Flood Control and Floodplains

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Flood Control and Floodplains

Introduction

With respect to environmental science, flood control refers to physical measures that are taken to minimize the chances of flooding. Typically, flood control involves a barrier to limit the overflow of water from a river or lake, or the inshore movement of storm-driven ocean water.

A floodplain is flat or almost flat land that lies next to a body of water that can flood. When the stream or river overflows, the excess water gathers on the floodplain. This spreading of the floodwater over a larger area can prevent the water from surging down a narrower corridor of a stream or riverbed, which could cause damage downstream.

Floodplains can also be deliberately constructed as a flood control measure in regions that are prone to flooding. For example, a recreational facility such as a park can be set aside from residential, commercial, or industrial use. When not flooded, the park is a useful community facility.

As Earth’s climate continues to warm, some regions are expected to experience a greater frequency of flooding. In these regions, flood control measures will become more important.

Historical Background and Scientific Foundations

Flood control in some areas of the world has been practiced for centuries. The best-known example is the Netherlands. Over 60% of the country, which borders the Atlantic Ocean, is below the normal level of the sea. As well, since the terrain is relatively flat, floodwaters can reach inland far more than in more rugged regions of the coastline of England, where cliffs can absorb and dissipate incoming waves. In the Netherlands, a national department is dedicated to flood protection, and similar departments are a feature of local governments.

Archaeological records from the Netherlands dating back to 500 BC document the construction of large earthen mounds on which villages and farms were located. These allowed life to proceed when the surrounding countryside was flooded, and so were a very early form of flood control.

Beginning in the thirteenth century, the construction of dikes (also known as dykes or levees) began. A dike is an embankment or a wall built of earth or other material. It forms a barrier that is intended to keep floodwater from moving from beyond the confines of the stream or river. A modern example of a dike is the structure that protects the French Quarter of New Orleans from the waters of the Mississippi River. The dike in this area is about 29 ft (9 m) above sea level and about 15 ft (4.5 km) above the surface of the river, and is wide enough to accommodate bicycle and walking paths.

The Netherlands also serves as a good example of several other flood control measures. One is known as a polder. These were first built in the Netherlands in the eleventh century. One type of polder, which is common in the Netherlands, involves the construction of a dike to separate the area from the sea, and the subsequent draining of the walled-off area. Polders can also be created in lakes, by walling off a portion of the lake and draining it or allowing the water to evaporate. Another type of polder is the construction of a dike around a naturally occurring floodplain or marsh, and the draining of water from these regions.

In the Netherlands, the estimated 3,000 polders must be drained regularly, since they are below sea level. A distinctive way this was once accomplished was by the use of windmills. A series of three or four windmills pumped water from the lowest lying areas to a higher elevation river or lake. Centuries ago, a windmill powered a water wheel or an Archimedes’ screw—a screw positioned inside a hollow tube—that moved the water upward. Both machines scooped up volumes of water and transferred them upward to the next collecting pond. Eventually the windmills were replaced by steam and diesel powered pumps, although some facilities in the Netherlands do continue to use modern and larger versions of the Archimedes’ screw.

Flood control can be done on a small scale or very large scale. An example of the latter, once again from the Netherlands, is the Zuiderzee Works, a flood control and land drainage system built in the early 1930s, which created over 618 square mi (1,600 square km) of land and a lake.

Another means of flood control that is similar in intent to a dike is a floodwall. As its name implies, the structure is a vertical barrier. In some locales, a floodwall is used as a temporary means of containing floods from a river that occurs following heavy rains or during a rapid snowmelt that can occur in the spring. Some floodwalls have gates that are routinely opened to allow vehicles to pass back and forth. When needed, the gates are closed, creating a barrier to the rising water.

Floodwalls tend to be used in places where space for a levee is too limited. They are more expensive to make than earthen dikes, but can be cost-effective in a city, where the property adjacent to the wall can be used for residential or commercial purposes. Cities such as New

WORDS TO KNOW

EROSION: The wearing away of the soil or rock over time.

HURRICANE KATRINA: A Category-3 hurricane that caused over 1,800 deaths and catastrophic damage to the Gulf Coast region of the United States in August, 2005.

ICE JAM: An stationary accumulation of river ice that restricts water flow and increases the risk of flooding.

STORM SURGE: Rise of the sea at a coastline due to the effect of storm winds.

