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Storage for One More Century

By Edwin Friend, P.E., P.G., PMP, Blaine Dwyer, P.E., M.ASCE, D.WRE, and Casey Dick, P.E., PMP

Since its completion more than a century ago by a previous owner, Denver Water's Antero Dam has experienced seepage problems, forcing the utility to limit the storage capacity of the associated reservoir. After the completion of a carefully planned four-phase dam and spillway rehabilitation project, the newly revamped Antero Dam is expected to operate safely for another 100 years.

Rehabilitation of aging dams must be approached with care, caution, and respect for the unknown. Those conducting such rehabilitation efforts may encounter unanticipated conditions, while construction activities can result in unexpected responses from the embankment. A decade ago, Denver Water set out to rehabilitate its then-century-old Antero Dam, a 45 ft high, 4,200 ft long earthen embankment that had experienced seepage issues since shortly after its construction by a previous owner. The focus of the rehabilitation involved dam safety improvements to correct overly steep slopes; seepage through alluvium, the embankment, and shallow bedrock; inadequate slope protection; a lack of a foundation seepage cutoff; and unfiltered seepage at the downstream toe. The project also sought to reduce the potential for scour of the unlined spillway channel that could have resulted in an uncontrolled release of the reservoir. The result was the development of a carefully planned, four-phase dam and spillway rehabilitation project that was completed on time and below the construction bid prices in a harsh, high-elevation climate.

Located on the South Platte River in Park County, Colorado, at an elevation of 9,000 ft, the Antero Dam and its reservoir are owned and operated by Denver Water, and recreation amenities are managed by Colorado Parks and Wildlife. Completed between 1908 and 1909, the dam's embankment consisted of a partial hydraulic fill core and sandy clay to clayey sand shells placed by means of horse and wagon. Antero Dam is situated in a wide, shallow valley with variable alluvial soils over highly variable sedimentary and volcanic geology. Concerns related to seepage began shortly after construction of the dam and prevented complete filling of the reservoir.

Since purchasing Antero Dam and its reservoir in 1924, Denver Water often has taken several proactive steps to ensure the continued safe operation of the facility. In fact, Denver Water implemented a self-imposed storage restriction that was more stringent than that mandated by the state. Reservoir storage has been restricted to less than one-third of the total storage capacity.

Denver Water's experience in obtaining permits for other new storage in the South Platte River basin showed that an asset such as Antero Reservoir is worth saving, especially if any environmental effects associated with the dam's rehabilitation could be avoided or reasonably mitigated.

On the dam's 100th birthday in 2009, Denver Water, in close collaboration with the Colorado Division of Water Resources-also known as the State Engineer's Office (SEO)-embarked on a comprehensive potential failure mode analysis (PFMA) and quantitative risk assessment (QRA) to identify and address all the asset's deficiencies, including seepage, stability, and spillway adequacy. RJH Consultants Inc., of Englewood, Colorado, was retained to perform a comprehensive data review and develop design components. Experienced in evaluating and designing earthen dams and their foundations, RJH was selected to compile and review the data and provide a fresh perspective on a dam that has been repeatedly investigated since 1964.

The initial seepage concerns arose shortly after the reservoir began filling. The highest water level on record occurred in 1912, with a gauge height of 25.1 ft. (Gauge height is the depth of water above the invert of the outlet works pipe). During early operation of the dam, observers reported springs or sand boils at the downstream toe. Since the first filling of the reservoir, the downstream area has been wet and marshy, with areas of standing water that vary by season and reservoir elevation. A document prepared by Denver Water for its board in July 1957-available in the Colorado State Archives-states, "There always has been some question as to the safe elevation which may be maintained in the reservoir." Because of concerns regarding the seepage and other dam safety issues, Antero Reservoir was restricted to gauge heights ranging from 14.2 to 16 ft, well below the design gauge height of 40 ft, between the 1950s and 1984.

