Changes in both regulatory oversight agencies and the rules themselves presented unique challenges for Fairfax Water and its partners when rehabilitating the Lower Occoquan Dam.
By Greg Zamensky, Mishelle Noble-Blair and Erica Zamensky
Greg Zamensky is the Americas practice leader for dams and flood protection with Black & Veatch. Mishelle Noble-Blair is a senior plant engineer for Fairfax Water. Erica Zamensky is a technical editor for Black & Veatch.
When Virginia’s Fairfax Water elected to surrender its federal license for power generation at the Occoquan River plant in 2008, the decision required the utility to correct a number of structural problems with the Lower Dam before regulatory oversight transferred from the Federal Energy Regulatory Commission to authorities with the state of Virginia.
The looming transfer of responsibilities required Fairfax Water to satisfy FERC dam safety guidelines, as well as changing state regulations from the Virginia Department of Conservation and Recreation. The challenge required Fairfax Water and its partners to overcome a number of obstacles en route to a solution.
The Occoquan River project
The Occoquan River project, located on the Occoquan River between Fairfax and Prince William counties in northern Virginia, was built in the early 1950s with an upper dam for storage and a lower dam for emergency supply.
The Upper Dam is an approximately 800-feet long by 70-feet high concrete gravity dam. The structure impounds 8.3 billion gallons of water from a 595-square mile drainage area, and consists of a left non-overflow section, powerhouse, spillway and right non-overflow section
Meanwhile, the Lower Occoquan Dam is a 485-foot-long by 30-foot-high structure consisting of an ungated ogee spillway, a raw water intake at the right abutment and a short non-overflow monolith at the left abutment.
The project was also home to a 1.35-MW hydropower plant that began operating in the late 1950s. Fairfax Water decided to surrender the operating license for this plant in 2008 due to the considerable capital investments it would have required to maintain its modest production.
Assessing Lower Occoquan
From 1990 through 2001, three dam safety reports were prepared encompassing both the Upper and Lower Dam in accordance with FERC’s Part 12, Subsection D regulations, which requires an inspection every five years.
During preparation of the first report in 1990, an incremental damage assessment was performed for both dams by Harza Engineering to identify the magnitude of Occoquan’s inflow design flood (IDF). Failure of the Lower Dam during larger storm events was shown not to increase the downstream hazard, judged as a greater-than-a-two-foot incremental rise in the downstream surface elevation.
As a result, the assessment concluded that 5% of the probable maximum flood (PMF) was the appropriate IDF for the Lower Dam. Harza’s FERC report also concluded the Lower Dam was stable under all loading operations, in good condition and well-maintained.
Subsequent Harza inspections in 1995 and 2001 provided similar conclusions regarding the Lower Dam’s overall condition. But, in 2005, a fourth 12D report, conducted by Schnabel Engineering, identified deterioration in the left abutment. It was originally thought the deterioration might only be surficial and not a dam safety issue, though a 2008 site visit by Black & Veatch came to a different conclusion.
Black & Veatch was selected as Fairfax Water’s “Basic Ordering Agreement”-contracted Dam Safety Engineer through the utility’s solicitation, evaluation and interview process. The company’s previous relevant experience, project team and interview results were compared to several other firms that submitted for the contract.
In June 2009, Black & Veatch executed an intrusive investigation that consisted of drilling three core holes (7 to 9 feet deep) in the left abutment. The downstream slope exhibited exposed coarse aggregate throughout its length, concrete deterioration resulting in areas of uneven flow, and areas of seepage along concrete lift joints.
Due to an unusually dry spring in 2009, flow in the Occoquan River was low, resulting in no flow over the spillway section for several days. During the concrete coring operation, Fairfax Water personnel conducted a routine inspection of the Lower Dam. The utility’s visual inspection of the downstream toe identified seepage exiting at the dam/foundation contact along a 50- to 75-foot length, totaling 30 to 40 gallons per minute.
Identifying the problems
By July 2009, Fairfax Water had numerous concerns and questions regarding the disposition of the Lower Dam — not the least of which was trying to ensure its responsibility to maintain an adequate dam safety program that protects the public from unacceptable risks within the regulatory frameworks of both FERC and the Virginia Department of Conservation and Recreation’s (DCR) Dam Safety Division.
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| This photo shows the Lower Occoquan Dam before the rehabilitation work was undertaken. |
While FERC provided regulation for the Occoquan River project beginning in 1990, Fairfax Water’s decision to surrender its federal license in 2008 meant a transfer of oversight to DCR in 2013. To meet its obligations during the pending transition, Fairfax Water hired Black & Veatch in 2008 to begin working to evaluate the Lower Dam’s hazard classifications, appropriate spillway design flood, and stability in normal and storm events.
Assigning a hazard classification
The evaluation process began by assessing the hazard potential classifications for the Lower Dam. FERC had attached a “high hazard potential” classification to both the Upper and Lower dams, likely because of the project’s proximity to the town of Occoquan. The Upper Dam is about 1.5 miles from the Occoquan and a half-mile from the Lower Dam.
