Debris-flow hazards in the Blue Ridge of Central Virginia

The June 27, 1995, storm in Madison County, Virginia produced debris flows and floods that devastated a small (130 km(2)) area of the Blue Ridge in th e eastern United States, Although similar debris-flow inducing storm events may return only approximately once every two thousand years to the same gi ven locale, these events affecting a similar small-sized area occur about e very three years somewhere in the central and southern Appalachian Mountain s. From physical examinations and mapping of debris-flow sources, paths, an d deposits in Madison County, we develop methods for identifying areas subj ect to debris flows using Geographic Information Systems (GIS) technology. We examined the rainfall intensity and duration characteristics of the June 27, 1995, and other storms, in the Blue Ridge of central Virginia, and hav e defined a minimum threshold necessary to trigger debris flows in granitic rocks. In comparison with thresholds elsewhere, longer and more intense ra infall is necessary to trigger debris flows in the Blue Ridge.