Crago, Richard, Department of Civil and Environment Engineering, Bucknell University, 1 Dent Drive, Lewisburg, PA 17837, rcrago@bucknell.edu.
Tropical Storm Lee in 2011 resulted in extreme flooding and sediment transport in Muncy Creek, a tributary to the West Branch of the Susquehanna River. Fortunately, high-resolution (one meter horizontal), aircraft-based LiDAR elevation maps (i.e., DEMs) were collected in 2006 and 2017 shortly before and after Lee. Large elevation changes due to sediment deposition or erosion should be detectable by taking the difference between elevation for a given 1-m cell in 2006 and in 2017. Thus, the variable Ediff = z2006-z2017 is a measure of erosion and negative values of Ediff correspond to deposition. Looking at reaches of all sizes and throughout the watershed, previous work at Bucknell showed that the standard deviation of Ediff measured underneath reaches was correlated with the sediment capacity index Tc=ln(A*S), where A is the watershed area feeding into the reach and S is the slope of the bed in the reach. This implies that high sediment motion (from high values of Tc) results in sediment redistribution within a reach, and variations in Ediff underneath the reach.
Muncy Creek was also studied in considerable detail by Kochel et al. (Geomorphic response to catastrophic flooding in north-central Pennsylvania from Tropical Storm Lee (September, 2011): Intersection of fluvial disequilibrium and the legacy of logging, Geosphere, 12(1), 2015) with aerial photography and field observations. Their work resulted in color-coded maps of the channel and floodplain, providing a picture of sediment motion at spatial scales far smaller than the reach scale. With their permission, their map for Muncy Creek was “rubber-sheeted” in ArcGIS Pro so that the Ediff prediction of sediment erosion or deposition could be compared to their observations. This combination of data helps to illustrate the sediment transport patterns. For example, when a channel was straightened by Ivan, the Ediff map shows deposition on the outside of the bend and erosion in the inside, pushing the channel so as to reduce its length.
A conclusion from this study is that reach-level measures such as Tc might correlate with actual sediment transport and re-distribution within a reach, but the physical processes that actually move the sediment act at much smaller spatial scales. Future work will extend this study further downstream.
Sediment transport, Muncy Creek, Sedimentation, Erosion