IMPLEMENTATION OF STRUCTURAL CONTROL MEASURES FOR AVALANCHE HAZARD MITIGATION ALONG TRANSPORTATION CORRIDORS

Transcription

1 IMPLEMENTATION OF STRUCTURAL CONTROL MEASURES FOR AVALANCHE HAZARD MITIGATION ALONG TRANSPORTATION CORRIDORS Re-Submission Date: November th, 00 Words and word Equivalents: Corresponding Author: Joshua T. Hewes PO Box 00 Flagstaff, AZ 0 Tel: () - Fax: () Joshua.Hewes@nau.edu Other Authors: Rand Decker PO Box 00 Flagstaff, AZ 0 Tel: () -0 Fax: () Rand.Decker@nau.edu Scott Merry c/o Rand Decker PO Box 00 Flagstaff, AZ 0 SMerry@kleinfelder.com Jamie Yount PO Box 00 Jackson, WY 00 Tel: (0) - Jamie.Yount@dot.state.wy.us Author Affiliations:. Northern Arizona University, Department of Civil and Environmental Engineering. Kleinfelder, Inc.. Wyoming Department of Transportation (WYDOT)

2 Hewes, Decker, Merry, Yount ABSTRACT Avalanche hazards along transportation corridors in the United States have traditionally been addressed via forecasting their potential and actively controlling them through explosive release while the roadway is closed. This approach reduces the threat of avalanches cascading onto the roadway and thus reduces danger to the traveling public. However, active control methods cannot always be implemented in a timely fashion, can be ineffective, and can have large associated economic impacts. An alternative to active control is passive, structural avalanche defenses. They are passive in that they do not require the efforts of winter maintenance personnel during winter storm periods. Structural defense measures include snow sails, snow supporting structures, and snow sheds. Despite their extensive use in Europe and their potential for effectively reducing avalanche hazards, there are very few examples to be found in the United States. The potential for negative impacts to the visual attributes of the landscape has been a significant reason for their lack of domestic use. This paper discusses several types of structural defense measures, criteria for their selection at a given site, and their relative effectiveness. Passive structural defense measures designed for implementation at the Avalanche on US Route /near Jackson, Wyoming are described. Details are given on important collaborations between landscape architects and engineers that led to successfully addressing National Environmental Policy Act (NEPA) requirements for retention of visual attributes at the Avalanche site in the presence of snow support structures deployed for the purpose of avalanche hazard reduction.

3 Hewes, Decker, Merry, Yount INTRODUCTION The mountainous regions of the western United States have experienced dramatic growth in the last several decades, and with this development has come an increased risk of motorist injury or death due to avalanches impinging upon roadways. Although this risk is increasing, there has been relatively little new research conducted on avalanche hazard mitigation techniques. Starting in the 0 s, the US Forest Service (USFS) served as the leader in avalanche forecasting, control, research, and education. This role resulted from the development of downhill skiing areas located on USFS land, and avalanche study centers were established at Alta, Utah and in Fort Collins, Colorado. During the period between the 0s and early 0s much was accomplished with respect to developing an understanding of avalanches, and methods of forecasting and active control measures using explosives or artillery were developed and refined. In the mid-0s the USFS ceased its involvement in avalanche research and since that time there has been no national organization coordinating avalanche work. Today, regional avalanche centers funded by state governments, and to a lesser extent by the federal government, are responsible for hazard forecasting and some active control activities for recreational areas as well as along transportation routes. Avalanche risk reduction measures employed by State Departments of Transportation focus almost exclusively on artificial release via detonation of explosives in the starting zone coupled with road closures during explosive control and to clear any resulting avalanche debris from the roadway. Although this approach provides for improved safety to the traveling public, the use of explosives can be problematic in both populated areas and where critical wildlife habitat exists. Moreover, active control is not always 0% effective. During extreme storm periods, avalanche control personnel may not have access to the starting zone, and hazard mitigation can only be accomplished by closing the road altogether. This is often an extremely costly option since shutting down transportation lines interrupts commerce and prevents access to mountain regions whose economies are dependent on tourism. In Colorado for example, Interstate Highway I-0 