The legacy of 11,000 watershed dams and their forthcoming rehabilitation
In 1948, the U.S. Soil Conservation Service, now known as the Natural Resources Conservation Service (NRCS), began a project to reshape the landscape of the United States to control flooding and prevent soil erosion. More than 50 years later, more than 11,000 dams protect 2,000 watersheds throughout 47 states, providing an estimated $1.5 billion in annual benefits. Originally constructed to protect pasture and farmland, many of these dams are now integral to communities, providing recreation, flood protection, and water supply benefits. However, their reliability is in question.
Factors, such as land development in watersheds, age of flood control structures, and sedimentation of waterways, are creating a need for dam rehabilitation. Dams in close vicinity to metropolitan areas are of specific concern because of increased population density and urbanization of upstream watersheds and downstream floodplains. This is a story about a race against time and how the NRCS is evaluating and rehabilitating this nation's massive watershed protection infrastructure.
The U.S. Department of Agriculture (USDA) estimates that the existing 11,000 flood-control dams provide approximately $1.5 billion in annual benefits, including, but not limited to, flood control, wildlife habitat, recreation, and erosion and sediment control. Since most structures were constructed with a 50-year design life, those constructed prior to 1956 have surpassed their original design life. Currently, these structures total approximately 6 percent of the nearly 11,000 watershed structures. Within the next 15 years, 62 percent of the watershed structures will have surpassed the 50-year design life. Major concerns with the aging structures include deteriorating components, sediment trapped in the reservoir, and watershed development.
Developing watersheds are a major concern near metropolitan areas, where the upstream watershed and downstream reach are no longer used for agriculture. Changes to the watershed often increase stormwater runoff, and development in the downstream valley can change the hazard classification of the structure, requiring more conservative design storms to be in compliance with state and federal design criteria.
Recognizing the benefits of the existing watershed structures and the costly improvements needed to extend the aging flood control structures' design life beyond the original 50 years, in 2000 Congress amended the Watershed Protection and Flood Prevention Act to provide technical and financial assistance to communities to address the public health and safety and environmental impact concerns related to the dams. The NRCS can provide as much as 65 percent of the project costs, while the local sponsor provides 35 percent of the project costs.
Early in the process, states evaluated the financial burden of the rehabilitation needs. Table 1 lists some of the results from the preliminary planning work conducted by various states. The information is based on financial data from 1999. Increasing construction labor and materials costs have certainly raised the present financial burden.
Table 1: Estimated cost by state for watershed structure rehabilitation
As of January 2005, approximately 38 dam rehabilitation projects were completed with the NRCS Watershed Rehabilitation funds, 67 structures were being studied for rehabilitation planning, and 27 structures' watershed rehabilitation plans were being implemented. In light of Hurricane Katrina, recent flooding events in the northeast, and dam failures in Missouri and Hawaii, people only now are realizing the importance of dams, their benefits, and the need for maintenance and rehabilitation.
For example, Gwinnett County, Ga., which currently operates and maintains 14 NRCS watershed structures constructed from the 1960s to the 1990s, has addressed rehabilitation needs. All 14 dams were designed and constructed as low- to medium-hazard structures and are located within the bustling suburbs of metropolitan Atlanta. Recently, 12 Gwinnett County watershed dams were reclassified as high-hazard structures in which a probable loss of life is predicted in the event of a dam breach. To address this issue, Gwinnett County undertook an aggressive capital improvement program to upgrade its NRCS dams to comply with the high-hazard structure design criterion. Based on watershed planning work, the county determined that four of the 12 high-hazard structures can be upgraded most efficiently using Watershed Rehabilitation funds. Gwinnett County completed the rehabilitation work on two of these four structures to date. Planning analyses for the remaining eight structures indicate that they can be rehabilitated more cost effectively by complying with Georgia's slightly less conservative Rules for Dam Safety.
The Yellow River Watershed Dam No. 14 in Gwinnett County, Ga., was rehabilitated by constructing a roller compacted concrete spillway over the entire dam (NRCS photo)
The Yellow River Watershed Dam No. 14 (Y-14) is one of two structures that was rehabilitated successfully using the Watershed Rehabilitation Funds. Golder Associates Inc. (Golder) began work on the project in the year 2000. The structure required a new spillway to route safely the probable maximum precipitation (PMP) design storm discharge of approximately 16,000 cubic feet per second (cfs) through the watershed. The Y-14 site is located within several subdivisions, and space was not available to widen the existing spillway. Because of these limited space constraints, the project required construction of a roller compacted concrete (RCC) spillway over the entire dam. The crest elevation of the new spillway was lowered approximately 1.5 feet below the old spillway crest elevation, and an ogee weir was constructed to provide additional discharge through the restricted spillway width. The valley downstream of the structure is much narrower than the spillway crest; therefore, the spillway was constructed with an arch shape to aid in convergence of spillway flows through the structure. Total project costs were approximately $1.7 million; it was completed in December 2003.
