First Hydro Plant with a Conduit Spillway: 4.96-MW Mio

Hydro Hall of Fame
 

For more than 100 years, the 4.96-MW Mio hydroelectric facility in Michigan, U.S., has been as dependable as sunrise and sunset. This dependability helped lead to Mio being inducted into the 2017 Hydro Hall of Fame.

The 4.96-MW Mio facility in Michigan has an under-sluice spillway built into its powerhouse foundation.

The 4.96-MW Mio facility in Michigan has an under-sluice spillway built into its powerhouse foundation.

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Consumers Energy’s hydroelectric facilities on the Au Sable River were constructed to power urban areas in southern and central Michigan in the U.S. The electricity the facilities produced eventually became instrumental in providing power to meet the state’s industrial needs.

The fourth of six hydropower facilities Consumers Energy built, and situated western-most upstream about 49 miles from Grayling, Mich., the Mio hydroelectric scheme is on the Au Sable River near the city of Mio.

Constructed between 1914 and 1916, Mio was the first hydroelectric plant in the world to use a conduit or under-sluice spillway. Before this, all dams included a massive above-ground concrete spillway that typically included a system of tainter gates to pass excess flows. Mio was constructed with one tainter spillway gate as a margin of safety for the new design.

William W. Tefft, a civil engineer and vice president at Consumers Energy at the time this facility was built, invented and patented the under-sluice spillway. Tefft’s innovation reduced tailwater erosion during spill operation, reduced construction expense and eliminated wintertime icing using tainter gates for spilling water.

The Mio under-sluice spillway was built into its powerhouse foundation, which eliminated the need for the above-ground structure.

The system was refined and used at subsequent Consumers Energy hydro projects, including the 8-MW Alcona hydropower project, built in 1924 and also located on the Au Sable River.

Video: Mio Dam: Allis Chalmers turbines were transported by railroad to Luzerne, Mich., and then loaded onto horse-drawn equipment and transported to the site

Video: Mio Dam: Allis Chalmers turbines were transported by railroad to Luzerne, Mich., and then loaded onto horse-drawn equipment and transported to the site. http://bit.ly/2AbyWYy

Looking back

In the early 1900s, most of the land along both banks of the Au Sable River from the unincorporated community of Oscoda where the river enters Lake Huron to the city of Mio was owned by lumber baron and Oscoda County resident Henry Loud and his sons.

One son, Edward Loud, was an officer in his father’s lumber company, Loud and Son’s. He teamed with William A. Foote, founder of Consumers Energy, to build six power-producing Au Sable River dams, all of which currently remain operational. They include:

  • 6-MW Five Channels, completed in 1912;
  • 9-MW Cooke, completed in 1912;
  • 4-MW Loud, completed in 1913;
  • Mio, completed in 1916;
  • 9-MW Foote, completed in 1918; and
  • Alcona, completed in 1924.

Mio presented unique challenges

Unlike the situation for the previous three dams built along the river (i.e., Cooke, Five Channels and Loud), there were no railroad lines for access to the isolated site where Mio was to be built.

According to local sources, the two Allis Chalmers turbines were transported using numerous local lumbering railroads to Luzerne and then loaded onto horse-drawn equipment and transported to the site. Other equipment — some weighing as much as 30 tons — was hauled by horse-drawn wagons during the summer.

The dam embankment is constructed of interlocking pieces of steel sheet piling topped with a reinforced concrete core wall surrounded by soil on the upstream and downstream sides, similar to other William Fargo-designed midwestern dams. The powerhouse was supported on wood pilings.

In 1915, during Mio’s construction, Foote, the founder of Consumers Energy, formerly Consumers Power, died at his home of a heart attack. Tefft, who was married to Foote’s daughter, grieved along with Foote’s younger brother, James B. Foote, who was chief engineer for Consumers Energy.

In 1916, Mio Dam was commissioned without fanfare. A monument to 20th century ingenuity, the powerhouse continues to operate with the same turbines and generators that were installed a century ago.

The 23-mile stretch between Mio Hydro and Alcona Pond was designated a National Wild and Scenic River in 1984. The watershed provides habitat for bald eagles and the endangered Kirtland’s warbler and numerous recreational opportunities to hike, camp, fish, paddle, picnic and bird watch.

