Dam Safety 101

Dam Safety: A National Concern

(A special thanks to the Association of State Dam Safety Officials who provided the following content)

Dams bring water, power, flood control, recreation, economic possibilities and many other advantages to people. But people must understand that safe operation and maintenance is key to sustaining these advantages and avoiding potential disaster.


Why Are Dams Built?

Water is one of our most precious resources. nothing can live without it. But there has to be the right amount of water in the right place at the right time. Throughout the history of humankind, people have built dams to maximize use of this vital resource. Today, dams are beneficial to communities and individuals for many reasons. Dams provide flood control, water supply for drinking, irrigation for farming, recreational areas, and clean, renewable energy through hydropower.

As populations have grown and moved to arid or flood-prone locations, the need for dams has increased. Millions of people, throughout the United States, depend on dams to bring them the benefits mentioned above.


Introduction to Dams

The purpose of a dam is to impound (store) water, wastewater or liquid borne materials for any of several reasons, e.g. flood control, human water supply, irrigation, livestock water supply, energy generation, containment of mine tailings, recreation or pollution control. Many dams fulfill a combination of the above functions.

Manmade dams may be classified according to the type of construction material used, the methods used in construction, the slope or cross-section of the dam, the way the dam resists the forces of the water pressure behind it, the means used for controlling seepage and, occasionally, according to the purpose of the dam.

The materials used for construction of dams include earth, rock, tailings from mining or milling, concrete, masonry, steel, timber, miscellaneous materials (such as plastic or rubber) and any combination of these materials.

Embankment dams-Embankment dams are the most common type of dam in use today. They have the general shape shown here. Materials used for embankment dams include natural soil or rock, or waste materials obtained from mining or milling operations. An embankment dam is termed an "earthfill" or "rockfill" dam depending on whether it is comprised of compacted earth or mostly compacted or dumped rock. The ability of an embankment dam to resist the reservoir water pressure is primarily a result of the mass weight, type and strength of the materials from which the dam is made.

Concrete dams-Concrete dams may be categorized into gravity and arch dams according to the designs used to resist the stress due to reservoir water pressure. Typical concrete gravity dams are shown here and are the most common form of concrete dam. The mass weight of concrete and friction resist the reservoir water pressure. A buttress dam is a specific type of gravity dam in which the large mass of concrete is reduced, and the forces are diverted to the dam foundation through vertical or sloping buttresses. Gravity dams are constructed of vertical blocks of concrete with flexible seals in the joints between the blocks.

Concrete arch dams are typically rather thin in cross-section. The reservoir water forces acting on an arch dam are carried laterally into the abutments. The shape of the arch may resemble a segment of a circle or an ellipse, and the arch may be curved in the vertical plane as well. Such dams are usually constructed of a series of thin vertical blocks that are keyed together; barriers to stop water from flowing are provided between blocks. Variations of arch dams include multi-arch dams in which more than one curved section is used, and arch-gravity dams which combine some features of the two types of dams.

Water Retention and Seepage

Because the purpose of a dam is to retain water effectively and safely, the water retention ability of a dam is of prime importance.
Water may pass from the reservoir to the downstream side of a dam by:
  • Passing through the main spillway or outlet works
  • Passing over an auxiliary spillway
  • Overtopping the dam
  • Seepage through the abutments
  • Seepage under the dam
Overtopping of an embankment dam is very undesirable because the embankment materials may be eroded away. Additionally, only a small number of concrete dams have been designed to be overtopped. Water normally passes through the main spillway or outlet works; it should pass over an auxiliary spillway only during periods of high reservoir levels and high water inflow. All embankment and most concrete dams have some seepage. However, it is important to control the seepage to prevent internal erosion and instability. Proper dam construction, and maintenance and monitoring of seepage provide this control.


Release of Water

Intentional release of water is confined to water releases through outlet works and spillways. A dam typically has a principal or mechanical spillway and a drawdown facility. Additionally, some dams are equipped with auxiliary spillways to manage extreme floods.
Outlet Works—In addition to spillways that ensure that the reservoir does not overtop the dam, outlet works may be provided so that water can be drawn continuously, or as needed, from the reservoir. They also provide a way to draw down the reservoir for repair or safety concerns. Water withdrawn may be discharged into the river below the dam, run through generators to provide hydroelectric power, or used for irrigation. Dam outlets usually consist of pipes, box culverts or tunnels with intake inverts near minimum reservoir level. Such outlets are provided with gates or valves to regulate the flow rate.

Spillways-The most common type of spillway is an ungated concrete chute. This chute may be located over the dam or through the abutment. To permit maximum use of storage volume, movable gates are sometimes installed above the crest to control discharge. Many smaller dams have a pipe and riser spillway, used to carry most flows, and a vegetated earth or rockcut spillway through an abutment to carry infrequent high flood flows. In dams such as those on the Mississippi River, flood discharges are of such magnitude that the spillway occupies the entire width of the dam and the overall structure appears as a succession of vertical piers supporting movable gates. High arch-type dams in rock canyons usually have downstream faces too steep for an overflow spillway. In Hoover Dam on the Colorado River, for example, a shaft spillway is used. In shaft spillways, a vertical shaft upstream from the dam drains water from the reservoir when the water level becomes high enough to enter the shaft or riser; the vertical shaft connects to a horizontal conduit through the dam or abutment into the river below.