carries traffic from the Denver Metro region through the Mountain Resort Regions, where it has been estimated that tourism-related transactions contribute $00,000 US dollars per hour to the local economy (). On December st, 00 New Year s Eve a 0-mile stretch of I-0 west of Denver was shut down for almost hours because of extreme avalanche danger, stranding 000 people who were forced to stay in Red Cross shelters overnight. Undoubtedly this served not only as a major inconvenience but also negatively impacted the tourist industry. In Washington State, Interstate Highway I-0 serves as the major transportation link between Seattle and Spokane, and carries,000 vehicles per day between the western and eastern portions of the State. I-0 passes through several areas of the Cascade Range that experience regular avalanche activity, and in order to keep it open year-round the Washington State Department of Transportation (WSDOT) has developed a comprehensive program to mitigate the avalanche danger. This program relies heavily on forecasting and active control to reduce avalanche hazard, although some passive defense measures are utilized. Epitomizing the potential for high economic impacts of avalanche hazard is the winter of which saw hours of closure of I-0 at Snoqualmie Pass, with an estimated cost to the State s economy of roughly $ million (00 US dollars, ). The long-term costs (direct and user delay related) of active control to mitigate avalanche hazard can be staggering and in some cases far outweigh the initial costs of structural passive avalanche defense. In Europe, structural defense measures are commonplace and have been utilized extensively for decades. Avalanche passive structural control represents the single largest public works expenditure in Austria. In the United States, the high cost of managing avalanche risk via active control has recently led the WSDOT to launch a project to widen and lengthen the existing snow shed at Snoqualmie Pass which currently only protects west bound lanes (). State DOT personnel in Wyoming have also begun embrace the notion that constructed defense measures can offer improved safety and reduced potential for economic impact compared to active control. Passive control projects for the Milepost Avalanche on US / near Jackson Hole, Wyoming have been funded by the Wyoming Department of Transportation, and these represent a significant advancement towards adopting avalanche hazard

4 Hewes, Decker, Merry, Yount mitigation policies that include passive structural control measures. This paper first describes common forms of structural avalanche defense along with their relative effectiveness and appropriateness, and then discusses the design, implementation, and effectiveness of structural control measures for the Avalanche in Wyoming. STRUCTURAL DEFENSE SYSTEMS Three basic schemes of passive avalanche defense exist and various considerations influence which is most appropriate for a given site. One approach is to reduce the build-up of snow in the avalanche starting zone area, and especially where the predominant mechanism of snow accumulation is via wind transport and deposition. Snow fence structures can be erected to disrupt wind patterns that transport snow from up-wind fetch regions to the starting zone. The effectiveness of snow fences is limited to cases where wind transported snow dominates build-up of the snowpack that can, subsequently, avalanche, and where there is a obvious prevailing wind direction. Snow baffles, also called snow sails, can be used where the primary avalanche mechanism is through the formation of wind-slabs (wind deposited snow slabs). A snow sail uses the energy of the wind and turbulence created by the sail to scour the snowpack in the vicinity of the sail, thus preventing continuous snow slabs from forming and reducing snowpack stresses in the starting zone. Snow sail effectiveness is limited to regions with wind slab avalanches and where snowpack depths are less than about.m (). Another type of constructed avalanche defense are snow supporting structures (SSS) which hold the snowpack statically in place within the starting zone. SSS are typically made of wooden poles or structural steel or aluminum members that form a plane to support the snowpack. The mass of snow uphill of any given structure is retained and tensile stresses within the snowpack which lead to snow slab fracture are relieved. Two types of SSS exist: snow rakes have vertical members supported on horizontal cross members while snow bridges have horizontal crossbeams that are supported by vertical girders which connect to foundations. Both systems utilize struts embedded in down slope foundations to transfer snowpack loads to the earth. Snow bridges are usually more effective than snow rakes (). A deployment of numerous SSS are distributed within