UNSTABLE SLOPE MANAGEMENT WHERE ROCKFALL IS FREQUENT, TIMPANOGOS CAVE NATIONAL MONUMENT

Timpanogos Cave National Monument (TICA) is a 0.5 square mile National Park Service (NPS) administered National Monument that lies near the mouth of American Fork Canyon in the Wasatch Mountain Range of north-central Utah. The park attracts over 100,000 visitors per operating season with its natural beauty and spectacular cave system. Access to the cave complex is a paved hiking trail located on the south wall of the canyon that gains 334 m. in elevation over 2.9 km. The Wasatch Range is steep and rugged due to intense regional uplift and close proximity to the Wasatch Fault. As a result of this setting, rockfall and other slope erosion processes are especially prevalent.

Daily minor rockfall is ubiquitous throughout the American Fork Canyon and significant rockfall and other slope movement occurs during seasonal storms in the mid-late summer. In 2013 flash flooding and debris flows trapped visitors and staff in the park for hours. Rockfalls have caused accidents and near misses at TICA since the park’s inception including a fatality in 1933. Incidents like these have prompted a culture of rockfall awareness and visitor education and the installation of rockfall shelters at the cave entrance and exit.

In order to quantify the hazard and risk posed to visitors and NPS employees, the Unstable Slope Management Program for Federal Land Management Agencies (USMP) was adopted to identify, categorize, and rate slopes along the cave trail and other areas in TICA. This program was adapted to suit the unique challenges of the Timpanogos Cave trail. Using the USMP rating criteria, 160 slopes were rated including 142 slopes along the cave trail and 18 slopes adjacent to buildings and picnic areas. Rockfall was identified as the primary slope hazard for 125 of the 142 cave trail slopes and all of the other slopes rated.

Results for the rated slopes show a wide range of total USMP scores. Known problem and “safer” areas predictably had higher and lower scores, respectively. These scores allow the possibility of hazard and resilience planning even along a corridor where slope erosion processes are pervasive.

This categorization and quantification of the geologic hazard potential at TICA allows for future mitigation, land use, and safety plans by helping to set priority areas and producing a frame work for future slope stability data collection.