Flood  relatively high flow of water that overtops the natural or artificial banks in any of the reaches of a stream. When banks are overtopped, water spreads over the floodplain and generally causes problems for inhabitants, crops and vegetation. Since floodplain is a desirable location for man and his activities, it is important to control floods so that the damage does not exceed an acceptable level.

Floods are more or less a recurring phenomenon in Bangladesh and often have been within tolerable limits. But occasionally they become devastating. Each year in Bangladesh about 26,000 sq km, 18% of the country is flooded. During severe floods, the affected area may exceed 55% of the total area of the country. In an average year, 844,000 million cubic metre of water flows into the country during the humid period (May to October) through the three main rivers the ganges, the Brahmaputra-Jamuna and the meghna. This volume is 95% of the total annual inflow. By comparison only about 187,000 million cu m of streamflow is generated by rainfall inside the country during the same period.

In Bangladesh, the definition of flood appears differently. During the rainy season when the water flow exceeds the holding capacity of rivers, canals (khals), beels, haors, low-lying areas it inundates the whole area causing damage to crops, homesteads, roads and other properties. In the Bangladesh context there is a relation between inundation and cropping.

Floods in Bangladesh can be divided into three categories: (a) monsoon flood - seasonal, increases slowly and decreases slowly, inundates vast areas and causes huge losses to life and property; (b) flash flood - water increases and decreases suddenly, generally happens in the valleys of the hilly areas; and (c) tidal flood - short duration, height is generally 3m to 6m, blocks inland flood drainage.

The combined annual flood wave from the Ganges, brahmaputra and Meghna rivers passes through a single outlet, the lower meghna tide levels in the bay of bengal, reducing the slope and discharge capacity of the Lower Meghna. The effects of these high river water levels extend over most of the country and are the main determinant of the drainage condition and capacity. The discharge from minor rivers is reduced and surface drainage by gravity is limited to land above the prevailing flood level. Flooding caused by this drainage congestion exists nearly everywhere except in the highland and hilly areas in the northern and eastern parts of the country.

In the northwest region an embankment protects the right floodplains of the tista and the Brahmaputra. In the north there are large areas of shallow flooding interspersed with more deeply flooded pockets in meander scars and old flood basins. In the south a highland area separates the Ganges from the deep flood basin in chalan beel. Nearly all the monsoon drainage of the northwest region east of the atrai river and south of the Tista river passes through this flood basin to the Brahmaputra. In the northeast region floodplains can be divided into three distinct areas - the Brahmaputra and Padma left floodplain; the old brahmaputra river valley separated from the Brahmaputra by the madhupur tract; and the Meghna river basin.

The Meghna basin is dominated by the great Sylhet depression where the surma and kushiyara rivers join to form the Meghna. High water levels in Meghna are controlled downstream by the water levels of the padma during the flood season. It fills rapidly with floodwater early in the monsoon and remains full until the Lower Meghna falls in the post-monsoon period. Drainage rates of this basin are low.

Hill catchment draining into the northeast and southeast regions is characterised by flash floods that are mostly of short duration but unpredictable in frequency and intensity. Several floods may occur in the flashy rivers in any water year.

Throughout most of the south-central and southwest regions, flooding is mainly associated with tidal influences, storm surges and poor drainage. The northern half of the south-central region is the principal floodplain of the Padma and Lower Meghna, while the southern half is the main network of estuary channels which distribute about 40% of the Lower Meghna flow to the sea. The drainage system of the southwest region consists mainly of silted former distributaries of the Ganges connected to the sea by a largely Moribund Delta. Consequently there is extensive shallow flooding.

The factors for causing floods in Bangladesh are: (a) general low topography of the country with major rivers draining through Bangladesh including a congested river network system, (b) rainfall in the upstream country or in the mainland, (c) snow-melt in the himalayas and glacial displacement (natural), (d) river siltation/lateral river contraction/landslides, (e) synchronisation of major river peaks and influences of one river on the other, (f) human intervention of the environment, (g) tidal and wind effects on slowing down the river outflow (backwater effect), (h) construction of barrages and protective works along the banks of the river - some are very close to both the banks - in the upper reaches thus making the passage of water flow downstream increasingly narrower and resulting in greater acceleration of water flow downstream presently than before. (i) deforestation in the upper reaches of the rivers is not only leading acceleration of water flow downstream but also lead deposition of loads in the river beds, resulting in reduced channel flow and consequent overland runoff water and (j) tectonic anomalies (earthquake) those change in river flow/morphology.

History of floods in this country is perhaps inseparable from the history of this land. In every century, the bengal delta witnessed the visit of nearly half a dozen floods, almost equal to the magnitude and intensity of those in 1987, 1988 and 1998 and as many with lesser magnitude.

