By Tim Cazier, P.E., Golder Associates, Inc.
Since the late 1800’s, the City of Glenwood Springs has relied almost exclusively on Grizzly Creek and No Name Creek for their drinking water. The original Grizzly Creek diversion system consisted of a diversion structure approximately four miles above the confluence of Grizzly Creek and the Colorado River, about 800 feet of 24-inch diameter riveted steel pipe, and a 4½-foot diameter tunnel through solid rock to No Name Creek. The original Grizzly Creek diversion structure was constructed of wood cribbing and concrete. During the past 100 years, the diversion structure has been washed out and rebuilt several times. In addition, sediment and debris have plugged the inlet on a regular basis. The last time the diversion structure washed out was in 1995 (see photo 1). The diversion pipeline has rested on concrete cradles, most of which are still in good condition. An additional 75 feet of the pipeline was hung on the side of a cliff.
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Photo 1Old Failed Grizzly Creek “Gabion” Diversion Structure — Large rock in stream center, above structure was removed to construct new diversion structure. (Note wood cribbing under gabions on left side) |
The City of Glenwood Springs desired a reliable structure that would be able to divert up to 20 cfs, pass an extreme flood event without damage and a durable pipeline that could resist damage from avalanches and rock fall. The city selected a design/build team with Golder Associates as the designer and Gould Construction as the builder to complete the project over a six-month period in 1999.
The site is located in the White River National Forest which required modifications to the US Forest Service special use permit before construction could begin. In addition, the historic nature of the pipeline made it eligible for consideration under the State Historical Preservation Society guidelines for structures with historical significance. This required the pipeline be surveyed and documented with archival quality photographs prior to construction.
The engineering challenges for the project were primarily cost and constructibility. A relatively maintenance free design was also highly desired by the city. All of these challenges were a result of the remote location. Helicopters were required to bring in materials and equipment. The unit cost of construction materials was dictated primarily by the cost of ferrying them in via helicopter and not the cost of the materials themselves. Concrete was chosen as the ideal material with which to build the diversion structure because it required significantly less volume and consequently, fewer helicopter trips. Excavation of the diversion structure foundation was accomplished using two small excavators (5,000 lbs. each) also flown in via helicopter. Coordinating flying schedules with local ski area and power line construction also reduced helicopter costs.
What to do with material from the diversion structure foundation excavation, turned out to be a problem that solved itself. The small excavators were too small to lift the spoil to any appreciable height, so excavated material was cast immediately downstream of the excavation. This additional material was enough to mitigate a problem resulting from the failure of the previous gabion diversion structure. The build up of cobbles behind the gabion diversion structure resulted in a steep drop when the gabion diversion failed. This was cause for concern with respect to what may happen to the new structure should the streambed degrade to historical grades and reduce the soil pressures on the toe of the new diversion structure. As a result of the foundation excavation, the streambed was graded to a constant grade about 100 feet downstream of the new structure thereby reducing the potential for severe scour at the toe of the new structure.
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Photo 2New Grizzly Creek “Ogee” Diversion Structure — Vent to left of structure indicates diversion pipeline location (Note self-cleaning drop inlet grate in diversion crest) |
Three control gates were needed in the design: one for the pipeline, a return-to-stream gate within the diversion inlet and flush pipe in the diversion structure itself. The flush pipe was designed to be opened in order to flush sediment that may build up in front of the secondary vertical inlet and lower the water level behind the diversion structure for maintenance purposes. Both the flush pipe and the return-to-stream pipes could be used to maintain low flows in Grizzly Creek during drought periods.
The old 24-inch riveted steel pipeline was replaced with 3/8-inch wall, 24-inch diameter steel pipe that will resist damage from rock fall. Demolition and removal (via helicopter) of the old pipeline was coordinated with the delivery of construction materials in order to reduce costs. A 50-foot long avalanche protection wall was constructed on the uphill side of the pipeline where the original pipeline had been severed by an avalanche.
The design/build team was notified February 8th of this year that the project has been selected as a winner of the Build America Award, Design Build New Category, for 2000 by the Associated General Contractors of America (ACG). The award will be presented during ACG’s 81st Annual Convention at the Build America Awards Dinner in Seattle, Washington on March 11, 2000.