Geology Report for the 2007 Santiago Fire Cleveland National Forest
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Geology Report for the 2007 Santiago Fire Cleveland National Forest Resource Specialty: Geology Fire Name: Santiago, Cleveland National Forest Incident #: CA-ORC-068555 Month and Year: November, 2007 Author(s) Name and Home unit Name: Alan J. Gallegos, Sierra National Forest Thomas E. Koler, Ph.D., P.G., Eldorado National Forest Redacted Version Nov 28, 2007 This is a redacted version of this report. The treatment costs and addresses were removed from this report so bidding for any contracts for treatment implementation would not be influenced. The location of T & E species was removed to protect them from potential human disturbance. I. Potential Values at Risk (identified prior to the on-the-ground survey) Potential values at risk from geologic hazards (i.e., landslides, debris flows, and rockfalls) are the health and safety for people, residences, roads and bridges within the wildfire area. Of particular concern is the potential risk for loss of life and limb. All of these resources are located in the canyon floors and lower canyon walls of Harding Creek, Modjeska Canyon (Santiago Creek), Silverado Canyon and its tributaries (i.e., Pine and Halfway Creeks), and Morrow Trail Watershed. II. Resource Condition Assessment A. Resource Setting Geology The Santiago Fire is located in the Santa Ana Mountains, which is within the Peninsular Ranges Province. The fire area on National Forest System lands is underlain with rocks from the Bedford Canyon Formation, Santiago Peak Volcanics, Ladd Formation, and Trabruco Formation. The Bedford Canyon Formation (Jurrassic) is located throughout the upper slopes of the fire and is a slightly metamorphosed assemblage of interlayered argillite, slate, greywacke sandstone, quartzite and small masses of limestone. The Santiago Peak Volcanics are located in Willams Creek in the Santiago Creek in the Laurel Springs area. The Ladd Formation and Trabruco Formation are located in the lower slopes of Santiago Canyon on the southeast side of Modjeska Canyon. The Ladd Formation consists of upper Cretaceous conglomerate, sandstone, siltstone and shale. The Trabruco Formation consists of nonmarine conglomerate with local sandstone and siltstone beds. Located throughout the steep, upper slopes of the Harding and Santiago drainages are young landslide deposits (late Pleistocene and Holocene). 1 Geomorphology The geomorphology of the upper slopes of the fire is dominated with slope stability processes including rotational/translational landslides, debris slides, debris flows and rockfalls. Although rotational/translational landslides, debris slides and rockfall processes were observed, the most common process is debris flow movement and deposition. Evidence of recent debris flow deposits were observed in many areas of the wildfire area. An excellent place for observing these recent debris flow deposits is in the Harding Creek above the Modjeska Reservoir. The dam at Modjeska Reservoir was built in 1919 and over the next several decades was partially filled. In 1968 the reservoir was finally filled as the result from high precipitation events. Sediment has filled the dam up to the spill way height. A debris flow deposits is located immediately above the dam in the lower, 2% gradient channel reach. Debris flow levees with unsorted gravels and cobbles were observed in a 150-foot reach of the channel. Disturbance of the channel within 150-feet of the dam has removed any evidence of debris flow activity near the dam. It is likely that debris flows are going over the dam. Three tributary channels and debris flow fans were reviewed above the Modjeska Reservoir. These fans have exposures of debris flow material in the incised channel of Harding Creek and the channels in the tributaries. One of these tributary channels had a very recent debris flow deposit on the fan surface, perhaps as the result of the 1998 storm event. Debris flow potential was evaluated by reviewing the fire area for recent debris flow activity, by debris flow modeling and by reviewing debris flow activity in nearby areas and applying this information to the Santiago Fire area. Most debris flows have been shown to initiate in burned areas by runoff and erosion and grow and size through erosion and scour by moving debris flows, as opposed to landslide-initiated flows with little growth (Santi, et al, 2007). Potential debris flows were modeled for the fire area by U.S. Geological Survey for a 2-year recurrence interval event of 1.4- inches of rainfall (Cannon, 2007). The results of the modeling show that most of the watersheds in the fire have less then 25% probability that a debris flow between 10,000-m3 and 100,000-m3 could occur. The Live Oak Canyon and Hickey Canyon watersheds have a 25% probability that a 1000-m3 to 10,000-m3 debris flow could occur (see Debris Flow Map) 1. Another consideration