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Rehabilitating Our Nation's Aging Flood Control Dams

by Larry W. Caldwell, State Conservation Engineer, USDA-NRCS, Oklahoma

Local watershed project sponsors, assisted by the USDA Natural Resources Conservation Service (NRCS), have constructed over 10,450 upstream flood control dams in 47 states under the PL-534, PL-566, Pilot, and RC&D water resource programs. Some of these projects are reaching the end of their 50-year design life. Many have significant rehabilitation needs. Some dams pose a threat to the public health and safety of community residents while others have potential for creating adverse environmental impacts in downstream floodplains that have been protected by the dams for the past 50 years.

These watershed projects, which represent a $14 billion infrastructure investment, have provided flood control, municipal water supply, recreation, and wildlife habitat enhancement on over 130 million acres in every state in the nation. They have reduced flooding to prime farmlands, highways, homes and businesses and have become an integral part of the communities they were designed to protect.

Many project areas are now in a far different setting than when they were originally constructed. Population has grown, development has occurred upstream and downstream from the dams, landuse changes have taken place, sediment pools are filling, structural components have deteriorated, and many do not meet state dam safety regulations that have been revised with more stringent requirements since the dams were built.

A major challenge exists as more than 2,000 dams need rehabilitation at an estimated cost of more than $540 million. Public safety, environmental concerns, funding, and liability are just some of the issues that must be addressed before these dams reach the end of their design life. There is currently no federal statutory authority for rehabilitation of these projects, and most local sponsors do not have the financial capability to address work needed to continue to protect their communities.

The USDA Natural Resources Conservation Service (NRCS) has provided technical and financial assistance to local sponsors in the development of water resource projects since the 1940’s. This assistance has been provided primarily through the following four programs:

The Flood Control Act of 1944 (PL-534) authorized 11 watershed projects in the United States. Since 1948, more than 3,400 flood control dams have been constructed in the 320 sub-watershed projects covering more than 35 million acres in 12 states.

The Watershed Protection and Flood Prevention Act of 1954 (PL-566) is commonly referred to as the Small Watershed Program. More than 6,300 flood control dams have been constructed in the 1,613 PL-566 projects authorized to date that involve over 109 million acres in every state in the nation.

The Pilot Watershed Program provided a transition between the PL-534 and PL-566 Acts. More than 400 flood control dams were constructed in the 62 pilot projects in 33 states covering almost 3 million acres. The RC&D Program has also provided technical and financial assistance to local sponsors in RC&D areas for the planning, design, and construction of more than 200 flood control dams since the 1960’s.

The 1995 national dams inventory is the source of this data. Some states have not updated this inventory; due to the small number of dams installed in recent years, the inventory is the best data available.

Flood control and watershed protection are the primary purposes for the vast majority of the watershed projects; however, others have included water management, municipal and industrial water supply, recreation, fish and wildlife habitat improvement, water quality improvement, water conservation and other related purposes.

Since 1948, these four watershed programs have resulted in the construction of over 10,450 flood control dams and more than 5,000 grade stabilization structures. Based on water resource appropriations since 1948, more than $8.5 billion (1997 dollars) of federal funds have been invested in these projects. In addition, over $6.0 billion is estimated to have been provided by local project sponsors. This has resulted in a $14 billion infrastructure investment across the nation. These projects provide over $1 billion in benefits annually.

Since 1948, these four watershed programs have resulted in the construction of over 10,450 flood control dams and more than 5,000 grade stabilization structures

These flood control dams typically consist of an earthen embankment; heights generally range from 20 to 80 feet. The dams have small drainage areas 1 to 10 square miles located on intermittent drainageways in the upper reaches of watershed tributaries. The inlet of the principal spillway generally reinforced concrete pipe 12 to 48-inches in diameter is placed at an elevation that provides storage in the reservoir for the anticipated sediment to be accumulated during the design life of the structure.

An auxiliary spillway (generally a vegetated channel) safely conveys runoff from storms that exceed the design storm. The detention storage available between the principal and auxiliary spillways provides temporary storage of runoff until it can be slowly released through the principal spillway pipe. With several dams in a watershed, this temporary detention of runoff controls flooding to downstream flood plain areas.

NRCS assisted project sponsors to develop the original watershed plan and provided technical and financial assistance to implement it. Most of the flood control dams were constructed with 100% federal costs for design and construction. Lesser cost-share was provided for structures with multiple purposes (water supply, recreation, etc.). Local sponsors had the responsibility for financing their share of the installation of the project (landrights, etc.) and 100% of the cost of operation and maintenance (O&M). After construction, dams then became the responsibility of the sponsors. There is no federal statutory authority and only limited state and local funding available for rehabilitation of projects that reach the end of their design life.