In 1979 an evaluation was performed in accordance with the dam safety program administered by the U.S. Army Corps of Engineers. This evaluation determined that the spillway had inadequate capacity for the probable maximum flood (PMF) and questioned whether the embankment would remain stable during PMF loading.

In 1984, the spillway channel was lowered by 7 ft to enable conveyance of the PMF. The 950 ft long excavated, trapezoidal spillway channel was 224 ft wide at the bottom and essentially flat along the flow path. No upstream control structure was constructed to maintain the reservoir pool elevation during spillway flows that would discharge to a narrow, steep, natural swale downstream. Material excavated from the spillway was placed along the downstream toe of the dam to create a 55 ft wide stability berm ranging in thickness from 2 to 10 ft. During berm construction, the subgrade in several locations was too wet and soft to place fill. In these cases, the contractor placed gravel encased in a geotextile fabric to bridge the soft and wet areas. These embankment modifications resulted in an increase of the reservoir's restricted level to a gauge height of 18 ft, which corresponds to a storage volume of 20,000 acre-ft.

Within three weeks of completion of the toe berm, seepage began exiting the top of the toe berm. A toe drain was installed along 20 percent of the total embankment length to collect this seepage. In 2005, when the reservoir was drained during a drought, the toe drain was reconstructed at a lower elevation, and flow in the drain increased.

In 1996, while the reservoir was drained, the outlet works was replaced along with 500 ft of embankment. During this construction, significant seepage was observed emanating from the exposed bedrock foundation. The reconstructed embankment section included a core trench into bedrock and a seepage collection system. Seepage flows into the drain system began after construction and continued to flow.

The highly complicated geology at the dam site consists of materials from older sedimentary rocks, including claystone, sandstone, and limestone, and younger volcanic rocks, including andesite, tuff, and various volcanic ash deposits. An understanding of the geology near the dam evolved dramatically over time as it was investigated by noted geologists using regional and site-specific information.

The site surface geology generally consists of fine- to coarse-grained interlayered and interlensed alluvial materials as well as intensely weathered and fractured bedrock of sedimentary and volcanic origin. The older sedimentary Maroon and Minturn Formations generally dip upstream between 40 and 50 degrees. Younger volcanic and sedimentary deposits-the Antero Formation and limestone-generally dip downstream up to 20 degrees.

Between 2009 and 2012, Denver Water considered alternative strategies to remove the Antero Reservoir storage restriction as the utility continued its commitment to provide a safe, reliable, long-term water supply to the 1.4 million people it serves. Denver Water evaluated overall dam safety, identified deficiencies, and performed investigations to provide data in areas in which information was missing. RJH performed a comprehensive review of the existing data and evaluations and developed a summary document that characterized the reliability of the various types of data. As this information was developed, Denver Water implemented a self-imposed restriction on the reservoir level, lowering the pool by 2 ft to a gauge height of 16 ft. This decrease in the reservoir level reduced the seepage and lowered piezometer levels.

In 2011, Denver Water engaged the SEO and several subject matter experts to perform a PFMA and QRA based on the summary document prepared by RJH. Denver Water used the results of the PFMA and QRA to focus remediation efforts on addressing the following potential failure modes (PFMs):

  • backward erosion piping through the foundation alluvium
  • alteration and degradation of the Antero Formation from weathering, resulting in an erodible material on the left abutment leading to internal erosion
  • seepage and internal erosion along the outlet works conduit
  • contact erosion at the bedrock-alluvium or bedrock-embankment contacts
  • embankment instability
  • backward erosion piping through the foundation alluvium under flood loading

Of these, the first two were judged to have the highest likelihood of occurring. Both were judged to be potential continuous seepage paths that could result in internal erosion of the foundation or embankment materials and lead to a catastrophic dam failure.

