Dams and Flood Control

Dams have substantially reduced the risk of catastrophic floods and saved lives throughout the world. As an added benefit, which helps to offset environmental costs, dams generate pollution-free hydroelectric power and provide a reliable water supply for consumption, irrigation, industrial use, and recreation.

Floodplain Risks

Human beings and rivers have competed for the use of floodplains for millennia. The floodplaina wide, flat, low-lying area adjacent to the riveris built by natural sedimentation as the river carries its load of sand, silt, and clay to the sea. During times of excess flow, the river spills over its banks and covers the floodplain with muddy water. When the water retreats, a new layer of fertile soil is left behind. Humans have long exploited floodplains for these rich soils in order to grow crops. Floodplains have also been attractive places to settle because nearby rivers provide accessible sources of usable water, transportation routes, and sewage disposal. In some rugged terrains, the development of communities in any area other than floodplains is almost impossible.

Unfortunately, floodplain development has often proceeded despite the risks involved, with disastrous consequences. In the United States, a few flood-related deaths occur almost every year, and sometimes one flood kills many. In other parts of the world, flooding is a major annual concern, made even more pressing as precipitation patterns are affected by global changes in the atmosphere due to global warming. For every death that occurs, many more are left homeless or experience property damage, hardship, or suffering. Heavy rains in the winter of 1926-1927 caused disastrous floods on the Mississippi River that killed 313 people and left up to 650,000 homeless. In 1993, flooding of the Red River and the Upper Mississippi drainage basin killed 50 people and caused some $12 billion dollars worth of damage. In early 2012, major flooding of river floodplains due to heavy rains occurred in Southeast Asia, Australia, Venezuela, and several other locations around the world.

Dam Construction

Human attempts to control floods with dams date as far back as ancient Egypt when, around 2700 BCE, a dam was built at Sadd-el-Karfara. The basic principle of flood control by the use of dams is to store floodwaters in the dams’ reservoirs, from which they can be released in a controlled manner over a period of time instead of allowing them to spread suddenly and disastrously over the natural floodplain and the valuable human-made structures therein. After the smaller tributary streams below the dam have passed their floodwaters safely, the main reservoir is drained in a controlled fashion. The overall effect is to lengthen the time of passage of the flood, while drastically reducing the peak flow.

Dams are constructed of three basic materials: earth, rocks, and concrete. Eighty percent of all dams in the United States and Canada are earthen dams. In these, sand and soil are compacted into a broad triangular embankment surrounding a watertight clay core. The upstream face must be reinforced with rock or concrete to prevent wave erosion. Earthen dams are the most practical in broad valleys and are relatively inexpensive to construct. Rockfill dams are similar to earthen dams, but the heavy weight of the rock requires a more solid natural foundation. The upstream side must be sealed with a watertight material to prevent water from leaking through the dam.

Concrete dams require narrow valleys with hard bedrock floors to anchor and support them. A concrete gravity dam uses its great bulk and weight to resist the water pressure. These dams can generally remain stable when floodwaters overtop them, but they are costly. Washington State’s Grand Coulee Dam, which is of this type, required 8.1 million cubic meters of concrete. A concrete buttress dam relies both on its weight and structural elements to support it: The watertight upstream face slopes underneath the reservoir, which helps to distribute the water pressure to the foundation and, in effect, uses the great weight of the water as an anchoring force for the dam. Buttresses on the downstream face of the dam both counteract the force of the water and help the dam to withstand minor foundation movements, a distinct advantage in earthquake-prone areas. Concrete arch dams have a convex upstream face that spans the steep valley walls. The water pressure transmits the force along the arch to the side abutments and foundation, bonding the dam to the canyon. These dams are less expensive to construct than gravity dams but are also more likely to fail in the event of a small rupture.

Outlet Works and Spillways

All dams must contain properly designed outlet works, which are gates or conduits near the base of the dam kept open to discharge the normal low-water flow of the stream. These gates can be operated manually or automatically, but they must be carefully regulated to control the reservoir flood storage in an optimal manner and to prevent overwhelming the spillway or overtopping the dam. This means that the engineers of the outlet works must have an intimate knowledge of the design flood (the statistical probability and approximate return period of the maximum flood for which the dam was designed), the reservoir capacity, characteristics of past flood behavior, downstream flood hazards, and accurate meteorological forecasts, and they also must have a good dose of intuition.

Finally, all dams must be constructed with a spillway. A spillway is generally a broad reinforced channel near the top or around the side of the dam that acts as a safety valve because it allows rising waters, which might otherwise overtop the dam and cause its collapse, to escape harmlessly. Outlet works cannot be depended on to relieve the rising waters because they may become either blocked with debris or inoperative during a flood. The spillway must be large enough to convey the maximum probable flood.

Multiple-Purpose Reservoirs

An additional benefit of dams is that they not only provide flood control, but the reservoir can also be used as a water supply for irrigation, industrial, and municipal purposes. When the water falls from the top of the reservoir to the dam base through turbines, it generates inexpensive and pollution-free electricity. The reservoir also can be used for fishing, boating, and swimming and can become a haven for certain kinds of wildlife.

