National Dam Safety Program. Thornhill Dam (VA 15505), New River Basin, Pulaski County, Virginia. Phase I Inspection report.
Abstract
Thornhill Dam is an earthfill structure 505 feet long and 25.1 feet high. The dam is owned and maintained by Glen O. Thornhill of Salem, Virginia. The dam is classified as a small dam with a significant hazard classification. The principal spillway is a 24-inch cast iron pipe drop-inlet that connects to an 18-inch corrugated metal pipe that passes through the dam at low level. Both the left and right emergency spillways are earthen open channels cut into natural ground at each abutment. The reservoir is used for recreation. Based on criteria established by the Department of the Army, Office of the Chief of Engineers (OCE), the Spillway Design Flood (SDF) is the 1/2 PMF. The spillways will pass 22% of the PMF or 44% of the SDF without overtopping the dam. The SDF will overtop the dam by a maximum 0.75 feet, reach an average critical velocity of 3.8 feet per second and flow over the dam for 1.75 hours. Flows overtopping the dam during the SDF are not considered detrimental to the embankment. The spillways are adjudged inadequate but not seriously inadequate. The visual inspection revealed no findings that proved the dam to be unsound. A stability check is not required. However, the inspection did reveal a shallow stable slough combined with a seep and wet spot in the vicinity of the spillway outlet pipe along the embankment and right abutment toe. There is no regular maintenance operation program or warning system.
Document Details
- Document Type
- Technical Report
- Publication Date
- Dec 01, 1980
- Accession Number
- ADA103502
Entities
Organizations
- Norfolk District, U.S. Army Corps of Engineers