Red Rock Hydro

[Picture of Red Rock Hydro] [Picture of Red Rock Hydro] [Picture of Red Rock Hydro] [Picture of Red Rock Hydro]

Project Location: Pella, Iowa

Client: WMMPA

Synopsis of work:

The unpredictable, rushing water of the Des Moines River is in the process of being harnessed to benefit a network of 60 municipalities. In August 2014, construction began on the Red Rock Hydro Project, located near Pella, Iowa, that includes a two-unit turbine powerhouse, intake structure, 26 T-shaped diaphragms, a 375-foot-long retaining wall, two steel-lined penstocks, and installation of turbines, generators and substation. When complete, the Red Rock Hydroelectric Project will produce up to 55 megawatts of electricity—enough to meet the needs of about 18,000 homes.

A significant milestone for the project in 2015 was completing the construction and overlapping placement of the 26 T-shaped diaphragms. One by one, crews trucked each large diaphragm cage 10 miles from the off-site staging area to the construction area on the upstream side of the dam. Cages weighed up to 98 tons with the largest measuring 132 feet long. Two massive cranes worked in unison to pick up each cage from its horizontal position on the truck and maneuver it to a vertical position for placement—a process that averaged 3.5 hours to complete.

Work for installing the first 21 cages was conducted from a platform at the 760-foot elevation on the upstream side. However, fluctuation of reservoir water levels due to rainfall in the Des Moines River watershed was an ongoing concern. The remaining five cages were installed from a temporary platform built 21 feet higher, which allowed construction at the site to continue as scheduled. The resulting 240-foot wall holds back the existing Red Rock earthen dam, and also provides a channel for water to enter an intake structure that will send water to the hydroelectric turbine and generator.

Downstream, Ames crews installed 85 five-foot-diameter secants, with excavation for the powerhouse performed in 10-foot lifts. Walers were installed with tie-back anchors drilled and grouted into the bedrock for stabilization, followed by consolidation grouting of the powerhouse outline to reduce water seepage into the rock excavation.

Work continues to be performed and carefully sequenced to meet its anticipated completion in the summer of 2018, without interfering with the existing dams operation.