Debris Flow Impacted Powerhouse
Situated in the southern Washington Cascades, Swift Dam provides clean hydropower for the region. Following a significant rain-on-snow event in the morning hours of January 8, 2009, a debris flow of approximately 2,500 cubic yards initiated on the slope above the Swift 1 powerhouse. The mobilized debris flowed down a 34° slope and then over a steep rock cut, picking up momentum. The debris struck an intermediate rock bench, which redirected the debris into the powerhouse, causing significant damage to the powerhouse, a rockfall barrier fence, and draped chain link mesh.
An initial assessment revealed additional cracks in the snow and overburden around the scarp. In order safely reenter the powerhouse, the risk posed by another unconstrained debris flow needed to be reduced. A system of draped Tecco mesh was selected to limit the velocity and constrain potential failures. The slope area covered is 55,000 square feet. Construction of the draped Tecco mesh was completed less than 14 days following the failure, including 5 days when wind prevented mesh placement by helicopter. Following installation, personnel could commence powerhouse repair.
Following the initial emergency response, additional mitigation efforts to prevent future debris flows were required. The Tecco mesh provided the adaptability to provide active resistance to sliding by nailing the mesh to the slope at an engineered spacing. Based on the slope materials encountered and computer modeling, a spacing of 15 feet on center within the failed zone, and 10 feet on center outside the scarp was constructed.
Following the initial early response, the remainder of the overburden slope behind the powerhouse was secured by an identical nailed Tecco system. In total, 720 nails between 5 and 15 feet deep, with about 90% at 10 feet, with approximately 100,000 square feet of slope covered with nailed or draped mesh.