Client:
Regional Water Rural Water Association

Avoca, IA

Key Features:

  • Designed to be navigable because the river is popularly used for paddle-craft. It also allows fish to navigate the structure, as required by the Iowa DNR.
  • The dam raises the water level in the river, which recharges the water levels in the aquifer that Regional’s well field draws from.
  • The dam includes six arching boulder weirs, made up of three-to-five-feet diameter boulders. This helps the structure resist erosion and increases longevity of the dam.

Key Experience:

  • Project was labeled substantially complete just six weeks after the contractor was authorized to proceed.
  • Within a few weeks of the dam being completed, Regional saw a water level increase of two to three feet in the closest wells
    to the dam.
  • Concurrent with the permitting effort, DGR helped Regional secure a $1,000,000 grant through the USDA Rural Development Emergency Community Water Assistance Grant (ECWAG) program.

Rock Riffle Dam

Witnessing the sharp decline in Regional’s wellfield capacity during 2020 and 2021, Regional and DGR Engineering (DGR) began conversations in 2021 with the intent of shoring up Regional’s wellfield capacity.

The principle behind a riffle dam structure is that the water in the river channel and water in the adjacent aquifer are hydraulically connected. Raising the level of the river raises the water level in the aquifer, giving wells more available drawdown to pump more water. The correlation between the river and the aquifer levels is dependent on the geologic conditions of the aquifer. DGR helped Regional secure the services of LRE Water, a hydrogeology firm which used previously-collected geologic records to set up a model, which confirmed that the aquifer and the river had a strong hydraulic connection.

Obtaining permits from the DNR to build the riffle dam required significant effort. Notably, the river programs and fisheries divisions were concerned respectively about impedance to recreational paddlecraft and fish passage. It was important that the structure was designed with a deep, center channel to allow navigation of the river. DGR used HEC-RAS river modeling to simulate multiple iterations of dam heights to determine the maximum height that could be used. Through the modeling and permitting process, a structure height was settled on to raise the upstream river level 2.83 feet.

The structure has netted Regional an approximately 100 GPM in additional wellfield capacity. This may not seem like much, but it makes a big difference in helping Regional meet its customers’ water needs. Perhaps more importantly, it helped stop the drought-induced aquifer decline and it buys Regional time as they and DGR pursue additional water supply solutions.