Given that the Lower Occoquan Dam was not yet subject to Virginia regulations, however, its classification under the state’s standards was unknown. And although Virginia’s 2009 dam safety regulations contained guidelines for identification of the IDF, they were not clear on the process by which hazard classification was judged.
Without a clearly defined process, Black & Veatch prepared a hydraulic model of the Occoquan River using the unsteady module within the U.S. Army Corps of Engineers’ river hydraulic model, HEC-RAS, to evaluate the inundation downstream during various storm events, with and without a dam failure.
These results indicated that:
- Flows from a sunny-day failure remained within the river banks; and
- The incremental increase in water surface elevation resulting from a Lower Dam failure during the 100-year, 50% and 100% PMF events was less than 2 feet throughout the inundated area.
In short, these studies suggested that failure of the Lower Dam represented a minimal increase in downstream flooding, even during the smallest storms, because of its size relative to the flow of the Occoquan River. The results were documented and provided to DCR for review and comment in August 2009. DCR reviewed the information while Fairfax Water continued to move forward with its studies.
Determining the IDF
Without waiting for specific direction from DCR regarding the hazard classification, Fairfax Water asked Black & Veatch to identify the likely IDF assuming a high hazard and significant potential designation from the state of Virginia. This led to the preparation of an incremental damage assessment (IDA) that illustrated the downstream effects of a Lower Dam failure during various storm events.
DCR’s 2009 dam safety regulations provided specific criteria for evaluating the results of an IDA. As defined in the regulations, an additional downstream threat to persons or property is assumed to exist when the incremental rise in water depth between failure and non-failure scenarios exceeds 2 feet or when the product of the incremental rise in water depth and flow velocity is greater than 7 feet. The regulations further stipulate that flow conditions at individual structures within the inundation zone should be recognized and considered. These structures included residences and businesses along the riverbanks.
The hydraulic model developed to evaluate the hazard potential classification was also used to perform the IDA. While Fairfax Water was waiting for a response from DCR regarding the hazard classification, the analysis began with a broad selection of storms — PMF, 1/2 PMF, and the 100-year storm — based on Virginia regulations.
From the study results, Black & Veatch concluded that the appropriate IDF for the Lower Dam would be the smallest storm allowable, depending on the hazard potential classification. In other words, if the dam was classified as “high hazard,” the IDF would be 1/2 PMF. If classified as “significant hazard,” the IDF would be the 100-year storm.
Assessing the Lower Dam’s stability
Based on stability calculations prepared in 1990 by Harza Engineering, the Lower Dam was shown to be stable under IDF loading conditions, defined as 5% of the PMF. The stability analyses also indicated the dam may become unstable at storm events exceeding 20% of the PMF. However, if DCR maintained the high hazard potential assigned by FERC, the Lower Dam would be judged unstable by state regulations, given the existing stability analysis results.
In January 2010, Black & Veatch began a re-evaluation of the Lower Dam’s stability due to the observed seepage under Monolith 3 in 2008 and observed inconsistencies between assumed and actual dam geometries Two loading conditions were examined — the 1/2 PMF and 100-year storm. The finite element computer program, ANSYS, was used to develop a finite element model and to perform a linear elastic analysis of the Lower Dam.
The 100-year loading condition resulted in small tensile stresses at the upstream base of the majority of the monoliths, with significant tensile stresses observed at Monolith 3 and the left non-overflow section, including a portion of the adjacent monoliths.
Using an iterative process, incremental portions of the tensile zone were removed to simulate cracking behavior resulting from the separation between the rock and the dam. The results indicated that significant tension remained throughout the foundation interface of Monolith 3 regardless of the assumed interface separation. Fortunately, the tension values dissipate quickly in the adjacent monoliths, and no significant tensile stresses were apparent.
The 1/2 PMF loading condition resulted in predicted tensile stress on the upstream heel of the dam for nearly the full width of the spillway and left abutment. In most areas, this “tensile zone” extended more than 10 feet back from the edge. Considering the magnitude of this separation, the study concluded that the dam would no longer have sufficient contact with the rock surface to function as a stable structure.
Regulatory complications
Although similar in broad terms, differences between FERC and DCR’s formulas in determining Lower Occoquan’s hazard classification made it difficult for Fairfax Water to establish a clear plan for the dam’s future.
Both FERC and DCR dam safety regulations allowed for a reduction of the IDF (or spillway design flood) based on the results of an incremental damage assessment. However, the lowest allowable IDF under DCR guidelines was limited to 1/2 PMF to PMF for high-hazard structures and 1/2 PMF to the 100-year flood elevation for significant hazard potential structures.
Therefore, FERC’s accepted IDF of 5% of the PMF for the Lower Dam would not be acceptable once DCR assumed regulatory responsibilities for the project, given its high-hazard classification from FERC.
