Hidden sediment sources: locating and studying road-draining gullies using a geospatial model and field measurements

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Title: Hidden sediment sources: locating and studying road-draining gullies using a geospatial model and field measurements
Author: Styers, Diane M.; Lord, Mark; Kinner, David; Gannon, John P. (JP); Wright, Charles
Abstract: Sedimentation is a major pollution problem in North Carolina according to the North Carolina Sediment Pollution Control Act. Gullies formed by road drainage are a potential source of sediment, especially in the mountainous western portion of the state (WNC). However, due to steep topography and dense vegetation, these features are difficult to locate. This study aimed to leverage high resolution LiDAR-derived digital elevation models (DEMs) from the state of North Carolina to locate road-draining gullies remotely using a geospatial model. Specifically, the goals of the project were to identify road-draining erosional features automatically, examine whether they are linked with particular environmental conditions, and determine how they may impact the landscape in terms of being connected sources of sediment. We used an empirical GIS approach to identify gully candidates within the two-county area which included slope as part of a multivariate logistic model. This approach highlighted areas that may be gullies but also identified broad areas of DEM cells that were not gullies. In order to answer the project questions, we visually examined the areas highlighted by the model and then determined areas that were gullies adjacent to roads using a field-based approach. We identified gullies in both Jackson and Haywood counties, and our initial conclusion is that they play a small role in the overall sediment budgets of the landscapes. However, this process is likely important on a hillslope or small watershed scale, especially because it is likely that more gullies may be located with an improved algorithm or more searching time. The promising aspect of this work is that it does appear we can more clearly answer questions about water and sediment connectivity on high resolution DEMs (0.5 m grid cells). Several environmental variables (e.g., landcover, geology and soils) were examined to determine any links to gully formation, and our initial conclusion is that these variables had little impact on the system’s ability to resist gully formation. The key variables that seem to emerge to explain gully locations are very local site conditions: slope and ditch length, the latter of which we estimated in the field. We hope to go back to determine if we can either improve our empirical approach or adopt another approach that improves our ability to automatically locate gullies relative to non-gully locations. We hope the results of this study have enhanced understanding of human-influence over drainage networks and erosion/sedimentation issues in WNC, and be useful to local governments and water quality advocacy organizations.
Publisher: NC Water Resources Research Institute
Date: 2021-03-15
Series/Report No.: UNC-WRRI;475
WRRI Project;18-08-W
URI: https://www.lib.ncsu.edu/resolver/1840.20/39043


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