APPENDIX J Hazards
Total Page:16
File Type:pdf, Size:1020Kb
APPENDIX J Hazards APPENDIX J-1 Alpine Sierra Avalanche Report AVALANCHE HAZARD STUDY Alpine Sierra Development Placer County, CA July, 2013 Prepared for: Chris Nelson Alpine Sierra Property Owner Prepared by: Larry Heywood Avalanche Specialist Homewood, CA Introduction This report discusses the results of the site specific Avalanche Hazard Assessment I conducted on the proposed Alpine Sierra subdivision development. This work was conducted at the request of the property owner Chris Nelson. The assessment and this report focus on the areas within the proposed development identified being located in Potential Avalanche Hazard Areas (PAHAs) in the Placer County Avalanche Ordinance and Maps. The County Ordinance defines PAHAs as areas subject to potential avalanche danger with an occurrence probability of greater than one in one hundred (100) per year. These three areas are 1) the intersection of the property entrance and the Alpine Meadows Road, 2) small rock outcroppings in the center of the property, and 3) a forested area in the southeast corner of the property. The intent of the Assessment and Report are to more accurately define the boundaries of these PAHAs than is provided in the County Avalanche Map. The results of the assessment for these areas are discussed below. The Assessment included both widely accepted practices in avalanche theory and a site specific study of the property. The field survey methods used in the Assessment included terrain analysis, forest cover analysis and vegetation analysis. The Assessment also included a review and analysis of topographic maps and aerial photographs of the area, a review of historical avalanche events in the area, a consideration of the effects of local weather and climate and the author’s extensive personal experience in studying and observing avalanches in the area. Placer County Avalanche Regulatory History Beginning in the early 1980s Placer County commissioned avalanche studies to define the avalanche hazards within the County and to map these hazards. In 1982 Norm Wilson conducted an evaluation, Avalanche Hazard Study, Placer County, Fall 1982. This Study included all areas of the County with potential avalanche terrain with the exception of Squaw Valley. The Study provided both background and discussion on avalanche hazards within the County and mapped areas subject to avalanche hazard. The avalanche hazard zones delineated on the Wilson map used the following criteria: o Red (high hazard) Zones: Areas where avalanche that could damage standard wood-frame structures and/or bury automobiles are expected to occur with a probability of one chance in twenty per year. o Blue (moderate hazard) Zones: Areas where avalanche that could damage standard wood-frame structures and/or bury automobiles are expected to occur with a probability of less than one chance in twenty per year, but more than one chance in one hundred per year. o Yellow (low hazard) Zones: Areas where avalanche that could damage standard wood-frame structures and/or bury automobiles are expected to occur with a probability of less than one chance in one hundred per year. o White (no hazard) Zones: Areas where, barring cataclysmic or unprecedented events, avalanche will not occur. The mapping and mapping criteria from this Wilson Avalanche Hazard Study became the basis for the county wide PAHA maps. 1 In 1987 Placer County commissioned Art Mears to do a second countywide avalanche evaluation. In this evaluation, Snow-Avalanche Hazard Analysis and Engineering Guidelines, County of Placer, California, Mears provides a discussion of methods used for determining design avalanche conditions and also avalanche mitigation design criteria. He also provided avalanche hazard maps of the entire county. The Mear’s maps used a two hazard zone classification with the following criteria: o High Hazard (Red) Zone: The reference pressure exceeds 600 lbs/ft2 on a large, flat, rigid surface normal to the flow direction. o Moderate Hazard (Blue) Zone: The reference pressure is less than 600 lbs/ft2 on a large, flat, rigid surface normal to the flow direction. The Mear’s Hazard Maps and criteria were not incorporated into the County PAHA Maps or the Avalanche Ordinance. The Placer County Code in Chapter 12 Article 12.40 Avalanche Management Areas defines and requires the following (partial excerpts) 12.40.010 Purpose. A. The potential avalanche hazard area, hereinafter referred to as PAHA, is intended to identify those areas where, after investigation and study, the county finds that an avalanche potential exists because of steepness of slope, exposure, snow pack composition, wind, temperature, rate of snowfall, and other interacting factors. B. Such PAHA is established: 1. To identify those areas with avalanche potential based on approved studies that designate a minimum probability of occurrence greater than 1 in one hundred (100) per year, or where avalanche damage is documented after adoption of the ordinance codified in this article. 2. To give notice to the public of such areas. 