2.5 AVALANCHE MTGATON 2.5.1 General considerations Several alternative forms of avalanche mitigation are in use around the world. The selection of the best form of avalanche protection in a given area depends on reliability, cost, and environmental factors. n summary, the various forms are listed below. a. Supporting structures. These are fences or rakes built in the avalanche starting zone on slopes steeper than 300. They anchor the snow to the ground and prevent avalanche release. They tend to be very expensive (> $300,000 per acre) and would'not be appropriate above Aspendell because of the very high cost of erecting structures in the remote starting zones. The upper starting zone area is estimated at approximately 30 acres, therefore total construction cost could be as high as $9,000,000..} " b. Deflecting structures. These are built in the upper runout zone and usually take the form of earthen or structural guide walls intended to change avalanche direction, generally through angles of less than 200. They could be used to reduce the hazard above the eastern portion of Aspendell where most damage has occurred historically, but they may increase the runout distance toward the Cardinal Road intersection and increase the hazard to buildings here. Therefore deflecting structures should not be used. c. Catching structures. These are usually earthen dams or fields of mounds built in a checker-board array i the upper runout zone. They are intended to reduce avalanche velocity and shorten the runout distance. With careful design they would be appropriate in reducing avalanche hazard above Aspendell. See Section 2.5.2 for details. d. Direct-protection structures. These consist of reinforcing individual buildings for avalanche impact and deposition loads. They could be used at Aspendell, but design of such structures requires determination of expected avalanche forces and dimensions. They will increase construction costs by ail estimated lo-to-30 percent. See Section 2.5.3 for details. e. Artificial avalanche release. This usually consists of delivering explosives to avalanche starting.. 13
zones and attempting to release avalanches in reduced volumes. The various explosives-release methods of controlling avalanches are not recommended above developed areas because of the uncertainties associated with this method. Sometimes they do not release avalanches at the time or size intended. Furthermore, avalanches released in this manner will sometimes be much larger than anticipated. Design of avalanche defenses depends on the design-avalanche characteristics. n general, knowledge of avalanche velocity, flow height, density, and impact pressure must be calculated in order to rationally design avalanche defenses. The two recommended forms of avalanche defenses which could be used at Aspendell, both of which depend on these parameters, are discussed below. 2.5.2 Catchinq structures Catching structures would be effective in reducing avalanche velocity and runout distance on the Aspendell alluvial fan. Figures 2-5 and 2-6 illustrate the locations and sizes of an array of earthen mounds and a catching dam. The proposed sizes and orientations of these structures would prevent avalanches similar to the 1986 event from reaching developed areas. However, an avalanche as large as the potential loo-year avalanche, although slowed considerably by the proposed structures, could overtop the barriers and reach buildings near Alpine Drive with reduced energy. An advantage to the mounddam array shown is the relatively low cost and the availability of local material in the alluvial fan for construction. Construction would, therefore, require no material to be transported to the site. Assuming, as shown on Figures 2-5 and 2-6, 36 mounds each 15 feet high and a 25-foot high dam 1,100 feet long, the approximate volume of earth moved for construction would be 30,000 cubic yards (20,000 yd3 for the dam and 10,000 yd3 for the 36 mounds). This volume estimate assumes a cut-and-fill geometry as indicated in Figure 2-6. Construction cost would depend primarily on local equipment operator rates. Final construction drawings should be completed bya local registered California engineer or surveyor as required by California law. Disadvantages include the obvious visual impact of such a large amount of earthwork immediately above town. Scars from mining activities throughout the mountains attest to the fact that construction activities may require long periods of time to revegette. Furthermore, the mounddam system will not effectively dissipate powder blast which will overrun the earthwork and may cause some damage within the developed area. 14
..:..
-- (A) EARTHEN MOUNDS - - UNDSTURBED GROUND SURFACE FGURE 2-6. Earthen mounds and a catch dam as shown in (A) and (B) and located in Figure 2-5 would reduce the avalanche hazard at Aspendell by approximately 80%. Fast-moving dry-snowpowder.avalanches could overtop this proposed defense system and reach the upper part of town. Mounds must be built as close together as possible and must have an uphill. "crest-tc-..",t elevation difference of 15 feet. The dam must have an uphill elevation difference of 25 feet. A detailed engineering and soils investigation may be required in any final-design stage. 16
2.5.3 Direct-protection structures Direction-protection structures are built immediately up slope from buildings or are incorporated directly into. the design of the building. An example of one type of direct-protection structure at Ketchum, daho, is shown in Figure 2-7. Avalanches reach these duplex structures from the left and will flow up onto the "ramp roofs." The roofs have been specially designed for avalanche impact and deposition loads. This particular design is especially useful because large impact loads on flat surfaces are avoided and the avalanche flow will not be deflected laterally toward adjacent property. This type of design is definitely applicable at Aspendell. Many other forms of direct protection are used throughout the world. The "splitting wedge" design in which the flow energy is dissipated laterally has been used at many loctions where lateral deflection is not a problem. At other locations where avalanche energy is not large, such as the lower end of the runout zone at Aspendell, walls facing the flow can simply be reinforced for avalanche impact and depositional loads. Reinforcing buildings for avalanche loads requires detailed avalanche dynamics calculations and structural engineering design. Design parameters that must be known are a. Building location, shape, and orientation: b. Design-avalanche building site; velocity and energy at the c. The design-avalanche loads "b. " which depend on "a" and n general, design loads must be resolved into 3 mutually perpendicular directions in order for structural details to be specified. Figure 2-8 illustrates the elevation and plan view of a flat surface subject to avalanche loads and lists the factors that corol the magnitude of the avalanche loads. Design-avalanche energy densities are shown on Figure 2-9. The energy densities are equal to kinetic energy per unit volume of avalanche. This energy-density parameter has the units of pressure ([ft-lbs]ft3 = lbsft2), therefore the pressure isobars on Figure 2-9 give a rough idea of the pressure on a structure. We strongly emphasize that these isobar lines are not design pressures. Therefore,they cannot be used directly for design of defense structures or buildings. Figure 2-8 lists the most important factors that must be coidered in direct-protection design. Costs for design and construction of direct protection will consist of the initial avalanche analysis and engineering (an 17
(A) ELEVATON VEW (B) PLAN VEW A V ALAN CHE DRECTON FGURE 2-. Direct-protection design for avalanches must consider forces in J mutually-perpendicular directions. Pn(normal), Pv(vertical), and p s (lateral shear). n general, these forces will depend on the desinavalanche dynamics and the structure details. Factors that must be considered in design include: * Avalanche velocity; * flow density; * flow height; * deposit geometry; * Snowpack depth at structure; * Structure location; shape; " orientation to avalanche; and * Local terrain irregularities. Because these factors will vary from one case to another, they must always be considered on a site-specific basis. A registered engineer and or architect should be consulted in final design. Ramp structures such as shown in Figure 2-7 are efficient in dissipating avalanche energy, therefore design loads are minimized. Vertical walls normal to the flow generally receive maximum loads. 19
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; engineer registered in California must be involved in design), and the additional construction costs. These costs will be highly dependent on the individual project, but may add lo-to-30% to the building cost... ") 21