CS14C - Concurrent Session 14C: 2003 Emergency at Mount Carmel Dam, North Dakota – DFIC Decade Series

On March 29, 2003, a local high school student taking pictures made a discovery that helped divert a potential disaster. Substantial erosion had occurred at the main spillway of Mount Carmel Dam, which is located near Langdon, North Dakota. Flows from the internal erosion (piping) event were discharging beneath the chute spillway structure in an uncontrolled release and exiting through a large hole that had developed at the bottom of the dam embankment. Swift action was required to safeguard the remaining reservoir supply, which served a population of over 5,000 people, and to prevent a complete failure of the dam. Emergency response included voluntary evacuation of two families, monitoring of downstream roads and bridges that might be impacted, and staging of equipment and supplies ready to be utilized in the event of a flood. Emergency earth and sheetpile cofferdams were constructed to prevent further erosion, and an out-of-use and buried spillway conduit was re-opened in order to safely lower the reservoir and avert larger consequences. Resulting forensic analyses and failure mode evaluations identified filter incompatibility, inadequate seepage cutoffs, and adverse effects due to freeze-thaw action as likely contributing factors. The damaged spillway structure was ultimately removed and replaced with one that addressed design flaws identified during the incident investigation. The incident at Mount Carmel Dam also instigated further evaluation at several similar spillway structures around the state. This case study will: (1) highlight the factors leading up to the erosion failure; (2) describe the incident, including the emergency response and investigation; (3) provide details about the reconstruction and repairs at the dam; and (4) highlight the impact to other similar spillway structures around the state. The authors will also attempt to identify and interview eyewitnesses or other impacted stakeholders to provide firsthand accounts associated with the event.