INVESTIGATION OF A RECENT LANDSLIDE IN FORT SMITH, ARKANSAS

A morphologically youthful slope failure was identified in a remote section of the City of Fort Smith Landfill, and permission was obtained to conduct an investigation. The slide is on a deforested, NW-facing 27% slope beneath a linear head scarp > 200 m long. One end of the head scarp curves abruptly downhill and transforms into a distinct lateral scarp > 100 m long. The surface of the slide is about 24,000 m² (6 acres) in area, and easily distinguished from adjacent intact slopes by its hummocky morphology and transverse ridges and valleys. The head scarp destroyed part of an unpaved landfill service road, and the toe emerged in a forested area about 120 meters downslope, tilting, uprooting, and killing trees. The slide is interpreted as a translational failure of unconsolidated, poorly-sorted colluvium above a shale-rich part of the McAlester Formation (a member of the Desmoinesian Krebs Group). The slide mass, exposed in the walls of two deep gullies, is composed of angular fragments of the underlying McAlester Formation in a moist, clay-rich matrix. The UAFS Unmanned Aerial Systems program obtained permits, created a flight plan, and acquired 791 images over the landslide with a drone-born RGB camera. DroneDeploy software was used to stitch the images together and produce a topographic map and several profiles. Analysis of the profiles indicates downslope translation > 7.5 m, maximum depth to the failure surface of 3 to 4 m, and volume of slide material approximately 60,000 m³. Historical Google Earth images indicate that most of the slide movement to date occurred between 2006 and 2016. Weather records indicate that Fort Smith’s average annual precipitation of 42 inches was exceeded during 12 of the 16 years from 2006 to 2021, with the 2015 total of 74 inches the highest on record since 1887. We conclude that the slope failure was triggered by a combination of tree removal, road construction, and abnormally high rainfall on thick clay-rich colluvium.