Flood

Flood is a temporary covering by water of land which is not normally covered by water. This shall include floods from rivers, mountain torrents, Mediterranean ephemeral water courses, and floods from the sea in coastal areas, and may exclude floods from sewerage systems.

In shortly we can define flood as an abnormal progressive rise in the water level of streams or rivers which may result in overflowing.

Types of Flood

Floods are mainly five types-

  • River / Monsoon flood
  • Flash flood
  • Ponding flood
  • Coastal flood
  • Urban flood

River / Monsoon flood

When river water rises and spills over the bank of the river and covers some area with river water which is normaly dry land this flood is called as river / monsoon flood.

Flash flood

Flash flood is a sudden flood in a low-laying area, most commonly caused by periods of perticularly heavy rainfall. Generally happens in the valleys of the hilly areas.

Ponding flood

Ponding is a type of flooding that can happen in relatively flat areas. Rain water falling in an area is normally stored in the ground, in canals or lakes, or is drained away. When more rain water enters a water system than can be stored, or can leave the system, flooding occurs.

Coastal flood

When coastal area is covered by sea which is normally dry land is called coastal flood.

Urban flood

Urban flooding is specific in the fact that the cause is a lack of drainage in an urban area. Flooding in urban areas can be caused by flash flood, or coastal flood, or river flood.

Causes of Flood

There are several causes of floods and differ from region to region. Some of the major causes are-

  • Heavy monsoon rainfall.
  • Low general topography.
  • Heavy siltation of the river bed reduces the water carrying capacity of the rivers/stream.
  • Blockage in the drains lead to flooding of the area.
  • Landslide blocking the flow of the stream.
  • Construction of dams.
  • Snow melting.
  • Rise of mean sea level.
  • Flow tide in the Bay of Bengal.
  • Deforestation and flood protection projects in the upper catchments outside Bangladesh.
  • Greenhouse effect.

Hazards Associated with Flood

A dangerous condition or threat for causing injury to life or damage to property or the environment is called hazard. Flood isn’t the stander condition of environment. It is threated not only human but also environment. Some effect of flood hazard is given below-

  • Displacement of Families in the river banks
  • Destruction of private property on the river banks
  • Destruction of Biodiversity on the river banks
  • Disruption of Transport and communication systems
  • Destruction of drainage and sewage system
  • Eroding of river banks
  • Adverse effect on Public Health

Flood Mapping in Bangladesh

Flood History of Bangladesh (1954-2016)

Year Flood Affected area Year

 

Flood Affected area Year Flood Affected area
Sq-Km % Sq-Km % Sq-Km %
1954 36,800 25 1976 28,300 19 1998 1,00,250 68
1955 50,500 34 1977 12,500 8 1999 32,000 22
1956 35,400 24 1978 10,800 7 2000 35,700 24
1960 28,400 19 1980 33,000 22 2001 4,000 2.8
1961 28,800 20 1982 3,140 2 2002 15,000 10
1962 37,200 25 1983 11,100 7.5 2003 21,500 14
1963 43,100 29 1984 28,200 19 2004 55,000 38
1964 31,000 21 1985 11,400 8 2005 17,850 12
1965 28,400 19 1986 6,600 4 2006 16,175 11
1966 33,400 23 1987 57,300 39 2007 62,300 42
1967 25,700 17 1988 89,970 61 2008 33,655 23
1968 37,200 25 1989 6,100 4 2009 28,593 19
1969 41,400 28 1990 3,500 2.4 2010 26,530 18
1970 42,400 29 1991 28,600 19 2011 29,800 20
1971 36,300 25 1992 2,000 1.4 2012 17,700 12
1972 20,800 14 1993 28,742 20 2013 15,650 10.6
1973 29,800 20 1994 419 0.2 2014 36,895 25
1974 52.600 36 1995 32,000 22 2015 26,000 18
1975 16,600 11 1996 35,800 24 2016 1,00,000 66

Source: Annual Flood Report, Flood Forecasting & Warning Centre, Bangladesh Water Development Board.

Flood Forecasting

Flood forecasting is the use of forecasted precipitation and streamflow data in rainfall-runoff and streamflow routing models to forecast flow rates and water levels for periods ranging from a few hours to days ahead, depending on the size of the watershed or river basin.

“Yellow” when the river is flowing in Low Flood: when river water level is at or above WL but below DL.

” Pink” when the river is flowing in Moderate Flood: when river water level is at or above DL but below level which is 0.5 m below HFL.

“Orange” when the river is flowing in High Flood: when the river water level is within 0.5 m of HFL and below HFL.

“Red” when the river is flowing in Unprecedented Flood: when the river water level is at or above HFL. HFL is updated on a yearly basis before start of the flood season.

Flood forecasting can be divided into two categories:

  • Flood forecasting in the rivers caused by upstream rise of river stage as well as rainfall in the basin.
  • Overland flow from upstream.

Measures of Flood

Floods are natural phenomena which cannot be prevented. But we can reduce flood damage. We found two ways to preventive and measures of flood damage. These are-

Structural Measure

  • A measure to control the physical process of flooding. It also prevents inundation.
  • Protects vulnerable areas up-to certain level of flooding.
  • Clear the river and water flow channel.
  • Construction culvert to save road during flood.
  • Planting trees near the river bank to save from erosion.
  • Sand bags and Stone are thrown at river bank to protect bank from erosion.
  • Construction dam to protect town from flood by changing the way of river.

Non-Structural Measure

  • Flood forecasting.
  • Early warning of flood.
  • Flood insurance.
  • Changes in cropping pattern.
  • Know about the safe zones during flood.
  • Emergency communication system during flood.

References

  1. Annual Flood Report by Flood Forecasting & Warning Centre, Bangladesh Water Development Board.
  2. American Journal of Civil Engineering. 2014; 2(1): 8-11 Published online February 20, 2014 (http://www.sciencepublishinggroup.com/j/ajce)
  3. A Review of Flood Management in Bangladesh: A Case study of 2004 Flood By Ainun Nishat, Country representative, IUCN-The World Conservation Union
  4. Article on Flood by Institute of Environmental Engineering, Technical University of Košice.
  5. Article on Natural Hazards and Earth System Sciences (2004) 4: 295–308 SRef-ID: 1684-9981/nhess/2004-4-295.

 

 

 

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