East Branch Dam – 1957 Internal Erosion Incident

East Branch Dam is located in North-central Pennsylvania on a tributary of the Allegheny River. This zoned earth embankment dam is approximately 180 feet high and 1,700 feet long and is controlled by the Pittsburgh District of the U.S. Army Corps of Engineers. The dam is operated to prevent flooding in downstream communities and also to enhance water quality by helping control water temperatures downstream of Ridgway, PA, where a paper mill discharges warmer water into the stream. The dam was constructed in the mid-1950’s. During a relatively moderate pool level that occurred after only a few years of operation, muddy water was observed flowing from a rock drain at the downstream toe of the dam. Emergency drilling through the embankment exposed a void at the base of the embankment near its contact with top of rock. This paper discusses the design and construction practices used at East Branch Dam, gives information related to the dam safety incident and emergency response, and theorizes about the possible causative mechanisms of internal erosion that nearly caused the dam to breach.