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