In 2005, Mio Dam was named a Michigan Historic Site.

Mio Dam, as it appears today on Au Sable River in Michigan.

Mio Dam, as it appears today on Au Sable River in Michigan.

Maintaining Mio

Similar to any mechanical equipment, maintaining Mio’s equipment requires attention to detail, technical expertise and knowledge of modern technology.

“Mio Dam is a testament to the engineers who designed it, the workers who built it and the employees who have kept it operating reliably and efficiently for 100 years,” said Bill Schoenlein, manager for Consumers Renewable Generation. “We carefully monitor the condition of all our hydropower dams and make necessary repairs to ensure the structures remain safe.”

Since the mid-1960s, Mio Dam has operated as a run-of-river dam. The river flow leaving the hydro plant and moving downstream matches the flow entering the pond upstream. Since then, automatic gate operators provided through North American Hydro have been added to the generators for run-of-river controls that automatically adjust the unit governors to maintain the steady 29 feet head elevation, and therefore maintain run-of-river flow through the powerhouse.

As a conservative safety measure, a static emergency overflow spillway with energy dissipation was added in 1988 to the embankment on the south side of the plant. If the pond level rises above, the spillway sill which is what the turbines can pass through the under-sluice spillway, the water overflows over the static spillway.

Mio Dam, at age 100, continues to serve.

Mio Dam, at age 100, continues to serve.

After 92 years of freeze-thaw cycles, the upper portion of Mio Dam’s concrete core wall was repaired in 2009 to replace deteriorated concrete sections. Consumers Energy also installed a cool water upwelling system in 2009 to reduce river water temperatures as negotiated with a comprehensive settlement agreement with multiple resource agencies. downstream to improve habitat for fish and other aquatic life.

The core wall repair work required Mio Pond to be drawn down to about 8 feet below its normal head level of 29 feet. While the pond was lowered, Consumers Energy, the Huron Pines Conservation and Development Council and the Michigan Department of Natural Resources and Environment teamed up to enhance fish habitat. Trees were placed in Mio Pond, bass spawning benches were installed and a rock reef was constructed.

Over the years, Consumers Energy has installed canoe portages and launches, barrier-free access sites, boardwalk structures, stairways, restroom facilities and parking lots for public use near Mio Dam.

All of the above projects were approved by the Federal Energy Regulatory Commission, which regulates the facility.

Pictured Circa 1915, the Mio hydropower facility was completed in 1916.

Pictured Circa 1915, the Mio hydropower facility was completed in 1916.

Centennial celebration

During its 100th birthday celebration in 2016, about 225 people took advantage of the rare opportunity for a public tour to step inside the Mio powerhouse and view the century-old turbines and generators.

Consumers Energy staff created displays for the milestone anniversary tracing the history of Mio Dam. The displays showed how energy is created from water and provided a better understanding of the contributions the dam makes to the area’s economy through tourism, recreation and property taxes.

Mio Dam displays can be viewed at the Lumberman’s Monument Visitor Center, along with those of Cooke, Five Channels and Loud dams and other Au Sable River hydropower facilities. The center is located on the River Road Scenic Byway and on the banks of the Au Sable River and Cooke Pond in Iosco County. The facility is owned and operated by the United States Forest Service, which is provided funding through the FERC license to operate Au Sable River hydro facilities.

A civil and hydraulic engineer, William W. Tefft, invented and patented the conduit or under-sluice spillway installed at Mio Dam.

A civil and hydraulic engineer, William W. Tefft, invented and patented the conduit or under-sluice spillway installed at Mio Dam.

“Mio Dam has been a reliable generator of clean, renewable energy for Consumers Energy customers for 100 years and will continue to serve customers in the future,” Schoenlein said. “Harnessing the energy of falling water to produce electricity was one of the greatest electrical achievements of the 20th century. It helped kick-start the economy, improved our quality of life and led to further development of the electric industry.”

In all, Consumers Energy has 13 hydroelectric plants operating on five Michigan rivers that have a combined installed capacity of about 132 MW. This source of renewable energy annually produces about 420,000 MWh — enough to serve more than 70,000 customers in the Great Lakes state.

Terry DeDoes is senior public information director for Consumers Energy.

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