the starting zone, typically in continuous linear rows based on efficiency considerations. Other factors such as aesthetics can dictate the use of staggered or echelon arrangements of individual SSS units. An example of a snow bridge supporting a snowpack is shown in Figure. FIGURE Snow bridge in situ retaining snowpack. The hazard to motorists in an avalanche pathway can be eliminated by using a snow shed that passes the avalanche over the top of the roadway. A snow shed is essentially an artificial tunnel that protects the roadway from the avalanche. These structures are typically used where the starting zone is very large, which makes the use of starting zone measures more costly. Consideration must be given however to the

5 Hewes, Decker, Merry, Yount impacts of passing the avalanche safely over the roadway, but then onto lands down slope of the roadway for example, allowing the avalanche to pass over the roadway and onto adjacent private property would likely not be an acceptable solution. The above structural defense measures rank in order of effectiveness (lowest to highest): wind disrupters (snow fences and sails), SSS, and snow sheds. While wind disrupters and SSS only reduce avalanche hazard since extreme events above the design snowpack depth are possible, snow sheds eliminate risk to the roadway altogether. The design and implementation costs of these measures rank as follows (lowest to highest): wind disrupters, SSS, and snow sheds. The costs of the technologies are roughly separated by an order of magnitude THE MILEPOST AVALANCHE, JACKSON, WYOMING US / is a four-lane primary regional trunk road into and out of Jackson, Wyoming, which is located less than one mile north of the Avalanche. US / is located at the valley floor m vertically below the Existing Avalanche starting zone. The Avalanche snow sails releases during periods of heavy snow (on the order of one inch per hour for eight hours or more at a minimum) and strong southwesterly winds ( mph, gusting to mph), and can also avalanche during those winters when depth hoar is present at the ground surface. On average, it avalanches to the road. to.0 times per year. In the past, a fraction of these avalanches have impacted motorists vehicles on the roadway, but to date, there have been no loss-of-life incidents. Because the starting zone of the Avalanche is managed as critical US / to big game habitat and the adjacent South Park Jackson, WY area of the Jackson Hole valley is now developed, hazard reduction via the regular use of explosives is not desirable. Figure is the FIGURE WYDOT Avalanche Atlas depiction of the WYDOT Avalanche Atlas depiction of the Avalanche. Avalanche. An optimal solution for avalanche hazard management at the Avalanche is an effective passive (constructed) system that performs stand alone and does not require WYDOT winter maintenance personnel efforts during storm fighting periods. There are three constructed avalanche hazard reduction technologies applicable to the Avalanche:. Wind/snow disrupters (snow sails) in the avalanche starting zone.. Snow supporting structures (snow bridges and rakes) in the avalanche starting zone.. Snow shed or gallery at the road. The first two structural measures are viable, but because of the presence of private property adjacent to US/, the passing of an avalanche over the roadway using a snow shed is not an option. Subsequent sections discuss implementation of snow sails, and then snow bridges, at the Avalanche. Implementation of Snow Sails A first attempt at reducing avalanche risk was made via a deployment of 0 snow sails adjacent to the starting zone of the Avalanche. The snow sails were installed in the summer and autumn of 00, and were trapezoidal in shape with dimensions of 00mm and mm horizontally at the top and bottom,

6 Hewes, Decker, Merry, Yount respectively. Vertically the sails measured 00mm and were mounted on mm tall masts. A gap of 0mm between the ground and the bottom of the sails was provided. The mast and booms were aircraft grade aluminum tubes, and each sail was anchored by up to twelve steel cables connected to earth anchor pins driven approximately mm into the ground. Sails were constructed from vinylized commercial truck tarpaulin material. Based on European guidelines, the snow sails were deployed so that the distance between any two sails was.0 to. times their top width. The snow sails are visible as black dots in the photograph shown in Figure near the upper and right hand side of the Avalanche starting zone. The cost to fabricate, transport, and install the 0 snow sails and leave pre-assembled sails in reserve at the WYDOT maintenance facility in Jackson was approximately $0,000. Development costs to design and test a proto-type during a pilot phase were $,000. The snow sails have proven to be only marginally effective at