In 2000, federal funding for an inflatable dam — a design that's been installed in one form or another in various climates around the world since the 1960s — was sought and denied. It took the widespread devastation wrought by 2008's 500-year flood to secure funding from the U.S. Economic Development Administration (EDA). The Iowa Community Development Block Grant (CDBG) Program covered the remaining 25% of the necessary funds.
Funding was contingent on Iowa State Historic Preservation Office requirements to develop photographic records of the construction process, preserve samples of the 1880s crib dam, and construct a scale model of previous dams for display. The history of the former dams has been captured through construction of a one-third-scale replica now on display in a park adjacent to the river.
Upon receipt of funding, local contractor Peterson Contractors Inc. was hired to demolish the existing dam and build the new dam. The former mass-concrete ogee-shaped structure was 310 feet long with a hydraulic height of 12 feet and an uncontrolled (ungated) spillway. A 36-foot-long level control gate section included four 12x6-foot vertical timber gates. The sill elevation was 11 feet lower than the spill-way. Aside from the hydroelectric turbines, the gates were the only means of controlling upstream pool levels.
Stanley Consultants designed the new dam with a gated spillway that maintains a set water level elevation established by the city and approved by the Iowa Department of Natural Resources. It adjusts as needed to changing flow rates. This is accomplished by continuously monitoring river water level with pressure transducers that feed information to a programmable logic controller (PLC), or computer. The computer senses when water level is deviating from the desired level, then either releases or supplies compressed air to the bladder, thus lowering or raising the gate. Crest gates were installed over the length of the former uncontrolled spillway. They are “hinged” at the bottom and rotate about this hinge to raise or lower, thus controlling discharge and pool level.
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.
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.
The top-discharging crest gates offer more available flow area without the flow restrictions inherent with radial gates. Bottom-discharging gates have more potential to be blocked by debris and ice, thus creating the potential for reduced discharge capacity, especially during periods of high river flows when discharge capacity is paramount. Crest gates require no operator's bridge or top-mounted hoist system, both of which can lead to debris/ ice blockage and add significant capital cost to a project.
Actuation (movement) of the gates is provided by air bladders. Air supply is provided by steel piping embedded with the underlying dam concrete. Pressurized air is supplied by redundant air compressors located within a public works building adjacent to the dam. Special steel plates are installed at the abutments to provide a smooth sealing surface for the gates. These abutment plates are heated to allow winter operation. The dam automatically and constantly compensates elevation for the gates to maintain upstream elevation, allowing the City of Waverly Electric Utility Board to operate the hydroelectric facility under a wider range of flows.
To reduce upstream flood levels, the crest of the existing dam had to be lowered 6 feet. Lowering the effective crest greatly increases spillway capacity and decreases upstream river levels during extreme flood events. But it also required the concrete crest to be partially demolished, significantly reducing the structure's overall weight and thus its stability (resistance to movement due to upstream water pressure). Stanley Consultants counteracted the loss of “ballast” with 25 rock anchors that were installed by drilling a hole through the dam and 60 feet into the underlying bedrock. A steel bar was then placed into the hole and grouted in place. Once the grout cured, the bars were tensioned and locked. Construction of the new dam also required that the existing concrete spillway be demolished and reconstructed one half at a time. The contractor 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 build much of the cofferdam with just two equipment operators: one working an excavator and one an end loader.
Construction of the first half required a cofferdam to be constructed two-thirds of the way across the river. The constricted flow resulted in the cofferdam being washed out twice during high water events.
Construction of the new dam began in October 2010 and continued through the winter of 2010–11 when the river is historically at its lowest flow rate. The east half was completed and operational by May 2011, at which point the city was already benefiting from the enhanced flood protection. Work on the west half began in June 2011 and was completed and operational by October 2011.
In spring 2011, while the east half was finished and completely operational and a cofferdam was protecting the work zone on the west half of the river, river flows increased from approximately 1,000 cfs to 9,000 cfs. This typically would have produced a 3-foot rise in the river immediately upstream and potentially washed away the cofferdam. Instead, even with just half the new dam operating, the water level upstream remained level.
Now fully operational, the dam protects hundreds of homes and businesses from a 100-year flood, preserves power generation at Iowa's oldest operating hydroelectric facility, and enhances recreational opportunities by maintaining a consistent pool elevation upstream of the dam.
—Cherry (firstname.lastname@example.org) is Public Works Director & City Engineer for the City of Waverly, Iowa; and Weber (email@example.com) is a project manager for Stanley Consultants Inc.