Flood-Control Technology

Summary

Flood-control technology deals with the myriad techniques employed to deal with water that overflows stream banks, thereby leading to deaths and injuries, property and crop damage, and severe erosion. The magnitude of the flood damage varies considerably, as it depends on the duration of the storm and the amount of precipitation. Each watershed has different physical features, such as size, slope, basin relief, impoundments, flood history, soil types, and drainage characteristics.

Definition and Basic Principles

Flood-control technology includes various structural and nonstructural measures that play a substantial role in mitigating flood damages. The methods vary from place to place, given the enormous heterogeneity of stream discharge, precipitation frequency and amount, and watershed factors, including slope, soil permeability, infiltration characteristics, degree of urbanization, varying land-management practices, governmental interest, and land-ownership practices.

The structural or physical measures that are employed include levees or flood walls made of earth or concrete, channel modifications such as deepening or widening the stream itself, building dams or reservoirs to hold additional water, watershed-improvement techniques that include reforestation and planting of vegetative cover in bare areas, flood proofing in lower-risk floodplain areas that include dikes, elevating the buildings, and waterproofing.

Nonstructural or preventative measures that could be adopted include floodplain regulations, such as zoning laws that stipulate where development may or may not occur, building codes that restrict basements and the permissible amount of impervious cover allowed on the site, open-space requirements for large-scale developments, and designing flood warning systems to provide advance notice about impending problems.

Background and History

The beginnings of speculation about water movement (the hydrologic cycle) began in ancient times by renowned philosophers such as Homer, Thales, Plato, and Aristotle in Greece, and later on by Lucretius, Seneca, and Pliny in ancient Rome. The field of hydrometry that pertains to streamflow measurement began in the seventeenth century, and improved techniques increased with each century. For example, flow formulas, measuring instruments, and stream gauging procedures became better established in the eighteenth and later centuries. Some of the many advances include civil engineer Theodore G. Ellis's and surveyor William Gunn Price's stream current meters in 1870 and 1885, respectively, Irish engineer Robert Manning's flow formula in 1891, civil engineer Allen Hazen's comments on increasing the use of statistics in flood studies in 1930, and German statistician E. J. Gumbel's suggestion for using frequency analysis for floods in 1941.

Significant changes at the governmental level in the United States assumed increasing importance as several hydrologic agencies were created in the nineteenth century: the Army Corps of Engineers in 1802, the Weather Bureau in 1870 (now called the National Weather Service), the US Geological Survey, and the Mississippi River Commission in 1879. In the twenty-first century, the Department of Homeland Security, along with these agencies, plays a major role in flood control.

How It Works

Floods. In the hydrologic cycle, atmospheric precipitation falls to Earth. It is either evapotranspired back to the atmosphere, absorbed by vegetation, or moved downslope as overland flow that eventually becomes large enough to form stream channels. Water generally stays within the channel for most of the year.

However, if the precipitation is heavy enough, the channel cannot transport all of this water, and the stream overflows its banks, creating floods. Floodplains are low areas on one or both sides of a stream and are common in humid regions. Small watersheds with steep slopes often have flash floods that move very quickly and have enough turbulence to damage buildings and vehicles easily. Since flash floods occur so rapidly, people cannot be readily warned, and drownings can occur.

Stream Discharge. In the United States, stream flow is measured in cubic feet per second at most gauging stations operated by the US Geological Survey (USGS). Most of the larger streams have higher discharges as they flow downstream. There are some exceptions, such as the Colorado River in the arid southwestern United States, where the demand for irrigation water and public supply dries up the stream at its mouth.

Flood Frequency. It would be wonderful if flood predictions were reasonably exact. However, the probability of knowing when floods will occur is a markedly difficult task as it involves frequency analysis, probability theory, data that need to be homogeneous and independent, and selected assumptions that existing streamflow data would be similar to future flows. This fallacious assumption implies that there will be no changes in future land use and climate.

The longer the period of record, the better the estimation of future flood flows. Accordingly, a “one-hundred-year flood” does not mean that the next one of that magnitude will occur in one hundred years. This means there is a 1 percent chance that a one-hundred-year flood could happen any year. Accordingly, a one-hundred-year flood could occur this year and again the following year, although the odds are against it. In hindsight, it would have been better if the term “one-hundred-year flood” had been expressed as a “one-in-one hundred chance flood” to reduce public confusion.

