M EMORANDUM DATE: OCTOBER 31, 2018 TO: Mr. William R. Short Acting State Geologist California Geological Survey 801 K Street, MS 12-20 Sacramento, CA 95814 FROM: Jeremy Lancaster (CGS - Acting Supervising Engineering Geologist) Team Lead SUBJECT: Holy Fire Debris Flow Inundation Hazard Assessment within Riverside County Cal OES Mission #: Deployment Date: 2018-SREOC-51612 September 9, 2018 Team Members (CGS): Jeremy Lancaster – Team Lead (PG, CEG) Don Lindsay – Co Lead (PG, CEG, CE, GE) Brian Swanson (PG, CEG) Pete Roffers (PG, GIS Support) Sol McCrea (CFM, GIS Support) Introduction The Holy Fire began on August 6, 2018 in Holy Jim Canyon on the upper west flank of the Santa Ana Mountains. The fire burned 23,136 acres of land within Riverside and Orange counties before it was fully contained on September 13, 2018. Eighty percent (80%) of the burn area was within the Cleveland National Forest and the remainder was within private State Responsibility Areas or Riverside County. The fire destroyed twelve residences in Orange County and six more in Riverside County. Governor Brown declared a state of emergency for Riverside and Orange counties on August 9th. A U. S. Forest Service (USFS) Burn Area Emergency Response (BAER) Team was mobilized to validate the Soil Burn Severity (SBS) map and to assess potential post-fire hazards and mitigation for areas under Federal responsibility (USFS, 2018). Most of the Holy Fire area had not burned within the prior 40 to 70 years and the BAER team found that 85% of the area burned at moderate or high severity soil burn severity, which is unusually high (Nicita and Halverson, 2018). The validated SBS map was submitted to the U. S. Geological Survey Mr. William R. Short October 31, 2018 Holy Fire Debris Flow Inundation Hazard Assessment Riverside County – Cal OES Mission Task No. 2018-SREOC-51612 (USGS) post-wildfire landslide hazards group, where models of potential debris flow probability and volume were generated for the Holy Fire on August 22, 2018 (USGS, 2018). This modeling indicates a high probability for debris flows in the major canyons within the burn area under expected rainfall intensities and for generation of substantial volumes of debris. Due to the increased potential for debris flows and flooding under post-fire conditions and the presence of extensive residential development down-slope of the burn area, particularly in Riverside County, CAL FIRE mission-tasked a California Watershed Emergency Response Team (WERT) to assess potential Values-at-Risk (VARs). CAL FIRE and CGS led the response team with support from DWR, and the team coordinated closely with the USFS BAER Team and with representatives of the Riverside County Flood Control and Water Conservation District (RCFCWCD) throughout the response effort. The WERT identified 79 VARs and presented a summary of findings to Riverside County personnel at a close-out meeting on August 30 to assist in the county’s initial response planning efforts. The WERT report was finalized and posted on the CAL FIRE website on September 28, 2018 (CAL FIRE, 2018a). Most of the VARs identified by the WERT are within Riverside County. Purpose and Scope Riverside County requested additional assistance through CalOES for CGS to prepare maps illustrating areas subject to high and low energy debris flow inundation in Riverside County, following the approach developed in the aftermath of the January 9, 2018 Santa Barbara debris flow event (see attachment in Thomas Fire WERT report (CAL FIRE, 2018b)). In response to the Cal OES mission task request, CGS assembled a team with a background in geomorphology and debris flow hazard assessmentPreli minar y , and GIS support personnel, to conduct the requested assessment. CGS met with RCDCWCD staff on September 11, 2018 and agreed on the following scope of work: • Task 1: Conduct field review and appraisal of existing available data. Identify and recommend acquisition of additional data. • Task 2: Review and provide input to preliminary hazard mapping developed by the county. • Task 3: Rapidly map the potential extent of debris flows and sediment-laden floods within and down gradient of the Holy Fire burn perimeter within Riverside County based on current conditions, including: Distribution, age and type of alluvial fan deposits. Identification and position of alluvial fan topographic apices, and possible hydrographic apices. Presence of features on the landscape suggestive of past debris flow activity. Map and catalog “accessible” locations that may impact the conveyance capacity of local channels (i.e. channel constrictions or “choke points”). Consideration of event-based inundation. scenarios developed for downstream urbanized areas 2 Mr. William R. Short October 31, 2018 Holy Fire Debris Flow Inundation Hazard Assessment Riverside County – Cal OES Mission Task No. 2018-SREOC-51612 • Task 4: Assist Riverside County in the development of a decision matrix based on four levels of risk tiered to four different triggering rainfall events that could be used to support operational response decisions. Develop