content of flood

MEANING:

A flood is an overflow or accumulation of an expanse of water that submerges land. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Flooding may result from the volume of water within a body of water, such as a river or lake, which overflows or breaks levees, with the result that some of the water escapes its normal boundaries. While the size of a lake or other body of water will vary with seasonal changes in precipitation and snow melt, it is not a significant flood unless such escapes of water endanger land areas used by man like a village, city or other inhabited area.

Floods can also occur in rivers, when the strength of the river is so high it flows out of the river channel, particularly at bends or meanders and causes damage to homes and businesses along such rivers. While flood damage can be virtually eliminated by moving away from rivers and other bodies of water, since time out of mind, people have lived and worked by the water to seek sustenance and capitalize on the gains of cheap and easy travel and commerce by being near water. That humans continue to inhabit areas threatened by flood damage is evidence that the perceived value of living near the water exceeds the cost of repeated periodic flooding.

TYPES OF FLOOD:

Riverine floods:

Slow kinds:

Runoff from sustained rainfall or rapid snow melt exceeding the capacity of a river's channel. Causes include heavy rains from monsoons, hurricanes and tropical depressions, foreign winds and warm rain affecting snow pack. Unexpected drainage obstructions such as landslides, ice, or debris can cause slow flooding upstream of the obstruction.

Fast kinds: include flash floods resulting from convective precipitation (intense thunderstorms) or sudden release from an upstream impoundment created behind a dam, landslide, or glacier.

Estuarine floods:

Commonly caused by a combination of sea tidal surges caused by storm-force winds. A storm surge, from either a tropical cyclone or an extratropical cyclone, falls within this category.

Coastal floods:

Caused by severe sea storms, or as a result of another hazard (e.g. tsunami or hurricane). A storm surge, from either a tropical cyclone or an extratropical cyclone, falls within this category.

Catastrophic floods:

Caused by a significant and unexpected event e.g. dam breakage, or as a result of another hazard (e.g. earthquake or volcanic eruption).

Muddy floods:

A muddy flood is generated by run off on crop land.
A muddy flood is produced by an accumulation of runoff generated on cropland. Sediments are then detached by runoff and carried as suspended matter or bedload. Muddy runoff is more likely detected when it reaches inhabited areas.
Muddy floods are therefore a hillslope process, and confusion with mudflows produced by mass movements should be avoided.

Other floods:
Floods can occur if water accumulates across an impermeable surface (e.g. from rainfall) and cannot rapidly dissipate (i.e. gentle orientation or low evaporation).
A series of storms moving over the same area.
Dam-building beavers can flood low-lying urban and rural areas, often causing significant damage.

Effects of Floods:

Floods make an enormous impact on the environment and society. Floods destroy drainage systems in cities, causing raw sewage to spill out into bodies of water. Also, in cases of severe floods, buildings can be significantly damaged and even destroyed. This can lead to catastrophic effects on the environment as many toxic materials such as paint, pesticide and gasoline can be released into the rivers, lakes, bays, and ocean, killing maritime life. Floods may also cause millions of dollars worth of damage to a city, both evicting people from their homes and ruining businesses. Floods cause significant amounts of erosion to coasts, leading to more frequent flooding if not repaired.However, floods do make a slight positive impact on the environment. Floods spread sediment containing beneficial nutrients to topsoil that might never arrive there otherwise.

Saint Francis Dam - March 12, 1928:

In 1928, the Saint Francis Dam in Los Angeles was filled with 12.5 billion gallons of water - enough water for one year for a population of 1.2 million people. However, on March 12, 1928, the dam broke, unleashing a 78 foot wall of water. It obliterated houses, ranches, automobiles, animals, and people. In roughly one hour, more than 500 people were killed and Santa Paula was overrun by water.

Grand Forks Flood - April 18, 1997On Friday, April 18, 1997, the Red River flooded over the dikes into Grand Forks, North Dakota. At the end of Saturday, the19th, the flood waters had spread over a large area of Grand Forks and East Grand Forks. Roughly 60,000 people were forced from their homes and downtown Grand Forks was left in flames. The damage was so extensive that it was weeks before people could return and rebuild their homes - and their lives.

CAUSES OF FLOOD:

Tropical Storms :

In 1969, a tropical storm caused extensive floods in Virginia. Hurricane Camille had spent most of its initial energy over the Gulf of Mexico. As it neared Virginia, it met a line of large thunderstorms coming down from the north. The warm, moist air carried by Hurricane Camille clashed with the cold northern storms, meeting over the Tye and Rockfish river valleys. Together, the two weather systems caused huge sheets of rainfall.

In some areas, hard rain accumulated 31 inches in only six hours. The Rockfish River rose 30 feet. Rivers and creeks in the region’s valleys could not hold all the water, so it spilled over the banks and covered the lowlands. Water then cascaded into the valleys. Rain softened a large chunk of a nearby mountain, causing it to slide into the valley, carrying boulders, mud, and thousands of giant trees. When the landslide ended, farms were buried under 30 feet of soil and earth. More than 125 lives were lost in the flood, while many others were left without homes.

