DAMS-PART 2

DAM BASICS

With the exception of the Great Wall of China, dams are the largest structures ever built. Throughout history, big dams have prevented flooding, irrigated farmland, and generated tremendous amounts of electricity. Without dams, modern life as we know it would simply not be the same

Since the first large-scale dam was built in Egypt more than 5,000 years ago, engineers have devised various types of dams to withstand the forces of a raging river

Arch dams
are good for narrow, rocky locations. They are curved, and the natural shape of the arch holds back the water in the reservoir. Arch dams, like the El Atazar Dam in Spain, are thin and require less material than any other type of dam

ARCH DAM - ELATAZAR DAM - MADRID SPAIN

 

Arch Dam: Forces
The arch squeezes together as the water pushes against it. The weight of the dam also pushes the structure down into the ground

 

Buttress dams

may be flat or curved, but one thing is certain: a series of supports, or buttresses, brace the dam on the downstream side. Most buttress dams, like the Bartlett Dam in Arizona, are made of  reinforced concrete

 

, BUTTRESS DAM - BARTLETT DAM - ARIZONA

 

Buttress Dam: Forces
Water pushes against the buttress dam, but the buttresses push back and prevent the dam from toppling over. The weight of the buttress dam also pushes down into the ground

 

Embankment dams

are the most commonly built dams in the United States. They are massive dams made of earth and rock. Like gravity dams, embankment dams rely on their heavy weight to resist the force of the water. But embankment dams are also armed with a dense, waterproof core that prevents water from seeping through the structure. Tailings dams -- large structures that hold back mining waste -- are a type of embankment dam

  Embankment Dam - NEW WADDELL DAM - MARICOPA COUNTY - ARIZONA 

Embankment Dam: Forces
Water pushes against the embankment dam, but the heavy weight of the dam pushes down into the ground and prevents the structure from falling over

 

Gravity dams

are massive dams that resist the thrust of water entirely by their own weight. Most gravity dams, like the grand coulee dam in Washington, are expensive to build because they require so much concrete. Still, many people prefer its solid appearance to the thinner arch and buttress dams

Gravity Dam
Grand Coulee Dam
Grand Coulee, Washington

 

Gravity Dam: Forces
Water pushes against the gravity dam, but the heavy weight of the dam pushes down into the ground and prevents the structure from falling over

Aerial view of the Grand Coulee Dam, Grand Coulee, Washington

 

All dams -- whether they're embankment, buttress, arch, or gravity -- must be maintained as they get older. Without proper maintenance, spillways can clog and concrete can crack. Some dams are even removed because they block the migration of fish

When should dams be taken down? When should they be repaired? Engineers must consider the services that each dam provides and the environmental impact that each dam creates before they make this decision -- and this isn't easy. Oftentimes, there is no right answer

 TO BE CONTINUED >>>

Dams -part 1

introduction

Dams are structural barriers built to obstruct or control the flow of water in rivers and streams. They are designed to serve two broad functions. The first is the storage of water to compensate for fluctuations in river discharge (flow) or in demand for water and energy. The second is the increase of hydraulic head, or the difference in height between water levels in the lake created upstream of the dam and the downstream river

By creating additional storage and head, dams can serve one or more purposes:

• Generating electricity
• Supplying water for agricultural, industrial, and household needs
• Controlling the impact of floodwaters; and
• Enhancing river navigation

They can be operated in a manner that simultaneously augments downstream water quality, enhances fish and wildlife habitat, and provides for a variety of recreational activities, such as fishing, boating, and swimming

Classes of Dams

Four major classes of dams are based on the type of construction and materials used: embankment, gravity, arch, and buttress.