Floodplain Development. The stream channel itself occupies a relatively narrow area within a floodplain. The channel is bordered by low-lying land that is called a floodway. As the distance away from the river increases, the slope of the land gradually increases, and a slightly higher landscape called a floodway fringe is created. A cross-sectional profile of this floodplain would then show a channel with a floodway on one or both sides of the channel and a floodway fringe farther away from the stream.

In a nonregulated development scenario that was common in earlier years and still exists in some areas, residential and commercial buildings were constructed in the floodway zone, which led to higher flood levels, structural damage, and potential loss of life. Low bridges crossing the stream in this zone could partially block the flow and increase flood levels.

The situation would be quite different in a regulated floodplain. Ideally, the floodway would be zoned for as much open space as possible, such as parks, picnic areas, and golf courses. This would facilitate easier passage of flood waters and restrict buildings to higher ground wherever possible.

Floodplain Extent. It has been estimated that one-hundred-year floodplains make up from 7 to 10 percent of the total land area of the United States. The floodplains with the largest areas are located in the southern portions of the country; those with large populations are located along the North Atlantic coast, the Great Lakes, and California.

Types of Floods. Damaging floods occur in varying locations. Flash floods are associated with quickly developing thunderstorms in mountainous areas. Even if relatively shallow, the rapid downslope movement of water can quickly move vehicles and their occupants to the extent that flash floods cause 50 percent of the flood deaths in the United States. If floodwater flows at a depth of only two feet, a trapped vehicle experiences a lateral force of 1,000 pounds and a buoyant force of 1,500 pounds, more than enough to flip the vehicle over and drown the passengers.

Regional floods are associated with level land that experiences long periods of heavy rain. The floodplain's damage to homes and properties can be enormous, as evidenced in the August 1993 flood in the lower Missouri and upper Mississippi rivers. About 69 percent of the levees built along the upper Mississippi River to protect the floodplain occupants were overtopped, drowning forty-eight persons, submerging seventy-five towns, destroying fifty thousand homes, and even excavating more than seven hundred coffins from a cemetery in Missouri.

Storm surges powered by tropical storms and hurricanes can wreak havoc along low-lying coasts. The settlers who founded New Orleans in 1718 along the lower Mississippi River experienced their first significant flood the same year. Over time, many other floods have occurred in New Orleans and the lower reaches of the Mississippi, aided by the slow sinking of the floodplain as river sediments compacted over time. The ability of New Orleans to catastrophically flood was on display during Hurricane Katrina in 2005. Although the city had a levee system constructed by the Army Corps of Engineers, its levees were overwhelmed by the waters of the hurricane.

Applications and Products

Structural Flood-Control Objectives. Most structural flood-control measures are physical and expensive. They include the construction of large reservoirs, diversion structures, levees, flood walls, channel alterations, modifications to bridges and culverts, and tidal barriers.

Flood-control reservoirs are built to store excess stormwater and release it later, reducing peak discharge. The economic value of protecting property that may be damaged in floods can justify the construction costs of a reservoir. In addition, the stored water can also be used for hydropower, water-supply purposes, and recreation. Diversion structures are designed to reduce peak flows by forcing floods to go to another location via channels or tunnels. Levees and flood walls can physically keep water away from floodplains, providing good potential for damage reduction. Levees are usually made of earth and flood walls of concrete. Both structures are usually set parallel to the stream.

Channel alterations to the stream can lower the height of water so that peak discharges are reduced at one point, but they can be increased downstream. The cost of these measures is substantial, but they can be justified in certain situations by their potential protection of valuable property. Possible long-term adverse effects of these structural measures include aggradation or degradation of the downstream channel and sediment deposition in downstream bypass channels or tunnels. Bridge and culvert modifications are helpful when the structures cannot handle flood flows. Repairing or raising the structures over the channel can lower damages by allowing more water to flow downstream. Tidal barriers along the coast can prevent high tides from moving upstream and damage developed areas. They are expensive to build and are generally used if large, urbanized areas need protection. Note that the potential for tidal flooding is expected to increase as sea level rises because of climate change.

Nonstructural Flood-Control Objectives. These measures are employed to lower flooding susceptibility, thereby decreasing possible damage. They include flood warnings, flood proofing, and a variety of land-use control procedures. Flood warnings can reduce potential loss of life or eliminate that possibility. Flood-proofing procedures include rearranging the working space in buildings, waterproofing outside walls, and raising the height of buildings that occupy particularly susceptible locations. Land-use controls include many kinds of action within the floodplain that can reduce flood-hazard potential. These controls include proper building codes, purchasing flood insurance, zoning restrictions, or buying land and property in the less vulnerable portions of the floodplain.

