Denver Water approved to expand reservoir behind Gross Dam

The U.S. Army Corps of Engineers has issued a record of decision and a 404 permit allowing Denver Water to proceed with a $380 million expansion of its Gross Dam and reservoir.

Denver Water says the additional storage capacity in Gross Reservoir will help prevent shortfalls during droughts and help offset an imbalance in the utility’s collection system.

“Denver Water appreciates the Corps’ dedication and commitment to careful study of the anticipated impacts of this project,” said Denver Water Chief Executive Officer Jim Lochhead. “We will complete this project responsibly, as evidenced by our actions during the public process and the resulting robust environmental protections we’ve agreed to along the way.”

Gross Dam was built in the 1950s and was designed to be expanded in the future to increase water storage capacity. The project is called a “dam raise” because engineers will increase the height of the dam by 131 feet, nearly tripling the reservoir’s storage capacity. The concrete gravity arch dam is 340 feet tall and 1,050 feet long.

When completed, Gross will be able to hold an additional 77,000 acre-feet of water. Current capacity of the reservoir is 42,000 acre-feet. Denver Water will use 72,000 acre-feet of the added capacity “as insurance against future challenges, including droughts, wildfire and future growth.” The additional water supply will come primarily during the high runoff months of May, June and July. Additional water will not be diverted for this project during dry years. The remaining 5,000 acre-feet will be set aside for an environmental pool that provides additional water for South Boulder Creek during low-flow periods.

The Corps action concludes more than a dozen years of study and debate, Denver Water says. According to a press release, the project earned key endorsements from Colorado Governor John Hickenlooper, state and federal lawmakers, major environmental groups, local mayors and city councils, chambers of commerce and economic development corporations, county elected officials and water interests.

Preconstruction activities are expected to begin in 2018, with the entire project expected to be completed in 2025, Denver Water says.

The current next step will be approval of the hydropower license amendment application by the Federal Energy Regulatory Commission. the license amendment allows Denver Water to expand the dam, add capacity to the existing 7.6 MW, and modify the recreation management plan to accommodate the new high water line.

The Gross hydroelectric project was commissioned in February 2008 after entering full operation in August 2007. The powerhouse contains two horizontal Francis turbine-generator units and is about 400 feet downstream of the existing valve house.

In May, HydroWorld reported Black & Veatch had been selected to provide owner’s representative and program management services for Denver Water during the Gross Reservoir expansion.

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