A benefit-cost analysis was then conducted to evaluate various concepts for rehabilitating the dam to current standards, addressing the outstanding PFMs, and extending the service life of the dam. Five concepts were evaluated:

  • construction of a sand filter into bedrock along the downstream toe
  • construction of a sand filter into bedrock along the downstream toe and the addition of a spillway weir
  • construction of a sand filter into bedrock along the downstream toe, a chimney drain, a toe drain, a blanket drain, a central barrier wall, flattened slopes, and a hardened spillway
  • construction of a toe drain, an upstream low-permeability core founded on treated bedrock, flattened slopes, and a hardened spillway
  • complete removal of the existing dam and construction of a new embankment and spillway

The estimated construction costs for these alternatives ranged from a few million dollars to more than $80 million.

The analysis included relative cost comparisons, evaluation of potential environmental effects and associated permitting implications, and the various levels of dam safety improvements. Each alternative was configured to minimize adverse environmental effects and associated permitting requirements while providing alternative levels of dam safety protection. Direct and indirect benefits were quantified, and the net present value method was used to identify the concept that had the largest benefit-cost ratio while adequately addressing the environmental issues and required levels of dam safety. Based on these considerations and on the limited construction seasons, Denver Water opted to conduct a phased rehabilitation. A summary of each phase, construction components, design firm, and prime construction contractor is provided in Table 1.

During preliminary design, a potential for significant spillway scour was identified. Denver Water, the SEO, and RJH performed a subsequent PFMA and QRA focused on spillway failure modes. The analyses concluded that the existing spillway may suffer extreme erosion that could potentially head cut (move upstream) to the reservoir pool and result in an uncontrolled release of the reservoir if subjected to a flow with a reoccurrence interval as short as a 100-year event. To address this new PFM, the spillway design was separated from the embankment portion of the project. The Denver office of HDR, which has its headquarters in Omaha, Nebraska, was engaged to further evaluate and design the spillway rehabilitation on the basis of its experience in hydraulic engineering and spillway design.

The project team designed the modifications to Antero Dam so that they would be confined within the footprint of the existing embankment and spillway, obviating the need to obtain a permit from the Corps in accordance with Section 404 of the Clean Water Act. By avoiding adverse environmental effects and associated requirements for formal compliance with the National Environmental Policy Act (NEPA), Denver Water likely saved several million dollars and several years of permitting processes. These estimates are based on the experiences of Denver Water and other municipal water agencies involving recent and ongoing NEPA compliance programs for Colorado water storage projects.

The design process culminated in a rehabilitation program consisting of three major components: the embankment, the spillway, and the outlet works. The embankment rehabilitation focused primarily on seepage and stability concerns. The spillway rehabilitation centered on arresting head-cutting erosion to prevent an uncontrolled release of the reservoir. The outlet works rehabilitation focused on raising the current intake tower to accommodate higher reservoir pool fluctuations and improve the operations and redundancy of the existing electrical and mechanical equipment. The general design concept sought to improve dam safety, enable safe reservoir storage, and provide another 100 years of service life.

Throughout the design process, Denver Water, RJH, HDR, and the SEO collaborated to identify and reduce design and construction risks. This strong working relationship produced thorough, carefully considered designs and resulted in a streamlined approval process with the SEO. This partnering enabled open communication and a productive design and review process. Table 2 presents the design components and associated PFMs that were addressed.

On the basis of seepage analyses results, the project team concluded that a combination of a toe drain, a sand filter trench excavated 2 ft into sedimentary bedrock and 5 ft into the volcanic Antero Formation bedrock, and a barrier wall excavated 5 ft into bedrock would be implemented. These measures combine to protect against internal erosion of the embankment, surficial foundation, and shallow bedrock foundation and adequately reduce exit gradients and the potential for the downstream toe to heave from seepage pressures that could result under full reservoir storage. Excavation depths greater than 5 ft into bedrock had limited effect on reducing exit gradients or seepage losses; therefore, a depth of 5 ft was determined to be the optimal depth to address dam safety concerns, recognizing that seepage would still occur in certain areas.

Read Companion Article: Cooperation Keeps Construction on Track 

The filter on the existing downstream slope was extended to 5 ft above the full reservoir elevation to protect against the internal erosion of the entire embankment and alluvial foundation during normal reservoir operations and half of the increase in reservoir level that is anticipated to occur during the inflow design flood (IDF). These components are discussed in further detail below.

