Storm-Induced Debris Avalanching and Related Phenomena in the Johnstown Area, Pennsylvania
Pomeroy JS. 1980. Storm-Induced Debris Avalanching and Related Phenomena in the Johnstown Area, Pennsylvania. U.S. Geological Survey (USGS) Professional Paper. 1191. US Geological Survey. Washington, DC
Several hundred debris avalanches, debris slides, slumps, earthflows, and combinations of the various types took place as a result of 30 cm of rain that fell during a 9-hour period, July 19-20,1977, in an area of about 60 to 70 km^2 that lies north, northeast, and east of downtown Johnstown, Pa. Before this rainstorm, the soil had been well saturated by above-normal rainfall earlier in the month. The most conspicuous mass-movement type was the debris avalanches, which reached a maximum of 300 m in length and 25 m in width and had head scarps in colluvium as high as 4 m. They were formed along mostly planar to gently concave upward colluvial 20" (35- percent) to 40" (85-percent) slopes. The less conspicuous slumpearthflows began on more moderate slopes. Because of their greater clay content, colluvial soils derived from the Allegheny and Conemaugh Groups of Pennsylvanian age were more susceptible to the rapid mass movement than were those formed from the older rocks of Mississippian and Pennsylvanian age. A relatively dense pattern of mainly debris avalanches along the steep northwestfacing slope above the Little Conemaugh River northeast of Franklin appeared at least in part to be controlled by lithologic factors coupled with an overdip slope conducive to the formation of seeps. The actual movement of regolith in the debris avalanching took place in two phases: first, limited planar or rotational sliding extending downhill a short distance away from the head scarp, and second, flowage caused by spontaneous liquefaction.
- Type
- Published Report
- Authors
- Pomeroy, J
- Date of Issue
- 1980
- Publisher
- US Geological Survey
- Units
- JOFL
- Keywords
- Appalachians, Avalanches, Cambria County, Debris Flows, Eastern U.S., Engineering Geology, Environmental Geology, Geologic Hazards, Geology, Johnstown, North America, Pennsylvania, Slope Stability, Storms