FERC provided regulatory oversight for the Occoquan River Project from about 1990 to the present. At the conclusion of the surrender process, the regulatory responsibility for the Upper and Lower Dams will transfer to the Virginia Department of Conservation and Recreation — Dam Safety Division (DCR).
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| *Specific value based on results of incremental damage assessment (4VAC50-20-52, 2009) |
The broad guidelines for the FERC and DCR dam safety regulation are very similar. However, in 2009, a specific difference existed in these regulations. Both the FERC and DCR dam safety regulations allowed reduction of the IDF (or spillway design flood) based on the results of an incremental damage assessment (or analysis). However, as demonstrated by Table 1, the lowest allowable IDF under DCR guidelines was limited to ½ PMF for high hazard potential structures and the 100-year flood for significant hazard potential structures.
Therefore, the FERC-accepted IDF of 5% of the PMF for the Lower Dam would not be acceptable to DCR given the high hazard potential classification. This is significant because the existing stability analysis indicated the Lower Dam becomes unstable at about 20 percent of the PMF — significantly less than the lowest acceptable IDF of 50 percent of the PMF.
In early 2010, the Virginia legislature voted to modify the dam safety regulations defining the required IDF. Specifically, the appropriate IDF for high hazard potential structures can range from the PMF to the 100-yr storm. With this provision in place by July 2010, the Lower Dam hazard classification was no longer the critical path — now knowing that the 100-year event was likely the IDF regardless of hazard.
Rehabilitation alternatives
In April 2010, Fairfax Water recognized a rehabilitation effort was needed given the condition of the dam and the results of the Black & Veatch stability study. However, DCR had yet to identify the hazard classification and, thus, could not evaluate the appropriate IDF.
As a result, Fairfax Water requested that Black & Veatch develop and evaluate rehabilitation alternatives that satisfied both the 1/2 PMF and 100-year storm loading conditions.
Considering the stability results and concrete condition, the rehabilitation approach for the dam centered on three areas:
- Cutting off seepage through and under the dam;
- Increasing the weight of the dam via mass or anchor; and
- Replacing the deteriorated concrete at the crest and downstream face.
Stability evaluations of the proposed alternative revealed that a reasonable configuration could be developed to satisfy the 100-year storm condition. These evaluations also indicated that satisfying the 1/2 PMF loading conditions would require similar attributes, plus post-tensioned anchors.
Given the significant cost difference between these alternatives — as well as the advancing surrender process — Fairfax Water needed to get a hazard and IDF determination from DCR.
Rehabilitating Lower Occoquan
The Virginia legislature voted to modify the dam safety regulations defining the required IDF in early 2010, meaning the appropriate IDF for high hazard structures could range from the PMF to the 100-year storm. With this provision in place by July 2010, the Lower Dam hazard classification was no longer the critical path, with Fairfax Water now knowing that the 100-year event was likely the IDF regardless of the rating.
DCR issued a letter stating that it was important to maintain the high hazard classification as assigned by FERC but did accept the results of the IDA to permit an IDF equivalent to the 100-year storm.
With the official notification received, Fairfax Water quickly moved to advance a rehabilitation plan, with the following elements included in the design:
— Removal and replacement of deteriorated concrete across the dam crest and downstream slope;
— Installation of an 18-inch-thick wall along the upstream face, keyed into the underlying rock; and
— Reconstruction of the river intake along the right abutment.
Black & Veatch prepared the design documents, received FERC approval and acquired the necessary construction permits for the project in 2011.
Fairfax Water solicited prequalified bidders to perform rehab of the Lower Dam in 2011, with the low bid coming from ASI Constructors Inc. of Pueblo West, Colo., for an amount of about $3.6 million. The company received a Notice to Proceed from Fairfax Water in February 2012.
FERC coordination included review of a temporary construction emergency action plan (TCEAP) and proposed cofferdams before construction could begin. ASI Constructors submitted a comprehensive TCEAP and cofferdam plan to FERC in April 2012. The two documents were coordinated in unison and provided a three-stage water diversion plan.
Setbacks including flooding pushed the completion of the project about eight weeks behind the originally planned date, but Fairfax Water accepted the completed project without liquidated damages in February 2013 based on the quality of the work done.
Fairfax Water reports that Lower Occoquan Dam is now in excellent shape following the rehabilitation project, and that it is essentially like having a new spillway.
Dam Safety in 2015: A Regulatory Perspective
By William H. Allerton, P.E., Director, Division of Dam Safety and Inspections, FERC Office of Energy Projects
The Federal Energy Regulatory Commission’s Division of Dam Safety and Inspections is focusing on strengthening existing initiatives and further developing emerging ones, such as risk-informed decision making and potential failure modes analysis.
The commission has a number of important actions and goals for 2015 that could have impacts on dam owners and operators, and Allerton shares them in greater detail at http://bit.ly/1Mrnian.
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