for debris flow activity is the debris flow that occurred during the Christmas Day Storm of 2003 in the Grand Prix/Old Fire area. The precipitation event for most of the fire area was a 2-year precipitation event that resulted in a 10-year discharge event. Major debris flows occurred as a result of the rainstorm and resulted in 16 deaths and substantial property damage. The lesson learned in this fire area is debris flows can occur from relatively small precipitation events and can occur on channel gradient stream reaches (USFS, 2003). B. Findings of the On-The-Ground Survey 1. Resource condition resulting from the fire and risk assessment of geologic hazards Modjeska Canyon, Santiago Creek 1 A rule of thumb for many decision-makers is that any probability of 10% and greater for slope movement by landslides and debris flows may be used in naming the unstable ground as “landslide-prone.” It is important to note that this is a subjective decision based on the level of risk the manager is willing to assume (Koler, 2005). 2 Debris flow deposits were observed in Modjeska Canyon near the upper home, beyond the Wildlife Sanctuary Gate (house number 29456). Two debris flow deposits that measured 30-inches and 24- inches thick were observed in the channel bank. An 8-inch soil has developed above the upper debris flow deposit. This soil is an A horizon layer over the upper debris flow deposit. It appears that debris flow deposits have been confined to the incised channel and have not deposited on the flood plain or terraces of the active channel. However, the stream channel immediately upstream from this residence is constricted with a narrow channel and very high canyon walls. In a high peak flow event (possibly similar to the 1969 and 1998 storm events) it is possible that in-channel debris flows will result in deleterious damage to this residence. Therefore the risk value is moderate to high (possible to likely likelihood and medium to major consequences, see Tables 1 and 2). Residence, Modjeska Canyon, Santiago Creek Several very small (e.g., 10- to 20-acres in size) sub-watersheds within the Modjeska Canyon area have houses at their outlets to Santiago Creek. Of these, one home of XXXXXXXXX on XXX Modjeska Canyon Road, was identified as a resource at risk. In 1998 a debris flow or several small debris flows failed within the small sub-watershed above the property and deposited against and inside her home. This resulted in significant repairs to the property. During field reconnaissance many debris flow areas were identified in this small watershed with various levels of activity ranging from dormant to active. Therefore the risk value for this property is high to very high (likely to almost certain likelihood and major to catastrophic consequences, see Tables 1 and 2). Confluence of Harding and Santiago Creeks The confluence of Santiago and Harding Creeks is located in the southeast quarter of Section 29, Township 5 South, Range 7 West (El Toro Quadrangle) within the community of Modjeska in Modjeska Canyon. This area is located approximately 1-mile downstream from the boundary of the Cleveland National Forest. This area was identified with resources at risk because the two drainages meet in a populated area with numerous homes, buildings, roads and bridges. Upstream within the two watersheds are highly unstable hillslopes with numerous debris flow basins (also called amphitheatres). Landslide activity ranges from dormant to active and numerous alluvial/debris flow fans populate the valley floors and lower valley walls in the two drainages. At the confluence the channels are incised with steep vertical banks ranging in height from approximately 8-feet to over 15- feet. Materials within the stream banks and channel are boulders with minor amounts of sand (<5%). The boulders are sub-rounded to round and range in size from several inches to over three feet in diameter. A majority of the boulders are over one foot in diameter. Antidotal information indicates that debris flow materials have been routed through the confluence during large storm events in 1969 and 1998. The amount of debris flow material dropped in the channel below the confluence was significant enough to require dredging. Therefore the risk value for these resources is high to very high ( likely to almost certain likelihood and major to catastrophic consequences ). Pine Creek Residence, Silverado Canyon Pine Creek is located in Sections 11, 13 and 14, Township 5 South, Range 7 West (Santiago Peak Quadrangle). Pine Creek is contained within Pine Canyon which has steep, precipitous valley walls and a narrow valley floor. Several houses are within the valley floor, most of which are high enough to not be at risk from debris flows. However, the house located the furthest upstream is at risk because it is situated within and adjacent to the stream channel. The burned area is approximately a quarter of a mile upstream from this residence.