Unfortunately, a multiple-purpose reservoir contains built-in conflicts of interest. Because a reservoir used to control floods must have storage space for the floodwaters, ideally the water level should be kept low, or the reservoir kept nearly empty. Conserving water for irrigation or domestic use, however, requires holding floodwaters in storage, sometimes for years. For hydropower generation, the reservoir must be kept as full as possible and certainly never emptied. The fish and wildlife that occupy these reservoirs are best served by maintaining a stable water level, as are recreational uses. Thus, the management of a multipurpose reservoir for flood control is a very complicated enterprise. The goal is to derive the maximum value from the water while keeping the threat of flood damage to a minimum. All forecasts of possible and probable floods must be carefully weighed against the need to keep the reservoir full for other purposes.

Integrated Flood-Control Programs

Flood protection is rarely implemented by the construction of a single dam. An integrated flood-control program also involves the construction of levees, floodways, and channel modifications. Levees are dikes or structures that attempt to confine the stream flow to its natural channel and prevent it from spreading over the floodplain. They have the advantage of increasing the flow velocity in the channel, which diminishes the deposition of sediment in it. The increased velocity, however, also increases the tendency to undercut and erode the levee. Levees block off the floodplain, but the increased volume of water in the channel raises the level to which the waters will flood. When levees are breached, the floodwaters often spill out over the floodplain suddenly, catching residents by surprise. A breached levee may also trap floodwaters downstream by preventing their return to the channel, thereby increasing the damage. A way to prevent the breaching of levees is to install emergency outlets (like dam spillways) to specially constructed floodways, or flood-diversion channels. These are a means for safely returning the river to its natural floodplain.

Channelization, or modifications to the stream channel, is also used in conjunction with flood-control dams. This generally involves straightening, deepening, widening, clearing, or lining the channel in such a way that the stream flows faster. In this way, potential floodwaters are removed from the area more quickly. The lower Mississippi River was shortened by 13 percent between 1933 and 1936 (by short-cutting meander bends), which reduced the flood levels by 61 to 366 centimeters (24 to 144 inches) for equal rates of flow.

Disadvantages of Flood-Control Systems

Channelization may have deleterious effects. Erosion of the channel from faster flows may drain adjacent wildlife habitats and may undermine levees and bridges. The rapid passage of floodwaters also increases the hazard to locations downstream of the channelized area.

Flood protection by means of dams and their attendant structures has some other disadvantages. The term “flood control” is often misinterpreted by the general public to mean absolute and permanent protection from all floods under all conditions. This leads to a false sense of security and promotes further economic development of the floodplain. Every levee and every dam has a limit to its effectiveness. To compound the danger, there is always the possibility that the dam could fail entirely. The main causes of dam failures are overflowing due to inadequate spillway capacity, internal structural failure of earthen dams, and failure of the dam foundation material. During the twentieth century, more than 8,000 people perished in more than 200 dam breaks. For example, in one of the most fatal events, an earthen dam across the Little Conemaugh River above Johnstown, Pennsylvania, was overtopped and failed on May 31, 1889, killing 2,209 people. To prevent such failures, geologists and engineers must cooperate closely to design the safest possible structure matching the geology of the dam’s foundation.

Dams can also cause both undesirable sedimentation and erosion of a riverbed. As the river enters the reservoir, it is forced to slow and drop its sediment load, which decreases the water-holding capacity of the reservoir and limits its usable lifetime. The resulting delta can grow upstream and engulf adjacent properties and structures. The opposite effect results from the discharge of the reservoir water, now deprived of its sediment load, back into the natural channel below the dam. Here, the water can rapidly erode the bed. In Yuma, Arizona, for example, 560 kilometers (348 miles) downstream of the Hoover Dam, the riverbed of the Lake Mead reservoir has been lowered by 43 meters (140 feet). As of 2021, the reservoir had dropped by 37 percent since it had been filled in 1935. If a river formerly supplied sediments to beaches, the deprivation of this material results in coastal erosion. The loss of topsoil deposition as a result of flood control on the Nile by the Aswan High Dam has required farmers to add expensive fertilizers to their crops.

Other harmful effects of the construction of dams and flood-control projects include an increase in the water’s temperature, salinity, and nutrient content (from the strong solar heating and evaporation in reservoirs and the return flow of irrigation waters). This decrease in water quality can result in undesirable weed growth in reservoirs and in fish kills. Fish that migrate up rivers to spawn, such as trout and salmon, are physically prevented from doing so by dams. Fish ladders around dams are expensive and have proved to be only partially effective. The loss of wet floodplain habitat to reservoir inundation has been detrimental to many water birds, some of which are already endangered species. Unfortunately, the reservoir itself does not usually substitute for this loss because its elevation must fluctuate.

Finally, dams are expensive to build and maintain, and they cause the loss of valuable farmland because of reservoir inundation and sometimes force the relocation of entire communities. Construction of the Three Gorges Dam in China, a project that cost some $24 billion, required the relocation of more than 1 million people from their homes and farms and of at least 116 towns and villages. It is now also thought to be responsible for affecting local weather patterns, increasing seismic activity, and destabilizing the surrounding land and causing landslides and other catastrophic failures. Such failures have claimed many lives in the Three Gorges area of China.