The design specified a trench excavation supported by bio-polymer slurry because by doing so, the work could occur in the presence of water in the reservoir, even with ongoing seepage, high groundwater levels, and limited construction space. The 24 in. wide trench was backfilled with fine aggregate that met the requirements of the ASTM C 33 Standard Specification for Concrete Aggregates , which is published by ASTM International, of West Conshohocken, Pennsylvania. Alternative construction methods would have required draining the reservoir.

The barrier wall construction method and backfill material types were evaluated to address the following potential concerns:

  • trench stability and slurry loss
  • the ability to process on-site soil into backfill
  • durability and resistance to internal erosion of the backfill

On the basis of these evaluations, the team determined that barrier wall construction would entail a 3 ft wide trench dug by means of an excavator. During excavation, a bentonite slurry would be used to support the trench, which would then be backfilled with a mixture of soil and bentonite.

Based on stability analyses, the project team required a minimum upstream slope for the embankment of 2.7:1 (H:V) and a minimum downstream slope of 2.25:1. Denver Water selected the upstream slope to be 3:1 because the flatter slope facilitated the use of smaller and thinner riprap for slope protection, reducing project costs. The selected downstream slope was 2.25:1.

While designing the slope protection for the upstream embankment, the project team evaluated whether to use material from an on-site terrace gravel deposit as the bedding for the riprap. Based on gradation data, the on-site terrace gravel did not meet the design requirements proposed by the U.S. Bureau of Reclamation in Chapter 7, "Riprap Slope Protection" (revised in 2014), of its publication Design Standards No. 13, Embankment Dams , nor did it meet the design requirements of the Corps's 2004 publication, EM 1110-2-2300, General Design and Construction Considerations for Earth and Rock-Fill Dams . However, both publications allow for deviation from the design criteria if the designs are functional and cost-effective and if thicker zones of bedding are used. If material finer than the No. 40 sieve is eroded, the resulting gradation of material would meet the design guidance for gradation. Estimated erosion would result in 70 to 80 percent of the material remaining in place.

Therefore, the terrace gravel deposit was considered suitable for a riprap bedding material source if the thickness was increased from 1 to 2 ft. This modification resulted in significant environmental benefits and cost savings by obviating the need to import quarry material from more than 25 mi away.

For the spillway modification, the project team considered a broad range of concepts, including a roller-compacted concrete crest and chute design, as well as alternative measures pertaining to the crest structure, flow control, channel configuration, construction materials, and peak design discharges. Through detailed two-dimensional flow modeling, progressive erosion simulation and analysis, and collaborative evaluation by Denver Water, HDR, and the SEO, it was concluded that an appropriate spillway configuration at this site would consist of the following primary components:

  • a 110 ft wide concrete control weir with flanking containment dikes to minimize the initiation of erosion within the spillway
  • a hardened, 210 ft long, riprap-lined primary spillway chute that would significantly reduce the potential for erosion initiating within the spillway and convey flows that measure as high as the expected runoff associated with 50 percent of the probable maximum precipitation (PMP)
  • a 292 ft long, 2 ft wide, and 15 ft deep scour cutoff wall designed to halt the progression of head cutting to the reservoir during the IDF event and prevent the breach of the spillway and uncontrolled release of water from the reservoir

The spillway design was intended to avoid major erosion damage from flows associated with a 50 percent PMP event throughout the 185 sq mi upstream watershed and allow tolerable damage to the containment dikes and possibly to the spillway chute from rainfall exceeding the 50 percent PMP event. These potential damages were determined to be acceptable based on erosion and risk analyses that demonstrate the spillway can provide an acceptable level of safety without compromising the main dam or resulting in an uncontrolled release of the reservoir.

