Paper No. 11
Presentation Time: 11:05 AM
LIDAR-BASED GEOMORPHIC MAPPING IN FORESTED LANDSCAPES: USING TREES TO YOUR ADVANTAGE
LEGG, Nicholas T.1, MEIGS, Andrew
1, GRANT, Gordon E.
2 and KENNARD, Paul
3, (1)Department of Geosciences, Oregon State University, 104 Wilkinson Hall, Corvallis, OR 97331, (2)Pacific Northwest Research Station, USDA Forest Service, 3200 SW Jefferson Way, Corvallis, OR 97331-8550, (3)Mount Rainier National Park, 55210 238th Ave E, Ashford, WA 98304, leggn@geo.oregonstate.edu
Bare-earth digital terrain models (DTMs) based on airborne LiDAR data are now widely employed in geomorphologic studies. Similarly, forest scientists employ digital canopy models (DCMs) generated from the same datasets. In this study, we combine DTMs and DCMs to map geomorphic surfaces of differing relative age in a densely forested landscape shaped by glacial outburst floods and debris flows on decadal timescales. Where spatial boundaries in tree heights are found to correspond with landform boundaries, distinctly aged geomorphic surfaces can readily be identified. Shade intolerant Douglas Fir (
Pseudotsuga menziesii) common to the study area rapidly colonize fresh geomorphic surfaces, which allows tree height to be used as a proxy for geomorphic age. Using a simple GIS approach, DCMs are created by differencing the highest hit (or first return) and the bare-earth DTM rasters. DCMs are then overlain at 50% transparency on hill-shaded and elevation-contoured bare-earth DTMs.
The method was developed to study valley bottom evolution in response to debris flows in catchments flanking Mount Rainier. With this technique, we are able to: 1) map geomorphic features of differing relative age; 2) provide a rational basis for designing a dendrogeomorphic sampling strategy based on identifying abandoned channels with adjacent old-growth trees that record evidence of debris flow disturbance; and 3) map extents of individual recent debris flow events. The LiDAR data used for this study was collected in 2007-2008, in part, as a response to a storm event in 2006 that unleashed major debris flows from multiple drainage basins. The DCM/DTM method clearly differentiates thinned stands where 2006 debris flows spilled from channels into adjacent forests. Examples from the Kautz Creek drainage basin reveal recent events, known historical events, as well as previously unrecognized pre-archival debris flow events. This method provides a simple, first-order tool to develop high-resolution chronologies of geomorphic surface evolution in areas with minimal anthropogenic disturbance.
