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Rapidan Dam

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Coordinates: 44°05′35″N 94°06′32″W / 44.093°N 94.109°W / 44.093; -94.109
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Rapidan Dam
Blue Earth River below Rapidan Dam
Rapidan Dam is located in Minnesota
Rapidan Dam
Location of Rapidan Dam in Minnesota
CountryUnited States
LocationBlue Earth County, Minnesota
Coordinates44°05′35″N 94°06′32″W / 44.093°N 94.109°W / 44.093; -94.109
PurposePower
StatusPartially failed
Construction began1908; 116 years ago (1908)
Opening date1910; 114 years ago (1910)
Owner(s)Blue Earth County Government
Dam and spillways
Type of damConcrete Gravity
ImpoundsBlue Earth River
Height87 ft (27 m)
Length475 ft (145 m)
Spillways5 steel Tainter gates and 2 timber gates
Reservoir
CreatesRapidan Lake
Catchment area2,430 sq mi (6,300 km2)
Surface area318 acres (129 ha)
Turbines2[1]
Installed capacity6 MW[1] (not currently operating)[2]

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.[1] Power was no longer generated at the dam due to damage from flooding in 2019 and 2020.[2]

Blue Earth County operates the Rapidan Dam Park & Campground on the west embankment of the dam for camping, hiking, river access, and recreational activities.[3] The Rapidan Dam Cafe and Store, which operated on the west bank of the river until 2024, was known for their homemade pies.[4]

History

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People viewing construction site of Blue Earth Dam, Rapidan Township, Minnesota

Rapidan Dam was constructed from 1908 to 1910[5] by Consumers Power Company (successor of Washington County Light & Power), which became Northern States Power Company in 1916. 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.[6]

1965 gate failure

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In 1965 a record-breaking 500-year flood damaged the dam.[7][8][5] 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.[6]

In 1970 (or 1969[6]), the dam was acquired by Blue Earth County, which has operated the facility since then.[9]

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.[10]

1983 retrofit

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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.[6]

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.[11]

In 2019 the dam was damaged by flooding and stopped producing electricity. In 2021 the county commissioned two studies from Barr Engineering Co.[12] to explore the options for the dam’s state of disrepair. Two options were studied: dam repair or dam removal.[13] From April to June 2022, the county collected community feedback regarding these options.[14] 69% of responses supported repairing the dam and 18% supported removing the dam.[15] As of April 2023 the National Inventory of Dams rated the dam in poor condition and its hazard potential as "significant".[16]

2024 storm damage

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The dam on June 25, 2024, showing the Dam Store at the edge of the precipice before its collapse

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".[17] Dam operators first alerted authorities at around 10:36 a.m. CDT.[18] A flash flood warning was enacted and residents were urged to evacuate.[19][20] Officials had said this is a low-risk dam if there was a catastrophic failure.[21] 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 was predicted to be environmental, as a large amount of sediment had built up in the reservoir, and much of it would eventually wash down river.[22]

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.[23] On June 25, 2024 at around 9:30 p.m., the home of the Rapidan Dam Store owners collapsed into the water. On June 28, 2024, Blue Earth County bought the Rapidan Dam store, and demolished it.[24][25][26] The store has temporarily re-opened in Mankato.[27] With the continuing erosion, and the release of sediment resulting from the collapse, officials are monitoring the County Highway 9 bridge that spans the river just upstream from the dam, particularly one pier whose pilings were exposed by the high water flows and shifting sediment.[28][29]

The Blue Earth County Board of Commissioners has voted to remove the dam and replace the bridge.[30][31]

