Hurricane Isabel

Landslide Summary—Impacts and Lessons from Hurricane Isabel
September 18-19, 2003

Hurricane Isabel was expected to trigger landslides as it came ashore and passed through mountainous areas; however, the observed landslide impacts were much less severe than anticipated. The speed of Hurricane Isabel as it passed from south to north from North Carolina through Virginia accelerated slightly so that rainfall totals at most locations were less than expected. The heaviest recorded rainfall reported on the IFLOWS network was along the Blue Ridge in central Virginia. The maximum rainfall reported by the NOAA-NWS IFLOWS system network was 20.2 inches during 24 hours at the Upper Sherando station of Sherando Lake (20 km south of Waynesboro, Virginia). However, this rainfall total was questioned because of possible malfunctioning of the rain gage and is still being examined by NOAA-NWS. This maximum value could not be verified from other local measurements. Elsewhere along the central Blue Ridge, up to 11 inches of rainfall was recorded by the IFLOWS network. Major flooding occurred in the Sherando region and the larger Back Creek basin. Ground reconnaissance by geologist Scott Eaton did not yield evidence of any landslide and debris-flow activity in the Sherando Lake region. Rainfall totals of 11 inches over 24 hours from Hurricane Isabel would not have reached the critical thresholds of rainfall necessary for triggering debris flows previously developed in a region of central Virginia (Wieczorek et al., 2000). Extensive flooding from Hurricane Isabel resulted in erosion of channel banks and subsequent reactivation of a landslide detected by Scott Eaton along the western flank of the Blue Ridge northeast of Waynesboro. Although aerial imagery must still be examined for verification, the U.S. Forest Service expects that some landslides could have been reactivated along the western flank of the Blue Ridge in George Washington and Jefferson National Forests between the cities of Buena Vista and Glasgow, Virginia, 45 km southwest of the Sherando region. Preliminary reports by the National Park Service indicate no landslide activity to the north of Sherando, along the Blue Ridge in Shenandoah National Park.

Wieczorek, G.F., Morgan, B.A., and Campbell, R.H., 2000, Debris-flow hazards in the Blue Ridge of central Virginia: Environmental & Engineering Geoscience, v. VI, n. 1, p. 3-23.