reducing the occurrence of natural avalanches. No avalanches to the road were observed in the first season of operation, but during the winter an avalanche that occurred naturally reached the road during a period of very heavy snow and strong winds. Subsequent seasons have indicated that the snow sails can be effective at reducing the occurrence of avalanches at the site for conditions that are moderate, but not during periods of intense snow and wind, which are of most concern to the WYDOT. The snow sail deployment at the Avalanche was an initial attempt to address the avalanche hazard with the most cost effective constructed solution. Implementation of Snow Supporting Structures The next logical escalation in efforts to reduce the avalanche hazard at the Avalanche was to explore the design, costs, and USDA Forest Service administered National Environmental Policy Act (NEPA) requirements for a deployment of SSS in the Avalanche starting zone. Design Specifications United States domestic experience in the design and implementation of SSS for mitigation of avalanche hazard in transportation applications is limited. Whereas in-depth national specifications for the design of bridges, highways and other transportation facilities have been around for decades, no federal requirements or guidelines for design of snow bridges or rakes exist. The preeminent treatise on the implementation of SSS for avalanche hazard mitigation is the Defense structures in avalanche starting zones Technical guideline as an aid to enforcement or the Swiss Guideline (). Although this is a fully mature and comprehensive design guideline, it does not prescribe all aspects of design of snow bridges. Furthermore, it references European design and material specifications which are not necessarily appropriate or applicable in the US. Thus, the Swiss Guideline serves as an invaluable starting point for the development of the American voice for the design of SSS, but the resulting design for the Avalanche is not a simple importation of the European snow bridge. US structural design specifications used for the Avalanche include: Building Code Requirements for Structural Concrete (), Specification for Structural Steel Buildings (), Manual for Design and Construction Monitoring of Soil Nail Walls (), Manual for Railway Engineering (). The American Society for Testing and Materials (ASTM) standards were used for material specifications for the project. Snow Load Environment The primary loads to be resisted by SSS consist of the down slope component of the snowpack s weight and the loads that result as a consequence of the snowpack s slow, viscous deformation under its own weight. These latter loads include the motion between the snowpack and the ground surface (glide) and the internal deformation of the snowpack (creep). They are accounted for in design through the application of an equivalent static load, expressed as a fraction of the static snowpack weight. The snowpack weight is based on the design snow depth which in the Swiss Guideline is that with a recurrence interval of 0 years. The Swiss and other European countries have recorded snow depth measurements for up to seven decades; conversely no recorded snowpack depth information was

7 Hewes, Decker, Merry, Yount available for the Avalanche. Thus, the design snow depth was selected based on anecdotal evidence from WYDOT and the nearby Snow King Resort, and was set at.0m. Snowpack glide and creep are a function of the ground surface roughness characteristics, solar exposure, and snow density. In the Swiss Guideline, the influence of glide on structure loads is accounted for by multiplying the down slope component of the snow weight by a glide factor, N. At the Avalanche, the ground consists of primarily smooth grass and the slope is facing west-southwest, both of which contribute to a high tendency for glide. Based on the Swiss Guideline, a glide factor of N=.0 was selected, which implies quasi-static equivalent creep and glide loads three times larger than that due to weight of the snowpack alone. This latter load, the component of snow weight that acts parallel to the slope and that is resisted by the snow supporting structure, is calculated based on the slope inclination,, taken as for the Avalanche. In the Swiss Guide, a nominal snow density of 0. ton/m is assumed for an elevation of 00 m a.s.l, and is increased % for every 0 m increase in elevation. Total snow loads are also dependent on the boundary or end conditions at either end of any given snow bridge (across slope). If a snow bridge unit is positioned in the snowpack a significant distance from any other units, glide and creep of the snowpack around the ends of that structure imparts larger loads to the unit than if it abuts another unit immediately adjacent to it. These increased loads are termed end