Embankment

Embankment dams typically are constructed of compacted earth, rock, or both, making them less expensive than others that are constructed of concrete. Consequently, more than 80 percent of all large dams are of this type. Embankment dams have a triangular-shaped profile and typically are used to retain water across broad rivers

Gravity

Gravity dams consist of thick, vertical walls of concrete built across relatively narrow river valleys with firm bedrock. Their weight alone is great enough to resist overturning or sliding tendencies due to horizontal loads imposed by the upstream water

Arch

Arch dams, also constructed of concrete, are designed to transfer these loads to adjacent rock formations. As a result, arch dams are limited

to narrow canyons with strong rock walls that can resist the arch thrust at the foundation and sides of the dam

Buttress

Buttress dams are essentially hollow gravity dams constructed of steel-reinforced concrete or   timber

karun 3 dam, IRAN

Planning for Dams

Careful planning throughout the siting, design, and construction of dams is necessary for optimal utilization of rivers and for preventing catastrophic dam failure. These planning phases require input from engineers, geologists, hydrologists, ecologists, financiers, and a number of other professionals

Designers must first evaluate alternative solutions and designs for meeting the same desired objective, whether the goal is to allocate water supply, improve flood control, or generate electricity. Each alternative requires a comprehensive cost-benefit analysis and feasibility study for evaluating its physical, economic, ecological, and social impact

Once an alternative has been selected, a number of important considerations enter into the design and construction of the dam. These include

• Hydrological evaluation of climate and streamflows
• Geologic investigation for the foundation design
• Assessment of the area to be inundated by the upstream lake (also called a reservoir) and its associated environmental and ecological impacts
• Selection of materials and construction techniques
• Designation of methods for diverting stream flow during construction of the dam
• Evaluation of the potential for sediments to accumulate on the reservoir bottom and subsequently reduce storage capacity; and
• Analysis of dam safety and failure concerns

 

When a dam is put into operation, or commissioned, water is released from the upstream reservoir over a spillway or through gates in a manner to satisfy intended objectives. Operating rules for maximizing power generation, for example, include maintaining hydraulic head. In contrast, water levels in flood control reservoirs must be periodically reduced to allow for new storage during anticipated periods of flood hazard. Operating issues, however, can easily become complex and highly politicized and may be difficult to resolve. This is particularly true for river systems containing several reservoirs, for dams serving multiple purposes, and in cases where adverse social, ecological,  and environmental impacts are significant

 

Overview of Dam-Building

The first dam for which reliable records exist was built on the Nile River sometime before 4000 B.C.E. near the ancient city of Memphis. Remains of other historic dams have been located at numerous sites bordering the Mediterranean Sea and throughout the Middle East, China, and Central America. The oldest continuously operating dam still in use is the Kofini Dam, which was constructed in 1260 B.C.E. on the Lakissa River in Greece

Today, there are approximately 850,000 dams located around the world. Of the more than that are categorized as large dams, more than half are located in China and India. It is estimated that 24 countries currently generate more than 90 percent of their electrical power from dams, and 70 countries rely on dams for flood control 

Dams in the United States

Large-scale construction of dams occurred in the United States during the post–World War II years and reached its peak in the 1960s. The organizations that have been primarily responsible for dam-building are the U.S. Army Corps of Engineers, the Bureau of Reclamation (part of the U.S. Department of the Interior), and a number of public and private utility developers

Since the nineteenth century, the U.S. Army Corps of Engineers has been engineering rivers to accommodate river traffic, control floods, produce electricity, and provide irrigation waters. Four of the largest dams

Arizona's Glen Canyon Dam on the Colorado River shows the curvature in arch dams that provides structural stability. The rock walls of the deep canyon absorb a majority of forces that result from the upstream reservoir, Lake Powell

constructed by the Corps include Garrison, Oahe, Fort Peck, and Fort Randall Dams

The second group, the Bureau of Reclamation, was established in 1902, when Congress passed the National Reclamation Act. The Bureau was initially charged with developing irrigation and power projects in seventeen western states and has been responsible for the construction of more than six hundred dams and reservoirs, including the massive Hoover, Shasta, Glen Canyon, and Grand Coulee Dams

The third organization responsible for dam construction encompasses various power administrations, such as the Tennessee Valley Authority, the largest public power company in the United States, as well as others operating under the Federal Power Act of 1920, which provided for the licensing of privately built dams to produce electric power. In part because of this mid-twentieth-century dam-building era, the U.S. dam population has approached 75,000. More recently, however, the rate of dam construction in the United States is exceeded by the rate of decommissioning. In many cases, maintenance costs for aging infrastructure, significant social and ecological impacts, high construction costs, and the reduced availability of suitable sites have made alternatives to dams more viable