Gateway Trail Project

Transcription

1 Gateway Trail Project Debris Flow Hazard Assessment By: Juan de la Fuente April 30, 2010 Background- On April 22, 2010, the Shasta-Trinity National Forest (Mt. Shasta-McCloud Unit) requested a geologic assessment of the debris flow hazard associated with a trail proposed for construction by the Gateway Trail Project. The segment in question is about one mile SW of McBride Springs Campground in SE ¼ of Section 3 (Figure 1). Refer to the soils and hydrology reports in the project files for additional background information. Debris flows are sediment laden flows which can travel rapidly down channels, and are capable of carrying large boulders at relatively low gradients (10% or lower). As a result, they can be very destructive to channels and infrastructure. They are often triggered by debris slides, which are shallow, rapidly moving landslides. Debris flow hazards are present around the entire perimeter of Mt Shasta Volcano, primarily in channelized upper slope positions, and also on alluvial fans below. In this setting, they can be initiated by: a) Sustained warm winter rains on a snowpack (such as occurred in January, 1997); b) Intense convective summer storms; c) High summer temperatures which cause rapid glacial melt; d) Volcanic eruptions which melt snow and ice. The hazard associated with volcanic eruption in the project area has been classified as Intermediate potential hazard from mudflows associated with eruptions (Miller, 1980). The terms mudflow and debris flow are used synonymously in this report. Purpose- The purpose of this investigation was to assess the debris flow hazard to trail users at the Proposed Trail Crossing of Debris Flow Crossing in Figure 1. This channel carried a large debris flow during the flood of January, Methods- Field- On April 23, 2010, a field visit was made by Juan de la Fuente and Steve Bachmann. It included visits to the southern half of Section 3 ( Terminus of 1997 Debris Flow, Figure 1) adjacent to Road 40N88X (One Mile Road), and to the SW ¼ of Section 2, where the Everitt Memorial Highway crosses the debris flow channel. The area near Everitt Memorial Highway had substantial snow cover, but the lower site was mostly free of snow. We did not visit the actual site where the trail would cross the debris flow track due to the snow cover. We plan to visit the site with Recreation personnel when the snow is gone. We also discussed field observations Steve had made immediately after the debris flow in 1997.

2 Review of Documents & Maps- The Mount Shasta Gateway Trail Recreational Trails Program Proposal, and associated hydrology report were reviewed. S.O. Geology files were searched, but no information was found regarding the 1997 debris flow. Air Photo Review- Resource air photos: The following air photos were reviewed. One flight was before the 1997 debris flow, and the other was after it: Color, Scale 1:16,000 USDA-F , numbers 698-(48-49) and 1598-( ) flown ; Color Infrared Scale 1:58,000 HAP 83 F (74-76) flown on ; Findings Debris Flow- The 1997 debris flow was initiated by a shallow debris slide (shallow, rapidly moving landslide) at an approximate elevation of 6000 feet (Figure 1). This elevation coincides with the zone (4,000-6,000 feet) which experienced the highest concentrations of landslides during the 1997 flood in nearby areas (USFS 1998, page 31). Examination of post-flood 1998 air photos (Figure 4) reveals a number of large conifers still standing within the raw debris slide scar. This suggests that the slide was very shallow, with the deepest part near the top, and debris probably overrode natural ground in the lower 2/3 of the slide, and thereby allowed the trees to survive. The debris slide is about 170 feet wide and 700 feet long. When the debris slide entered the stream below, it initiated a debris flow which traveled rapidly downslope. In several locations, it left the channel as it went around bends, such as at Everitt Memorial Highway, and at its confluence with Cascade Gulch below the highway (Figure 1). In these areas, it cleared all vegetation in a swath up to 300 feet wide (Figures 2, 3). This suggests that the debris flow likely attained velocities of at least 20 miles per hour, and possibly much higher. Slope gradients at the terminus of the 1997 deposit were measured in the field at about 8%. The gradient of the channel, at the base of the debris slide is about 25%, and the gradient of the debris slide itself is about 80% (from USGS 7 ½ minute topographic map). Other Channels- This investigation focused on the unnamed channel which experienced a debris flow in However, it should be noted that all channels on the flanks of Mount Shasta, such as Cascade Gulch, Avalanche Gulch and Diller Canyon also experience periodic debris flows. Hazard to Trail Users- The frequency of debris flows in the channel crossed by the trail is likely on the order of once in a hundred years, or possibly longer. The most likely trigger would be a sustained warm winter storm preceded by an unusually wet rainy season (such as 1997) during the winter or spring. A summer convective storm could also trigger such an event. Once initiated, a debris flow similar in size to the 1997 event would likely behave in a similar way, and travel at a speed of miles per hour in upper channel reaches, slowing to a few miles per hour where the fan gradient lowers to 8%. Debris flows of this type can often be heard before they reach lower reaches, providing some warning. However, other factors, such as wind can obscure the sound. Few if any people would be on the trail in winter or spring. A debris flow triggered by a summer convective storm would have the highest potential for affecting trail users. A small event of this type was reported in this channel in the summer of 2003