effects. Based on the Swiss Guideline, a snow bridge separated from adjacent bridges by more than.0m will have an end effect factor. This was calculated as f r =. for the Avalanche starting zone site. This factor increases the equivalent snow loads (snowpack weight plus glide and creep effects) in a discrete end-effect region at the side (or end) of a snow bridge grate. Snow Bridge Unit Design Traditional European design of snow bridges is based on linear and continuous rows of structures separated by a given distance in the line of slope and distributed over the avalanche starting zone. Although this is the most effective deployment configuration for retaining the snowpack, it was not acceptable for the Avalanche based on its visual attributes. Because of this, design of a single, standalone snow bridge unit was pursued. This unit design was developed so that it can be used anywhere in the deployment configuration, irrespective of whether it is grouped with other units or whether it is an isolated single unit. The primary advantage of this approach is simplicity in the design, fabrication, and construction stages of implementation. Another significant driver for the unit design was the desire to minimize construction effort on the steep slope by allowing a fully assembled unit to be transported via helicopter and lowered into position on the slope. The Swiss Guideline is not a cookbook for snow bridge design where all dimensions, member sizes, connections details, etc. are given by a simple recipe. In fact, it explicitly states that The present guideline allows considerable leeway in laying out and dimensioning the structures. What follows is a brief summary of the process by which the characteristics of the Avalanche snow bridge unit were selected. A side elevation view of the snow bridge unit is given in Figure. The unit consists of two girders to which crossbeams are connected this forms the grate which supports the snowpack. The girders are supported by struts that connect to lower foundations downhill from the grate. The length from the top of the girder to the ground, measured along the axis of the girder, is set such that the vertical height of the snow bridge equals the design snowpack depth, H K. No recommendation is given in the Swiss Guideline on where along the girder length the strut should attach. The 0.m from the top of girder to the strut attachment point was determined based on a consideration of balancing the girder negative internal moment demand at the strut attachment point with the positive girder moment demand in the span between the strut attachment point and the foundation, leading to an efficient use of the girder members. The Swiss Guideline recommends that girders be laid downhill from perpendicular to the slope. The angle between the strut axis and the slope is not discussed in the Swiss Guideline and an analysis of the influence of this angle on both foundation forces and the strut axial force was conducted.

8 Hewes, Decker, Merry, Yount From a structural engineer s perspective, it is obvious that as the strut-to-slope angle (interior angle between strut and slope) becomes larger, the distance between the upper and lower foundations becomes smaller with the consequence that foundation forces increase to satisfy rotational equilibrium. Since large foundation forces require more robust foundations, large strut-to-slope angles are undesirable. The impact of reducing the strut-to-slope angle is that distance between foundations becomes greater which leads to smaller foundation forces. However, this implies a longer strut which increases the strut unbraced length and tendency for member buckling. The angle selected provided a reasonable balance between the opposing needs of minimizing foundation forces and minimizing the strut size. The total weight of any given unit structure also needed to be limited so that it could be safely handled by commonly available helicopters. With the height of the snow bridge selected as previously described, the width as measured across the slope became a critical variable. Preliminary engineering of the snow bridge indicated a maximum grate width of.m in order to comfortably meet heli-tac lifting weight limits. Figure shows a drawing of the snow bridge viewed downhill and perpendicular to the grate surface. Wx girder Wx girder 0.0m 0.0m.m.m MCx crossbeam MCx crossbeam.m.m Concrete footing.m HSS XX strut CL Ground anchor HSS XX strut CL Ground anchor (vertical) 0 Concrete footing HSS x x pressure bar (a) CL Ground anchor (b) FIGURE (a) Unit side view on CIP concrete footings, (b) Unit side view on ground anchors. The distance between the end of a crossbeam and the girder is not addressed in the Swiss Guideline. This overhang length was selected with consideration of the different distributed snow pressure patterns acting