Spillway flow test conducted for Garrison Dam Safety Modification Project

RIVERDALE, N.D. - A functional spillway flow test was conducted at the Garrison Dam in late June marking a first for the Garrison Dam Safety Modification Project. Engineers with the U.S. Army Corps of Engineers, Omaha District conducted the test to evaluate the long-term performance and reliability of the structure while ensuring it is ready for future flood events.

Matt Stobbe, USACE Omaha District dam safety engineer, said the comprehensive flow test is designed to gather data on how the spillway performs under operational conditions.

“We opened up 23 of our spillway gates as part of a spillway flow test,” Stobbe said. “This is actually a first for the project.”

As part of the testing, engineers deployed six GoPro cameras in the spillway’s subdrain system and installed six staff gauges in the spillway to measure water depth. The team also used an unconventional but effective tool to measure water velocity: oranges.

“We threw oranges into the flow of water,” Stobbe said. “We know the distance between each of those staff gauges, and we can then time the orange between the first and the second gauge to calculate the velocity.” Engineers combined the velocity measurements with water-depth data to calculate the total flow moving through the spillway.

According to Stobbe, the spillway gates were previously operated during flood-control events in 2011 and for operational releases in 2018 and 2019. However, this is the first time the gates have been opened specifically for a functional flow test.

The testing effort is focused on monitoring the performance of the spillway’s subdrain system and the spillway chute itself while also verifying that all gates remain fully operational. The flow test represented an additional gate operation beyond the routine operational and maintenance exercises conducted every three years.

“We’re doing all this to really ensure the long-term performance and reliability of the spillway structure itself,” Stobbe said. “We’re testing to make sure that all the Tainter gates are operable so that we’re prepared for the next flood event.”

The project required extensive coordination among multiple agencies and organizations. Stobbe credited partners including the North Dakota Game and Fish and Western Area Power Administration, as well as operations personnel at the project site, for helping make the test possible.

“The team was outstanding,” he said. “We needed a lot of coordination across multiple different agencies in order to get this done.”

Officials said the data collected during the test will assist engineers in making design decisions during the preconstruction engineering and design phase of the project, ensuring the spillway continues to function safely and effectively for years to come.