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.