As an increasing proportion of US dams are ageing, more and more are being retired from active service. Roger J. Austin* takes a closer look at the recently published Guidelines for the Retirement of Dams and Hydroelectric Facilities, which aim to provide a technical approach for assessing the engineering, sediment management and environmental aspects of retirement evaluations.
OVER the past decade, the number of retirement evaluations of dams and associated hydroelectric facilities in the US has increased substantially. For example, in the State of Wisconsin alone, over twenty dams, with the majority non-hydroelectric projects, have been retired since 1987. On March 19, 1996, the Federal Energy Regulatory Commission (FERC) issued an order approving a dam removal plan for Stronach Dam on the Pine River in Manistee County, Michigan (FERC, 1996a). Meanwhile, a coalition of stakeholders and government agencies are seeking the removal of the Edwards Dam on the Kennebec River in Maine in a relicensing proceeding currently underway.
A few small hydroelectric dams licensed by the FERC have been removed pursuant to agreements reached with the owners, including Fort Edward Dam on the Hudson River (FERC, 1973, 1978) and Mussers Dam in Pennsylvania (FERC, 1992, 1993). The potential removal of the Elwah and Glines Canyon dam in Washington has drawn nationwide attention as retirement of these two dams, which are approximately 30m in height, would be some of the largest dams ever removed. The increase in retirement evaluations has been attributed to the economics of continued operation, dam safety concerns, and increasing fish passage.
Currently in the US there are no nation-wide standards or guidelines for retiring dams and associated hydroelectric facilities. However, in July 1997, the American Society of Civil Engineers (ASCE) published the Guidelines for the Retirement of Dams and Hydroelectric Facilities.
In the United States in the late 19th and early 20th centuries, dams were constructed to power industry and provide water supply, recreation, and flood control for a growing economy. The addition of hydroelectric power to these facilities began in the early 1880s. By 1910, more than 435 hydropower facilities were operated in conjunction with water resource projects. The economics of flood control, water supply, and power generation have motivated several periods of large growth in dam construction and changes in the power supply mix. In addition many projects were constructed as multipurpose projects, providing flood reduction benefits, recreational uses, with power production as an added benefit. The corresponding pattern of dam construction is shown in figure 1.
By 1996, over 75 000 dams existed in the US. Figure 2 shows dam ownership by type and indicates that the private sector is the largest owner/operator of dams. Information from the National Inventory of Dams (USACOE, 1996) describes the current primary purposes of dams over 1.8m high, as shown in table 1. Many of these dams are multipurpose, combining several water resource functions as a part of their operation.
As dams and other parts of the nation’s infrastructure are ageing, attention is being paid to restoration, repair, and retirement evaluation of these facilities. Technical innovation and environmental and socio-economic factors have influenced these evaluations. Owners, multipurpose users, regulatory agencies, and other interested stakeholders involved in water resources are comparing the economics of continued operation with partial or full retirement options.
The retirement evaluation process will not be the same in every case. Generally the size and nature of the impoundment and its prominence in the watershed influence the complexity of retirement studies and actions. Retirement can be defined by levels of removal, namely partial retirement and full retirement.
Partial retirement may involve one of two options:
• Retirement of only the hydroelectric facilities, if present. In this option, the dam and other structures are retained. Some rehabilitation of the structures for safety or maintenance may be required.
• Partial retirement of the project facilities. This option may include the hydroelectric facilities (if present) and/or reduction in height or breach of the dam. In this case, the dam is either reduced or eliminated, while some of the ancillary facilities may remain intact.
Full retirement is defined as a removal of the project and all appurtenant structures. This may include rehabilitation or restoration of the affected project area.
A retirement evaluation includes an engineering review, identification of sediment management alternatives and an environmental assessment, which depend upon the complexity of the project. For example a partial retirement consisting of staged breaching will include evaluations and iteration of the engineering, sediment, and environmental aspects. The engineering review can determine the feasible options for demolition techniques. These demolition techniques may or may not release sediments downstream which were previously contained within the impoundment. A sediment management plan, in this case, will include a determination of the volume of sediments and the demolition levels for which sediment release is possible. Any ‘uncontrolled’ release of sediments can have several environmental impacts such as negatively impacting turbidity levels or raising the future flood stages of the river. This constant interplay between engineering reviews, sediment management alternatives, and environmental assessments is evident in the case studies presented in the guidelines.