See also

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References

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  1. ^ a b c "Rapidan Dam". Blue Earth County, MN - Official Website.
  2. ^ a b Exemption surrender process for the Rapidan Dam (Report). Blue Earth County. March 2023.
  3. ^ "Rapidan Dam Park & Campground".
  4. ^ Rathburn, Betsy (February 1, 2021). "A Secluded Restaurant In The Minnesota Countryside, Rapidan Dam Store Is One Of The Most Charming Places You'll Ever Eat". OnlyInYourState®. Retrieved June 29, 2024.
  5. ^ a b U.S Army Corps of Engineers (2009). "Rapidan Dam Investigations Blue Earth River Feasibility Study Ecosystem Restoration" (PDF). p. 9. Retrieved June 24, 2024.
  6. ^ a b c d Waterpower '83, International Conference on Hydropower, September 18-21, 1983, Hyatt Regency/Knoxville, Tennessee: Environmental impacts research and development dam safety general sessions. Vol. 3. Tennessee Valley Authority. 1983. pp. 1536–1538. Retrieved July 6, 2024.
  7. ^ Orenstein, Walker; Lawrence, Jp (June 26, 2024). "Rapidan Dam near Mankato will likely survive this flood. But what about the next?". Star Tribune. Retrieved July 7, 2024.
  8. ^ Anderson, D.B; Burmeister, I.L (1970). "Floods of March-May 1965 in the Upper Mississippi River Basin" (PDF). USGS Publications Warehouse. United States Government Printing Office. p. A62. Retrieved July 8, 2024.
  9. ^ Moser, Riley (June 24, 2024). "What is the history of the Rapidan Dam in southern Minnesota? - CBS Minnesota". www.cbsnews.com. CBS Minnesota. Retrieved June 24, 2024.
  10. ^ heatherharren (December 30, 2015). "A Brief History of Rapidan Township". Blue Earth County Historical Society. Retrieved June 26, 2024.
  11. ^ Barr (2021). "Rapidan Dam Repair Feasibility Study". pp. 5, 10–11. Retrieved June 24, 2024.
  12. ^ "Rapidan Dam Repairs Executive Summary". Blue Earth County. Retrieved June 25, 2024.
  13. ^ "2021-Rapidan-Dam-Repair-Feasibility-Study_Nov_2021_Final". blueearthcountymn.gov. barr.com. Retrieved June 24, 2024.
  14. ^ "Rapidan-Engagement-Summary-Report_Web". blueearthcountymn.gov. Retrieved June 24, 2024.
  15. ^ Grey, Meghan (August 17, 2022). "Survey results released on future of Rapidan Dam". KEYC. Retrieved June 26, 2024.
  16. ^ Moser, Riley (June 25, 2024). "Where is the Rapidan Dam and what is its history?". CBS News. Retrieved June 26, 2024.
  17. ^ Krohn, Tim; Murray, Robb; Arola, Brian; Lagerquist, Mike (June 24, 2024). "UPDATE: Rapidan Dam in 'imminent failure condition'; Mankato floodwalls expected to handle any surge". Mankato Free Press. Archived from the original on June 24, 2024. Retrieved June 24, 2024.
  18. ^ Swanson, Stephen (June 24, 2024). "Rapidan Dam breached by flood waters in southern Minnesota; nearby residents warned to potentially evacuate". CBS News. Retrieved June 24, 2024.
  19. ^ Madani, Doha (June 24, 2024). "Rapidan Dam in Minnesota is in 'imminent failure condition,' officials warn". NBC news. NBC. NBC. Retrieved June 24, 2024.
  20. ^ Keegan, Mitch (June 24, 2024). "BREAKING: Concerns over the integrity of the Rapidan Dam". KEYC-TV. Retrieved June 24, 2024.
  21. ^ Fischenich, Mark (July 18, 2021). "Ask Us: Rapidan Dam collapse wouldn't bring a wall of water to Mankato". Mankato Free Press. The Free Press. The Free Press. Retrieved June 24, 2024.
  22. ^ Thompson, Howard (June 25, 2024). "Rapidan Dam partial failure: What happens if the dam collapses?". FOX 9. Retrieved June 26, 2024.
  23. ^ Keegan, Mitch (June 26, 2024). "BREAKING: House next to Rapidan Dam falls into Blue Earth River". KEYC News. Retrieved June 26, 2024.
  24. ^ "Video: House near Rapidan Dam falls into Blue Earth River". MPR News. Video by Brycen Keech. June 26, 2024. Retrieved June 27, 2024.{{cite news}}: CS1 maint: others (link)
  25. ^ "Rapidan Dam house falls into Blue Earth River, caught on video". Fox9. Fox. June 26, 2024. Retrieved June 26, 2024.
  26. ^ WCCO Staff (June 28, 2024). "Video shows Rapidan Dam store being torn down, days after partially collapsing in southern Minnesota - CBS Minnesota". www.cbsnews.com. Retrieved June 29, 2024.
  27. ^ Nace, Aki (October 7, 2024). "Rapidan Dam Store temporarily reopens in Mankato - CBS Minnesota". www.cbsnews.com. Retrieved October 8, 2024.
  28. ^ Krueger, Andrew (June 27, 2024). "Owners of flood-imperiled Rapidan Dam Store vow to reopen; community rallies to support family". MPR News. Retrieved June 27, 2024.
  29. ^ "Rapidan Dam partial failure sent more than a century's worth of sediment pollution downstream". MPR News. July 5, 2024. Retrieved September 26, 2024.
  30. ^ Goldberg, Michael (September 1, 2024). "Rapidan dam and bridge, damaged during flood, will be demolished". hometownsource.com. Associated Press. Retrieved September 26, 2024.
  31. ^ Krohn, Tim (September 24, 2024). "County accepts bid to remove damaged highway bridge by Rapidan Dam". Mankato Free Press. Retrieved September 26, 2024.

Sources

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