Prevention of Floodplain Misuse

One of the best methods to avoid the economic, social, and environmental disadvantages of dams is to prevent floodplain misuse. This is often done through the use of flood zoning laws, which restrict or prohibit certain types of development in flood-hazard areas. Flood insurance laws, building codes, and tax incentives have a similar effect. Appropriate uses of flood hazard zones include parkland, pastureland, forest, or farmland.

If necessary, existing buildings can be flood-proofed or relocated. Flood-proofing involves reinforcing the building against flood damage, physically raising the elevation of the building above flood levels, or both. Land-treatment procedures can improve the ability of the natural ground surface to retain water and release it slowly to streams. These techniques include reforestation, terracing, and building contour ditches and small check dams. In urban areas, rooftop or underground water retention tanks and porous pavements can be installed to reduce the risk of flooding from excessive precipitation.

Mississippi River Project

The Mississippi River Project is an example of a massive flood-control project. Levees were first constructed along the banks of the Mississippi River in 1727 to protect the city of New Orleans, and they continued to be constructed (privately and haphazardly) into the 1830s. Major funds were granted in 1849 and 1850 by Congress to the U.S. Army Corps of Engineers for a flood-control study. After the Civil War, the entire 1,130-kilometer (702-mile) lower Mississippi Valley was organized into levee districts. In spite of the levees, major floods struck the Mississippi River in 1881, 1882, 1883, 1884, 1886, 1890, 1903, 1912, 1913, and 1927. The worst of these came in the spring of 1927 after a long winter of heavy rains, when the levees failed in more than 120 places. Many people were killed or left homeless. This inspired Congress to pass the Flood Control Act of May 15, 1928, which provided for levee expansion and improvement, dams and reservoirs, bank stabilization coverings (revetments), floodway diversion channels, artificial meander cutoffs, and emplacement and operation of river gauging stations (to continuously monitor the water level). The first test of the system came in 1937. Although the potential for another disastrous flood was just as great, the damages caused in 1937 were far less severe than a decade prior.

Work continued on the Mississippi. A major contribution was the 1944 project on the Missouri River, a tributary of the Mississippi River system. On the Missouri, six huge dams and a 1,500-kilometer (932-mile) chain of reservoirs and levees have been built. From 1950 to 1973, the combined effect of natural events and human-made structures produced a lull in the floods. The calm was broken when a massive flood in 1973 moved down the Mississippi, a significant test of the control projects. The maximum flow was approximately 56,800 cubic meters (2,005,873 cubic feet) per second, enough to supply New Orleans with its daily water needs in less than 10 seconds.

Despite the flood-control facilities, losses were great. Thirty-nine levees were breached or overtopped. Property losses were estimated at $1 billion. In spite of this, the river crested 21 centimeters (8 inches) lower than it had in 1927 in Cairo, Illinois, and 207 centimeters (82 inches) lower than it had the same year in Vicksburg, Mississippi. Engineers estimated that flood-control works had reduced damages from a possible $15 billion. Still, the flood left 23 dead and another 69,000 homeless.

Changing global weather patterns, due primarily to the effects of global warming, have resulted in the occurrence of increasing amounts of rainfall in many flood-prone parts of the world, including the Mississippi River drainage basin. This effectively guarantees that higher and more frequent floods will occur in those regions. A major flood in 1993 extending from central Manitoba in Canada, along the Red River, and south throughout the Mississippi River drainage basin resulted in numerous deaths and more than $12 billion in damages. The event made obvious many weaknesses and shortcomings in the flood control system and underscored the need for continuous monitoring and maintenance of the system. It is a certainty that the potential for even larger floods exists.

Importance of River-Management Policies

Where humans have been short-sighted in settling and building cities on floodplains, wisely built and well-managed dams and their attendant structures are an excellent way to reduce the flood hazard, though they cannot eliminate the risk entirely. The expenses that have been saved have been great, and no price can be put on the lives saved by flood control programs. At the same time, however, the environmental losses, which are harder to measure, also have been great. As dams age and as weather patterns change due to climate change, communities are faced with decisions to close, remodel, or break down dams altogether. These decisions can have lasting impacts on communities and their neighbors for decades to come, and thus policies are often necessary to safeguard people, property, and nature. Wild, natural rivers are rapidly becoming one of nature’s rarest possessions in the United States. As the demands for electric power generation and water resources continue to increase, managing the rivers wisely is in the nation’s best long-term interest.

Principal Terms

channelization: the practice of deliberately rerouting a stream or artificially modifying its channel by straightening, deepening, widening, clearing, or lining it

floodplain: a wide, flat, low-lying area, adjacent to a river, that is generally inundated by floodwaters

flood zoning: passing laws that restrict the development and land use of flood-prone areas

levee: a dike-like structure, usually made of compacted earth and reinforced with other materials, that is designed to constrain a stream flow to its natural channel

outlet works: gates or conduits in a dam that are generally kept open so as to discharge the normal stream flow

spillway: generally, a broad reinforced channel near the top of a dam, designed to allow rising waters to escape the reservoir without overtopping the dam

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