The preparation of the watershed work plans involved an economic analysis to compare long-term benefits and costs of the project to assure it was economically feasible. The period of time considered in the economic analysis was called the “evaluated life”. The majority of the earlier projects had an evaluated life of 50 years. After the early 1960’s, most projects were evaluated for a life of 100 years. The dams within the projects were designed with a design life equal to the project evaluated life. It is estimated that more than two-thirds of the dams constructed to date were components of projects planned prior to the early 1960’s, which means they were planned and designed for a 50-year life. For many dams, that 50-year life is at or nearing the end.

Current Situation
More than two-thirds of the small flood control dams are more than 30 years old. Thirty-five dams have already reached the end of their design life. More than 450 of the dams will reach the end of their design life within the next 5 years. More than 1,800 dams will reach the end of their design life within the next 10 years.

Survey of Rehabilitation Needs
In April 1999, a rapid survey of rehabilitation needs in 22 states was conducted. More than 10,000 of the small flood control dams are located in these 22 states. The survey concluded that more than 2,200 of the dams have rehabilitation needs that are estimated to cost more than $540 million.

Over 650 of these dams pose a threat to public health and safety. Most of these dams were designed to protect agricultural areas in downstream floodplains. Since construction of the dams, homes and businesses were built downstream from the dams. Since the dams now pose a potential threat to life and property if the dam should fail, most do not meet the higher design standards required by state dam safety laws. Thus, the majority of these 650 dams need to be rebuilt and upgraded at an estimated cost of almost $400 million.

The survey concluded that more than 2,200 of the dams have rehabilitation needs that are estimated to cost more than $540 million.

The remaining 1,600 dams have rehabilitation needs to extend the life of the dams and avoid future environmental damage and loss of flood control. It is estimated this rehabilitation work will cost more than $150 million. It is emphasized that this survey was just a preliminary “snap-shot” picture of known information today; a detailed field assessment of the nationwide situation must be completed to obtain a more accurate, complete analysis prior to implementation of any rehab program. It is anticipated that the numbers of dams and cost of rehabilitation will increase when detailed on-site assessments of the dams are made.

Common Rehabilitation Needs
Many of these dams can function indefinitely beyond the original design life with continued maintenance and rehabilitation of the structure. The following are some of the issues that must be addressed for this aging infrastructure:

  1. Replacement of deteriorating components, such as principal spillway pipes, slide gates, and trash guards. Over 1,800 of the dams have metal conduits which are generally considered to have a life expectancy less than 50 years.
  2. Unanticipated development downstream from the dam increases potential for loss of life or significant economic damage in the event of a dam failure which results in a higher hazard classification than considered in the original design. Also, upstream development has increased runoff volumes from the original design assumptions due to parking lots, streets, roof tops, etc.
  3. Reservoirs are filling with sediment. Since reservoirs are designed to store the sediment anticipated to accumulate during the design life of the dam, all reservoirs will be filled with sediment at some time in the future. If modifications are not performed, continued delivery of sediment to the site will encroach on the flood detention storage resulting in more frequent flows through the emergency spillway, increased maintenance needs, and the increased threat of dam failure. If the dam fails, the stored sediment will be released into downstream riparian areas that will be damaged by pollutants that are attached to the sediment particles.
  4. Some dams do not meet current state dam safety regulatory requirements. Typically, these requirements have increased since the original construction as a result of federal legislation and/or state laws. Essentially all of the state dam safety laws were written or significantly revised after dam safety concerns were raised in the 1970’s. Over 70% of these project dams were in place by that time. Since many laws were retroactive, conflicts with the design of the existing dams and the new dam safety rules were inevitable.
  5. Since 50 years have elapsed since many of the projects were planned, there are many resource needs of the watershed that were not addressed by the original work plan. By the time the National Environmental Policy Act (NEPA) became law in 1969 and was functional in its present form in 1974, approximately 70% of the structures installed to date had been constructed.
  6. In some cases, there may be a lack of adequate landrights under current easements to conduct future rehabilitation work. Water rights issues will be critical in rehabilitation alternatives, especially in western states. Landuse control (upstream and downstream from the structure) needs to be addressed prior to development of a rehabilitation plan.

Common Rehabilitation Approaches
There are many approaches to address rehabilitation of aging small flood control dams. It must be emphasized that the alternatives to be considered and the final approach selected must be determined on the site specific economic, environmental and social merits of the project; there is no one solution for rehabilitation for all small flood control dams. Common approaches normally considered involve: dredging the reservoir; raising the dam; and removing the dam (sometimes referred to as decommissioning).

