Rapidan Dam

The Rapidan Dam is a concrete gravity dam located on the Blue Earth River in Rapidan Township southwest of Mankato, Minnesota in the United States. The dam was constructed from 1908 to 1910 to generate hydroelectric power. The dam and reservoir are owned by Blue Earth County, and the power plant and dam were operated by Eagle Creek Renewable Energy under an agreement with the county. Power was no longer generated at the dam due to damage from flooding in 2019 and 2020.

Blue Earth County operates the Rapidan Dam Park & Campground on the west embankment of the dam for camping, hiking, river access, and recreational activities. The Rapidan Dam Cafe and Store, which operated on the west bank of the river until 2024, was known for their homemade pies.

History
Rapidan Dam was constructed from 1908 to 1910 by Consumers Power Company, who became part of the Northern States Power Company soon after construction in 1911. The equipment used to generate power was: two 750kw pressure cased Francis turbines. with a 50kw exciter.

The hydroelectric equipment at the dam generated power from 1911 to 1965 for Northern States Power with an average annual production of 7.3 Gigawatt-hours.

1965 gate failure
In 1965 a record-breaking 500-year flood damaged the dam. The flood brought high water levels and heavy ice downstream to the dam, and "ripped out" 6 of the 7 tainter gates. This was mostly caused by the tainter gates being positioned so that the bottom edge of the gate was submerged during a high water condition. The failure of the gates caused the water level in the reservoir to drop 9 feet and exposed 400 acres of mud flats.

In 1970 (or 1969 ), the dam was acquired by Blue Earth County, which has operated the facility since then.

Two years later in 1972 Blue Earth County ruled that the dam should be rebuilt or torn down due to concerns over the impact of a collapse.

1983 retrofit
In 1983 the county awarded a contract to the Johnson Bros. construction company to restore the dam so it could generate power. Work started in May 1983. The original corbels on the dam were not reinforced so they did not meet 80s standards. There was concern that they were vulnerable to shearing. During the retrofit, 193 anchors were installed to strengthen the corbels. New tainter gates were designed and installed to replace those damaged in 1965. The new gate design was stronger than the original to prevent it from failing due to pressure from water and ice again. In July 1982 a specification was issued for pumping equipment and bids were received in October of that year. Allis-Chalmers gated tubular turbines were chosen. These were rated for a generator output of 2.5 megawatts of power each. Concerns were raised that a vortex could form due to the design of the new intake. The design was modelled at Saint Anthony Falls Hydraulic Laboratory and shown to cause a vortex. The design was already under construction by this point so a change was made to add a guidewall, round the noses of the piers and add a plate with holes behind the trashracks. Work also involved the repair and maintenance of trashracks, stairs, doors and a roof. Part of the retrofit involved financial computer modelling of the dam's operation using 40 years of historical data.

In 2002 it was discovered that the foundation of the dam needed emergency repairs which were carried out to stop the dam failing. This included filling concrete in a void between several buttresses and part of the base slab at the toe of the spillway. This cost $1,034,000.

In 2019 the dam was damaged by flooding and stopped producing electricity. In 2021 the county commissioned two studies from BARR Engineering to explore the options for the dam’s state of disrepair. Two options were studied: dam repair or dam removal. From April to June 2022, the county collected community feedback regarding these options. 69% of responses supported repairing the dam and 18% supported removing the dam. As of April 2023 the National Inventory of Dams rated the dam in poor condition and its hazard potential as "significant".

2024 storm damage
Following flooding on the river due to heavy rain, on June 24, 2024, debris on the river gathered behind the dam and caused the river to back up. The dam was declared to be in an "imminent failure condition". Dam operators first alerted authorities at around 10:36 a.m. CDT. A flash flood warning was enacted and residents were urged to evacuate. Officials say this is a low-risk dam if there was a catastrophic failure. They indicated that the campground would need to be evacuated, and then the only risk would be to people located on the river itself. The largest damage is predicted to be environmental, as a large amount of sediment has built up in the reservoir, and much of it would eventually wash down river.

The river suffered a side breach due to high water flows and washed away an Xcel Energy substation which provided power to around 600 customers; it also destroyed a shed and a home. On June 25 at around 9:30 p.m., the home of the Rapidan Dam Store owners collapsed into the water. On June 28, Blue Earth County bought the Rapidan Dam store, and demolished it. With the continuing erosion, officials are monitoring the County Highway 9 bridge that spans the river just upstream from the dam.