LIDAR DETECTION OF UNSTABLE SLOPES IN ROCK ISLAND COUNTY, IL

Unstable slopes are problematic in developed areas, and the ravines of the Quad Cities metropolitan area of Illinois and Iowa pose a challenge for property owners and local governments. City planners rely on accurate, updated hazard maps. Formerly, land hazard mapping has been limited by the resolution of topographic maps and aerial photographs, resulting in significant field work necessary to identify unstable slopes. This study employs LIDAR data as a high-resolution alternative to standard topographic maps, testing the results with the published map, Geologic Conditions Affecting General Construction in Rock Island County, IL. The terrain consists of deep ravines dissecting the upland areas between the Rock River to the south and the Mississippi River to the north. Shales of the Pennsylvanian System are exposed in many ravines, overlain by Illinoian till (Glasford Fm.) and Wisconsinan silt and loess. This combination is responsible for ongoing soil creep and intermittent slumping and sliding, with failure often initiated in the lower shale layers due to undercutting by small streams. LIDAR data were acquired online through the Illinois State Geological Survey’s Geospatial Clearinghouse’s county data. Data were then converted using Geographic Information Systems (ArcMap10) to view the data as a digital elevation model. Data were then surveyed for areas of irregular hummocks along bluffs and in ravines. The denoted areas of interest were marked on a map, visually field checked, and compared to the published map. Results confirm that unstable slopes can be readily identified with LIDAR interpretation, although field verification remains a necessary task for those creating hazard maps. The findings of this study agreed with the hazardous areas previously mapped, however no new sections were distinguished.