The FY 2000 Appropriations Bill included authorization of $8,000,000 for pilot rehabilitation watershed projects in Mississippi, New Mexico, Ohio, and Wisconsin.

National Pilot Rehabilitation Project
In July 1998, NRCS announced that funding would be provided for a national pilot rehabilitation project in Oklahoma. Authority was provided in the FY 1998 Appropriations Bill.

The objectives of this project were:

  • Facilitate and provide technical assistance to a locally-led planning effort to address all resource needs within the watershed.
  • Document a streamline process to develop a rehabilitation plan.
  • Rehabilitate selected existing dams.

The 19,650-acre Sergeant Major Creek Watershed in Roger Mills County in Oklahoma was selected. This project involves six small flood control dams that were built from 1948 to 1963. One dam has had a hazard classification change due to development downstream. The sediment pool of another dam is used as a sole source water supply for the town of Cheyenne, Oklahoma (population 1000).

The planning effort has been led by a 15-member local coordinating group, composed of landowners within the watershed, representatives from the City of Cheyenne, and other interested citizens. The group identified local community and resource needs to be addressed:

  • Rehabilitation of high priority flood control dams (Sites 1 and 2)
  • Protection of the city drinking water supply.
  • Improvement of rangeland conditions and wildlife habitat.
  • Rehabilitation of aging conservation practices.
  • Education of oil and gas company representatives in erosion control methods around well sites.
  • Solving storm water runoff problems within the city.

Grants have been applied for, workshops and demonstration projects are underway, and a comprehensive inventory of rangeland conditions in the entire watershed has been completed.

The watershed planning effort has resulted in the original work plan being supplemented. An environmental assessment has been completed and a Finding of No Significant Impact (FONSI) issued to cover the rehabilitation of two of the oldest dams. Site 2 was originally designed and constructed in 1949 as a low hazard dam. Downstream development has since occurred that has resulted in this site being reclassified as a high hazard. Rehabilitation of the dam involved installation of a new principal spillway conduit, inlet, and impact basin, widening the emergency spillway, raising the top of dam, installing a foundation drain, and flattening the backslope. The rehabilitation was completed in February 2000. The design for the rehabilitation of Site 1, originally built in 1948, has been completed; construction will commence in late winter 2000.

Four-state Pilot Rehabilitation Projects in FY 2000
The FY 2000 Appropriations Bill included authorization of $8,000,000 for pilot rehabilitation watershed projects in Mississippi, New Mexico, Ohio, and Wisconsin. NRCS in these four states is working with local project sponsors, state dam safety officials, and community leaders to identify high priority projects that will address rehabilitation of flood control dams that are in critical need of rehabilitation. Most projects have been selected, and detailed planning has begun. Priority projects include those where there is population at risk, the condition of the structure is impaired, municipal water supply is potentially impacted, or potential adverse environmental impacts could result if the dam should fail. During FY 2000, community leaders and local project sponsors will work with NRCS technical staff to develop a plan for rehabilitation. These plans will address other resource needs in the community area, the National Environmental Protection Act (NEPA), possible cultural/historic preservation issues, and an economic analysis of the costs and benefits of the various rehabilitation alternatives. By late calendar year 2000, designs of the reconstruction of selected dams should be complete and construction should begin. The construction of most of these pilot projects should be completed during FY 2001.

Opportunities for Future Joint Cooperative Efforts
There are numerous opportunities for residents, community leaders, agencies, and interest groups to work closely together during rehabilitation of aging watershed projects. This will allow many projects that were initially single-purpose flood control to be rehabilitated and to also consider all of the resource needs within the watershed. Examples of these opportunities are: 1.Wildlife and Wetland Enhancement; 2.Municipal Water Supply; 3.Rural Fire Protection; and 4.Floodplain Management and Hazard Mitigation. Through these efforts, future flood damages can be reduced by identifying potential threats and implementing measures to avoid these threats.

With everyone working together, rehabilitation of existing watershed projects can proceed in a timely manner. It will be a huge challenge to keep ahead of the needed rehabilitation of the more than 2,200 dams known to date as well as the growing number anticipated in the future. A similar challenge was met more than 50 years ago to plan and construct the original projects; another is needed to accomplish the current challenge with watershed rehabilitation. L&W

For more information, contact Larry W. Caldwell, State Conservation Engineer, USDA-NRCS, Stillwater, OK, (405)742-1254, fax: (405)742-1201, e-mail: Larry.Caldwell@ok.usda.gov.

©2000, 1999, 1998 Land and Water, Inc.
Tuesday, November 14, 2000 - http://www.landandwater.com/features/vol44no3/vol44no3_2.html