National Dam Safety Program. Unimin fresh Water Pond Dam (Inventory Number Va 06917), Potomac River Basin, Frederick County, Commonwealth of Virginia. Phase I Inspection report

摘要

Unimin Fresh Water Pond Dam is a 45.5 foot high by 627 foot long earthfill embankment, with a 40 foot wide vegetated earth emergency spillway located adjacent to the right abutment. The principal spillway consists of a square concrete tower with two rectangular openings, 4.8 and 10.2 feet by 4.4 feet, respectively. The tower connects to a 36-inch diameter outlet pipe. The dam, located approximately 1.1 miles south of Gore, Virginia, is used for water supply by the owner, Unimin Corporation. Unimin Fresh Water Pond Dam is an 'intermediate' size - 'high' hazard structure as defined by the Recommended Guidelines for Safety Inspection of Dams. The dam and appurtenant structures were found to be in good overall condition at the time of inspection. Past maintenance of the dam has been inadequate. A stability check of the dam is not required. Using the Corps of Engineers' screening criteria for initial review of spillway adequacy, the Probable Maximum Flood (PMF) was selected as the spillway design flood (SDF). The SDF was routed through the reservoir and found to overtop the dam by a maximum depth of 0.8 feet with an average critical velocity of 4.1 feet per second. Total duration of dam overtopping would be approximately 0.4 hours. The spillway is capable of passing up to 80 percent of the PMF without overtopping the crest of the dam.