Dams Are a Vital Part of the National Infrastructure

Dams provide a life-sustaining resource to people in all regions of the United States. They are an extremely important part of this nation?s infrastructure—equal in importance to bridges, roads or airports, other major elements of the infrastructure. They can serve several functions at once.


Ownership Makes Dams a Unique Part of the National Infrastructure

Dam owners are solely responsible for the safety and the liability of the dam and for financing its upkeep, upgrade and repair. While most infrastructure facilities (roads, bridges, sewer systems, etc.) are owned by public entities, the majority of dams in the United States are privately owned. Many different types of people and entities own and operate dams. About 58 percent are privately owned. Local governments own and operate the next largest number of dams, about 16 percent. State ownership is next with about four percent. And the federal government, public utilities and undetermined interests each own smaller numbers of dams.

Safety Concern Spurs Need for Regulation: Dam Safety Regulation in the US

Safety is key to the effectiveness of a dam. Dam failures can be devastating for the dam owners, to the dam's intended purpose and, especially, for downstream populations and property. Property damage can range in the thousands to billions of dollars. No price can be put on the lives that have been lost and could be lost in the future due to dam failure. Failures know no state boundaries—inundation from a dam failure could affect several states and large populations.

Early in this century, as many dams failed due to lack of proper engineering and maintenance, it was recognized that some form of regulation was needed. One of the earliest state programs was enacted in California in the 1920s. Federal agencies, such as the Corps of Engineers and the Department of Interior, Bureau of Reclamation built many dams during the early part of the twentieth century and established safety standards during this time. Slowly, other states began regulatory programs. But it was not until the string of significant dam failures in the 1970s that awareness was raised to a new level among the states and the federal government.


State Regulation Today

Today, every state but Alabama and Delaware have dam safety regulatory programs. State governments have regulatory responsibility for 95% of the approximately 78,000 dams within the National Inventory of Dams. These programs vary in authority but, typically, the program activities include
1) safety evaluations of existing dams,
2) review of plans and specifications for dam construction and major repair work,
3) periodic inspections of construction work on new and existing dams, and
4) review and approval of emergency action plans.


Federal Regulation Today

There are several federal government agencies involved with dam safety. Together, these federal agencies are responsible for five percent of the dams in the U.S. They construct, own and operate, regulate or provide technical assistance and research for dams. Included in this list are the Departments of Agriculture, Defense, Energy, Interior, Labor and State (International Boundary and Water Commission), the Federal Energy Regulatory Commission, Nuclear Regulatory Commission and the Tennessee Valley Authority. The Federal Emergency Management Agency administers the National Dam Safety Program, a program established by law in 1996 to coordinate the federal effort through the Interagency Committee on Dam Safety, to assist state dam safety programs through financial grants, and to provide research funding and coordination of technology transfer.


Top Issues Facing the Dam Safety Community

  1. Risk of Failure

    Driving every other issue and all activities within the dam safety community is the risk of dam failure. Although the majority of dams in the U.S. have responsible owners and are properly maintained, still many dams fail every year. In the past several years, there have been hundreds of documented failures across the nation (this includes 250 after the Georgia Flood of 1994). A life was recently lost in New Hampshire as a result of a dam failure. Dam and downstream repair costs resulting from failures in 23 states reporting in one recent year totaled $54.3 million.


    Dam failures are most likely to happen for one of five reasons:

    Overtopping caused by water spilling over the top of a dam
    Structural failure of materials used in dam construction
    Cracking caused by movements like the natural settling of a dam


    Inadequate maintenance and upkeep

    Piping-when seepage through a dam is not properly filtered and soil particles continue to progress and form sink holes in the dam
    Historically, dams that failed had some deficiency, as characterized above, which caused the failure. These dams are typically termed "unsafe." Currently, there are about 2,000 "unsafe" dams in the U.S. There are unsafe dams in almost every state. (A majority of states and federal agencies define an "unsafe" dam as one that has been found to have deficiencies that leave it more susceptible to failure.)


  2. The Increasing Hazard

    Dams are innately hazardous structures. Failure or mis-operation can result in the release of the reservoir contents
    --this includes water, mine wastes or agricultural refuse
    --causing negative impacts upstream or downstream or at locations remote from the dam. Negative impacts of primary concern are loss of human life, economic loss including property damage, lifeline disruption and environmental damage.
    Some dams are considered to have a greater hazard potential than others. There are approximately 10,000 state-regulated "high-hazard" potential dams in the U.S. "High-hazard" is a term used by a majority of state dam safety programs and federal agencies as part of a three-pronged classification system used to determine how hazardous a dam's failure might be to the downstream area.

