Planning for dry weather
Last summer, Golden, Colo., completed construction of a 697-million-gallon water reservoir to augment the city’s water storage capacity and to protect the city from the effects of droughts. The $11.5 million Guanella Reservoir was built on the site of a sand gravel quarry and features a 34-foot-high dam.
Before the construction of the Guanella Reservoir, Golden only had 586 acre-feet of supplemental water storage available in the Urad reservoirs, which are located about 35 miles west of the city. During the summer of 2002, Colorado’s worst drought in history produced record low flows in Clear Creek, the city’s main water source. By late August, most of the supplemental storage in the Urad reservoirs was gone, and Golden was using one of its last remaining Clear Creek direct flow water rights.
On Sept. 6, the Colorado State Engineer’s Office ordered the city to stop diverting water from Clear Creek because Golden’s water right was superseded by other water users who diverted water downstream of Golden’s water treatment plant. The order essentially shut off the city’s remaining water supply, forcing it to implement drastic conservation measures and purchase about $145,000 of water from other Clear Creek users.
Since the 1980s, the city planned to build supplemental storage in an active gravel mine located about 25 miles west of Golden in Clear Creek County. However, construction was not scheduled to start until 2004 and would last two years, says Dan Hartman, Golden’s public works director. The drought changed the city’s plans. “The Golden City Council determined that the Guanella Dam needed to be designed and constructed by the end of 2003 and be ready for filling by March 2004,” he says.
Golden pursued a construction manager/general contractor construction approach that allowed building to begin several months sooner than with a typical design-bid approach. The city contracted with Englewood, Colo.-based W.W. Wheeler and Associates as the prime engineering consultant responsible for design, construction engineering and overall project management. Wheeler subcontracted geotechnical, structural and electrical engineering services to the Denver office of San Francisco-based URS.
Guanella Dam, an off-channel, zoned embankment dam, was built using fine-grained silts and clays from the gravel mining operation for the dam core and on-site soils for the dam’s outer shells. The dam is 1,925 feet long and transitions to a 1,975-foot-long homogeneous flood levee constructed along the northern reservoir rim. Water from the West Fork of Clear Creek enters the reservoir from a 2,200-foot-long inlet pipeline. The dam includes a 35-foot-high combined service spillway and low-level outlet intake tower; and a 54-inch-diameter, concrete-encased steel outlet conduit under the embankment to release low-level outlet and service spillway discharges from the reservoir.
A 2,160-foot-long, 85-foot-deep, soil-bentonite seepage barrier wall was built into a fine-grained silt layer that underlies the dam. The wall provides the dam foundation cut-off and controls alluvial groundwater adjacent to the reservoir. The design uses a unique bank storage water right that allows conjunctive use of surface water and alluvial groundwater and minimizes evaporation losses.
The combined total of the open water and bank storage in Guanella Reservoir is 2,140 acre-feet. The reservoir was filled to its full capacity by December 2004.
The reservoir can be filled and drained by an automated control system located in the Golden water treatment plant or at the dam site. When water is released from the reservoir, it travels down the West Fork of Clear Creek to its junction near Empire, Colo. From there, water is conveyed the remaining 24 miles to the Golden treatment plant. The Guanella Reservoir will provide Golden with critical drought protection that will allow it to withstand two successive dry water years equal in magnitude to the 2002 drought.
