Engineers Complete Project to Make Pueblo Dam Safter: Denver Post

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Special to The Denver Post / Sean Cayton Blake Greenhalgh walks through the spray near a flow- by the Bureau of Reclamation. The test was the final one ing spillway at Pueblo Dam during,a flow test Thursday for a $29 million project aimed at making the dam safer. PUEBLO Engine ers complete project to make Pueblo Dam safer By Erin Emery Denver Post Southern Colorado Bureau Tom Musgrove stood on the back side of the Pueblo Dam on Thursday and grinned as engineers opened the gates at the reservoir, releasing thousands of gallons of water for a final test of a $29 million construction project aimed at making the dam safer. Water gushed from the chutes of the spillway outlets at a rate of 50 mph, slammed into newly installed concrete blocks and shot straight up into the air, creating 35 -foot gey- sers. "It's like Old Faithful," Musgrove said. In 1997, the U.S. Bureau of Reclamation, the federal agen- cy that ensures dams are safe, recommended that the Pueb- lo Dam be modified. Safety inspectors found shale seams, which can slide, in the ground beneath the dam. "They found that the Pueblo Dam had the potential to slide forward because there was nothing buffered up against it," said Kara Lamb, spokeswoman for the bureau. Using computers, engineers in Denver created hydraulic models and decided to bolster the dam by adding 20 feet of roller- compacted concrete to a stilling pool on the down- stream side of the dam. Before the project, the stilling pool — 550 feet wide and 80 feet long — was 30 feet deep. It's now only 10 feet deep. In essence, Lamb said, construction workers built "a giant doorstop" that prevents dam movement. The project took three years, Lamb said, because that very sturdy compacted concrete was laid an inch or so at a time. "You've got to lay it down, roll it, lay it down, roll it and then install the rock bolts," Lamb said. Engineers added 22 giant concrete blocks in the stilling pond and one outside the three chutes to blunt the force of the water released from the spillway outlets. The stilling pool collects water and calms it before it is sent down the Arkansas River. "It looks turbulent, but there's a lot of air — that's good," said John Trojanowski, design team leader for the Pueblo project. "It hits the blocks and that breaks up the flow." Water was released at a rate of about 5,000 cubic feet per second — less than the allowed maximum of 6,000 cfs. The U.S. Army Corps of Engineers says the start of flood season in Colorado is Friday, and Thursday's release was scheduled to provide room for spring flood control. None of the water was designated for use by farmers, mu- nicipalities and businesses downstream. Even before construction, the Pueblo dam never showed signs of movement. But the bureau felt it was a worthwhile project since thou- sands of people live downstream from the dam in Pueblo and southeastern Colorado. The dam at Horsetooth Reservoir west of Fort Collins is another in need of repair. This fall, the bureau will begin a project that will cost well over $100 million to improve four earthen dams, which have had problems with seepage. Workers will add a filter system and will strengthen the dams, which were built in the mid - 1940s. Water delivery will not be affected but recreation will. Because workers will strip off the face of the northern dam, making it weaker, the water level will have to be low- ered. That will render all five boat ramps at the reservoir useless. Lamb said the bureau is working with Larimer County to provide access for boaters during the three to five years the dams will be under construction.