Flood control refers to all methods used to reduce or prevent the detrimental effects of flood waters (Wikipedia). Some of the common techniques used for flood control are installation of rock berms, rock rip-raps, sandbags, maintaining normal slopes with vegetation or application of soil cements on steeper slopes and construction or expansion of drainage channels. Other methods include levees, dikes, dams, retention or detention basins. After the Katrina Disaster that happened in 2005, some areas prefer not to have levees as flood controls. Communities preferred improvement of drainage structures with detention basins near the sites.
Causes of Floods
Floods are caused by many factors: heavy precipitation, severe winds over water, unusual high tides, tsunamis, or failure of dams, levels, retention ponds, or other structures that contained the water.
Periodic floods occur on many rivers, forming a surrounding region known as the flood plain.
During times of rain or snow, some of the water is retained in ponds or soil, some is absorbed by grass and vegetation, some evaporates, and the rest travels over the land as surface runoff. Floods occur when ponds, lakes, riverbeds, soil, and vegetation cannot absorb all the water. Water then runs off the land in quantities that cannot be carried within stream channels or retained in natural ponds, lakes, and man-made reservoirs. About 30 percent of all precipitation is in the form of runoff small and that amount might be increased by water from melting snow. River flooding is often caused by heavy rain, sometimes increased by melting snow. Aflood that rises rapidly, with little or no advance warning, is called a flash flood. Flash floods usually result from intense rainfall over a relatively small area, or if the area was already saturated from previous precipitation.
Severe winds over water
Even when rainfall is relatively light, the shorelines of lakes and bays can be flooded by severe winds—such as during hurricanes—that blow water into the shore areas.
Unusual high tides
Coastal areas are sometimes flooded by unusually high tides, such as spring tides, especially when compounded by high winds and storm surges.
Effects of Floods
Flooding has many impacts. It damages property and endangers the lives of humans and other species. Rapid water runoff causes soil erosion and concomitant sediment deposition elsewhere (such as further downstream or down a coast). The spawning grounds for fish and other wildlife habitats can become polluted or completely destroyed. Some prolonged high floods can delay traffic in areas which lack elevated roadways. Floods can interfere with drainage and economic use of lands, such as interfering with farming. Structural damage can occur in bridge abutments, bank lines, sewer lines, and other structures within floodways. Waterway navigation and hydroelectric power are often impaired. Financial losses due to floods are typically millions of dollars each year.
Control of Floods
Some methods of flood control have been practiced since ancient times.These methods include planting vegetation to retain extra water, terracing hillsides to slow flow downhill, and the construction of floodways (man-made channels to divert floodwater). Other techniques include the construction of levees, dikes, dams, reservoirs or retention ponds to hold extra water during times of flooding.
Methods of Control
In many countries, rivers prone to floods are often carefully managed. Defences such as levees, bunds, reservoirs, and weirs are used to prevent rivers from bursting their banks. When these defences fail, emergency measures such as sandbags or portable inflatable tubes are used. Coastal flooding has been addressed in Europe and the Americas with coastal defences, such as sea walls, beach nourishment, and barrier islands.
A dike is another method of flood protection. A dike lowers the risk of having floods compared to other methods. It can help prevent damage; however it is better to combine dikes with other flood control methods to reduce the risk of a collapsed dike.
A weir, also known as a lowhead dam, is most often used to create millponds, but on the Humber River in Toronto, a weir was built near Raymore Drive to prevent a recurrence of the flooding caused by Hurricane Hazel in 1954, which destroyed nearly two fifths of the street.
London is protected from flooding by a huge mechanical barrier across the River Thames, which is raised when the water level reaches a certain point.
Venice has a similar arrangement, although it is already unable to cope with very high tides. The defenses of both London and Venice will be rendered inadequate if sea levels continue to rise.
The largest and most elaborate flood defenses can be found in the Netherlands, where they are referred to as Delta Works with the Oosterschelde dam as its crowning achievement. These works were built in response to the North Sea flood of 1953, in the southwestern part of the Netherlands. The Dutch had already built one of the world's largest dams in the north of the country: the Afsluitdijk (closing occurred in 1932).
Flood blocking the road in Jerusalem
Currently the Saint Petersburg Flood Prevention Facility Complex is to be finished by 2008, in Russia, to protect Saint Petersburg from storm surges. It also has a main traffic function, as it completes a ring road around Saint Petersburg. Eleven dams extend for 25.4 kilometres and stand eight metres above water level.
