GEOMORPHIC CHANGE DETECTION AT THE CHALK CLIFFS NATURAL DEBRIS FLOW LABORATORY USING MULTI-TEMPORAL TERRESTRIAL LASER SCANNING DATA (Invited Presentation)

Debris flows can produce significant changes in the morphology of steepland channels and alter the boundary conditions for future events. High resolution topographic data that quantify changes in channel and hillslope form caused by sequential debris flows in natural channels are rare at the reach scale. At Chalk Cliffs, Colorado, a bedrock-dominated basin where debris flows are triggered by runoff during summer rainstorms, a high rate of debris flow occurrence (1-4 flows per year) permits the evaluation of morphological changes in a debris-flow dominated channel from high-resolution terrestrial laser scanning (TLS) data. We used high resolution (2-cm) digital terrain models from seven TL S surveys between May 2009 and July 2010 to document topographic changes caused by six summer debris-flow events. From these data, we identified changes in channel morphometry and calculated the net volume of material deposited and eroded from the channel and adjacent slopes for each event. Results indicate that debris flows triggered by storms with low cumulative rainfall (< 10mm), lower average intensities (<10mm/hr) and lower 10-minute rainfall intensities (< 20mm/hr) were largely depositional in the surveyed reach of channel, and provided an important source of sediment for larger, more erosive flows. In addition, slope failures adjacent to the channel, which were very difficult to recognize from field observations alone, were identified as significant sources of sediment for several events. Analysis of relations between the magnitude of erosion and deposition yielded no strong correlations with channel slope or roughness suggesting that point-specific predictions of erosion or deposition cannot be adequately derived from statistical analyses of pre-event channel topography alone. Instead, our results indicate that rainfall characteristics control the density and volume of debris flows in basins where flows are initiated by surface water runoff. These flow properties determine locations of deposition and erosion, and, ultimately, total travel distance. Our results suggest that accurate estimates of rainfall characteristics, particularly those measured over short durations, are critical for predictions of runoff-generated debris-flow erosion, deposition, and travel distance.