Detecting and characterizing the hillslope to channel transition in high erosion rate, bedrock landscapes

<p>Channel networks exert a key control on drainage basins shape and dynamics in mountainous landscapes, including the transfer of water and sediment throughout basins, and thus hydrosedimentary hazards. Landscape dissection by channels results from the competition between hillslope processes and channelized erosion processes such as overland flow or debris flows. Although the competition between fluvial processes and hillslope processes has been modeled in simple cases, more data is still needed to constrain the localization and characteristic of this transition, especially in channels where debris-flows occur. High resolution LiDAR DEMs open new perspectives for the extensive extraction of channel heads. Several channel extraction methods exist but none is yet robust on fast eroding landscapes where channels are not always fluvial and sometimes initiate in rough bedrock areas.<br />Therefore we developed the CO&#178;CHAIN method which identifies the hillslope to channel transition in drainage basins based on relative changes of local and upstream measures of flow convergence. We calibrate CO&#178;CHAIN by fitting its results to channel head mapping made by geomorphologists on four contrasted basins in the United States and France. Compared to state-of-the-art channel extraction methods, and without any recalibration, it achieves similar and higher accuracy in moderate and high erosion-rate basins, respectively.<br />This allows us to identify the first order &#160;channels (the most upstream part of mountain channels) to better understand how their morphology (slope, minimum drainage area, length) are linked to catchment mean erosion rates. We applied our method to a few LiDAR DEMs of mountain catchments with varied mean erosion rates and where debris flows have been identified. It appears that, &#160;where channels begin, the drainage area and &#160;slope &#160;are correlated and this correlation depends on the catchment mean erosion rate and the local hilltop curvature. In order to understand what controls the transitions between hillslopes and debris flow channels, we also studied preliminarly the distribution of hillslope length and slope gradient and of channel gradient &#160;throughout the catchments. This could give us insight into the processes shaping the bedrock channels and allow us to test long-term geomorphic models for debris flow erosion.</p>