on the grate. Because some SSS units will be isolated (separated by say or so meters from other units) while others will be grouped in a line across the slope, a unit must be designed to accommodate any of the various snow pressures that can occur. A unit that is in the middle of three units placed together in a line, for example, will not experience the increased end effects loading due to creep and glide effects. For this uniform pressure distribution, a given optimal crossbeam overhang length could be determined. However, for units that have a one or both ends not bounded by another unit, end effects are significant and the crossbeam experiences greater equivalent snow pressures at its free end(s) than along its interior span. Hence, a different optimal overhang length could be determined compared to crossbeams that do not suffer end effects. In this context, optimal means efficient use is made of the crossbeam structural section, which in turn means that local maximum internal moment demands along the crossbeam are similar (or equal). Hence, the crossbeam overhang length was chosen so that the

9 Hewes, Decker, Merry, Yount maximum positive crossbeam moment in the span between the girders and for the case of a uniform snow pressure pattern was equal to the negative crossbeam moment at the girder and under the non-uniform snow pressure pattern associated with end effects. The optimal overhang of the crossbeam, expressed as a fraction of the total grate width, was 0.. Because the magnitude of end effects depends on the arrangement of units (across slope spacing) and glide factor, both of which are unique to a given site, the optimal crossbeam overhang length is a unique design parameter for any given avalanche site. Crossbeam spacing along the height of the grate was selected based on recommendations in the Swiss Guideline, which provides a range of acceptable openings in between individual crossbeams. CL Structure 0.m.m.m 0.m 0.0m typ. spacing 0 0 MCx crossbeams FIGURE Downhill and perpendicular view of snow bridge grate. Snow Bridge Foundations and Construction Ground conditions at the Avalanche vary from rock outcropping to areas with several feet of soil. The Swiss Guideline suggests that several different foundation types may be used, including cast-in-place concrete foundations, prefabricated foundations, micropiles, and ground anchors. However, it states that the preferred method of connecting snow supporting structures to the slope is via micropiles (a soil nail in compression) and anchors (a soil nail in tension). This approach was used for the Avalanche. As is the domestic norm, the distinction between a soil nail (a length of steel reinforcing bar inserted into a hole drilled into the earth and grouted in place to develop the bar s axial capacity) in compression versus tension is not made and that vernacular is maintained in the remainder of this paper. The term ground anchor is used to describe a soil nail, whether in compression or tension. The desire to simplify construction and the corresponding plan to install fully assembled SSS units via helicopter significantly influenced the selection of foundation type and connection details. Because heli-tac time can be expensive, a quick method of securing completely assembled units on the slope after being lowered into position was needed in order to minimize total airtime and thus costs. Another key issue in the development of foundation connections was the need to provide for reasonable tolerances for the installed location of ground anchors. An initial connection system was developed that allows for both angular and linear dimension errors in location of ground anchor bars. It also provides for rapid capture and securing of a unit as it is lowered by helicopter into position on the slope. Figure a shows a detail of the concrete strut foundation, which consists of the ground anchor bar, a series of steel plates, steel anchor rods and nuts, and cast in place concrete. The construction process is envisioned as follows. All ground anchors are installed and then soil is excavated around each for the concrete footings. A spherical nut is installed on the ground anchor bar, and a steel plate with a hole cut out at its center ( Plate A in Figure a) is lowered onto the nut. The mm diameter ASTM F anchor rods and Plate B are attached to the strut base plate before the unit is lifted from the staging area. Plate B has a mm diameter hole cut out at its center, and the ground anchor bar has a diameter on the order of mm. As the unit is lowered into position on the slope, ground crews guide each ground anchor bar up through the center hole in Plate B. When the weight of the unit is resting completely on the ground anchor bars via contact between Plate A and Plate B, Plate C is

