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 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 1960s.
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.
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 1960s, 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 1960s, 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.
Survey of Rehabilitation Needs
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 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
Common Rehabilitation Approaches
National Pilot Rehabilitation Project
The objectives of this project were:
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:
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
Opportunities for Future Joint Cooperative Efforts
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.