Geologic Publications for Mount Rainier

Geologic hazard and floodplain management: Mount Rainier General Management Plan

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Author(s): Jon L. Riedel

Category: PUBLICATION
Document Type: Unpublished Internal Document
Publisher: National Park Service
Published Year: 1997
Volume:
Number:
Pages: 99
DOI Identifier:
ISBN Identifier:
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Abstract:
Mount Rainier National Park (MORA) is in the process of developing a 20-year General Management Plan (GMP) in time for its 100th anniversary celebration in 1999. This report was prepared to assist planners in managing geologic hazards and floodplains in front country development and visitor use sites. NPS management policy in regard to geologic hazards focuses on saving human life, and avoiding hazard if possible. Further, where facilities must be located in hazard areas, design and siting should include mitigating measures to minimize risk to life and human property.

At MORA, however, most developed areas are in mapped volcanic hazard zones. The designation of Mount Rainier as a Decade Volcano Study Area by the National Research Council in 1994 underscores the seriousness of the volcanic hazards at Mount Rainier. Due to NPS management policy and the considerable hazards at Mount Rainier, a geologic hazard mitigation approach is presented that avoids unrealistic closer of large areas of the park. On a short time scale of 0-5 years, this approach emphasizes education and contingency planning for response to hazards as means of mitigating volcanic hazards at the park. GMP hazard mitigation is focused on longer time scales. The recommended approach is that no new housing, administrative facilities, concessions or overnight visitor facilities be constructed in high hazard zones.

A risk analysis of 23 visitor and administrative sites was conducted to identify the most hazardous and risky sites in the park. This analysis considered hazard, value and vulnerability at each of these sites. Components of hazard in the risk formula included both deterministic and probabilistic factors, while emphasizing the hazard presented by debris flows. Results indicate that White River Campground, Longmire and Cougar Rock Campground are the three sites at highest risk in the park by a large margin. It is recommended that hazard mitigation in the GMP focus on these three areas.

White River Campground is by far the most hazardous and risky site at MORA with a hazard score two times greater than that of the next most hazardous site, Camp Schurman. High hazard score at White River Campground is due to the site's proximity to the volcano, location below fractured, hydrothermally weakened rocks on Little Tahoma Peak, and position next to the floodplain of White River.

Nonvolcanic geologic hazards are also a concern at MORA. Hazards such as rock falls, snow avalanches and landslides occur at sites scattered throughout the park.

The risk analysis and field studies also showed that the portion of Tahoma Woods north of highway 706 is an appropriate place for future developments. Trenches dug in spring 1995 indicate that this site has not been inundated by a debris flow in the past 10,000 years. Further, Tahoma Woods is outside case II and case III debris flow inundation zones, which have the most frequent recurrence intervals.

Floodplain management at MORA floows the NPS Floodplain Management Guideline (1993). Ten of 23 developed sites, which are primarily day use areas and entrances, are actions that are excepted from compliance with the guideline. Preliminary floodplain assessments at 13 other sites indicate that only three sites are within regulatory floodplains. Detailed floodplain studies were conducted at Longmire, Carbon Entrance and Ipsut Campground to provide information that will allow these sites to be in compliance with the guideline. Walk-in sites at Ipsut and Loop-C of Ohanapecosh campgrounds are recommended for temporary seasonal closure during periods of high river flow in spring and early winter.

Floodplains at MORA are as dynamic as at any NPS area, due to the movement of vast amounts of water and glacial sediment carried by the large rivers down the steep slopes of the volcano. It is estimated that because of rapid rates of deposition and erosion, typical floodplain mapping techniques would be inaccurate in as little as 10 years after completion. Therefore, it is recommended that floodplain boundaries be drawn conservatively, without the use of expensive hydraulic modeling techniques. Further, stream gaging stations place on the large rivers of the park would provide important information to managers on rates of stream channel deposition and channel instability.

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Suggested Citations:
In Text Citation:
Riedel (1997) or (Riedel, 1997)

References Citation:
Riedel, J.L., 1997, Geologic hazard and floodplain management: Mount Rainier General Management Plan: Unpublished Internal Document, National Park Service, 99 p..