Camille was an example of one of the most common causes of floods - heavy rains that come with tropical storms. Such storms form over the warm waters of the tropics, so they are full of moisture. When the right conditions form, bringing these giant storms toward land, many inches of rain usually fall. The heavy precipitation is too much for the streams and rivers to handle, causing water to overflow and produce inland floods. Many of these tropical storms form over the Gulf of Mexico.

Seasonal Flooding :

In May 1990, heavy rainstorms caused floods in Texas, flooding more than 200 square miles along the Trinity River. A record 100,800 cubic feet of water passed through Lake Livingston Dam, destroying crops far east. More than 700,000 acres of Louisiana farmland were covered with water.

According to some scientists, the record floods of 1990 were caused by global warming, the theory that the earth’s atmosphere traps heat near the earth, slowly warming the earth. This greenhouse effect may have heated the water in the Gulf of Mexico, causing it to evaporate faster. With more water vapor in the lower atmosphere, small storms escalated into large systems with lots of moisture. These storms moved over the southern United States and released a torrent of rain that led to massive flooding. If the global warming theory is correct, sea levels will rise three to five feet in 60 years. Coastal areas may be partially submerged underwater or easily flooded.
Other seasonal weather conditions can also cause floods.

In September of 1982, large amounts of rain fell in Utah, followed by heavy winter snows. In May, an unexpected heat wave melted the snow, causing water to cascade down the mountain slopes. The water raced into Salt Lake City and through much of the plains.

The next winter brought another heavy snowfall. Now that they were prepared for what was to come, residents filled over one million sandbags and placed them on riverbanks to keep them from overflowing in the spring. In May, a heavy thunderstorm hit, starting a new series of floods.
The water swelled the Great Salt Lake, bursting through dikes and flooding wetlands, marshes, roads, parks, and homes. Fortunately, very few people or livestock died because of the warning people had beforehand. The flood had also built slowly, giving people lots of time to prepare.

Coastal Flooding :

The earth has seven oceans that cover almost three-fourths of its surface. Naturally, wind and other events caused ocean water to sometimes overflow. When this happens, flooding on the shores occurs. Ocean storms can dump lots of water on a coast, raising the sea level in that area. These are known as storm surges, and cause coastal flooding.

Coastal flooding usually occurs as a result of severe storms, either tropical or winter storms. Ocean waves intensify on the open ocean, and these storms make surface water much choppier and fierce than normal. Raging winds can create huge waves that crash on unprotected beaches.

In the winter of 1978, the northeastern United States coast saw severe flooding that resulted from high winds, high tides, and a storm surge. High winds coincided with unusually high waves. South of Boston, Massachusetts, waves came over a seawall built to protect houses in the region. These wild waves destroyed the houses into rubble. Low-lying towns were also flooded with ocean water and ice.

IMPACT OF FLOOD:

'Flooding is a natural and recurring event for a river or stream which is a result of heavy or continuous rainfall exceeding the absorptive capacity of soil and the flow capacity of rivers, streams, and coastal areas. This causes a watercourse to overflow its banks onto adjacent lands. Floodplains are, in general, those lands most subject to recurring floods, situated adjacent to streams and rivers.

These are therefore "flood-prone" and are hazardous to development activities if the vulnerability of those activities exceeds an acceptable level; therefore affecting livelihood of the region. Eastern Gangetic Flood Plain particularly Bihar has been found to be most prone to frequent flood severely affecting agro-ecosystem and livelihood of the region. The use of remotely sensed data facilitates the synoptic analyses of flood at different spatio-temporal scale depending upon the spatial and temporal resolution of the sensor in use. In this study, an attempt has been made to make an use of coarse spatial (1 km) combined with high temporal (10 days) resolution component of SPOT VEGETATION data to monitor the inundation and recession pattern of flood (i.e. flood dynamics) during the major flood event year 2003 in Eastern Gangetic Flood plain covering Bihar state only.

Ten days composite vegetation products taken for the period May 2003 to April 2004 were preprocessed and analyzed. It is very well known that NDVI data from a satellite sensor is primarily related to vegetation changes and follows annual cycle of growth and decline. But clouds and poor atmospheric conditions usually depress NDVI values causing sudden drops in NDVI which are not compatible with the gradual process of vegetation growth and is considered as noise and removed using FASIR (Fourier Adjusted Sun Zenith Angle Corrected Interpolated and Reconstructed) Technique.Extent of flooded area was delineated at every 10 days interval resulting in inundation pattern till it attains the peak followed by the recession pattern of the flood. The processed VEGETATION data products were also analyzed for mapping cropping pattern of the year. Crop damage due to flood was assessed and spatio-temporal distribution of crops of the area was linked to flood dynamics for understanding its impact on agro-ecosystem of the region. It was revealed that multitemporal data though having coarse spatial resolution was especially advantageous to study the dynamic feature like flood and crop at regional scale. Information on flood dynamics led to know its impact on the flood plain in terms of damage to agricultural crops and adoptive strategies for agricultural practices to live with flood.