10 Hewes, Decker, Merry, Yount lowered onto the ground anchor bar and finally the top spherical nut is installed. The tolerance for misalignment of ground anchors is provided by the difference in diameter of the hole in Plate B and the ground anchor bar. By this mechanism a tolerance of 0mm from ideal center in any direction for each ground anchor bar is provided. Height adjustments can be made via the spherical nuts and ground anchor bars as well as by the steel mm diameter anchor rods and associated nuts. A similar foundation connection detail is used at the girder (uphill) foundation. CL Strut Strut PL xx CL Strut CL Strut to CL opposite strut C: PL.xx B: PL xx Øx long ASTM F GR anchor rod TYP. A: PL. xx ASTM A Heavy hex nut TYP Steel spherical seat nut 0 mm Ø Pin ASTM A B HSSxx strut Concrete footing CL Ground anchor (a) Note: All dimensions in mm Ground anchor coupler (b) 0 FIGURE (a) Strut foundation for soil sites, (b) Strut foundation for rock sites The foundation and connection to the snow bridge described previously allows for reasonable misalignment of ground anchors while also providing for fast installation. However, where soil over rock in the Avalanche starting zone is thin, making excavation for the concrete footing difficult, different snow bridge foundation connections are required. Figure b shows the strut foundation system which utilizes one ground anchor to secure each strut. The strut connects to the foundation via a pin and shackle connection. A shackle built from steel plates and a bar coupler is installed onto the ground anchor bar. A steel plate welded to the bottom of the strut is fastened to the anchor bar shackle via a mm diameter steel pin. Because the strut carries large axial compression loads (on the order of kn) and since perfect alignment of the strut and ground anchor axes is not likely to occur, an additional structural element is required to connect to the strut and ground anchor. This element connects the downhill and uphill foundations and ensures that the strut ground anchor is not overloaded due to off-axis or transverse loading. Figure b shows a side elevation view of a snow bridge unit to be used where rock prevents installation of the concrete footing foundation. Girder foundations use two ground anchors and a connection similar to that used at the strut base. These foundation connections will require very tight geometric control of the location of each ground anchor. However, using a jig or template on the slope during anchor installation will lead to more effective installations of fully assembled snow bridges.

11 Hewes, Decker, Merry, Yount Deployment Criteria and Organic Configuration Any given snow bridge unit supports a portion of the snowpack equal to its across slope width and an uphill zone of influence. The length of this zone of influence from any given snow bridge is a function of the same parameters that control the structural design, and is dominated by the design snow depth and slope angle. For the Avalanche this length is.m. The Avalanche starting zone is approximately m long (up/down slope) and has an average width of.m laterally for a net area of 0 square meters or 0.0 hectares. The snow bridges in the Avalanche starting zone are to be implemented as single units that are.m wide laterally and hence support a tile of snowpack.m wide and.m in length, or as double units separated by a 0.0m gap for a net lateral effective width of.m, or as triple units with a resulting effective lateral width of.m. Though supporting a wider tile of the snowpack, double and triple units are limited in the same fashion as single units, to an uphill effective zone of influence of.m. Based on this geometry and the areal extent of the Avalanche, it will take approximately 0 snow bridge units in various combinations of singles, doubles and triples to adequately cover the Avalanche starting zone. Despite being valuable information, the resulting number of 0 snow bridges in the Avalanche starting zone says little about where, exactly, they are to be place and the resulting cumulative appearance they ll have once they are installed. Moreover, the appearance of a deployment of snow bridges in the Avalanche starting zone is not a trivial matter. The appearance of 0 SSS in the Avalanche starting zone can be dramatically different, depending on the configuration chosen for their deployment. The Swiss Guideline suggests four different SSS deployment configurations. Each of these results in a relatively orderly and repeating pattern of snow bridges in landscapes. This is problematic, as these patterns do not mimic visual elements of natural the landscape. For this reason, deployments of SSS tend to have high visual impacts on the landscape, especially as seen from a distance. The National Environmental Policy Act (NEPA) dictates that activities and facilities on Federal lands must minimize or