flows, about 80% of the sediment generated comes from the channel, and 20% from the hillslope in Colorado. • Subsequent Storms- Once a channel network experiences a debris flow, and rills strip soil from steep hillslopes, the likelihood of of a subsequent debris flow in the same channel network is much lower; Exceptions could occur where large landslides develop on channel banks, or where significant tributaries within the network do not experience the initial debris flow episode. • Debris Replenishment Rate in Channels- The replenishment of debris in channels, and in gullies on hillslopes occurs over time, but we don’t have a good picture of how rapidly this occurs; • Revegetation- Revegetation effectively stops dry ravel and vulnerability of rills and gullies to further erosion. • Loss of Water Repellancy- Fire-associated repellancy decreases over time; • Factors Affecting Post-Fire Debris Flows- Key factors appear to be: time since last fire, time since last debris flow, and nature of vegetation burned in fire (grass versus chaparral versus oak woodland versus conifer stand). • Debris Flow Risk in 2005- Potential for debris flows has apparently decreased since the fall of 2003 due to: a) revegetation (stops dry ravel, adds ground cover, replaces lost root support); b) Reduction in soil water repellancy; c) Reduction of debris available in channels and on steep hillslopes brought about by the post-fire dry ravel, rilling and debris flows. Over time, the available debris will increase once again.

9. III. Summary of Previous Work Observed Post-Fire Storm Responses: From Discussions With BAER Teams and USGS Personnel- • Virtually all of the steep, basins within the Old and Grand Prix fire area meeting certain burn severity and slope gradient criteria, experienced debris flows in response to the relatively low intensity storms in December of 2003. • Models addressing post 2003 fire debris flow potential did good job at predicting which basins would experience debris flows. • Most debris flows did not have landslide sources; • Large channels which did not experience debris flows did experience mobilization of bed material and local realignment, resulting in damage to facilities. • Some relatively small drainages (150 acres) experienced sizeable debris flows which damaged property (eg. Greenwood avenue). • Most watersheds which experienced debris flows in the December 2003 storms did not exhibit repeat debris flows in the larger storms of 2004 and 2005, but rather experienced sediment-laden flood flows. • There were fewer landslide sources for debris flows following the December storm than in storms the second year (Sue Cannon, personal communication, 2005). • Abundant shallow debris slides (soil slips) occurred in the foothills at lower elevations the second winter following the fire (2004-2005). • Debris flows traveling down some alluvial fans followed streets, and were confined by block walls, allowing them to travel much further than they would have on an unaltered natural fan. • Fatalities were concentrated in alluvial fans at mouths of canyons and narrow terraces in confined channels.

10. IV. Findings Results of 2005 Air Photo Mapping (USFS): . • SUMMARY • Altered channels and landslides are clearly much denser in burned areas than in unburned; to a lesser degree, they were also denser in high/moderate burn severity areas than in low severity. • Shallow debris slides were concentrated in lower elevation foothills in granitic rock, and densities in high/moderate severity burn areas were higher than in low severity. • There were a small number of reactivated deep seated landslides producing debris slides and flows. • Altered channels extend long distances down some alluvial fans into lower gradient areas. • Some bedrock units exhibited elevated landslide densities • Small alluvial terraces within large confined canyons appear to be particularly susceptible to inundation in many areas, likely involving debris flow processes. • Field verified debris flows and air photo identified altered channels occurred with few exceptions within the channel network identified by the 2 meter DEM (1/2 acre contributing area). Debris slides occurred on steep slopes (within a relatively narrow slope gradient range when classified with 2 meter and 10 meter DEMS). There may be some useful links between post-fire response and elevation plus pre-fire vegetation. • Revegetation which occurred from 2003-2005 (primarily grass) concealed some small features on air photos

11. Map 3

12. IV. Findings Results of 1972 Air Photo Mapping (USFS): The Forest Service mapped approximately 21,000 acres in the Warm & East Twin Creek watersheds inside the boundaries of the Old Fire of 2003. Altered channels and landslides associated with previous storm years (eg. 1969) were mapped on color, 1:16,00 air photos taken in 1972. The mapping area included a 7,000 acre fire which burned in 1970. • Majority of features (landslides and altered channels) are found near roads or in the 1970 burned area. • State Highway 18 runs through the 1972 mapped area and appears to have influenced the density of features outside the 1970 fire perimeter. • Majority of the landslides occur in the Cretaceous plutonic bedrock (Kcc: Monzogranite of City Creek or KJc: Quartz monzodiorite of Crestline), not the Quaternary sedimentary units. • Landslide features are all in one of two land use/vegetation classes: chaparral or forest. • All landslide features occur on 30º-60º slopes (majority on slopes between 30º-45º). • All landslide features are on south facing slopes (90º-270º aspect). • Features within the 1970 burned area show a statistical correlation between feature type & elevation, feature type & slope, and feature type & the presence of a debris flow. • Bedrock type, slope, aspect, burned areas and roads appear to be the key factors controlling the location of features.

