Exploring Hydrologic Controls on Patterns of Debris Flow Initiation in the Colorado Front Range during the September 2013 Flood Event

At least 1,100 debris flows occurred throughout the Colorado Front Range in response to 5-days of extreme rainfall in September 2013. Among these debris flows, about 78% were initiated on south-facing slopes (SFS). In the Front Range, SFS have sparser forest cover than north-facing slopes (NFS), which may correspond with the lower root density and the effective soil strength. However, in-situ soil moisture observations suggest that soil saturation occurred more commonly on SFS during the storm, which would promote slope instability and subsequently, debris flows. This soil moisture pattern contrasts with the pattern observed under dry conditions, which exhibits wetter locations on NFS. The objective of this study is to assess the hydrologic mechanisms that produced saturated conditions on the SFS slopes and the resulting debris flows. We used the Equilibrium Moisture from Topography, Vegetation, and Soil (EMT+VS) model to simulate fine-resolution soil moisture patterns (10 m grid cells) for the flood period across seven Front Range watersheds (Cache la Poudre, Big Thompson, Little Thompson, Saint Vrain, Left Hand, Fourmile and North Boulder). Five hypotheses were tested with the model. Specifically, we ask if saturation on SFS was produced by: (1) higher rainfall rates, (2) lower interception rates, (3) lower porosity, (4) thinner soils or (5) reduced percolation to groundwater. The hypotheses were constrained by available data from the Front Range, and they were tested by comparing the generated soil moisture patterns to soil moisture observations and the debris flow initiation sites. Preliminary results suggest that some hypotheses likely had little effect on the saturated pattern, while others likely played a major role. Results of this analysis suggest that the EMT+VS model may be able to predict soil moisture patterns that facilitate mass movements.