a. The historic avalanche areas, or areas in which there exists clear evidence of previous avalanches, are mapped.

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1 1 OBJECTIVES AND LIMITATIONS OF STUDY ;J'-o~ 1 j 1.1 BACKGROUND AND STUDY OBJECTIVES The communities of Aspendell, Habegger's, and Sage Flat are located in the eastern Sierra Nevada west of the towns of Bishop and Big Pine, in Inyo County, California (Figure 1-1).These developed areas are located adjacent to steep slopes which occasionally receive heavy snows and avalanches. Major snow avalanches have occurred within and adjacent to all three of these areas, most recently in 1969 and Buildings intended for human occupancy have been destroyed in 1969 and Both year-round and seasonal populations are increasing~ll three areas. In some cases development is expanding directly into areas previously affected by avalanches. A natural consequence of increased development in avalanche areas is an ~ncrease in "hazard," which, of course depends on both the natura1 avalanche conditions and the human exposure to this condition. Avalanche hazard can be reduced through appropriate land-use and construction practices. These practices can be referred to collectively as "mitigation," rational procedures through which avalanche hazard can be reduced. In order to identify the appropriate mitigation procedures, Inyo County has undertaken this study which has accomplished the following objectives. '.. '-') a. The historic avalanche areas, or areas in which there exists clear evidence of previous avalanches, are mapped. b. The potential avalanche areas are also mapped. These are areas in which avalanches may occur in approximatelya loo-year period even through there may be no historic record of avalanches in these areas. c. Avalanche mitigation concepts through which the avalanche hazard can be reduced are discussed. The report has been organized so each of the objectives "a" through "c" are applied to each of the three areas separately. Chapter 2 discusses the hazard and mitigation in Aspendell, Chapter 3 concentrates on Habegger's, and Chapter 4 on Sage Flat. 1.2 STUDY LIMITATIONS The study has important limitations which must be understood by those utilizing the material presented here. a. The study maps areas subject to reasonably foreseeable avalanches. These are areas which, based on the experience of the author in the Sierra Nevada and other mountain areas of the world, may produce 3

2 ,-. avalanches once in 100 years, on the average. Many of the avalanche runout areas within the mapped limits are reached much more often than that, as the historic record shows. This is true because as one approaches the base of a mountain, avalanche frequency increases. On the other hand, there may be some areas that have not been indicated as avalanche-prone in this study where extremely rare events may occur. In general, any slope in excess of approximately 25 may produce avalanches when very unstable snow conditions exist. These areas are omitted because avalanches will be so rare as to not warrant consideration in the land-use and planning process. b. The maps should not be used for recreational or back country purposes. An area designated as "hazardous" for construction purposes where objects such as dwellings are exposed continuously may not constitute a significant hazard to back country use which, in contrast, is transient and exposed rarely. c. The potential avalanche runout limits are based upon a large body of empirical knowledge gained about major avalanches in the Sierra Nevada and elsewhere. Although the potential runout limits are "theoretical" they are based on the best available methods and procedures in use throughout the world. As more research is done and observations are obtained, details about defense-structure design may change. The runout-distances probably will not change significantly, however, because topographic barriers constrain avalanche flow in the study area. d. This is not an engineering-design study. Final design of all protection structures require input of registered engineers or other professionals as required by California law. 1.3 THE AVALANCHE PHENOMENA. Inyo County extends into steep mountainous terrain subject to heavy rain and snow, high winds, and rapidly changing weather conditions. This combination of weather and terrain favors frequent and sometimes severe snow avalanching. Typically, avalanches will be small, occur on steep slopes or in unpopulated back country areas, and will go unnoticed by residents. However, during unusual and severe conditions, massive volumes of snow will simultaneously release from steep slopes (typically 'in the 30O-to-4So range), will develop into major dry- or wet-snow avalanches, and will extend unexpectedly long distances across!ow-gradient or level terrain. The major events of interes~ in mountain land planning usually have return periods of 50 or 100 years. Therefore many decades may pass between major avalanches. Avalanche runout zones (areas in which avalanches decelerate and stop), sometimes ~ience residential development during the 4