3. To minimize health and safety hazards, disruption of commerce, and extraordinary public expenditures. 4. To promote the general public health, safety, and welfare. 12.40.020 Scope A. These regulations do not apply to existing structures or existing parcels in the PAHA zones except as noted below. Nothing in these regulations is intended to preclude the development of any parcel, provided proper design and construction safeguards are taken. B. These regulations shall apply to the following activities: 1. Conditional use permits or their modifications. 2. General plan amendments. 3. Rezoning applications. 4. Parcel splits and subdivisions. 12.40.030 PAHA boundaries identified. A. The PAHA boundaries shall be those specifically identified areas within the county subject to potential avalanche danger, as shown on any study or mapping commissioned by the board of supervisors and maintained by the Placer County planning department to include any amendments authorized through the 2 appeals process of the county, which designates an occurrence probability greater than one in one (100) per year, or documents avalanche damage after that adoption of this ordinance. B. Applications for amendments to the PAHA boundaries shall be submitted to the Placer County planning department and processed in the same manner as administrative review permits pursuant to Chapter 30 of the Placer County Code. Approval of boundary amendments will be in a form that may be recorded by the owner. C WARNING. Avalanches occur naturally, suddenly, and unpredictably. The PAHAs designated are based on the limited scientific evidence available which is not definitive in nature. The studies and reports do not represent or imply that compliance with the provisions of this article will protect from avalanche danger, nor does it represent or imply that areas outside the PAHA are free from avalanche danger. 12.40.060 Construction requirements. A. Within any areas designated as a potential avalanche hazard area, a building permit for construction regulated by Section 12.40.020 (Scope), will not be issued for a new building or an addition unless a California licensed architect or engineer experienced in snow design, in conjunction with a recognized avalanche expert or team of experts, certifies that the structure will be safe under the anticipated loads and conditions of an avalanche. B. The avalanche expert(s) must utilize both widely accepted practices in avalanche theory and a site specific study for the proposed construction. Any study commissioned by the board of supervisors to identify potential avalanche forces must be considered in the site specific study. C. An expert or team of experts shall mean individuals with existing demonstrable recognition as “experts” among the community of avalanche practitioners. It shall be the responsibility of the project architect/engineer to demonstrate this recognition. 12.40.040 Notice Requirements A. Duty of Owners. Each owner of real property containing any structure or building within a PAHA area shall post the following notice at a prominent location within the main winter entry area of such structure or building. Such notice may be augmented by the owner to include whether the property is in a high or moderate hazard zone, as long as this information is consistent with the official records retained by the planning department. Avalanche Path and Runout Delineation The Placer County Potential Avalanche Hazard Map (PAHA) identifies a number of avalanche paths within the study area. This Map was developed from an Avalanche Hazard Study conducted by Norm Wilson in the early 1980s. The Wilson study did not take into consideration site specific terrain characteristics. Of primary concern in land use planning is a determination of the maximum size avalanche that individual paths can produce (the design avalanche) and the return period of the design avalanche for individual avalanche paths. The Placer County Avalanche Ordinance requires that areas with an avalanche occurrence of greater than one in one hundred (100) be identified. Identifying these areas is the focus of this Study and the accompanying PAHA Map. This Study employed a number of widely accepted methods to identify these areas and to 3 predict the return probabilities. It is widely accepted that the best methods of determining runout distances and return period of avalanche occurrence is (1) long-term observations of avalanche deposits; (2) observations of damage to vegetation, ground or structures; or (3) searches of the historical record. Below is a short discussion of the methods used: Terrain and Forest Cover Analysis Snow slab avalanches originate on a wide variety of terrain. The main requirement is steepness. Dangerous slab avalanches are most likely to start on slopes in the 30 to 45 degree range. Most large long return period avalanches begin in the steepness range of 30 to 40 degrees. For slopes of less than 30 degrees, shear stress on the bed surface is not enough to cause shear failure and avalanching. For slopes steeper than 45 degrees sluffing routinely prevents slabs from forming.