Following a week of daily low intensity rainstorms, a heavy rainstorm that dumped approximately 3 inches of rain between 6:30 and 7:30 pm on July 21, 2007 mobilized several debris flows (“mudslides”) in Weldon Gulch that impacted the town of Alpine, CO. Several structures, roads, and utilities were damaged and two nearby county roads had to be closed. The town and upstream communities were subsequently evacuated. Although no injuries or loss of life resulted, approximately 65 people are directly affected by the debris flow and subsequent cleanup and mitigation activities will take time.
Following the event, CGS mobilized geologists to the site on July 24-25 to assist in determining what happened during this event. CGS geologist Karen Berry met with emergency management officials and county representatives to investigate the damage to the town infrastructure. As a separate investigation, CGS geologist TC Wait worked with a team of landslide experts from the USGS to do reconnaissance work to identify and characterize the geologic characteristics of the debris flows.
Sediment and debris inundates a home from the Alpine debris flow. Photo by TC Wait
The town of Alpine is located on a debris fan at the base of Weldon Gulch along Chalk Creek west of Nathrop. The fast moving debris flows initiated on the steep slopes of Weldon Gulch below treeline and followed the topography down the gulch and across the fan. Most of the fan was well vegetated, indicating that a large debris flow has not impacted the area in some time. Historic cemetery markers dating back to the late 1800s indicate that the cemetery area had not been impacted within at least the past 130 years, but was buried by new debris less than a foot deep during this event.
No one was hurt during this event and Alpine residents manage to retain their unique sense of humor. Photo by TC Wait.
4/12/2011 3:12 PM
Aerial view of Weldon Gulch and the debris fan where Alpine is located. Debris flow scars are visible along the left side (east facing slope) of Weldon Gulch below treeline. Photo by TC Wait
The alpine debris debris flows tore through this garage removing all the contents inside (including a jeep) and knocking a nearby 500-gallon propane tank from its foundation. Photo by TC Wait.
The debris flow closes CR292 east of the Iron City Campground. Photo by TC Wait.
Although this event may be the first time the town of Alpine was affected by a debris flow, it is not a geologically uncommon event in this area. The fan that the town is located on is composed almost entirely of old debris flow material and the debris flow channels above town appear to run frequently – just not making it all the way down to the town. The densely treed areas on the higher slopes likely help stop some of the flows from making it to the fan. Other areas in the Collegiate Peaks with similar geology and topography (Cottonwood Pass, Chalk Cliffs, etc.) have resulted in debris flows following localized heavy rain over recent years. The debris flow hazard remains for Alpine, although the frequency of this sort of event is not known.