recommendations for additional work Develop this project report Task 1 of this assessment began with reconnaissance field work from September 10 to 12, 2018, and was followed by discussions with the county and recommendations for additional information, including, high resolution lidar, aerial photography, and hydrology data. Task 2 was completed at the request of RCFCWD on 9/26/2018 by reviewing the “draft Holy Fire Risk Map” prepared by the county. After historic aerial photos of the area were assembled and lidar imagery was obtained and processed by RCFCWCD to produce detailed, post-fire topography, CGS completed Task 3 field mapping between October 10 through 12. An overview of geomorphic conditions by canyon and table of aerial photo review notes is attached in Appendix B and C, respectively. The results of the above listed tasks are included in this report and its associated attachments. As with the January 2018 Santa Barbara County response, the primary objective of Task 3 is to rapidly identify channel constrictions where avulsions may occur during storm events and to qualitatively delineate areas of potential inundation by high and low energy flows. Procedures for this task include: • Review of historic aerial photography to Prelie minar y valuate geomorphic expression of past flood and debris flow events in order to characterize recent flood and debris flow activity • Review of historic aerial photography and locations where manmade alterations may influence flow paths of debris flows. • Map and catalog channel constrictions (natural or manmade) or bends that may limit, or block, the conveyance capacity of local channels, which could force future flows out of their channels (avulsion) and generate debris flows and debris-laden flooding away from established flow paths. • Consideration of mapped channel constrictions and existing channel capacities by reviewing HEC RAS models at channel constriction locations. • Preparation of maps illustrating areas subject to potential high and low energy flows. The purpose of this rapid approach is to provide a general understanding of the potential extent of debris flow inundation based on geomorphic factors. Quantitative modeling of potential flow paths was not a part of the scope of work. Concurrent with the preparation of the qualitative high and low energy map, CGS provided assistance to Riverside County emergency management personnel, RCFCWCD, and CalOES in the development of a decision matrix based on four levels of risk tiered to four different triggering rainfall events that could be used by emergency response managers and personnel to develop appropriate operational needs and logistics. The rainfall thresholds in this matrix were based on published relationships from previous events, debris flow thresholds identified by the USGS for the Holy Fire area combined with input from the NWS, and input from Riverside County. 3 Mr. William R. Short October 31, 2018 Holy Fire Debris Flow Inundation Hazard Assessment Riverside County – Cal OES Mission Task No. 2018-SREOC-51612 Background The following sections provide general background information on fire-induced effects on runoff and the unique characteristics and hazards associated with debris flows, mud flows, and flash floods. The observations and results obtained during this assessment are discussed thereafter. Discussions pertaining to the geologic, geomorphic, and climatic characteristics within the impacted areas and upslope source areas are provided separately in the WERT and BAER Team reports already completed for the Holy Fire (CAL FIRE, 2018a; USFS, 2018). Fire-induced Impacts on Runoff Vegetation on natural, unburned slopes supports and protects the soil, creates a litter covered surface that acts to reduce raindrop impact and soil detachment, intercepts rainfall and provides evapotranspiration that reduces the availability of water to generate runoff. When the vegetation is burned during wildfires, the benefits provided by the vegetative cover are lost and runoff in the watershed is increased, both in overall streamflow volume and peak flow. Thus, post-fire flows are flashier than normal with more frequent flood events, especially in the first two to five years following a fire (Cannon et al., 2008; USGS, 2005). For this reason, post-wildfire runoff can be disproportionately large for the size of the watershed (Moody et.al., 2013). In general, the denser the pre-fire vegetation and the longer the fire residence time, the more severe the effects of the fire are on soil hydrologic function. This is because aside from consuming vegetation and vegetative litter, fire can promote the formation of water repellent layers at or near the surface of soils which subsequently increases runoff. The two primary ways in which soil infiltration is affected by fire is by soil sealing and the creation of water-repellent (hydrophobic) soils near the surface. Soil sealing is caused by the infilling of surface voids in Preli minar y the soil by fine-grained clay and ash exposed and mobilized by raindrop impact after fire (Larsen et al., 2009).