Another elaborate system of floodway defenses can be found in the Canadian province of Manitoba. The Red River flows northward from the United States, passing through the city of Winnipeg (where it meets the Assiniboine River) and into Lake Winnipeg. As is the case with all north-flowing rivers in the temperate zone of the Northern Hemisphere, snowmelt in southern sections may cause river levels to rise before northern sections have had a chance to completely thaw. This can lead to devastating flooding, as occurred in Winnipeg during the spring of 1950. To protect the city from future floods, the Manitoba government undertook the construction of a massive system of diversions, dikes, and floodways (including the Red River Floodway and the Portage Diversion). The system kept Winnipeg safe during the 1997 flood which devastated many communities upriver from Winnipeg, including Grand Forks, North Dakota and Ste. Agathe, Manitoba.
In the U.S., the New Orleans Metropolitan Area, 35% of which sits below sea level, is protected by hundreds of miles of levees and flood gates. This system failed catastrophically, with numerous breaks, during Hurricane Katrina in the city proper and in eastern sections of the Metro Area, resulting in the inundation of approximately 50% of the Metropolitan area, ranging from a few inches to twenty feet in coastal communities.
In an act of successful flood prevention, the Federal Government of the United States offered to buy out flood-prone properties in the United States in order to prevent repeated disasters after the 1993 flood across the Midwest. Several communities accepted and the government, in partnership with the state, bought 25,000 properties which they converted into wetlands. These wetlands act as a sponge in storms and in 1995, when the floods returned, the government did not have to expend resources in those areas.
In China, flood diversion areas are rural areas that are deliberately flooded in emergencies in order to protect cities.
Many have proposed that loss of vegetation (deforestation) will lead to a risk increase. With natural forest cover, flood duration should decrease. Deforestation amplifies the incidents and severity of floods.
Flood clean-up safety
Clean-up activities following floods often pose hazards to workers and volunteers involved in the effort. Potential dangers include electrical hazards, carbon monoxide exposure, musculoskeletal hazards, heat or cold stress, motor vehicle-related dangers, fire, drowning, and exposure to hazardous materials. Because flooded disaster sites are unstable, clean-up workers might encounter sharp jagged debris, biological hazards in theflood water, exposed electrical lines, blood or other body fluids, and animal and human remains. In planning for and reacting to flood disasters, managers provide workers with hard hats, goggles, heavy work gloves, life jackets, and watertight boots with steel toes and insoles
Europe is at the forefront of the flood control technology. With many countries around Europe at or below the sea level, the problems of floods and rising sea levels are ever increasing. Countries like the Netherlands with projects such as the Zuiderzee works and the Delta works could prove to be important models for other countries around the world to follow. These sorts of humongous projects could be key in combating the increasing effects of global climate change such as: rising sea levels, an increase in the frequency and severity of some natural disasters, and even increased durations of dry or rainy seasons.
The tremendous amount of damage that Katrina did to New Orleans could have been mostly prevented if New Orleans had such an intricate flood control system as the Netherlands. The result of Katrina was that the state of Louisiana sent politicians to the Netherlands to take a tour of the complex and highly developed flood control system in place in the Netherlands. Many countries around the world are also at or below sea level and the worst part about that is the fact that a significant amount of the global population lives on or near to the coastal shores. Even though many of these projects around the world are designed to fight floods like a 100 or even 10,000 year flood; these projects can still prove to be key instruments in the fight against global climate change. The Netherlands is the world leader in flood control and has been battling the sea for centuries and new ways to deal with water are constantly being developed and tested. Projects such as the underground storage of water, storing water in reservoirs in large parking garages, and even something as simple as turning a playground during normal conditions into a small lake during heavy rainfall weather all show how the Netherlands is actively trying to combat the increasing dangers of rising sea levels. In Rotterdam there is even a project to construct a floating housing development of 120 acres, which of course will be unaffected by rising sea levels. These flood control systems do not always have to be solely to prevent floods but can also be used to combat droughts. China has recently gone to the Netherlands and requested their help in combating the large scale drought that is occurring around China. The Dutch are going to help China develop a drought warning system as well as new water management programs and contribute to flood defense research. Flood control will become an ever increasing issue in world politics and as more and more countries start feeling the effects of a global increase in the sea level then it will be time for action and the Netherlands will certainly be at the forefront of this action and furthermore used as an example for many countries when it is time for them to start dealing with issues of the sea.
Benefits of Flooding
There are many disruptive effects of flooding on human settlements and economic activities. However, flooding can bring benefits, such as making soil more fertile and providing nutrients in which it is deficient. Periodic flooding was essential to the well-being of ancient communities along the Tigris-Euphrates Rivers, the Nile River, the Indus River, the Ganges and the Yellow River, among others. The viability for hydrologically based renewable sources of energy is higher in flood-prone regions.