Cracking Up: The Story of the 2014 Wanapum Dam Incident

In February 2014, an observant technician inspecting the dam noticed a displacement of the bridge over the spillway at Wanapum Dam. Subsequently, a two-inch wide crack was found in one of the concrete monoliths on Wanapum Dam, a 186 ft high dam on the Columbia River in central Washington. The crack ran across the 65-foot width of one of the 13 spillway-pier monoliths. The dam is owned and operated by Public Utility District No. 2 of Grant County (PUD) and is part of the Priest Rapids Hydroelectric project. The project had operated without incident since its original construction in 1963 until the 2014 incident. The crack and deterioration of the spillway prior to displacement was not obvious, as it was located below the reservoir surface and the dam appeared to be in excellent condition. The PUD activated the Emergency Action Plan, drew down the reservoir and immediately began a forensic investigation. They determined that tensile stresses in the upstream face initiated that crack and were then exacerbated by concrete fatigue and uplift intrusion.
The crack was repaired with the installation of thirty-five sixty-one strand tendon anchors through the upstream face of the dam at depths up to 260 ft. An additional sixty-nine anchor bars were installed in the ogee spillway in the upstream side of the gates. To relieve hydrostatic pressures, lift joints drains were installed from the drainage gallery in each monolith.
Although the incident did not breach the spillway or result in an uncontrolled release of the reservoir, it was recognized as a significant incident that resulted in dam safety, environmental, cultural, and economic impacts. The incident shed light on the importance of evaluating stability of concrete gravity dams and led to new considerations about analyzing stability of dams and spillways along planes within the dam.