Mapping Ski Hill Avalanche Terrain in the Vancouver, B.C., Area

By Shirley A. McLaren

Introduction
The Canadian Avalanche Association (CAA) reports that, 20 years ago, most avalanche incidents were industrial accidents that occurred during road building and while laying railroad tracks across Canada. Today, field observations show that people trigger most avalanches during recreational activities. Recent avalanche deaths of out-of-bounds snowboarders and a hiker in British Columbia indicate that people remain at high risk from avalanches. The North Shore Mountains in the Greater Vancouver area are no exception. These mountains include the Cypress, Grouse, and Seymour Mountain ski resorts, which are experiencing rapid growth in winter visits. Therefore it is vital to determine where are the avalanche-prone areas, and can these areas be defined and mapped?

Suitability Mapping of Avalanche Trigger Sites
Terrain and climate factors influence both where avalanches are triggered and where they run out. The most conservative estimate of avalanche risk is the assumption that a snow pack will be periodically unstable and periodically prone to avalanche. Therefore, mapping the most suitable locations for triggering an avalanche - based upon climate and terrain parameters - can be an important part of avalanche safety and avalanche mitigation techniques. If both the public and avalanche forecasters know where the most suitable trigger sites are located, then avalanche terrain can either be avoided when snow conditions are optimal, or else explosive controls can be implemented. The CAA reports that many skiers and climbers fail to recognize avalanche terrain. The most suitable avalanche trigger sites, therefore, are mapped across the Cypress, Grouse, and Seymour Mountains of the North Shore, based upon the collection of 12 years of avalanche incident data in Canada.

Method
Suitability mapping, using a GIS, is a powerful method for assessing and aggregating various factors that control the degree of suitability for fitting a particular condition to a specific location. Multi-criteria evaluation (MCE) is a common method of suitability mapping. In MCE, each factor is a layer of geographic information - normally in raster format - that is weighted and mapped to produce a single map that defines the suitability of each pixel or cell.
      In the book Avalanche Accidents in Canada 1984-1996: Volume 4, the type and number of recreational avalanche incidents across Canada are well documented. Avalanche incident statistics reveal that many terrain factors cause avalanches to occur in specific areas. Most avalanches begin under the following conditions:
• On slopes between 25-40 degrees - the slope factor
• On slopes downwind or parallel to the prevailing wind direction - the aspect factor
• On convex slopes, steep planar slopes, and along ridges - the terrain shape factor
• At or above the tree line, especially on rocky exposed surfaces - the groundcover factor.
      Each factor was mapped across the North Shore Mountains by using Landsat 7 imagery and a DEM for input into a MCE analysis.

Cartographic Model
Idrisi32 GIS software was used because it has sophisticated procedures for satellite data classification, as well as excellent raster data processing capabilities with advanced MCE functions.
      In summary, the DEM was created from topographic data at a 30-meter resolution rate to match the resolution of the Landsat 7 image. The DEM was used to create the slope, aspect, and terrain shape factor maps. Taken on July 12, 1999, the Landsat 7 ETM image was imported into Idrisi32 and geo-referenced to the DEM using 19 control points. The satellite data was used to classify forested and non-forested areas of the North Shore Mountains to create the groundcover factor map. A constraint map (urban and water areas not considered by the MCE analysis) was digitized from a false-color composite image of the project area.
      In the MCE analysis, the four factors - slope, aspect, terrain shape, and groundcover - may work in isolation or together in defining the degree of suitability for avalanche initiation. The factors are said to fully "trade-off." In other words, a factor with high suitability in a given location can compensate for other factors with low suitability in the same location. The degree to which one factor compensates for another is determined by its factor weight. Put into terms of an equation, the sum of the factor weights must equal one. Each factor is given a weight based upon its relative importance to the other. The relative importance of the slope, aspect, terrain shape, and groundcover factors for triggering an avalanche is not fully understood, so they are arbitrarily given an equal weighting of 0.25.
      The factor maps were standardized to a continuous scale of suitability from zero to 100, and weighted equally for input into the MCE module in Idrisi32. The MCE analysis also determines the degree of suitability on a scale of zero to 100 for avalanche initiation sites, where the higher the value, the greater the suitability. The result is a suitability index map - a weighted and summed index of the influence of these factors. The final map was filtered (generalized) to remove isolated pixels by using a 5x5-pixel-size mode filter.

Avalanche Trigger Site Factors
The category weighting for each factor was based upon reported avalanche incidents in Canada between 1984 and 1996. Each category was weighted according to its percentage incidence. In each factor, the sum of the weights equals one. Not all factors are weighted by the same number of avalanche incidents, but rather by the amount of incidents where the factor was reported.

Slope Factor
The primary terrain feature for inducing an avalanche is a steep slope. The majority of recreational accidents in Canada involve dry slab avalanches, which rarely start on slopes less than 25 degrees. The slope angle of the start zone is between 25 degrees and 40 degrees for 83 percent of the 184 recreational accidents that reported their slope angle. One of the reasons many incidents occur between 25 degrees and 40 degrees is because people prefer to ski or ride slopes within this range.

