| Table of Contents | Appendix A | Appendix B | Appendix C | Appendix D | Appendix E | Appendix F | Appendix G |
| Appendix H | Appendix I | Appendix J | Appendix K | Appendix L | Appendix M | Appendix N |

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Appendix C — Yosemite Valley Geologic Hazard Guidelines

Background

Rockfalls and other associated forms of mass movement, such as rockslides, debris flows, and rock avalanches, are natural processes that continue to shape Yosemite Valley. During historical time (1850-present), more than 400 rockfalls or other forms of mass movement have been documented. Several people have been killed by such geologic hazards and many others injured. Many trails, roads, and buildings have also been destroyed or seriously damaged by such processes.

For land-use planning in Yosemite Valley, the U.S. Geological Survey and the National Park Service have cooperated to document potential geologic hazards, primarily rockfalls, debris flows, and rock avalanches (hereafter referred to as rockfall) in the Valley. The documentation consisted of a review of archival records, aerial photographic interpretation, and field mapping (USGS 1992). Most recently the National Park Service requested the U.S. Geological Survey to conduct additional field work and to assess the previous information in order to produce a report on the rockfall potential within the Valley (USGS 1998). In the report, there were two areas of potential rockfall identified. The first area lying closest to the Valley walls was identified as the Talus Slope Zone, where the majority of materials are deposited during a mass movement event. The second area identified was the Rock Fall Shadow Line Zone which extends out from the Talus Slope zone and is the area in which individual rocks may travel out from the Talus. These zones indicate the closer one approaches the Valley walls, the greater the potential for damage by a mass movement event.

The frequency and magnitude of rockfall within the Valley can and does vary considerably. Singular events involving stones or rocks less than one cubic meter occur on a fairly regular basis depending upon weather conditions, freeze/thaw conditions, moisture conditions, and rock composition/condition. Events of greater magnitude up to 100,000 cubic meters may occur on an interval of over ten years (USGS 1998). The location of these movements can also be fairly random within the Valley.

It is not possible to avoid all rockfall related risks in a narrow valley like Yosemite Valley. This means that some facilities located in the Valley will be exposed to risk of damage by rockfall. The National Park Service is currently revising its Management Policies pertaining to geologic resources and hazards. Excerpts from the most recent Draft Management Policies, January, 2000 state that:

• The National Park Service will work to protect park visitors, staff, and infrastructure from geologic hazards.

• The National Park Service will allow natural geologic processes to proceed unimpeded. Geologic processes will be addressed during planning and other management activities in an effort to reduce hazards that can threaten the safety of park visitors and staff and the long-term viability of park infrastructure.

• Park managers will work closely with specialists at the U.S. Geological Survey and elsewhere, and with local, state, and federal disaster management officials, to devise effective geologic hazard identification and management strategies. Although the magnitude and timing of future geologic hazards are difficult to forecast, park management will strive to understand future hazards and, once understood, minimize their potential impact on visitors, staff, and developed areas. The National Park Service will work to avoid placing new facilities in geologically hazardous areas. Managers will examine the feasibility of phasing out, relocating, or providing alternative facilities for park developments subject to hazardous processes.

• The National Park Service will strive to avoid locating new facilities in areas where they may be damaged or destroyed by natural geologic and hydrologic processes, unless no practicable alternative exists and safety and hazard probability factors have been considered.

Using this management guidance, the following guidelines were developed for new and existing structures within Yosemite Valley.

Guidelines

The 1916 Organic Act requires the National Park Service to provide for public enjoyment of the parks while conserving the scenery, natural and historic objects, and wildlife of parks in a manner that will leave them unimpaired for the enjoyment of future generations. Balancing these policies requires National Park Service managers to exercise judgment and discretion, particularly when making decisions about visitor safety and protection. Therefore, Yosemite National Park’s decisions about locating and relocating facilities are an exercise of discretion. Each decision will be based on the park’s balancing of the policy of promoting public enjoyment including minimizing safety hazards, with other policies such as: minimizing human intrusion on natural and historic resources and wildlife; conserving cultural resources, scenery, aesthetics, and visitors’ natural park experiences; minimizing environmental impact; and operating within limits of available financial and human resources.

In evaluating the uses of existing facilities within the Valley, the National Park Service should first determine the historical significance of each facility and determine its Occupancy Category (figure C-1).

Existing Facilities

1. The National Park Service should work to remove structures or uses in the Essential and Hazardous categories from the Talus Slope and Shadow Line zones, unless no practicable alternative exists and if safety and hazard probability factors have been considered. If historic structures are identified as Essential or Hazardous Occupancy categories, or if the National Park Service determines there are other policy reasons for leaving the structures, the structure may remain if contingency planning is completed to provide for the function in the event a rockfall or other geologic incident occurs.
2. The National Park Service should evaluate structures and uses in the Special Occupancy category in the Talus Slope zone. Such evaluations should include safety and hazard considerations and other policies. If the evaluation shows there are policy reasons to retain these structures and/or uses in their existing locations, they may remain at the discretion of the National Park Service.

3. The National Park Service should evaluate safety and hazard considerations and other policies relevant to structures and uses in the Standard and Miscellaneous Occupancy categories. If the evaluation indicates there are policy reasons to retain these structures and/or uses in their existing locations, the structures may remain at the discretionof the National Park Service.

In evaluating the placement and uses of new facilities within the Valley, the National Park Service should first determine those facilities’ Occupancy Category (figure C-1).

New Facilities

1. The National Park Service should place structures in the Standard Occupancy category outside the Talus Slope zone, unless no practicable alternative exists and all safety and hazard probability factors have been considered.

2. Miscellaneous structures may be placed in any area if there is no practicable alternative. Before locating new miscellaneous structures in the Talus Slope zone, the National Park Service should conduct a site-specific review that includes safety and hazard considerations.

Figure C-1
Occupancy Categories for Yosemite National Park Facilities

Essential Facilities

Fire station (National Park Service & concessioner)
Visitor protection/search and rescue
Medical clinic
Jail
Court
Communications center
High voltage

Hazardous Facilities

Fuel storage

Assembly facilities (occupancy load of greater than or equal to 300)

Eating and drinking establishments
Auditoriums
Visitor center

Education facilities (occupancy load of greater than or equal to 300)

Assembly facilities (occupancy load of less than 300)

Eating and drinking establishments
Auditoriums
Visitor contact stations

Education facilities (occupancy load of less than 300)
Offices
Post office
Retail sales
Maintenance facilities
Hotels
Dormitories
Multi-family
Single-family housing

Miscellaneous Structures

| Table of Contents | Appendix A | Appendix B | Appendix C | Appendix D | Appendix E | Appendix F | Appendix G |
| Appendix H | Appendix I | Appendix J | Appendix K | Appendix L | Appendix M | Appendix N |