3 (Steve Bachmann, personal communication ). This storm mobilized sand and gravel but had minor channel effects compared to the debris flow of It was part of a series of storms in July and August, 2003 which affected channels from Mt. Eddy through Mt Shasta, to Haight Mountain (Miller et. al, 2004). As a means of providing context, the Everitt Memorial Highway already exposes more people to debris flows at this channel than would the trail. Recommendations- Trail Design- During actual trail layout and design, the crossing of the 1997 debris flow channel should be examined more closely for geologic considerations to see if any specific design measures are needed to prevent diversion of a future debris flow. Debris Flow Frequency Historical air photos (1944 to Present) should be examined for this area to provide a better understanding for debris flow frequency on the SW flank of Mt Shasta. This information would be of great value in future project work in this area. Public Awareness- Public awareness of debris flow hazards on the flanks of Mt Shasta should be emphasized. Important points to convey are that debris flows in this setting typically occur under the following conditions: 1. Sustained warm winter storms following above average precipitation; 2. Intense convective storms; 3. Sustained high summer temperatures in streams fed by glaciers; 4. Volcanic Eruptions. Also, when faced with an imminent debris flow in a field setting, the best action to take is to avoid channels and alluvial fans (particular the heads of fans), and to move to higher ground. Discussion Snow cover prevented a close field examination of the trail crossing on the debris flow channel. However, this does not detract from the assessment of debris flow hazard addressed by this investigation because the source of this hazard is a considerable distance upslope. Future debris flows would most likely originate from the steep canyon walls above the Everitt Memorial Highway. The main issue to address at the trail crossing site is to assure that trail design and construction is done in a way to prevent any diversion of a future debris flow. The most likely trigger for a debris flow in the same channel affected by the 1997 event would be a sustained warm winter storm during an overall wet year. The steep gorge near the initiation site of the 1997 event could produce a similar debris slide in a winter storm of similar magnitude in the future. The 1997 flood has been classified as a year return interval storm, but the recurrence interval of the 1997 debris flow was probably longer than that (>100 years?). This estimate is based on the observation that the 1997 debris flow stripped many large conifers (>100 years old) from the floodplain (compare Figure 2 to Figures 3 & 4. This debris flow recurrence interval could be refined by examining

4 historical air photos back to 1944 and tracking disturbances in the channel and changes in vegetation over time. A debris flow caused by rapid summer melt of a glacier, such as at Whitney Creek in 1997 (de la Fuente and Bachmann, 1999) could not occur in this channel because it lacks a glacier directly upslope. A convective summer storm could generate a debris flow, but no large events of this type (other than a much smaller runoff event in summer 2003) have been identified historically in this channel (Steve Bachmann, Personal Communication, ). Lastly, there is always a potential for a debris flow triggered by a volcanic eruption (snowmelt), but this potential is also relatively low. On average, Mt Shasta has erupted once per 600 years over the past 4,500 years, and a small eruption occurred 200 years ago (Miller, 1980). While precise predictions are not possible, this historical information suggests that the interval between eruptions is likely to be on the order of several hundred years. Atachments- Maps, Air Photos, and Photographs are included in a separate electronic file. References- de la Fuente, Juan, Bachmann, Steve Whitney Creek Debris Flow of August 20, 1997: Triggering Mechanisms, Transport Processes, and Debris Sources. In: National Association of Geoscience Teachers Far Western Section Fall Field Conference September 17-19, Miller, Alisha, Tannaci, Tuli, de la Fuente, Juan, Jasso, Abel, and Bachmann, Steve Geomorphic and Hydrologic Effects of High-Elevation Summer Storm Events. In: Advancing the Fundamental Sciences Proceedings of the Forest Service National Earth Sciences Conference, San Diego, CA, October, 2004 (Vol. 2, page 559). Miller, C. Dan, Potential Hazards from Future Eruptions in the vicinity fo Mount Shasta Volcano, Northern California. Geological Survey Bulletin 1503 (pages & Plate 3). USFS The Flood of 1997, Klamath National Forest Phase 1 Report. Internal USFS Document by Juan de la Fuente and Don Elder (pages 13-14).

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