Orleans, Louisiana, utilize floodwalls in the metropolitan area. It was the failure of some of these floodwalls as well as levees that caused the massive flooding of the city in the aftermath of Hurricane Katrina in 2005.

Another related type of flood control that can be used in coastal areas is a seawall. A seawall is a barrier to incoming waves that is built in the water near the shoreline. Modern sea walls can be constructed of concrete that is reinforced with metal rods, boulders, or cages filled with rocks or smaller stones. A wall can be vertical

on both sides, or vertical on the landward side and sloping on the seaward side.

The vertical wall is designed for locations where the shoreline is exposed, and where standing waves develop. Standing waves are waves that oscillated up and down, and which are caused when similarly sized waves are moving in opposite directions. This can occur when waves reflecting back from the shore encounter incoming waves. Shorter and thinner walls may be present farther out in the water from the vertical wall, to help dissipate some of the incoming wave’s energy.

A curved or sloping wall, which tapers from a wider base to a narrower top, tends to be built where waves crest and tumble forward just before reaching the shoreline, due to the slope of the seabed. The wall’s design is intended to dissipate the energy in the crashing wave and to direct the resulting spray up and out, rather than in toward the shore.

In areas where the waves are less intense, the sloping wall can be made of rubble. This is less expensive to build and can let some of the incoming water seep through, which can be an advantage if the water collected on the landward side can be used. Sea walls, particularly the sloping walls, may also have interlocking blocks positioned along the sloped surface. The blocks help dissipate even more energy.

Another form of flood control is a weir; a wall-like structure over which water can tumble, and which is used to raise the level of a small watercourse such as a stream. A weir can also be used to hold back water, which can be released by opening a gate (sluice gate) in the weir; this design is very similar to a dam, but is not used to generate any energy from the flowing water.

A weir is not itself a sufficient flood control, since floodwaters would just cascade over the weir and continue downstream. But, if positioned upstream from a wider portion of the stream, a weir can allow a pond to form. The increased volume of the pond can help trap floodwater.

Floodplains can be an efficient flood control, since they allow the waters overflowing from a stream or river to spread out in a wider area. If development has been restricted in the region—which is typically done in areas that are identified as being prone to flooding—the resulting flood does little damage.

A floodplain can be a natural feature, or can be created. The latter can help reduce flood-related damage and casualties in urban areas.

Impacts and Issues

As exemplified by the Netherlands, flood control measures can be very successful. In another example, an earthquake on the seabed near Sumatra, Indonesia, on December 26, 2004, spawned a series of tsunamis. The waves that topped 100 ft (30 m) in height at coastlines near the earthquake’s epicenter devastated coastal regions of Indonesia, Sri Lanka, Thailand, and India, killing more than 225,000 people. However, the Indian coastal district of Puducherry was not damaged at all, due to the huge stone sea wall that had first been built in

the 1730s, when the region was a French colony. The 27-ft (8-m) high seawall protected the coastline and the residents from the tsunami waves.

As Earth’s climate continues to warm, the result in some regions, according to the 2007 report of the Intergovernmental Panel on Climate Change (IPCC), could be increased rainfall and flooding in regions of Asia and elsewhere. In these regions, flood control measures will become more important as a means of protecting people and land.

However, if flood control technology is improperly used or neglected, it can be ineffective. For example, in the aftermath of Hurricane Katrina in 2005, the failure of levees and floodwalls in New Orleans resulted in the flooding of 80% of the city. Results from the five investigations carried out determined that the soil foundation of some floodwalls has become saturated with water, making them less stable. When stressed by the surge of water accompanying the storm, some of the walls were pushed along with the flow. As well, it was determined that anchoring the walls more deeply could have made them stable enough to withstand the floodwaters. Other cases of inattentive inspection and repair of levees and floodwalls were documented. The U.S. Army Corps of Engineers ultimately accepted the blame for the faulty design of some dikes, acknowledging that some dikes were not built high enough and were constructed from “erodible materials.”

Although flood control measures can benefit humans, they may adversely affect other species. For example, a 20-mi (32-km) long seawall constructed at South Korea’s Saemangeum wetland—one of the largest wetlands on the globe—to reduce tidal surges and create land for agriculture, has dried much of the wetland, destroying an important resting spot on the migratory route of a number of species of birds. In particular, the spoon-billed sandpiper and Nordmann’s greenshank are now in danger of extinction if the habitat cannot be at least partially restored, according to Britain’s Royal Society for the Protection of Birds.