Gwinnett County is currently in the design phase for the remaining two structures, which will use NRCS watershed rehabilitation funds. Approximately 90 homes are at risk in the event of a dam failure of the Yellow River Watershed Dam No. 15 (Y-15) structure. The project has been ongoing with Gwinnett County since 1999. Previous consultants identified that a 660-foot-wide RCC spillway with an ogee weir was required to route the PMP storm through the watershed safely. The homeowner's association for the residents that live around the lake contested the proposed work and delayed the project for several years. Golder was retained by Gwinnett County in mid-2004 to design the proposed RCC spillway.
Golder identified that the proposed spillway width could be reduced by approximately 30 percent by evaluating impacts to the 100-year floodplain downstream of the structure and by using a weir designed specifically for the Y-15 structure. Standard hydraulic design methodologies could not be used to assess the proposed design; therefore, the NRCS reviewers suggested that a model study of the proposed spillway be conducted to evaluate its performance. The model was constructed at the U.S. Bureau of Reclamation Hydraulics Laboratory in Denver at a scale of 1:24. The study concluded that several minor adjustments were needed to the design, but in general, the remedial solution was acceptable. The Y-15 project is anticipated to start construction this fall and be completed by fall 2007. Estimated project costs are $3.3 million.
An aerial view of the Y-15 structure in Gwinnett County, Ga., shows the proximity of homes in the downstream valley (NRCS photo)
Fortunately, not all rehabilitation projects require such massive structural spillways. Generally, dams located in rural areas are more effectively rehabilitated by simpler means. Widening the existing earthen spillway or raising the crest of the dam are common solutions when the watershed is not overly populated. Gwinnett County is implementing these solutions at several of its NRCS structures to comply with Georgia's Rules for Dam Safety, not the NRCS design criteria. These options require evaluating whether the existing spillway system is capable of passing the design storm flows without damaging the structure—erosion of the earthen spillway or overtopping of the dam. In many cases, the original NRCS design criteria has been found to be conservative, and simply adding a wave wall to the crest of the dam or re-grading the spillway to a lower crest elevation is sufficient to bring a dam into compliance with the state's design criteria.
A study of the proposed Y-15 spillway used a 1:24 scale model that was approximately 24 feet wide (NRCS photo)
The NRCS design criteria is restrictive on the use earthen spillways in that erosion of the spillway is not permitted for the NRCS stability design hydrograph (SDH), which is for a design storm of smaller magnitude than the freeboard hydrograph (FBH) storm. Seldom does compliance with the SDH require a wider spillway than compliance with the FBH. The intent is to protect the earthen spillway from erosion during more frequent storms. Project sponsors such as Gwinnett County may find that compliance with state-mandated design criteria is attained more easily, but funding is not available through the Watershed Rehabilitation Program for the work.
For more than 50 years, the NRCS has combated the "national menace" of soil erosion and controlled flooding by implementing structures in the headwaters of large watersheds. Nearly 11,000 dams were constructed that reshaped the surface of North America and the development of communities across the country. The benefits of these projects are innumerable and worth re-investment for future generations.
Gregg Hudock, P.E., is a senior water resources engineer at Golder Associates, Inc., at its Atlanta headquarters. He can be reached at email@example.com. Russell Morgan is the NRCS National Watershed Rehabilitation program manager. He can be reached at firstname.lastname@example.org.
Sidebar: Flood and stream flow information
Floods are the most frequent of all catastrophic natural hazards, costing an average of $6 billion in losses annually and threatening lives and property in every state. The U.S. Geological Survey (USGS) has made the National Flood Summary information from 1970-1998, including maps and data, available on the Internet at http://ks.water.usgs.gov/Kansas/floodsummary. The website provides a tool to compare current or possible flood conditions with historical flood information for this 28-year time period by state and year, including magnitude, cause, loss of life, damage, and cost.
For more than 125 years, the USGS has conducted research and operated the national stream gaging network of more than 7,000 gaging stations that provide real-time information to emergency responders and long-term data to floodplain managers. These components work together to provide the information—before, during, and after each flood—to manage flood risks and to expand our understanding of flood processes and characteristics. Real-time stream flow information is accessible at http://water.usgs.gov.
Flood information also is available on WaterWatch, a USGS web site that displays maps, graphs, and tables describing current, recent, and past stream-flow conditions for the United States. WaterWatch summarizes flow conditions, including recent and historical peaks for rivers and streams, and also depicts and quantifies the severity and spatial extent of hydrologic hazards such as floods and droughts. WaterWatch can be accessed at http://water.usgs.gov/waterwatch. Included in the National Flood Summary website (http://pubs.water.usgs.gov/sir2005-5194) is the latest volume of the National Flood Summary series describing significant floods in the United States and Puerto Rico from 1994-1998.
— Source: U.S. Geological Survey