The ABC’s of Dam Construction
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What Has Happened at Stevens Reservoir?
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Rome wasn’t built in a day, as the saying goes, and neither are dams.
The Stevens Reservoir enlargement project process began nearly twenty years ago in 1989 with preliminary engineering and survey work. In 1997, PAWSD finalized negotiations on the acquisition of properties required for the enlargement. By raising the dam ten feet, the reservoir will be enlarged from 624 acre-feet to 1,775 acre-feet, comprising 162 acres of surface area. Until this project is completed, the PAWSD service area cannot be expanded to include new development – the District simply does not have the water supply and drought safety margin. In 1999, with a short exception in 2001, the Board of Directors placed a moratorium on new District inclusions in order to ensure that the vacant properties within the District, which the District is obligated to serve should they be developed, would have available water.
Stevens Lake dam in 2007, after breach and prior to construction
Although it sounds simple to raise an existing dam, thus enlarging an existing reservoir, in truth it is a complex and costly process. After the land needed for expansion was acquired, the permitting required by the Army Corps of Engineers and the Forest Service began (Forest Service permitting was necessary in order to replace a portion of the Dutton Ditch with a pipeline which transports diverted water from Four Mile Creek to Stevens and Hatcher Reservoirs, thus insuring a more dependable raw water supply). During this time, resurveys showed that more land was required for the full enlargement. Negotiations on this land lasted for years and in the end, when the property was finally acquired in 2008, the price per acre was 10 times more than it had been eleven years prior!
Meanwhile, anticipating acquisition of remaining needed land, PAWSD geared up to begin dam construction. Davis Engineering completed the dam design and submitted plans to the State for approval and permitting. Weeminuche Construction Authority, the same contractor that is completing the Animas-La Plata project in Durango, was hired and began preliminary site work. EcoSphere Environmental and Aquahab were hired to work with the District on Corps-required wetlands mitigation issues, and biological and botanical compliance requirements.
Steven's Reservoir in 2007 after draining
When an existing dam is raised, its reservoir must first be drained completely. In early 2007 the existing reservoir was drained. The dam was breached in the summer of 2007 to begin the necessary site drying-out process. For every one foot in height this dam is raised, it must be 3 feet wider on both sides of the base. Therefore, in order to raise the dam ten feet it must be 60 feet wider at the base. Also, in order to accommodate the increased volume of water, the dam has to be lengthened from approximately 1,300 linear feet to 2000 linear feet. In order to construct the increased dam size, the dam site must be dry. Also, in order to attain the earthen embankment material required for the dam, the reservoir must be dry so that the basin can be used as a source “borrow pit” for this material. Therefore, it was important to drain the lake well ahead of anticipated construction.
When building an earthen-fill dam, soil moisture content is everything. The State of Colorado requires an optimum moisture content (percent moisture) in the soil after compaction, with an allowance of one percent lower to two percent higher. For the Steven’s project, the acceptable range is approximately 14 percent to 17 percent. Western Technologies, Inc. is the on-site geotechnical firm that tests the soil, both in the borrow pit and at the dam site. These reports must be furnished to the State. The operators turn the soil in the borrow pit until it reaches the optimum moisture content, and then deliver it to the dam site where it is spread and compacted in 6 inch lifts, and tested again. If at any point the soil fails the test – and relative humidity in the air is part of the soil moisture equation – the soil must be scarified and turned until it dries.
Dam construction in August
An abundance of summer rain wreaked havoc on the construction schedule. Approximately 70 work days were lost due to weather. As the days became cooler and shorter, it became more and more difficult to work the moisture out of the soil. Once frost set in, at least an hour of work in the morning was required to remove the frost protection (approximately two feet of dirt to cap what has already been compacted and tested) and another two hours in the evening to replace the frost protection. Extra crews and equipment were brought in, and work weeks were extended to seven days. The autumn proved relatively warm and dry, and on November 24, 2008, the dam was completed!
But the project was not...wetlands mitigation was required by the US Army Corps of Engineers (ACOE). This work, consisting of preparing 27 acres around the lake for planting followed by planting 1300 shrubs and trees and 78,000 plant plugs, was conducted in stages. The wetlands mitigation was completed on June 10, 2010 and will be followed by three to five years of US ACOE monitoring.
As this project demonstrates, from land acquisition and permitting to final State dam approval, construction, and refilling, with unforeseen weather and even legal delays, enlarging a dam and reservoir can take decades. In the meantime, if there is not adequate, dependable water supply, no new development can be accommodated. Therefore, it is the PAWSD philosophy to plan early and revisit plans often so that, as in the case of Stevens Reservoir, the water will be available when it is needed.
Our Water is Our Future
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