Dams 101 - Association of State Dam Safety Officials
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Water is one of our most precious resources; our lives depend on it. Throughout the history of humankind, people have built dams to optimize the utilization of this vital resource.
Dams are crucial in providing a life-sustaining resource to communities across the United States. Their significance in our national infrastructure is comparable to that of bridges, roads, airports, and other major systems. They serve multiple functions, including supplying water for domestic, agricultural, industrial, and community uses; controlling floods; providing recreational activities; and generating clean, renewable energy through hydropower.
As populations have expanded and settled in either arid regions or areas prone to flooding, the necessity for dams has escalated.
Potential Benefits of Dams
Renewable, clean energy: According to the U.S. Department of Energy, hydropower generated more than 7% of the United States' electricity in a recent year, significantly contributing to renewable electricity generation.
Flood control: Dams, when constructed with assistance from the Natural Resources Conservation Service, provide an estimated $1.7 billion in annual benefits by minimizing flooding and erosion damage, offering recreational opportunities, supplying water, and enriching wildlife habitats. For instance, dams operated by the Tennessee Valley Authority generate electricity and avert around $280 million in flood-related damages each year.
Water storage: Dams create reservoirs that serve numerous purposes, including fire control, irrigation, recreation, domestic and industrial water supply.
Irrigation: Approximately ten percent of America's cropland relies on water sourced from dams.
Navigation: The U.S. Army Corps of Engineers oversees navigation projects across 41 states, managing around 12,000 miles of waterways while facilitating 15% of U.S. freight transported via inland waterways through operating 275 locks and maintaining 926 harbors.
Recreation: Dams are the foundation of prime recreational areas throughout the U.S., with around ten percent of the population visiting at least one facilities managed by the U.S. Army Corps of Engineers annually.
The primary purpose of a dam is to store water, wastewater, or other liquid materials for diverse reasons, such as flood control, human water supply, irrigation, livestock hydration, energy production, mining tailings containment, recreation, or pollution management. Many dams are designed to fulfill several of these functions.
There are classifications of manmade dams based on several criteria, including construction material, construction method, dam slope or cross-section, resistance to water pressure, seepage control, and sometimes the dam's intended purpose.
Dams may consist of materials such as earth, rock, mining or milling waste, concrete, masonry, steel, timber, or other miscellaneous materials (such as plastic or rubber) and any combination thereof.
Types of Dams
Embankment Dams: These are the most prevalent type of dam today, constructed from materials like natural soil, rock, or mining waste. Embankment dams are categorized as either 'earthfill' or 'rockfill' dams based on their composition, and their ability to withstand reservoir water pressure largely relies on the weight and properties of the materials used.
Concrete Dams: These can be further categorized based on their design, which is intended to counteract the stresses from reservoir water pressure. The common types of concrete dams include strong gravity dams, buttress dams, and arch dams.
Gravity Dams: These are the most typical form of concrete dams, employing their mass and friction to counter reservoir water pressure. They are built with vertical concrete blocks, with flexible joints between them.
Buttress Dams: A buttress dam exemplifies a specific variety of gravity dam where the concrete mass is reduced and forces are transmitted to the foundation via vertical or sloping buttresses.
Arch Dams: Usually thin in cross-section, arch dams carry forces from the reservoir water laterally to the abutments. Their shape might take the form of a circular segment or ellipse, often including various adaptations like multi-arch and arch-gravity designs.
With the paramount goal of water retention, dams must manage how water passes from the reservoir to the downstream side, typically via:
- Main spillway or outlet works
- Auxiliary spillway
- Overtopping of the dam
- Seepage through the abutments
- Seepage beneath the dam
Overtopping is particularly undesirable for embankment dams as it risks the erosion of the dam’s materials. Normally, water should flow through the main spillway or outlet works, with auxiliary spillways engaged only during periods of excessive water inflow.
Release of Water
Controlled water releases come through outlet works and spillways. Typically, a dam has both a principal or mechanical spillway and a drawdown facility, potentially incorporating auxiliary spillways to tackle extreme flooding situations.
Outlet Works: These facilitate continuous or as-needed water withdrawals from the reservoir and include methods to reduce the reservoir level for maintenance or safety. Water can be directed to the downstream river, utilized for hydroelectric power generation, or for irrigation purposes. Outlets usually consist of pipes, culverts, or tunnels with inlets situated near the minimum reservoir level.
Spillways: Commonly, spillways are ungated concrete chutes located over or alongside the dam. Movable gates can be placed above the crest to manage water discharge efficiently. Smaller dams may have combined pipe and riser spillways to control primary flows and an earth or rock-cut spillway for rare high floods.
According to the National Inventory of Dams (NID), there are over 90,000 dams cataloged in the U.S., classified by the potential hazard in case of a failure. High-hazard dams could result in fatalities, while those classified as significant hazards might not cause loss of life but pose considerable economic risks. As illustrated by NID’s map, the numerous dams across the U.S. highlight the critical goal of ensuring their safety.
Ensuring dam safety is crucial for their effectiveness. Failures can have devastating consequences for owners, objectives, and especially for populations living downstream. Property losses can vary from thousands to billions, and the toll on human life is immeasurable. Failures do not adhere to state boundaries; flooding due to a dam failure can impact multiple states.
In the early 2000s, numerous dam failures due to inadequate engineering and maintenance necessitated regulatory oversight. The first state program debuted in California, followed by federal agencies like the Corps of Engineers and the Bureau of Reclamation, which not only constructed many dams but also established safety norms. Gradually, other states adopted regulatory measures, but significant dam failures in the later years drew increased attention and scrutiny.
State Regulation Today
At present, every state except Alabama has implemented a dam safety regulatory program, overseeing roughly 70% of approximately 90,000 dams identified in the National Inventory of Dams. While these programs vary in authority, typical responsibilities include:
- Evaluating the safety of existing dams
- Reviewing plans and specifications for dam construction and repairs
- Conducting periodic inspections of new and existing dams
- Approving emergency action plans
Federal Regulation Today
Several federal agencies play a role in dam safety, managing about 5% of U.S. dams through construction, ownership, operation, and regulation. These include departments focused on agriculture, defense, energy, interior, labor, and state matters, as well as regulatory commissions. FEMA oversees the National Dam Safety Program, aimed at fostering collaboration among federal efforts and supporting state programs through financial aid and research funding.
Federal Agencies
Around 16% of ASDSO membership comprises federal agency representatives, with around 14% of U.S. dams under federal jurisdiction.
FEMA, part of the Department of Homeland Security, manages the National Dam Safety Program to coordinate federal dam safety initiatives and assist states in enhancing their safety regulations. Other federal agencies working alongside FEMA include NOAA, the National Weather Service, and the U.S. Geological Survey.
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