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Embracing the Cedar River

Embracing the Cedar River

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    Air bladders inflate and deflate to raise or lower the dam's crest gates. Pressurized air, supplied by compressors inside a Waverly Public Works building adjacent to the dam, flows through steel piping embedded in the dam's concrete. Steel plates installed at the abutments provide a smooth sealing surface for the gates and are heated to allow winter operation. Photo: Stanley Consultants

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    William Harmon built Waverly's first dam in 1853. The stone and timber structure powered a sawmill that provided lumber for the growing town. By the 1880s entrepreneurs had built a more uniform and durable crib dam to harness the Cedar River's power for industrial use. The structure was made using large timbers connected with steel spikes to form “cribs,” then hand-filled with stone to add weight. Photo: City of Waverly

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    Photo: City of Waverly

    OUT WITH THE OLD. The city's mass-concrete ogee-shaped dam was 310 feet long and 12 feet high and since it had no gates was considered “uncontrolled.” A 36-foot-long level control gate section included four 12x6-foot vertical timber gates. Aside from hydroelectric turbines, the gates were the only way of controlling upstream pool levels. Result: persistent flooding.

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    Photo: Stanley Consultants

    IN WITH THE NEW. An “inflatable” dam maintains a constant water elevation under variable river flows by automatically raising or lowering the crest gates. In addition to saving at least 450 homes and businesses from flooding, the design allows the City of Waverly Electric Utility Board to operate its hydroelectric facility with more reliable pool levels.

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    UNCOMMON SOLUTION TO A COMMON PROBLEM

Launch Slideshow

Four steps to cost-effective dam replacement

How a community of 10,000 cut construction costs without compromising safety.

Four steps to cost-effective dam replacement

How a community of 10,000 cut construction costs without compromising safety.

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    Photos: Stanley Consultants

    The existing dam’s concrete spillway was demolished and reconstructed one half at a time. Peterson Contractors Inc. built a causeway and cofferdam using sand from a nearby migrating sandbar. This eliminated the need to haul in hundreds of loads of material and made it possible to construct much of the cofferdam with just two equipment operators.
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    Air bladders consist of reinforced, layered rubber, approximately ¾ inch thick. Each bladder has special fittings that connect to embedded air piping via flexible connections.
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    Since much of the spillway concrete was removed, the dam required measures to maintain its stability (resistance to movement due to water pressure). The solution consisted of rock anchors drilled through the dam and into the underlying bedrock. The weathered and fractured bedrock below the dam was improved via a foundation grouting program.
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    Steel panels provide the strength necessary to hold back water and protect the rubber bladders from potentially damaging ice, debris, and sunlight. Steel anchor bolts, embedded into the underlying concrete, secure the panels, hinge, and bladders to the dam.

By Mike Cherry, PE, and Martin Weber, PE

WAVERLY DAM REPLACEMENT

Owner: City of Waverly, Iowa, Public Works
Engineering and design: Stanley Consultants Inc., Des Moines, Iowa
Gate supplier: Obermeyer Hydro Inc., Fort Collins, Colo.
Contractor: Peterson Contractors Inc., Reinbeck, Iowa
Cost: $4.4 million
Funding: U.S. Economic Development Administration (75%); Community Development Block Grant (25%)

For more than 150 years, the Waverly Dam has been an integral part of picturesque Waverly, Iowa; population 10,000.

In 1853 city founder William Harmon built his family a cabin on the Cedar River's heavily wooded east side and a dam of stone and timbers to power his sawmill, which would provide lumber for the growing town. Additional water-powered mills followed, including a flour mill and a woolen mill. By the 1880s entrepreneurs had built a more uniform and durable crib dam to harness the river's power for industrial use. The crib dam was constructed using large timbers connected with steel spikes to form “cribs,” then hand-filled with stone to add weight.

Waverly boasts the oldest operating hydroelectric plant in the state (circa 1908), which annually saves the city $100,000 on electricity. Over the years the river's value to the city's industrial and economic development led to extended development on the adjacent floodplain and a need for more power.

In 1915, the original crib dam was replaced with a concrete dam and elevated spillway. Unfortunately, the new structure plagued residents with decades of persistent and regular flooding. Memorable flood events occurred in 1917, 1933, 1945, 1948, 1961, 1965 (twice), and 1993 (twice). A 100-year flood hit in 1999 and a 500-year flood struck in 2008, destroying hundreds of homes and businesses.

After the 100-year flood of 1999, Waverly's Department of Public Works retained Stanley Consultants Inc., which had designed improvements to the dam in the 1940s, to perform a flood protection study. The engineering firm identified two methods of flood protection: levees or an “inflatable” dam.

Surprisingly, at public forums the inflatable dam concept held the popular appeal with residents. Despite the hardship it has caused, most embrace the Cedar River as a recreational and aesthetic asset and feel their city should be “one with the river.” They considered levees to be the equivalent of turning their back on this resource and creating a visual obstruction that would put the community at odds with it. An inflatable dam, on the other hand, would maintain recreational opportunities and hydroelectric functions while also preserving the community's historical look and feel.