mitigate their impacts on the environmental assets of that site. The list of potential environmental assets is daunting and the process of identifying them and recommending various mitigation measures are the well known Environmental Assessments (EA) or Environmental Impact Statements (EIS) processes. In the case of the Avalanche, the Bridger-Teton National Forest conducted an EA for the snow sails, which are now installed in the Avalanche starting zone. The visual attributes of the Avalanche starting zone was the only environmental asset identified as being potentially impacted. The NEPA mitigation criterion for this attribute is known as visual retention. The visual attributes of the site found prior to construction must be retained or restored after construction. A collaborative work-process, utilizing expertise of the USFS landscape architect and avalanche experts from WYDOT and its contractor, was put in place for the purpose of assuring that a deployment of snow bridges in the Avalanche starting zone would both retain the visual characteristics of the site and perform their technical task of avalanche defense. Using suites of single, double and triple snow bridges, and the paired and triple clusters of small conifers found as visual elements in the adjacent landscape, the Avalanche snow bridges were deployed in a configuration that mimics the appearance of the conifer clusters. The resulting deployment configuration is shown in Figure, which illustrates how snow bridge units will be grouped together along the slope. It successfully achieves a configuration that mimics the natural landscape and has little or significantly reduced visual impact. This is a critical element for any future implementation of structural avalanche defense systems within the United States. When the Avalanche snow bridge deployment is coupled with reforestation, the potential to retain the visual attributes of the site is further enhanced. However, it should be noted that even though conifer reforestation assists in retaining the visual attributes of the Avalanche starting zone, it will not replace the snow bridges as an avalanche defense measure. The density of mature conifer stands in the surrounding area is not sufficient to preclude the onset of avalanches.

12 Hewes, Decker, Merry, Yount FIGURE Virtual rendering of snow bridge organic deployment configuration Cost Estimate and Implementation Cost estimates to fabricate, ship, transport and installed 0 snow bridges in the Avalanche starting zone is on the order of $. million US dollars. These net present costs can and will be influenced by today s volatile material and construction cost environment and can be expected to escalate over time. The one-time and fixed costs, including design but not the research elements of this effort, are amortized for an installation of 0 structures. The net unit cost per snow supporting structure installed in the Avalanche starting zone is $,00.0 US dollars, and with proper maintenance the service life of the installed defense system is on the order of 0 years. The snow bridge project for the Avalanche has not yet been funded by WYDOT for construction. ACKNOWLEDGEMENTS This research was funded by the Wyoming Department of Transportation through the Wyoming Research Center. The support of Jim Montuoro and Michael Patritch of WYDOT is greatly appreciated. REFERENCES. Development Research Partners. The Impact of I-0 Congestion on Colorado Denver to Grand Junction. Denver Metro Chamber of Commerce, April 00.. King County Regional Hazard Mitigation Plan. King County Office of Emergency Management, December 00.. I-0 Snoqualmie Pass East Corridor Program. Washington State Department of Transportation, May 00.. Yount, J., Decker, R., Rice, R., and Wells, L. Reducing Avalanche Hazard to US Route / in Jackson, Wyoming Using Snow Sails. Proceedings of the International Snow Science Workshop, ISSW, Jackson Hole, Wyoming, September, 00.. Chaudhary, V., and Gursharan, S. Structural Measures for Controlling Avalanches in Formation Zone. Defence Science Journal, Vol., No., 00, pp. -.. Defense structures in avalanche starting zones Technical guideline as an aid to enforcement (in English). Federal Office for the Environment, FOENWSL, Swiss Federal Institute for Snow and Avalanche Research SLF, Bern, Switzerland, 00.. Building Code Requirements for Structural Concrete (ACI -0).American Concrete Institute, Farmington Hills, Michigan, 00.. Specification for Structural Steel Buildings, th Edition (ANSI/AISC 0-0). American Institute of Steel Construction, Chicago, Illinois, 00.. Manual for Design and Construction Monitoring of Soil Nail Walls. Publication FHWA A-SA-- 0R. FHWA, US Department of Transportation,.. Manual for Railway Engineering, th Edition. American Railway Engineering and Maintenance-of- Way Association, Lanham, Maryland.