The monsoon phenomenon has been mentioned in the holy ramayana and mahabharata [Mahabharata] and other Vedic books. In the book Artha-Shastra [Artha-Shastra] written during the reign of Chandragupta Maurya (321-296 BC) by his minister Kautilaya, there is mention of the amount of rain at different places indicating that they had knowledge of rainfall measurements. The astronomer barahmihir [Barahamihira] (505-587 AD) used to predict rain. Astronomers Arya Bhatta and Brahmagupta also studied the monsoon. kalidasa, the famous sanskrit poet composed poems on monsoon clouds in his Meghdut and Ritusamahara. However, during the ancient times a lady named Khona [Khana] made most of the predictions on meteorology and agrometeorology. Even to this day the farmers of Bangladesh remember her verses. The Arabs used the knowledge of the changing pattern of monsoon winds very profitably for trade with India. The term 'monsoon' is derived from the Arabic word 'Mausam' meaning season. The first comprehensive report of Professor pc mahalanabish on floods in Bengal between 1870 and 1922 shows that moderate floods have occurred once in two years on an average, while severe floods have occurred once in 6-7 years on an average.

Flooding in Bangladesh is a recurring phenomenon. Recurrent floods between 1787 and 1830 changed the old course of the Brahmaputra. After a major flood in northern Bengal in 1922, a Flood Committee was formed and a report was published in 1927 on the north Bengal floods between 1870 and 1922. Statistical analysis of available records revealed that severe floods can occur every 7 years, and catastrophic floods every 33-50 years. Some severe monsoon floods of this region starting from the late 18th century are described here chronologically:

Chronology of big floods

Flood management Flood control and drainage projects heavily depended on dredging, embankments, polder and gravity drainage. Heavy dependence on structural means to manage floods, together with the effects of such other structures as roads, highways and railroads that obstruct flow of water in some cases aggravate the flood situation. Despite huge amounts of investment in flood control and drainage projects, the benefits have been less than satisfactory.

The floods of 1987, 1988 and 1998 were catastrophic, leading to widespread destruction, misery and loss of life. The severity of the floods of 1987 and 1988 led the government to look for a plan, which would in the long term provide a comprehensive and permanent solution to the recurrent flood problem. Several major studies were taken up in 1989. These studies led to formulation of the flood action plan (FAP) in 1989.

To reduce the losses from floods as well as to use the surplus water for irrigation, the bangladesh water development board, as part of structural measures for flood control, constructed a number of embankments and barrages and dug canals. Some major projects are: ganges-kobadak irrigation project (G-K Project), Dhaka-Narayanganj-Demra (DND) Project, Karnafuli Multipurpose Project, Coastal Embankment Project, Tubewell Project in northern Bangladesh, Brahmaputra Right Embankment Project, Chandpur Irrigation Project, Meghna-Dhonagoda Project, Manu River Project, Khowai River Project, Pabna Irrigation Project, Gumti Project, Muhuri Irrigation Project, Tista Barrage Project (Phase-I), Dhaka Integrated Flood Protection Project, System Rehabilitation Project, Early Implementation Project.

However, an alternative strategy for mitigating flood hazard, the concept of social adjustment, also known as non-structural measures can be important. These include: (a) Dissemination of meteorological forecasts, short and long range warning system including the height to which the flood water is likely to rise in the next few hours or so and a programme of speedy evacuation. (b) Land management for reduction of runoff water. In this case a programme of afforestation and reforestation together with animal grazing controls to increase absorption and reduction of runoff water could be undertaken. (c) landuse change and enactment of building codes, diversification of agricultural production, that is, identification and planting of flood resistant crops and adjustment of planting season. (d) Floodplain zoning, involving landuse zoning to control development and restrictive development regulations, should ensure that any development meets certain standards and that they take into consideration the threat to a site.

Non-structural measures can be implemented at nominal costs and in a very short time resulting in positive benefit to the floodplain users.

Since flood management is an integral part of overall water resource management, regional cooperation in this area will help create a better climate for the much-needed joint strategies for sustainable utilisation of the water ecosystem. [Sifatul Quader Chowdhury and Md Sazzad Hossain]

Bibliography  M Aminul Islam, Environment Land Use and Natural Hazards in Bangladesh, University of Dhaka, 1995; Qazi Kholiquzzaman et al ed, Perspectives on Flood, University Press Limited, Dhaka, 1998; Thomas Hofer, Floods in Bangladesh: A Highland-Lowland Interaction, Geographica Bernensia, University of Berne, Switzerland, 1998; Bangladesh Water Development Board (BWDB), Annual Flood Report 1998, Flood Forecasting and Warning Centre, BWDB, 1999.