Aspect Factor
The orientation of a slope to the prevailing wind is an important factor in the triggering of avalanches. Lee slopes (away from the prevailing winds) tend to experience rapid accumulations of snow, and can develop dangerous cornices. Windward slopes experience a shallower and more compacted snow cover. The avalanche incident data show a distinct distribution pattern across all slope orientations. Most avalanche incidents occur on northeastern-, eastern-, or southeastern-facing slopes because of the southwesterly, westerly and southerly prevailing winds in western Canada. The resulting snow slabs on these lee slopes tend to be unstable.

Terrain Shape Factor
Avalanches tend to begin on recognizable geo-morphological features, such as convex slopes, planar slopes, or along ridges with cornices. They also begin at changes in slope profile, or at changes in groundcover. The shape of the terrain influences the shape of accumulated snow and where snow pack weaknesses will occur. In the avalanche accident report, 86 incidents recorded specific terrain features in the starting zone. Where more than one terrain feature was involved, i.e., at a small convex slope at the top of a gully, only the predominant feature was noted. The greatest number of avalanches (37 percent) started on convex slopes, while 26 percent started along ridges or peaks, 24 percent started on planar slopes, and 13 percent started in gullies. These percentages are reflected in the form of category weights.

Groundcover Factor
Avalanches are greatly affected by groundcover type because this feature affects the stability of the overlying snow pack. The rougher the ground surface, the more snow depth is required before an avalanche will take place. When the snow pack is thick, avalanches are not influenced by surface roughness. Groundcover becomes an important factor when the snow pack is thin, especially in early winter or in late spring.
       In the avalanche accident report, of the 80 incidents that reported the avalanche start zone with respect to the tree line, 89 percent occurred at or above the tree line, and 11 percent occurred below the tree line (in dense forest). The groundcover is divided into two categories. Forested areas (below the tree line) are given a weight of 0.11 (11 percent of avalanche incidents) and non-forested areas are given a weight of 0.89 (89 percent of avalanche incidents). Non-forested areas include snow cover, exposed rock, clear cuts, scree, forestry roads, and power lines.

Results
A suitability index map for avalanche trigger sites on the North Shore Mountains is displayed in Figure 9, and a perspective view is displayed in Figure 10. The suitability map is grouped into three classes, and the area of each class is given in Table 1. On these maps, the higher the suitability index, the more suitable a site is for an avalanche trigger zone.
Table 1: Area of Suitability Indices
      Areas "unsuitable" for avalanche trigger sites (values 0-9) occur at the base of river valleys and along the beaches of Indian Arm. The "least suitable" areas for avalanche initiation (values 0-28) are restricted to the valleys between the mountains, and on densely forested slopes at lower elevations. "Moderately suitable" areas (values 28-48) tend to occur on slopes of all aspects and on forested slopes between the critical angles of 25 degrees to 40 degrees. The moderately suitable areas cover the most acreage at approximately 190 square kilometers. The "most suitable" areas occur at mountain peaks, along ridges, and on less densely forested slopes and bare slopes, at all aspects. The most suitable class covers 56 square kilometers, or approximately 14 percent of the project area.

Avalanche Trigger Zones in the North Shore Mountains
Some residential areas in northern and western Vancouver are adjacent to "moderately" to "most suitable" avalanche trigger zones. These include Hollyburn Heights in West Vancouver, Forest Hills in North Vancouver and Indian Arm in Deep Cove. The creeks draining into Indian Arm are recognized debris-flow hazards, and there is growing evidence that a strong link exists between avalanche events and debris-flow events in the Coastal Mountains of British Columbia.
      The three ski hills, Cypress, Grouse and Seymour, contain "moderately" to "most suitable" avalanche trigger sites. The "most suitable" areas are at higher elevations and at all aspects. North of the Mount Seymour ski hill, trails in the Mount Seymour Provincial Park travel through "most suitable" avalanche start zones. The Elsay Lake Trail is closed to the public during the winter months because of the avalanche risk.
      There are "most suitable" avalanche trigger zones on the south-facing slopes above Eastcap Creek. Landsat data reveals that these areas are harvested forest blocks. There is evidence that harvested areas can become avalanche trigger zones, even where there is no history of avalanche activity. Regenerated cut blocks and the mature tree stands below them are damaged by avalanches that begin in harvested terrain. These avalanches may also damage logging roads, power lines, and streams.

Conclusions
The mapping of avalanche trigger sites as a function of location - using GIS and satellite imagery - is a powerful tool for the initial assessment of avalanche terrain over mountainous areas. Suitability mapping of avalanche terrain may be used as the following:
• A base map for plotting avalanche incidents, testing the validity of the suitability analysis
• A reconnaissance mapping tool for defining potentially hazardous areas, both in regions with no avalanche frequency data and in alpine areas experiencing increased recreational use
• A component of an avalanche risk analysis for assessing avalanche safety over large geographic areas
• An efficient and understandable way to warn people about the location of avalanche terrain.
      Selective suitability mapping of individual mountains and slopes is possible with high-resolution satellite data and precise topographic data. In a GIS, suitability maps can be updated on a timely basis to show changes in the landscape through forestry practices, erosion, natural deforestation, and urban development. Suitability mapping of avalanche trigger sites offers an efficient way to warn people about the location of avalanche terrain.

About the Author:
Shirley McLaren is a project manager with Triathlon, a mapping and photogrammetry firm located in Richmond, B.C., Canada. She may be contacted via e-mail at: [email protected]

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