Thomson Canal Embankment Failure of 2012

ABSTRACT ONLY - In the week leading up to June 19th, 2012, the area around Duluth, MN had seen steady rains totaling 2-4 inches, saturating the ground, and priming the St. Louis River basin for runoff. The storm that hit overnight on June 19 - 20 dropped an additional 5-10 inches of rain, resulting in record flooding in the region. The St. Louis River topped its historic maximum flood level by 0.8 feet (an estimated 0.2% annual exceedance probability), overtopping and failing the Thomson Hydroelectric Project power canal embankment. The official failure mode was determined to be flood induced overtopping of the canal embankment, but this description oversimplifies the complexity of what actually occurred to fail the embankment at a flood flow that was analyzed as being well below the capacity of the dam. The power canal embankment overtopped and failed due to numerous confounding issues that included - debris clogging, personnel access, design oversights, operational issues, maintenance and more. This case study truly exemplifies the principle of an uncommon combination of common factors leading to failure. In this paper we will identify each of the contributing factors and look at how they contributed to increase the probability of failure of the structure.