FIELD OBSERVATIONS OF DEBRIS FLOWS AT CHALK CLIFFS, COLORADO: PART 1, MONITORING STATION DATA FROM A 2 JUNE 2009 EVENT

Chalk Cliffs, a band of hydrothermally altered quartz monzonite in the Rocky Mountains SW of Buena Vista, Colorado, has one of the highest frequencies of debris flows in the state. These debris flows, like many in the region, are generated by surface-water runoff over loose channel material. The relatively high frequency of debris flows at Chalk Cliffs makes it an ideal site to study debris-flow mechanics in a natural setting. We began an intensive monitoring program in a 0.3 km² basin at Chalk Cliffs in 2008. The current monitoring effort combines repeat high-resolution measurements of topography with high-frequency measurements of hydrologic and debris-flow parameters at a series of seven stations in the basin. The topographic measurements consist of 2-cm-resolution terrestrial LiDAR scans of the debris flow channel made before and after each event. Measurements at monitoring stations within the surveyed reaches include precipitation, flow stage, bed pore pressure, bed normal stress, and video and still photography. Additional measurements of rainfall and soil moisture are made on hillslopes within the basin. The combined data sets provide a near-comprehensive depiction of the initial conditions, overall basin response, final morphometry, timing, and flow dynamics of natural debris flows.

Here we present station data for one of three debris-flow events documented by the current monitoring program. On the afternoon of 2 June 2009, debris flows occurred in two channels at the head of the basin. The flow in each channel was recorded separately by individual stations. The flows were triggered by a relatively modest rainfall of 10 mm over 3 hours and resulted in substantial deposition that was documented by LiDAR surveys one day after the event. Monitoring data and video show multiple surges with dry, coarse-grained granular debris-flow fronts, watery tails, 1.3 m/s front velocities, and excess pore-fluid pressures. Continued debris-flow monitoring at Chalk Cliffs should provide detailed documentation of the wide spectrum of responses that have been observed previously at the site. This documentation will help develop and constrain new modeling approaches for debris flows generated by surface-water runoff.