Tropical storms and the flood hydrology of the central Appalachians

Flooding from Hurricane Fran is examined as a prototype for central Appalachian flood events that dominate the upper tail of flood peak distributions at basin scales between 100 and 10,000 km(2). Hurricane Fran, which resulted in 34 deaths and more than $3.2 billion in damages, made land fall on the North Carolina coast at 0000 UTC, September 6, 1996. By 1200 UTC on September 6, Fran had weakened to a tropical storm, and the center of circulation was located at the North Carolina-Virginia border. Rain bands surrounding the tropical depression produced extreme rainfall and flooding in Virginia and West Virginia, with the most intense rainfall concentrated near ridge tops in the Blue Ridge and Valley and Ridge physiographic provinces. The most severe flooding occurred in the Shenandoah River watershed of Virginia, where peak discharges exceeded the 100-year magnitude at 11 of 19 U.S. Geological Survey stream-gaging stations. The availability of high-resolution discharge and rainfall data sets provides the opportunity to study the hydrologic and hydrometeorological mechanisms associated with extreme floods produced by tropical storms. Analyses indicate that orographic enhancement of tropical storm precipitation plays a central role in the hydrology of extreme floods in the central Appalachian region. The relationships between drainage network structure and storm motion also play a major role in Appalachian flood hydrology. Runoff processes for Hurricane Fran reflected a mixture of saturation excess and infiltration excess mechanisms. Antecedent soil moisture played a significant role in the hydrology of extreme flooding from Hurricane Fran. Land use, in particular, the presence of forest cover, was of secondary importance to the terrain-based distribution of precipitation in determining extreme flood response.