Background
Various areas of North Dakota experienced a severe snow event in April of 2022. The snow accumulations reached anywhere from 12 inches all the way up to 30 inches. Due to the spring rains, the snowfall melted at a rapid pace, initiating major flooding. As a result, 10 flood control dams in the Tongue River Watershed nearly reached their capacity. In several instances, capacity was exceeded, and the auxiliary spillway, a secondary spillway in case of a large flood event, was activated.
The Problem
The community of Cavalier, ND requested assistance from HEI in emergency response efforts to combat flooding in the community on Sunday, May 1, 2022. HEI’s team quickly began assisting by surveying flood levels on the ground, providing in-the-field elevations to community leaders, field checking modeling results, and advising the City on where to construct emergency flood control measures.
After successfully assisting Cavalier, HEI heard the news that that the Bourbanis Dam was starting to fail on Monday, May 2. The earthen auxiliary spillway had begun to erode, leaving a nearly 450-lineal-foot headcut near the crest of the spillway. HEI’s staff quickly sprung into action to assess the damage and remained on-site to monitor additional progression of the erosion.
The emergency action plan (EAP) entered into Stage 3 after a meeting with local, state, and federal officials. As a result, ND Highway 5 was closed downstream of the dam, and at-risk residents were required to evacuate. The North Dakota National Guard (NDNG) delivered large sandbags by helicopter in order to stabilize the erosion.
The Emergency Response
On May 3, HEI staff assisted the NDNG and their Blackhawk helicopters with placing one-ton sandbags to help stabilize the damaged area of the spillway. HEI collaborated with the Natural Resources Conservation Service (NRCS), North Dakota state dam safety officials, and the Pembina County Water Resource District (PCWRD) to develop a plan to add additional capacity to speed up the drawdown. The team decided that adding capacity through temporary pumping was the best option.
Chinook helicopters from the Minnesota National Guard (MNNG) were used to set the pumps and hoses on the top of the dam because the Blackhawk helicopters, used by the NDNG to set the sandbags, could not carry the weight of the two pumps. Two more lighter, submersible tractor pumps were brought to a site on the opposite side of the dam, which added pumping capacity. The submersible pumps were pushed to their limit, and the on-site team constantly addressed issues associated with failing lines.
HEI’s team provided 24-hour inspection from May 2 through May 8. The North Dakota Department of Water Resources installed a mobile telemetry unit so water levels could be monitored online in real time. By May 8, levels had receded below the auxiliary spillway elevation. For the next few weeks, the dam was closely monitored, until local, state, and federal authorities felt it was reasonable to lower the risk to Stage 2.
Decommissioning the Bourbanis Dam
The partial decommission of Bourbanis Dam was fast-tracked for final plans, permitting, bidding, and construction starting in July 2022 with a target completion of December 2022. HEI provided technical services for design, state permitting, bidding, and construction administration to meet this timeline. HEI is working with the PCWRD and NRCS to develop the best long-term solution to address future flood concerns within the Tongue River Watershed.
