Debris Slides and Related Flood Effects in the 4-5 August 1938 Webb Mountain Cloudburst: Some Past and Present Environmental Geomorphic Implications

On the night of 4-5 August, 1938 a cloudburst occurred over Webb Mountain, Tennessee, that lasted three hours with a rainfall in excess of twelve inches. This sudden deluge caused considerable debris sliding and flooding which resulted in the loss of eight lives and extensive property damage in the narrow valleys below the slide area. Over 100 individual slide scars were identified in the study area, 40 of which occurred in the Matthew Creek watershed.

The debris slide movement is thought to have been initiated by sliding at the head of the scar, with the mass of moving rock, soil and forest debris remaining intact and then developing into a debris flow as it progressed downslope. The major volume of material was carried out of the upper stream channels and slide tracks and deposited in the lower reaches of the streams.

Intense summer rainstorms are the major cause of debris slides in the Appalachian Highland south of the glacial border. Complex cultural and physical factors interact to affect an individual slide location. Precipitation is a critical localizing factor, as well as the rate of water infiltration which increases the soil pore pressure very rapidly during intense rainfall, such that soil and regolith fail by sudden reduction in their shearing strength.

Careful field investigation was the method of obtaining detailed data on the slide areas. All slides were plotted on a large map of the entire area. The Matthew Creek slide area was the center of detailed study.

Evaluation of several types of imagery indicated the Ekatachrome Infrared Aero film to be the most useful. The look angle, altitude and vegetational foliage are important factors in defining the slide scars with aerial photography.

The accumulation of data over longer time intervals will improve recurrence interval projections. Further debris slide studies will add to the understanding of slide localization and distribution factors and areas of slide potential can better be determined.