Case Study: Avalanche – Silver Plume

By Jill Carlson

On March 23, 2003, a large avalanche occurred about one mile west of the Town of Silver Plume. The avalanche brought trees, rock, soil and snow to the valley floor, knocked down overhead utility lines, blocked the I-70 frontage road, damaged the town’s water treatment plant (WTP), and dammed Clear Creek. The dam was breached using explosives before the plant’s electric pump motors were flooded. With damage to the WTP’s chlorine contact tank and building, Silver Plume residents had to boil their tap water for over a month.

The avalanche occurred three days after near-record snowfall. It was triggered by additional snow loading in the starting zone caused by a change in wind direction, and began in a known avalanche path above timberline on Pendleton Mountain.

Its unusually large volume and velocity caused it to unexpectedly reach the valley floor, along a path not previously identified as an avalanche chute. Rick Gaubatz, the Town’s water commissioner, counted 110 rings in a spruce tree that was found in the avalanche debris at the damaged WTP, indicating that an avalanche of similar magnitude had not occurred in the immediate area in at least 110 years.

Snow and debris ran up the abutment of eastbound I-70, but did not reach I-70 traffic lanes. However, a very strong air blast would have been felt by anyone passing by on I-70. Fortunately, the avalanche occurred at about 2:30 a.m., and no one was injured. The existing WTP is now known to be located in an avalanche hazard area.

The Town of Silver Plume is planning to move the WTP, considered a critical facility, to a new location. The proposed site is approximately 250 feet west of the existing site, below an old tailings pile from the Johnny Bull Mine.

The Colorado Geological Survey, at the Town of Silver Plume’s request, provided post-avalanche assistance and completed a report for the town concerning geological hazards resulting from this avalanche. We evaluated three new or newly identified geological hazards, including avalanche, debris flow (often referred to as mudslide), and rockfall, at four critical locations: the existing water treatment plant, the proposed WTP relocation site, the I-70 frontage road, and I-70. A fourth natural hazard, in-stream flooding, could occur as a consequence of future avalanches or debris flows.

Avalanche Hazard

Since a path has been cleared to the valley floor, future, smaller avalanches are more likely to reach the frontage road, the existing WTP, and I-70. Our avalanche modeling results indicate that very high velocities and impact pressures should be expected in this avalanche path. The velocities and impact pressures are probably too high for structural protection methods at the existing WTP; it is more cost-effective to relocate the plant outside of the known avalanche hazard area. For highway protection, CDOT is considering explosive control to reduce the risk of large avalanches and avalanche related accidents along this stretch of I-70.

The proposed WTP site is better protected from avalanche damage than the existing WTP site because of a resistant rock outcrop, located about 750 feet directly uphill from the proposed site, which diverts avalanche flows to the east. The limited slope length between the outcrop and the proposed WTP site reduces the ability of an avalanche starting within this segment to attain high speeds and therefore high impact pressures.

Debris Flow Hazard

As a result of the avalanche, the slope above the existing WTP was denuded of thick stands of mature vegetation, with many trees and shrubs uprooted. The soil has been destabilized by the removal of much of its root structure. A heavy rain in the next few years, before vegetation firmly re-establishes itself, is likely to saturate the soil immediately above the soil-rock contact, and may cause the soil mass to lose strength and to slump, slide, or fail catastrophically. We strongly recommended that personnel should avoid the area below the denuded March 23 avalanche path during and for several days after a heavy rainfall or snowmelt. The town and CDOT will need to have heavy earthmoving equipment, and a disposal site, available to clear the debris and reopen the frontage road and possibly I-70. It is possible that Clear Creek could be dammed by a debris flow as it was by the avalanche; this would add significant urgency to the Town’s earthmoving effort in order to reduce the risk of water damage either to the existing or relocated WTP.

Debris flow risk is considered to be low and not to pose a significant threat to personnel and structures at the proposed WTP relocation site for several reasons. The slope above the proposed WTP site is not located within the March 23 avalanche path; therefore, it was not stripped of its vegetative cover. The existing trees, shrubs and roots serve to anchor the thin veneer of topsoil in place. As with avalanche hazard, the slope height above the site is limited to several hundred feet, so the volume of potential debris that could be mobilized is much smaller than that above the existing WTP. The slightly convex shape of the slope above the proposed WTP means that flows would not be concentrated toward the WTP.

Rockfall Hazard

The talus (boulder) fields on the slopes below Pendleton Mountain indicate that rockfall is an ongoing process in this area. As indicated by the boulders that have been raveling off of the slope since the avalanche, the rate of rockfall has increased below the path of the 2003 avalanche as a result of surface disturbance and loss of vegetation. The rockfall hazard has increased along the frontage road and at the existing WTP; however, this increase is temporary. Considering the low volume of traffic on the frontage road, the absence of a well-defined source area, and the low occurrence rate of rockfall, rockfall mitigation to protect the frontage road and existing WTP should not be considered a high priority.

Moving the WTP, at first glance, worsens the rockfall problem. A large outcrop lies from approximately 550 to 750 feet above the proposed WTP site, and represents a significant potential rockfall source area that could cause damage to the relocated WTP. We modeled the rockfall hazard at the proposed WTP site using the Colorado Rockfall Simulation Program. Our modeling results indicate that most blocks that spall off of the outcrop become trapped in the large talus field immediately below the outcrop. This is consistent with field observations. Buildings associated with the hundred-year-old Johnny Bull Mine, located immediately above the proposed WTP relocation site, do not show evidence of damage by rockfall; also we did not see any boulders on the ground in this area. The rockfall hazard at the proposed WTP site is considered to be low.

Summary

The avalanche that occurred on March 23, 2003 and damaged the Town of Silver Plume’s water treatment plant produced several consequences that were not immediately apparent:

• In addition to the newly identified avalanche hazard, several geological hazards were created or exacerbated by the avalanche, including debris flow, rockfall, and flooding hazards.
• Avalanche, debris flow, and rockfall hazards can be mitigated with structural protection such as deflection berms, catchments, and fences; but, depending on slopes and space constraints, these can be much more expensive solutions than controlled releases, warning systems, and relocation of facilities.
• After evaluating the rockfall hazard at the proposed water treatment plant relocation site, we feel that moving the WTP to a location outside of the March 23, 2003, avalanche path is a good example of “mitigation by avoidance.”

CGS’ assistance on this project benefited the Town of Silver Plume and CDOT by providing information that will allow needed activities to be carried out safely, and provides a basis for their planning and continued use of the area. Funding was provided by the Colorado Office of Emergency Management, CDOT, and CGS.