Climate Change. There is growing concern that global climate change could substantially impact flooding in coastal locations. For example, since 1870, the sea level has increased between 8 and 9 inches. The overall sea ice cover in the Arctic has been decreasing by about 12.2 percent per decade. Estimates indicate that average global surface temperatures from 1990–2100 will increase between 3.1 and 10.5 degrees Fahrenheit. As a result, sea level could rise between 8.3–18.5 inches by the end of the twenty-first century. Some of the climatic models employed suggest that rates of sea-level rise will double along the shorelines of certain portions of the eastern United States and the western North American and Arctic coasts. Studies indicate heavy downpours have increased in frequency worldwide and are expected to intensify. Because of this, the number of one-hundred-year floods is expected to increase by 45 percent by the year 2100.

Flood Mapping. It would be beneficial to be able to generate flood maps in advance of approaching storms. Experimental work has been developed by the USGS and the National Weather Service (NWS) to make forecasting and mapping of potential floods readily available to local officials. The goal is to have NWS provide storm forecasts for a particular area, thereby enabling the USGS to generate maps showing potential flooded areas on the floodplain, arrival times, and the depth of the flood itself. Combining new methodologies and techniques, such as light detection and ranging (LIDAR), advanced computer programs, and geographic information systems (GIS), has led to the strong possibility of creating useful flood-forecast maps.

Flood Warning Systems. A useful addition to the regulation of floodplain development occurs when flood-warning systems are employed by local governments. These systems use radar, rainfall, and streamflow gauging that are connected by satellite transmitters to relay real-time data to computers at some central site, which in turn make the data available to interested parties via the Internet. Automatic warnings are then dispatched to emergency-management officials who can then institute procedures, based on the predicted flood levels, which range from selective road closures to complete evacuation.

Careers and Course Work

A surprising number of courses and future jobs are included in the field of flood-control technology. The most obvious ones include civil engineering for the construction of dams and levees, hydraulic engineering to handle channel deepening where appropriate, and surveying. However, many other skills are needed, and it would benefit those interested in this field to study geology, physical geography, hydrology, meteorology (storms), and water resources management. Other valuable courses of study include environmental planning, computer science, economics, and environmental law.

Many governmental organizations, including the Army Corps of Engineers, the American Water Resources Association, and FEMA, have job postings on their websites that encompass a range of employment opportunities. These agencies employ civil engineers, scientists, natural resource specialists, administrators, hydrologic technicians, and ecologists.

Social Context and Future Prospects

Floods have occurred on numerous occasions over many millennia. Societies have designed ways to store some of this water, in particular, by collecting basins or small reservoirs and, in selected cases, creating diversion canals. In other cases, the floods have damaged settlements and farms. What makes the situation even worse is the population growth on floodplains, the inhabitants of which have the mistaken notion that structural techniques can contain floods. Too many damaging floods have occurred in too many countries that render naive the presumption that one is safe behind the levee. The United States is no exception.

One step in the right direction has been the growing recognition that floodplain development carries a substantial risk: predicting flood heights is fraught with uncertainty because of a complicated mix of storm tracks, soil-moisture conditions, available upstream reservoir storage, and the elevations of commercial facilities and homes. One can now recognize the benefits of nonstructural measures, such as zoning lands in the one-hundred-year floodplain as unsuitable for development. To make matters more complicated, how does one factor in shifts that could occur over many decades, such as climate change, river behavior, and sea-level rise that would impact coastal areas and the existing homeowners in the floodplain who simply enjoy living close to rivers?

Two things will help this situation. One better explains the flooding danger, given the probability of certain events occurring in any year. These odds could change as floodplain development continues or decreases. The second element is a better discussion of future weather patterns, as unpredictable as they are, that could occur globally.

Advances in technology continue to be crucial. Scientists have experimented with green infrastructure to prevent localized flooding caused by heavy rains. This includes permeable pavement and rain gardens, which are shrubs and bushes planted in small depressions. Other advances include enhanced early warning systems, improved forecasting, the creation of smart infrastructure, especially in urban areas, using nature-based flood control approaches incorporating the existing natural landscape, providing education and community engagement, and pursuing policy decisions and regulations monitoring development in flood-prone areas. Artificial intelligence, the Internet of Things, and data analytics also advance the prediction, monitoring, and management of flooding and flood control techniques. 

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