3 long periods between major events. This is the case in Inyo County (especially at Aspendell), where extensive building in the avalanche runout zone of the "Hambone Canyon" avalanche occurred between the large avalanches of 1969 and See Chapter 2 for details. New buildings within Inyo County avalanche run out zones have not been sited or designed to avoid or minimize the avalanche hazard. Therefore occupants of these buildings may be exposed to great personal risk during the usual avalanche conditions, which may occur during any winter or spring. The major avalanches of concern here result from failure of large volumes of wet or dry snow, generally from 30O-to-45 slopes located 1,000-to-3,000 feet above the developed sites. The ~ flowing avalanches may reach maximum velocities of 100-to-130 mph during extreme conditions and can extend, at locations similar to Aspendell, for 1,500 feet or more over low-gradient terrain. These avalanches are very destructive and damage :or destroy buildings through crushing, and relocation. High velocity dry-flowing avalanches are usually accompanied by a powder-avalanche component of lesser destructive potential but even longer runout-distance potential. Powder avalanches may completely envelope large structures subjecting them to forces similar to those produced by a strong wind of increased density. Powder avalanches typically blowout windows or doors and may lift roofs off buildings. Large powder avalanches (such as those in portions of the Habegger's and bage Flat areas) may flatten buildings with destructive forces in excess of those associated with well-developed hurricanes. Wet-snow avalanches ("wet slides") will also reach developed areas in Inyo County. These slides will rarely exceed 50 mph even on steep slopes, but can advance for long distances across moderate terrain because of the reduced frictional resistance at the avalanche boundaries. Large wet slides may also be very destructive as thick ridges of snow subject structures to crushing overburden pressures. Wet slides will often spread into separate: flow fingers in the runout zone. The direction of each finger of the flow is difficult to predict in advance. The destructive effects of snow avalanches are well known throughout the mountainous regions of the world. The Eastern Sierra, with its impressive topography and heavy snowfall, is no exception. Unfortunately, development is extensive within many runout zones in ~lono, Placer, and Nevada Counties. Other areas in the United States with avalanche problems include Vail and Aspen, Colorado; Alta, Snowbird, and Sundance, Utah; Sun Valley and Ketch~m, Idaho; Jackson, Wyoming; Juneau and Anchorage, Alaska. All of these areas have formally identified the avalanche areas; some have enacted zoning restrictions in the identified areas. - 5

4 ~ 2.1 ASPENDELL DESCRIPTION 2 ASPENDELL Aspendell is a rural residential community with 162 developable lots and 85 single-family structures. Approximately 250 people inhabit the community at various times of the year. The community is located at an elevation of 8,400 feet, as shown on Figures 1-1 and 2-1. The earliest use of the area was by mining interests and the current Cardinal Lodge immediately south of Aspendell was the headquarters building for the mining company. The lodge is believed to have been converted to its present use in the early 1940'5. The Aspendell subdivision was conceptualized and s~urveyed for development in State Highway 168, a paved, 2-lane paved road, links Aspendell with Bishop, 19 highway miles to the east. Fire protection is provided by the Aspendell Volunteer Fire Department, and the community is served by a sewer and water service. Subdivision streets are paved to County standards and are maintained by the County. All utilities are underground. 2.2 AVALANCHE TERRAIN.~.. '. ) The community of Aspendell had been reached by avalanches originating in Jawbone (Hambone) Canyon and by avalanches starting on a steep WNW-facing slope immediately southeast of the community, (the "East Slope"). Either of these two avalanche areas can damage structures within developed areas or areas proposed for development within Aspendell. The avalanche areas are described below. Jawbone Canyon, referred to locally as "Hambone" Canyon, will accumulate deep windblown snow deposits on North-northeast- (NNE) facing slopes in the upper basin at lo,400-to-ll,000 feet elevation (Figure 2-1). This upper-basin terrain is the primary avalanche "starting zone," or area in which the unstable snowpack breaks loose and accelerates during avalanche conditions. During conditions of widespread snow instability, approximately 30 acres (300,000 yd3) of snow will release from this upper starting zone, accelerate down the canyon, and deposit snow and debris on the alluvial fan upon which a portion of Aspendell is located. The steep slope of the alluvial fan enables avalanches to advance for long distances, sometimes at moderate velocities. Avalanche-dynamics calculations conduc;':.ed for this study indicate that during design ("loo-year"j avalanche conditions, a maximum velocityof 95 mph will be obtained in the upper canyon, but the velocity will have decreased to 50 mph at the top of the alluvial fan, a short distance above the community. This "design- 6

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6 .,.;; avalanche" velocity was used to determine th.e sizes and placements of earthen avalanche defense structures as described in Section 2.5. The East Slope can produce two smaller avalanches which can enter the east portion of Aspendell. The two avalanche areas begin in steep bowl-shaped depressions in the terrain between 9,200 and 10,000 feet elevation, accelerate on the steep open slope and run out onto the relatively flat ground. Avalanches on the East Slope will not be as large as those produced in Jawbone Canyon, but they will attain a velocity sufficient to reach town and damage exposed structures. Avalanches from the East Slope may reach speeds in excess of loo mph on the steep slopes. Powder blast and low-density flowing snow will reach and potentially damage structures. As defined in the Aspendell, Habegger's, and Sage Flat areas, "historic" avalanche areas have been determined by two methods: a. From eyewitness accounts, photographs; newspaper articles, and -')... b. From unmistakable field evidence, such as damaged trees and distribution of old vegetation and other debris. Large avalanches are known to have occurred on the Aspendell alluvial fan in 1952 (approximately), 1969, and The 1952 avalanche predated the community, but is known to have crossed Highway 168 south of town. We have no eyewitness accounts of this avalanche reaching any of the presently-developed areas, but this is understandable because detailed observations of this area did not exist at the time. Because this avalanche crossed Highway 168, however, it does provide a minimum estimate of the runout potential immediately south of town. A large avalanche also occurred during the heavy snow winter of 1968/69. This avalanche reached the intersection of Alpine Drive and Sage Drive and damaged a house. Debris from this avalanche may also have reached the mailboxes on the west side of Highway 168. Although this avalanche may have reached other undeveloped portions of Aspendell and the terrain south of the community, we have no eyewitness accounts from these areas. A large avalanche also occurred on February 16, This avalanche is well documented by obselvations of several residents. One house south of Alpine Drive was destroyed (Figures 2-2 and 2-3) and three other houses on Sage Drive suffered minor damage. Avalanche debris blocked the intersection of Sage Drive and Alpine Drive and debris was transported nearly 3