The spillway design was based on both 1-D and 2-D numerical computer hydraulic models and empirical design guidance that were used to evaluate the expected spillway discharge rating, flow characteristics, and reservoir routing. The 2-D model was also used to evaluate flow velocities within the approach channel, across the spillway weir and containment dikes, and within the new spillway channel under the 50 percent PMP and IDF event conditions.

Erosion of overburden and upper-bedrock surfaces under various flow conditions were assessed incrementally using the 2-D model and the estimated rock mass erodibility indices for each material by progressively removing material calculated to be susceptible to erosion and scour and then recomputing the hydraulic conditions throughout the flow path. The "end-state" (final) eroded surface was calculated as the surface that would resist further erosion under the calculated hydraulic conditions. The slope and length of the selected spillway chute design, the elevation and location of the scour cutoff wall, and the progression of erosion of bedrock materials and overburden were all considered in the development of the selected design.

The outlet works intake tower, originally constructed in 1996, was modified to accommodate the increased reservoir storage and maintain compliance with the SEO's dam safety regulations. The modifications to the intake tower included raising the structure by 13.5 ft, installing a three-phase power supply, installing new gate stems and associated supports, installing new electric gate actuators, relining the 36 in. diameter and 120 in. diameter outlet pipelines, upgrading instrumentation and controls, replacing the existing bubbler system, and installing a pedestrian access bridge to connect the dam crest to the tower platform.

Antero Dam was successfully rehabilitated using innovative design and construction techniques tailored to avoid or minimize untoward environmental effects and permitting requirements and address unknown conditions associated with complex geology and a 109-year-old embankment. Harsh winter conditions required a carefully planned and phased multiyear construction program, with specialized methods to construct the filter trench and barrier wall, which were affected by various reservoir levels.

Observations and measurements made during the first filling of the reservoir show that the rehabilitation appears to be functioning better than anticipated. Areas that were typically wet and marshy no longer are, and the phreatic surface has dropped up to 3.4 ft compared to prerehabilitation levels.

Before installation of the barrier wall, the seven toe drain outfalls produced water. Now only two of the outfalls flow, and their combined discharge is typically about 99 percent less than previous flow rates. These observations show that most of the historic seepage appears to have been conveyed through the embankment, alluvium, and shallow bedrock, and this seepage has been effectively reduced by the barrier wall.

When rehabilitating any aging dam, unanticipated site conditions and dam behavior should be expected. Although unplanned events happened during the rehabilitation of Antero Dam, issues were resolved through close collaboration of the owner, designers, regulators, and contractors, and the finished project meets the dam safety and storage capacity criteria set during the initial benefit-cost analysis.

All four construction phases were completed under budget, for a total cost of $19.66 million, which includes construction and engineering costs. This results in a cost of about $980/acre-ft for 20,000 acre-ft of storage. If the reservoir level were to be increased to the gauge height of 26.5 ft, for a total of 44,000 acre-ft of storage, the cost per acre-ft would decrease to about $450.

The success of this project largely resulted from the collaboration of the owner, designers, regulator, and various contractors. All parties approached each phase of the design and construction with the goal of delivering a successful project to improve dam safety and maintain the potential to store additional water in the future. CE

Edwin Friend, P.E., P.G., PMP, is a principal and project manager for RJH Consultants Inc., of Englewood, Colorado. Blaine Dwyer, P.E, M.ASCE, D.WRE, is a vice president and project manager based in the Denver office of HDR. Casey Dick, P.E., PMP, is a project manager for Denver Water. 



Owner and outlet works designer Denver Water, Denver
Embankment designer RJH Consultants Inc., Englewood, Colorado
Spillway designer Denver office of HDR, Omaha, Nebraska
Phase I prime contractor Geo-Solutions Inc., New Kensington, Pennsylvania
Phase II prime contractor Tezak Heavy Equipment Co. Inc., Cañon City, Colorado
Phase III prime contractor SEMA Construction Inc., Centennial, Colorado
Phase IV prime contractor Moltz Construction Inc., Salida, Colorado

© ASCE, Civil Engineering, April, 2019



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