13. IV. Findings Status of USGS Mapping (2004 Air Photos): Scheduled for completion 2006: Preliminary Observations: • Altered channel density is considerably higher in burned relative to unburned areas. • There is a marked greening of slopes (germination of grass) on the March photographs (compared to February), which may mask some evidence of altered channels and dry ravel. • There were fewer landslide sources associated with debris flows which occurred the first year after the fire than in the second year. Status of 3-Dimensional Imaging: Preliminary stratification of watersheds was conducted with the 3-D imaging tool available for this project. Promising tool, for doing identified by slope, dissection, and roughness. but came too late in the process to fullyl integrate into the Hazard Map development. Status of 2005 Digital Orthophoto- • Map became available at end of project. • Will use to refine location of landslides and altered channels;

14. Streams Generated from 2m DEM & 2005 Mapping Map 12

15. IV. Hazard Map Development of Hazard Classes: Hazard Classes: Susceptibility to debris flows and landslides was characterized in three simple categories (low, medium or high). Classification was based on previous work in the vicinity and similar areas, and on the findings of the airphoto inventories. Debris Flow Hazard: • Slope Gradient • Channelization • Landform • Altered channels were assumed to have high potential for debris flow • Altered channels were mostly coincident with stream network (2 meter DEM 0.5 acre area) or on steep slopes • Altered Channels on alluvial fans and terraces in confined channels • Therefore: All above characteristics assigned high hazard for debris flow Debris Slide and Rock Fall Hazard: • Slope Gradient • Landform • No data available for “runout” or downslope effects on flats; • Debris slides on steep slopes (90% on slopes > 40% and 63% on slopes > 65%. • Limited correlation of 2005 slides to bedrock, veg etc. • Therefore used slope >65% as high hazard, 45-65% for Moderate Hazard • No data available to refine for rock fall hazard; Deep Seated Landslide Hazard: • Areas having experienced deep seated landsliding were assumed to be high hazard • Existing bedrock map identified older landslide deposits; • Studies in other areas show a good correlation of current deep seated landsliding occurring on older landslide deposits.

16. IV. Hazard Map Planned Use of Maps: • U.S. Forest Service Use: • Post-fire assessments • Project Planning for Prescribed Fire, etc. • TMDL’s • County: • Planning for Future Urbanization • Disaster preparedness Limitations of Maps: • Not site specific • Post-Fire for only one large fire event is analyzed Future Updates to Map: • Preliminary Hazard Map in December 2005 • Updates in subsequent years • Will be done in conjunction with geomorphic mapping, and acquisition of high resolution DEM’s

17. V. Conclusions . • Determining Extent and Nature of Storm Effects- Area-wide air photos or similar imagery, are essential to document post fire response. Color infrared air photos at a scale of 1:40,000 scale provide a good view of effects over the entire burn area. • DEM- A high resolution Digital Elevation Model is essential in Southern California to development of a debris flow and landslide hazard map. Slope gradient and location of the channel network appear to be the best indicators of debris flow and debris slide potential. • Past Response- Understanding of previous landscape response to severe storms and fires is essential. It must be recognized that the pattern of landslides and altered channels identified by this study following the 2003 fires may not be representative of responses to future storms. • Bedrock, Geomorphic and Soils Mapping- Detailed bedrock, geomorphic, and soil mapping may shed light on debris slide and debris flow occurrence, particularly when examined in concert with slope gradient and other factors. Acknowledgements • The following persons provided valuable advice and comments: • Sue Cannon: USGS, Golden Colorado • Dave Lidke: USGS, Golden, Colorado • Bill Hicks: USFS Retired • Mark Reid: USGS, Menlo Park • Steve Ellen: USGS, Retired • Jim Mckean: USFS Boise Idaho • Tim McCrink: CGS Sacramento, California • The following persons assisted in photo mapping, digitizing, and GIS analysis: • Don Elder: USFS • Krista Mondelli: GeoCorps America Intern • Ashlee Dere: GeoCorps America Intern • Richard Van de Water: USFS • Bob Jester: USFS • Bonnie Allison: USFS