Paper No. 5
Presentation Time: 9:20 AM
EXPLORING KNICKPOINT RETREAT MECHANICS WITH REPEAT TERRESTRIAL LIDAR SCANS
RENGERS, Francis K., Department of Geological Sciences, University of Colorado, 2200 Colorado Ave, Boulder, CO 80309-0399, TUCKER, Gregory E., CIRES & Department of Geological Sciences, University of Colorado, 2200 Colorado Ave, Boulder, CO 80309-0399, PHILLIPS, David A., UNAVCO, 6350 Nautilus Dr, Boulder, CO 80301 and OKAL, Marianna, UNAVCO, 6350 Nautilus Drive, Boulder, CO 80301, francis.rengers@colorado.edu
In arid landscapes, gullies are the major conduits for sediment transport, and their morphology is largely impacted by the upstream migration of steep vertical drops (knickpoints). Traditional efforts to measure knickpoint retreat using methods such as erosion pins and repeat aerial photography suffer from poor spatial and temporal resolution, and are therefore insufficient to diagnose the driving processes or the space-time variability in erosion rates. Terrestrial LiDAR scanning (TLS) provides the opportunity to detect morphologic change at very high spatial resolution. We report results from a study that investigates dynamics of gully knickpoint evolution using repeat terrestrial LiDAR scans to document patterns of morphological change on a sub-annual timescale at centimeter-scale resolution. The study focuses on a ~2.5 m high gully knickpoint in an alluvial channel on the high plains of eastern Colorado (40 miles east of Denver, CO). We conducted TLS surveys biannually from 2006 to 2011 to track knickpoint erosion. TLS measurements are complemented by measurements of rainfall, flash-flood depth and discharge, and soil moisture.
Our TLS observations suggest that knickpoint erosion occurs in patches when blocks of sediment detach from the face, as opposed to failure of sediment across the entire wall of the knickpoint. The blocks tend to be on the order of 50 cm which agrees with analysis of historical aerial photos showing an average annual knickpoint retreat rate of 50 cm per year. Material detached from the knickpoint is quickly carried away by ephemeral flows. In situ measurments of rainfall and soil moisture indicate that most knickpoint is associated with convective thunderstorms.
Based on our observations, we have developed a three step conceptual model for the mechanism of knickpoint retreat: 1. Partial wall failure leaving behind an overhanging block, 2. Toppling of the overhanging block, and 3. Downstream scour to move the sediment away from the base of the knickpoint. We hypothesize that overhangs develop because dense grass roots are strong enough to hold back a portion of the soil. This piece fails once the weight of the overhanging block exceeds the root strength. These results allow us to more fully understand the alluvial system dynamics that are dependent on knickpoint migration.
