COMPLEX RESPONSE OF CHANNEL MORPHOMETRY TO MULTIPLE DEBRIS FLOWS DURING THE 2009 FIELD SEASON AT CHALK CLIFFS, CO

Debris flows are a common event in many headwater streams and geomorphologically important to sediment transfer and channel changes within drainage basins. However, it is difficult to capture topographic changes resulting from multiple surges within a debris flow as well as between several debris flows. A tributary stream to Chalk Creek, CO is investigated in the current study. Over the last decade this stream has experienced one to four debris flow events annually. Four field sampling campaigns were conducted in the summer and fall of 2009. A Leica ScanStation 2, in conjunction with a robust local control network, were used to capture channel dimensions and hillslope topography along five stream reaches prior to the debris flow season and after three debris flows. Point cloud data from the scanner permit the generation of high-resolution digital terrain models (DTM). DTM-of-difference analyses and measures of slope, roughness, and channel dimensions were employed to detect changes in channel morphometry and calculate the net volume of material deposited and eroded from each channel reach. The first debris flow occurred on unsaturated bed material and resulted in aggradation along 3 of the 5 reaches. One reach, a bedrock step, remained relatively unchanged, while the final reach saw significant erosion along boulder steps in the channel and an associated mass failure adjacent to the stream bank through this section. The second debris flow resulted in net aggradation along all of the reaches. The third and largest debris saturated bed materials and the flow produced net erosion along all reaches. Significant channel changes were associated with the headward erosion of debris flow snouts and bank failures associated with undercutting of angle-of-repose slopes during debris flow erosion. Analysis of the potential relationships between the magnitude of erosion and deposition magnitudes yielded no strong correlations with channel morphometry measures. This would indicate channel point- and reach-scale cannot be used to adequately predict erosion and deposition. Instead, a combination of high-resolution topographic changes and process information from in-situ measurements of flow dynamics are necessary to better understand the relationships between hillslope-channel coupling in headwater streams.