7 ...1 FIGURE 2-2. The February, 1986 avalanche from Jawbone (Hambone) Canyon completely destroyed this house on Alpine Drive and moved it at least 200 feet to the northwest. ),1 " FIGURE 2-3. This photograph, taken 2 months after the 1986 avalanche from Jawbone Canyon occurred, shows the complete destruction from avalanche impact and crushing. 9

8 to the north end of Sumac Road. The historical record of avalanches from Jawbone Canyon was extended by a search of the alluvial fan for avalanche debris and by study of 1955 aerial photographs. The alluvial fan is littered with vegetative debris transported from higher elevations by avalanches. Debris was found on most of the fan, attesting to the fact that avalanches can be widespread. The debris is much more heavily concentrated on higher portions of the fan above the powerline, and becomes less distinct toward Highway 168. In planning the present Highway 168, Caltrans obtained aerial photographs of the alluvial fan in 1955, which predates construction of Aspendell east of the Highway. These photographs are of very high resolution, and when viewed in stereo, areas of avalanche runout into the aspen forest on the no~th side of Aspendell can be clearly distinguished. In particular, there exists several areas in which the aspen sizes are uniformly smaller (and younger) than the sqrrounding forest. This indicates a sudden removal of the forest and a gradual regrowth of the trees. These trees had not attained the size of the surrounding forest at the time of the photograph. The avalanche or avalanches determined through air photo study probably occurred at least 10 years before the 1955 photos. / The historic record of avalanches in Aspendell indicates that.three large avalanches occurred (1952, 1969, and 1986) and that at least one additional large avalanche before 1955 swept over the area that was to become Aspendell. The avalanche magnitude that has been observed, therefore, has an apparent "return period" of only 10 to 15 years, (4 events in roughly 40 to 50 years). The runout limits of all these avalanches have been combined on Figure 2-4, a 1" = 200' scale topographic map which was produced by Caltrans from the 1955 (pre-contruction) photos. We have superimposed the historic avalanche limits and present roads and houses on this photo. Building and road positions are correct :to within 0.15 inch (30 feet on the map scale). Avalanches which are larger than those documented are possible on the Jawbone Canyon alluvial fan. These avalanches will travel at least as far as the historic limits (Figure 2-4), but may also reach locations in which no documented evidence for past avalanches exists. This is true because no significant topographic barriers exist on the alluvial fan which may deflect or channelize avalanches. The Jawbone Canyon alluvial fan is typical of many small drainage basins that produce avalanches throughout the Sierra Nevada and other mountain ranges of the world. Alluvial fans, such as this 10

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11 j one, tend to disperse avalanches laterally into random directions because the fan surface lacks topographic barriers to flow. Furthermore, avalanches can travel for long distances because the fan surface is relatively steep (8 to 12 over much of the surface), therefore frictional resistance and gravitational forces may be nearly in balance. An area missed by avalanches during one event may be reached by the next one. Finally, a study of 90 "design-magnitude" ("loo-year") avalanches in the Eastern Sierra Nevada indicates that large avalanches similar to Jawbone Canyon usually flow in random and unpredictable directions and reach to the end of the alluvial fan. The potential avalanche determined runout limits, therefore, have been a. by study of avalanche performance in the Eastern Sierra Nevada, and b. through personal experience with the size and shape of major avalanche runouts on alluvial fans throughout the Sierra Nevada and other mountain areas. -'1." 'J These potential runout limits are also shown on Figure 2-4. The probability or return period of the "potential" avalanche runout can be estimated from the statistical data base of major avalanches obtained in the Sierra Nevada. Because the data base itself consists entirely of 100-year return period avalanches, and because this data base was used to infer the potential avalanche extent at Aspendell, then the potential avalanche runout must also have a 100 year return period (a constant 1% probability each year). This is an order-of-magnitude return period estimate. Ther"efore, the actual return period may lie between 30 and 300 years. A more accurate estimate of the return period cannot be made without a detailed historical record at least 200 years long. As shown on Figure 2-4, the potential runout distance is not significantly greater than the historic avalanche. The primary differences between the historic and potential runout limits are: a. The areas between the historic runouts are affected by the potential avalanche; b. The southwest part of Aspendell (near Cardinal Road), an area in which little historic data was obtained, is within the potential runout; and c. Avalanche runout from the into the east part of Aspendell. "East Slope" is mapped 12