FIELD OBSERVATIONS OF DEBRIS FLOWS AT CHALK CLIFFS, COLORADO: PART 2, CHANGES IN CHANNEL MORPHOMETRY OBTAINED FROM TERRESTRIAL LASER SCANNING

Debris flows can produce significant changes in the geometry of steepland channels and subsequently alter the boundary conditions of future events. Topographic data that quantify changes in bed geometry caused by sequential debris flows in natural channels are rare. At Chalk Cliffs, CO, a short debris flow recurrence interval permits the evaluation of geometric changes in a debris flow dominated channel using high-resolution terrestrial laser scanning (TLS) methods. Here, we present the results of two TLS surveys: 28 May – 1 June 2009 that documented the pre-event channel configuration, and 4 June 2009 that documented the changes in channel form in response to a debris flow that occurred between surveys.

On 2 June 2009 a debris flow was initiated from 10 mm of rainfall over a period of 3 hours. The debris flow event was largely depositional, with 60 percent of the 157 meter surveyed channel length experiencing net aggradation. A total of 58m³ of material was deposited over the surveyed channel length, with a maximum thickness of 110 cm. Debris flow features, such as lobes and levees were easily identified from the TLS survey data. Lobes occupied 41 percent of the channel and represented 76 percent of the total volume of material deposited during the event. Lobes had frontal slopes of varying gradient, yet were always steeper than the finer-grain material they impounded. Debris flow levees occupied 42 percent of the channel length, and contained 24 percent of the deposited material. Scouring of channel material was minor and localized, with a total volume of 8m³ removed and maximum depths of 70 cm spatially coincident with an area where material was transported into the channel from a hillslope failure not evident during field reconnaissance.

Preliminary analyses of changes in channel morphology suggest that aggradation cannot be adequately predicted from pre-event channel slope or topographic roughness alone. Our findings highlight the importance of having both high temporal resolution monitoring data that capture debris flow dynamics, and high spatial resolution topographic data for process-form modeling. The unique combination of both datasets at Chalk Cliffs presents an opportunity to make more defensible links between debris flow process and observed form in natural channels.