DESIGN OF ROCK OVERHANG REMOVAL FOR THE UNION CITY DAM EMERGENCY SPILLWAY

The Union City Dam, owned and operated by the U.S. Army Corps of Engineers, is located on French Creek near the town of Union City, Pennsylvania. The dam is a zoned earth embankment built in 1970 for flood control. Flow through the dam occurs via an uncontrolled lower outlet works consisting of a concrete intake structure, conduit, and stilling basin. The dam spillway consists of a concrete weir which empties into the emergency spillway. The spillway is concrete lined on the floor and walls and contains baffles on the downstream end that serve to dissipate hydraulic energy before water enters into the unlined rock channel. The rock channel extends for 1,250 feet to its confluence with the main channel. The upper 700 feet of the rock channel, with sidewalls ranging from 50 to 60 feet high, consists of sandstone overlying shale. The left sidewall exhibits sandstone overhangs ranging from 2 to 10 feet. The right sidewall has less prominent sandstone overhangs that range from 1 to 4 feet.

The rock overhangs have resulted in rockfalls that were both a safety threat to personnel and caused damage to the concrete baffles of the emergency spillway. Existing reports and design drawings along with current field reconnaissance data were used to develop plans for mitigation. The mitigative design includes removal of the rock overhangs to the contact between the sandstone and underlying shale, leaving a 2 foot bench on top of the shale to allow for future weathering.

The rock to be removed, primarily durable sandstone, will be placed in the spillway channel to serve as an energy dissipater and reduce the erosion rate of the shale in the spillway channel floor. The in-place quantity of rock to be removed is estimated to be 4,000 cubic yards. Assuming a bulking factor of 45%, the spillway volume will be reduced by 5,800 cubic yards after the rock is placed in the channel. This approach was deemed desirable as limited hauling of spoil would be required. Based on historical flood events and reservoir capacity analysis the spillway capacity reduction has been deemed acceptable and not detrimental to project operation. The total cost for the rock overhang removal is estimated to be $160,000.