Assessing Rock Wall Permafrost Changes in Colorado’s Front Range and Their Implications for Rock Fall

Abstract:

Only a handful of investigations have tried to map the extent and changes of alpine permafrost in the United States, and these have mostly focused on measurements from alpine meadows and simply extrapolated those ground temperatures to the higher altitude rock walls. To our knowledge, bedrock temperatures in alpine regions are not being systematically monitored anywhere in the United States. However, observations from around the world have shown how degrading permafrost in steep bedrock can increase the risk of rock fall – something that is critical for risk assessments around alpine reservoirs, infrastructure, or popular recreation spots. Here, we present initial results from a rock temperature network that we established in 2019 around the Niwot Ridge Long Term Ecological Research site near Boulder, Colorado. We placed 20 temperature sensors at 10 different sites with various aspects and elevations ranging between 3500 m and 4000 m above sea level. We assess rock thermal conductivity by combining measurements from sensor at ~5 cm and ~10 cm below the surface. In two locations, we instrumented blocks in talus fields, with one sensor measuring rock temperatures and one measuring air temperature. We also re-instrumented a borehole installed in the early 1970s. We use these data to calculate active layer thickness, assess diurnal and seasonal temperature variabilities, validate ground surface temperatures modeled from local air temperature measurements, and compare our results with global and regional permafrost models.