Floods and Flood Control Essay

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F loods occur when a body of water, e.g., a stream, lake, or sea, overtops its normal channel or basin because of excessive inflow of water or geophysical or atmospheric activity. The two broad categories of floods are stream flooding and coastal flooding. Most types of floods are a very normal part of the hydrological rhythms of water bodies. In fact, many riparian and coastal systems generally depend upon cycles of flooding to maintain a healthy ecology. Human societies in coastal and riparian regions have evolved agricultural and resource extraction systems dependent upon cycles of flooding to maintain productivity and sustainable livelihoods. Since the onset of the Industrial Revolution, urbanization, and increasing populations, however, the equation between human societies and floods has changed into a largely negative one.

Types of Flooding

Of the two broad types of flooding, stream flooding impacts more people than coastal flooding because of the higher concentration of human populations in river valleys. The world’s great river valleys-the Nile, Tigris-Euphrates, Indus, Ganges, Huang He, Yangtze and Mei Kong were not only cradles of human civilization but also have extremely high population densities. Natural causes for stream and river floods include seasonal snow melt or precipitation, glacial slip and/or landslides in the headwaters. Glacial slip happens when a glacier in the headwaters of a stream breaks off and slips into the main channel, causing temporary damming of the channel. The dam inevitably breaks from the pressure, causing a wall of water to flow down the channel. Glacial slip and landsliding can cause some of the most sudden and devastating floods.

Sudden, high-level flooding is also called flash flooding, and can also be caused by intense precipitation. Flash floods have the most adverse impact for life and property. If the ground is fully saturated as a result of prolonged precipitation, even very small amounts of precipitation on saturated ground in a watershed can cause intense flood events.

Beyond the natural causes of flooding, human impacts on watersheds, such as urbanization, deforestation and agricultural activity further accelerate flood peaks. Smaller amounts of precipitation in a watershed naturally flows downhill as surface runoff. A considerable amount of precipitation is interrupted by a well-vegetated watershed, and is stored in the plants or reevaporated into the air, percolated into the ground, or directed into the water table. Urbanized surfaces with concrete and asphalt cause precipitation to flow out of the watershed as surface runoff, causing higher flood peaks downstream. Furthermore, agricultural or deforested surfaces lack vegetative cover to intercept the precipitation, or have been compacted to the extent that their absorptive capacity is much lower than a natural surface, causing similar impacts as urbanized surfaces. Human modification of watersheds all over the world is becoming a more important cause of downstream flash flooding.

Manipulation of Waterways

Human manipulation of streams for irrigation, flood control and navigation have also impacted the pattern of flooding in world’s streams and rivers. Increasingly, it is difficult to find streams or rivers that have not been manipulated. Part of the problem is increasing human occupancy of floodplains. The spatial extent of a floodplain is by definition the area along the stream channel, which may be flooded in the normal rhythm of flooding in a stream. Humans have occupied floodplains since the dawn of civilization because of the availability of fertile soils, hydropower, and river transport. Many preindustrial societies were well adapted to the cycles of flooding along their streams, but with higher population levels, urbanization, and industrialization, modern societies are increasingly inflexible in the face of cycles of nature. Consequently, modern societies have increasingly tried to control and tame streams and rivers instead of trying to adapt to their rhythms, with disastrous results.

Development of dams, canals, barrages, and other diversion structures on rivers for irrigation purposes is today considered an important and legitimate use of water resources-to an extent. As more and more water is withdrawn from streams, their normal flows are curtailed, leading to increasing deposition of silt in the stream channels, leading to lowering of the channel capacity of the streams. Even moderately high flows of water, which otherwise could have been accommodated in the stream channel, end up overtopping the banks of the channel, causing damage to life and property. Neither the Colorado River delta nor the Syr and Amu Darya deltas in Central Asia makes it to the sea in most years because of excessive water withdrawals upstream, causing loss of livelihoods for the people living in the delta and immense damage to ecosystems.

Dams are a popular means of water development for irrigation, power generation and flood control. Whereas dams have proved themselves effective in controlling low to moderate intensity floods, their impact on river geomorphology downstream has not been very positive for flood peaks. Dams serve to lower the channel capacity of the streams. Since all dams are subject to failure, in the event of large inflows of water, they can actually serve to accentuate flood peaks downstream from the dam site.

Levees are another popular means of flood control and maintaining navigability of streams. On many of the world’s great rivers, from the Mississippi to the Indus to the Huang He Rivers, levees line the channels, facilitating human encroachment upon the flood plains. Levees, depending upon their capacities, are effective in retaining surplus water within stream channels in the event of low to moderate flooding. Levees ensure that rivers stay within a course that a society deems convenient. However, in the long run, as rivers and streams continue to deposit silt within their levee-enclosed channels, stream beds can become higher than the surrounding flood plain. In the event of a levee breach, the flood peaks in the flood plain can be much higher than expected. All levees are designed for certain flood peaks; a levee may be designed for a flood peak equivalent to a flood with a 100 year return period, or 1% chance of happening in any given year. The return period calculation based on historic floods is nothing more than a projection based on past record. But the role of levees in controlling low to moderate flood peaks, coupled with the inflated sense of certainty that the “return period” statistic conveys, instill a false sense of security among the floodplain residents. The result is more intense human use of floodplains, more levee building and greater devastation when the levees inevitably fail or are overtopped. Furthermore, levees require large capital outlays to build and maintain. Most societies can simply not afford the expense for their maintenance and hence must seek adaptation to flood hazard rather than outright control.