    While the definition varies from place to place, it generally means if failure of a high-hazard dam occurs, there probably will be loss of life. It must be emphasized that this determination does not mean that these dams are in need of repair--these dams could be in excellent condition or they could be in poor condition. "High-hazard" just reflects the dam's potential for doing damage downstream should it fail.

    High-hazard potential dams exist in every state and affect the lives of thousands downstream. The current issue and debate is over the increasing number of these high-hazard structures--not because more high-hazard dams are being built, but that more development is occurring downstream. Dam safety regulators generally have no control over local zoning issues or developers' property rights. So this issue continues to worry regulators as the trend persists.


  3. Financing for Maintenance, Upgrade and Repair

    Dams must be maintained to keep them safe. Occasional upgrade or rehabilitation is necessary due to deterioration, changing technical standards and improved techniques, better understanding of the area's precipitation conditions and increases in downstream populations and changing land use. When a dam's hazard classification is changed to reflect an increased hazard potential, the dam may need to be upgraded to meet an increased need for safety.

    The lack of funding for dam upgrade has become a serious national problem, especially within the private sector. Unfortunately, operation, maintenance, and rehabilitation of dams can range in cost from the low thousands to millions. And owners are responsible for these expenses (58 % of dams in the U.S. are privately owned). Many owners cannot afford these costs. Funding assistance, through government or private sources, is minimal at best. (Although, a handful of states offer loan programs.)

    In 2001, the American Society of Civil Engineers released its Infrastructure Report Card. In this report, dam safety was given a "D" partially due to the lack of funding available to support the repair and upgrade needs of the nation's dams.
    In 2002, ASDSO concluded that it would take approximately $10 billion to rehabilitate the nation's most critical (high-hazard) dams that are in need of rehabilitation.


  4. Lack of Adequate Authority and Resources for State Dam Safety Programs

    Although most states have legislative authority to carry out a comprehensive dam safety program, many are lacking in specific areas. Some states are unable, by specific language in their law, to regulate certain types of dams, allowing these structures to fall between the regulatory cracks. Other states have limited ability to enforce the law. In some states, officials have no recourse if dam owners do not carry out safety repairs ordered by the state.

    Many states are simply under-resourced for carrying out the letter of the law. State budgets for dam safety range from $0 to $6.4 million. But the average annual state dam safety budget is about $454,000. The average number of regulated dams per state is approximately 2,000.

    The average number of dam inspectors per state is less than eight; this means that each dam inspector is responsible for overseeing the safety of about 250 existing dams, plus the additional responsibilities of overseeing new construction.

    The industry has determined that, in general, ten state regulators are necessary per 250 dams to do the best job possible in carrying out the regulatory mandate set out in most state dam safety laws. (Model State Dam Safety Program, Association of State Dam Safety Officials, 1998) This would mean that the average program regulating 1,500 dams would need about 60 professionals as opposed to six.
    There is, therefore, a serious need, in almost every state, to pump additional state resources into these programs.


  5. Lack of Emergency Preparedness In Case of Failure

    Emergency preparedness is lacking. Only 33 percent of non-federally owned dams considered high hazard in the U.S. have emergency action plans. That means that most dam owners and local authorities are not prepared for a sudden dam failure and the ensuing downstream consequences.


  6. Lack of Public Awareness

    Intersecting almost all the issues above is the issue of public education about dams. The ordinary citizen is unaware that the beautiful lakes on which he or she boats, skis or fishes are only there because of manmade dams. Developers build in dambreak flood inundation areas knowing nothing about the potential that upstream dam has to cause devastation should it fail. In fact, some developers and zoning officials are completely unaware of dams within their community. Even if citizens understand and are aware of dams, they still can be overly confident in the infallibility of these manmade structures. Living in dambreak flood-prone areas is a risk.

    Many dam owners do not realize their responsibility and liability toward the downstream public and environment. Adequate understanding of proper dam maintenance and upgrade techniques is a typical problem among many owners across the United States.

    Some groups put forth the message that dams are bad for the environment and advocate their removal. This may mislead the public into thinking that taking care of our dams is a worthless cause. In some cases, dam removal is the best solution, but in all instances the consequences should be considered in coming to this decision.

The Good News

Through a public/private partnership, the outlook is improving for dam safety. The National Dam Safety and Security Act was passed in 2002. This legislation was enacted to assist states in improving their dam safety programs, to support increased technical training for state dam safety engineers and technicians, to pump money into dam safety research, and to maintain the National Inventory of Dams.

The partnership of public and private entities that was assembled to push for passage of the law and the ensuing partnership taking shape to implement the Act has formed the basis for a stronger dam safety community. Led by organizations, such as the Association of State Dam Safety Officials, the U.S. Society on Dams and the American Society of Civil Engineers; federal agencies, such as the Federal Emergency Management Agency (administrator of the NDSPA), the Corps of Engineers, the Department of the Interior and the Federal Energy Regulatory Commission, many projects and initiatives are underway to increase the safety of dams in the U.S.

The ball is rolling, but there is a long way to go.