Included in the guidelines are detailed descriptions of 12 case histories of complete dam and hydroelectric facilities retirement. These consisted of three earth dams, seven concrete dams and two timber dams. Reasons for removal were primarily dam safety considerations, economics of operation, or increasing fish passage.
A summary of the 12 case studies included in the guidelines is shown in table 2. Dams varied in height from 5.8 to 9.4m for earthfill and from 3.7-29m for concrete, and one of 15m for granite masonry. One timber crib dam averaged 5.8m with a maximum section of 9.4m.
This list does not exhaust the case studies of known retirement projects. However, the broad base of experience in these studies does reflect the nature and extent of the retirement process. The variety, nature, and timing of these case studies present the conditions associated with a retirement study and implementation, but do not represent the current and future expectations of retirement conditions. In each case, the owner was driven by certain conditions – safety, economics, or other circumstances – which bring the retirement decision to a conclusion.
Sediment management and control was an issue common to all of the case studies. Lessons learned were to avoid rapid drawdown and to remove the structures quickly once the drawdown was complete. Better sediment control features added significant cost and time to each project. Experience and a review of the selected case studies presented in the guidelines have shown that the cost of a sediment management strategy can be a dominant part of the total retirement cost. The relative magnitude of costs for environmental preservation, infrastructure removal, and sediment management for project retirement is shown in figure 3 (Pansic et al, 1995). The relationship is based on actual removal cost summaries of 23 dams in Wisconsin varying from 3-15m of hydraulic height.
The formulation and evaluation of sediment management plans depends on local conditions. Specific sediment management plans need to be tailored for each project depending upon sediment volume and the presence of contamination. Sediment management plans can be divided into four approaches:
• No action (leaving the sediment in place).
• Sediment removal by natural river erosion processes.
• Mechanized removal.
Sediment management can be an important part of the decision on retirement of dams. The volume of deposited sediment in the reservoir basin and its physical and chemical properties determine its potential impact. The removal of a dam without implementation of a sediment management plan exposes the deposited sediment to the erosive effects of water and can cause the sediment to be discharged downstream in uncontrolled amounts.
Uncontrolled releases of the sediment can can affect infrastructure, navigation, flooding, water quality, aquatic life and habitat, property interests, cultural resources, fisheries, wildlife habitat (including wetlands) and recreation. With proper analysis and care, reservoir sediment can be successfully managed in dam retirements.
With partial or full retirement of a dam, river flows may erode sediment from the reservoir basin. If large volumes of sediment are released, the downstream river channel will be affected. Retirement of a dam, without a sediment management plan, could have the following adverse effects:
• An increase in the sediment load in the downstream river can alter the river ecology established after the dam was constructed.
• Fines that are deposited in the downstream reaches of the river can negatively affect spawning gravel.
• Uncontrolled release of sediment can cause a sediment wave (or waves) to move down river over many years, increasing the elevation of the riverbed, and altering tributary confluences.
• An increased sediment load in the river can also increase the river’s potential to move in lateral directions.
• Increased riverbed elevations can increase flood potential and block water intakes.
However, with an effective sediment management plan, releases of sediment in a controlled fashion can generate beneficial results such as releasing coarser grained gravels, woody debris, nutrients, and other materials which can provide benefits to fish, wildlife, and aquatic communities. Increased braiding of a river course can create new spawning and rearing habitat for salmon and other fish species.
Selection of a sediment management plan is closely related to the overall retirement study.
Selection requires concurrent consideration of the findings of the engineering and environmental studies. Quantification of costs, benefits, and risks should be included in the decision-making process for a preferred project retirement alternative.
Although the number of US dam retirement evaluations has been increasing steadily over the last decade, currently there are no nationwide standards or guidelines for a retirement evaluation. The ASCE guidelines provide a technical approach for assessing the engineering, sediment management, and environmental aspects for a retirement evaluation. The guidelines are further supplemented by 12 case studies of actual projects which have been retired.