Coastal Flooding

Coastal flooding is growing in importance due to the increasing number of humans living in coastal regions. With increasing globalization, many of the world’s largest metropolitan areas are in coastal areas. Furthermore, in countries like the United States, Netherlands, Bangladesh, and China, along with island nations, very high proportions of the populations live in coastal regions.

The main cause of flooding in coastal regions is tropical storm activity. Intense tropical storms are called hurricanes, typhoons or cyclones, depending on the region. Tropical storm activity is largely limited to tropical and subtropical regions, though storm systems can and do penetrate deep into the higher latitudes to cause damage.

Hurricanes can pack very high-velocity winds and intense rainfall, causing storm surges in coastal regions. Hurricanes draw their energy from the warms waters of tropical seas and they lose their energy very quickly once they come on land. Barrier islands and coastal mangrove swamps serve as natural barriers to storm surges. In the second half of the 20th century, however, there has been increasing human occupancy of barrier islands. In fact, in places like the southern United States, they are considered prime real estate because of their recreational amenities. Consequently, property damage has been increasing with every successive hurricane that has hit U.S. coasts. In developing countries, however, such as Bangladesh, people flock to the low-lying barrier islands because of economic necessity. With increasing populations and most of the prime inland land already taken over, the armies of landless have little choice but to move to the more dangerous, low-lying areas.

Mitigating Floods

There is no known way of controlling a hurricane. Some engineering interventions can be somewhat effective, such as reinforced building construction and sea walls. But even these solutions have their limits, with the result that even in weathier countries, the best course of action in the face of hurricanes is speedy evacuation. With developments in meteorology, satellite-based forecasts, and the diffusion of media, societies have a much greater capacity to predict and disseminate information about the onset of hurricanes days in advance of landfall. This is particularly true in developed countries. Once the warning has been received, these countries also have the infrastructure to evacuate large numbers of people from danger zones. Recent experience of hurricanes in the United States, however, has shown that there are large segments of the population who either do not get warned in good time, or do not have the resources to evacuate-e.g., the poor, ethnic minorities, single-parent households, and the elderly.

In poorer countries, the reach of electronic media is relatively limited, compounded by absence of infrastructure and resources for a timely evacuation, even if the warning reaches the at-risk populations in time. In these countries, everyone who is exposed to the risk of hurricanes is not necessarily equally vulnerable to them. Vulnerability is a key concept to understanding the pattern of damage from all types of hazards, including floods. Vulnerability is defined as the susceptibility of individuals or groups to be adversely affected by environmental extremes and their relative inability to recover from those adverse effects. In the less-developed countries, women, children, elderly, and poorer segments of the population are more vulnerable to hurricanes. Women, particularly in patriarchal societies, may not have the education or confidence to make decisions on behalf of their households in the event of an impending emergency. The elderly and children, with their weaker immune systems and limited mobility, may be more exposed to waterborne diseases that may break out in the aftermath of coastal flooding.

In case of both riverine and coastal flooding, mitigating vulnerability of the populations to the adverse effects of flooding is key to reducing damages. Many researchers have documented the profile and location of populations vulnerable to flood hazard and the complex causes for their vulnerability, demonstrating that (1) human manipulation of stream hydrology in the name of progress has created new spatial distributions of flood hazard; (2) hydrological manipulations have often benefited a few powerful segments of the society at the expense of exposing larger, weaker segments to flood hazard; (3) flood protection for urban areas often accentuates flood peaks for upstream and downstream rural areas; and (4) segments of population are differentially more vulnerable even at the same location because of their limited access to resources, lack of education, marginal social status, age, and gender.

A different stream of research, also known as pragmatist, has been concerned with human societies’ “range of choice” in the face of flood hazard, and was pioneered by Gilbert White in the first half of 20th century. According to pragmatist research, human societies have an infinite range of choices in the face of environmental hazards, which is curtailed by the exclusive focus on engineering solutions to hazards in general and flood hazard in particular. The key pragmatist insight considers multiple solutions through scientifically informed and reasoned public debate, including nonengineering solutions to flood hazard.

Nonengineering solutions found particular resonance in the policy field. In the United States, for example, public policy goes beyond building levees, dams and protective civil works to include flood insurance, flood plain mapping, flood warning, evacuation, flood-proofing of houses, and wetland restoration.

Civilization and Floodplains

Human civilization originated in the floodplains of the great rivers of the world and for good reason. Cycles of flooding replenished the fertile soils of great river valleys and allowed for increased agricultural production, as well as enhanced fisheries. Agricultural systems were well adapted to cyclical floods and almost depended on them for their sustainability. In the Nile River, the ancient Egyptian calendar was based on the cycles of the Nile’s floods. In the Mei Kong River basin, entire cultures have developed around flood farming and fisheries that are made available by the floods of the Mei Kong.


  1. Liakath Ali, Integrated Approach for the Improvement of Flood Control and Drainage Schemes in the Coastal Belt of Bangladesh (Taylor & Francis, 2002);
  2. Donald P. Batzer and Rebecca Sharitz (ed.), Ecology of Freshwater and Estuarine Wetlands (University of California Press, 2007);
  3. Karen O’Neill, Rivers by Design: State Power and the Origins of U. S. Flood Control (Duke University Press, 2006).

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