Floods in Bangladesh: Causes and possible solutions

The factors responsible for the recent low-frequency floods in Bangladesh are either short-term or long-term processes. Palpable phenomena that take place prior to and during floods are termed short-term processes. In contrast, the gradual phenomena which cannot be tied to the flood problem directly are termed long-term processes. Other significant causes do exist.
CAUSES OF FLOOD IN BANGLADESH: An increased amount of precipitation can cause flooding. An excessive monsoon downpour in the Ganges-Brahmaputra-Meghna Drainage Basin had caused the flood of 1988 in Bangladesh. However, it is still unclear whether heavy precipitation is actually an effect of other processes - the greenhouse effect or destruction of forests in the upstream region.
SYNCHRONISATION OF FLOOD PEAKS: The synchronisation of flood peaks for the three major rivers - Padma, Meghna and Jamuna - took place within a two weeks' time, causing a sudden increase of water level in virtually all areas of the country. While the synchronisation of flood peaks explains the cause of the 1988 flood, the augmented propensity for low frequency floods in recent years (1974, 1984, 1987, and 1991) is a mystery.
LOCAL RELATIVE RISE OF SEA-LEVEL: The elevation of land is measured with respect to the sea-level of an area. Hence, any change in the sea-level causes land elevation to change. Around the coastal areas of Bangladesh, the rate of local relative rise of the sea-level is 7mm per year. An increase in the sea-level raises the base level of rivers. This reduces the gradient of river flow. Eventually, the discharge of rivers decreases as the water flow becomes sluggish, creating a backwater effect further inland. This certainly seems to be one of the reasons for the increase in flood intensity in recent years in Bangladesh.
INADEQUATE SEDIMENT ACCUMULATION: The only way for land to counter the effects of a rising sea is for sediment to accumulate at a rate that is sufficient to keep pace with the rate of rising sea-level. For the last hundred years, average sediment accumulation rate is 5-6mm per year in the coastal areas of Bangladesh - not enough to keep pace with the rising sea-level. Consequently, net land elevations must have been decreasing over time with more flood inundations.
RIVERBED AGGRADATION: Due to a higher velocity, the large-grained sediments are deposited near the source area on the riverbeds, forming sand bars. The river gradient decreases rapidly if sedimentation continues on the riverbeds. Riverbed aggradations reduce the water carrying capacity of rivers, causing them to overflow their banks. The rise of riverbed levels must have contributed to the increased flooding propensity in Bangladesh.
DEFORESTATION IN THE UPSTREAM REGION: A rapid increase in population of South Asia has resulted in an acceleration of deforestation in the hills of Nepal and India to meet the increasing demand for food and fuel wood. Deforestation of steep slopes leads to accelerated soil erosion and landslides during monsoon precipitations.
DAMMING OF RIVERS: Damming of a river reduces the velocity of water flow downstream. As a result, the sediments carried by the river start to settle down faster on the riverbed, causing riverbed aggradations. This also reduces the water carrying capacity of the river. So far, India's Farakka Barrage has tremendously damaged the agriculture, navigation, environment, and hydrodynamic equilibrium in Bangladesh.
SOIL EROSION DUE TO TILLING:  Ploughing makes the land surface more susceptible to soil erosion. Surface erosion can easily wash away the topsoil from cultivated land. This reduces land elevation and increases flood intensity in an area. The land elevations in Bangladesh have been reduced over time due to cultivation. Besides, the tilling on the mountain slopes of the Himalayas is responsible for massive soil erosion in Nepal. Eventually, it causes rapid riverbed aggradations in Bangladesh.
SEISMIC AND NEO-TECTONIC ACTIVITIES: Bangladesh lies on the Indian Tectonic Plate which is pushing against the Asian Plate. It causes growth of the Himalayas and occasional earthquakes in the region. Earthquakes cause movement of the land while this can change the topography of the region and alter river courses. A sudden change in a river course can cause substantial flooding. The Old Brahmaputra River changed its course to its present location following an earthquake in 1897. Neo-tectonic activities are affecting river courses in the northern region of Bangladesh. The recent floods have been simultaneous with earthquake activities. For example, the floods of 1988 and 1991 coincided with minor earthquakes in northern parts of Bangladesh.
 GREENHOUSE EFFECT: Global warming has become inevitable. Bangladesh will face severe consequences from the rising sea-level. Environmental, economic, and climatic adversities along with the rising base level of rivers will change due to an increased sea-level. The greenhouse effect can increase precipitations and storms, further aggravating the flood problem.
POSSIBLE WAYS OF SOLUTION: Solutions to the flood crisis of Bangladesh can be divided into two basic types: structural solutions and geologic or alternative solutions. Structural solutions call for the engineering of structures such as embankments along rivers, dams, drains, reservoirs, and other structures designed to control the natural flow of rivers. In Bangladesh, structural solutions are in practice on a limited scale as part of a flood control project. The basic requirement for ending the flood problem is the knowledge of geology. The geological solution is farsighted in nature. Vertical and horizontal growth of the delta should increase at a rate that keeps pace with the relative sea-level rise in the region.
DREDGING AND RE-EXCAVATION OF RIVERS:  Constant dredging of the rivers and channels, and dispersion of the dredged sediments on the delta will not only increase elevation of the land, but also increase the capacity of the rivers. The rivers in Bangladesh carry ample sediment for dredging and dispersion on the flood plains despite the fact that the average sediment load of the Ganges-Brahmaputra river system has declined from 2.4 billion tons per year to 1.6 billion tons per year since the diversion of the Ganges through the Farakka Barrage. The recent increase in sediment discharge has caused the river bed level to rise about 5-7 metres, thus increasing its flooding propensity. If dredged and dispersed properly, this surplus sediment can help the delta to grow.
PREVENTING LAND DEGRADATION: Suspended sediments adhere to the stems of plants. Farmers can be advised to leave a few inches of stem that remains of their rice crops during the summer harvest. They should be informed about the problems of soil erosion.
FLOOD PREPAREDNESS: An understanding of how individuals have adapted to and are affected by floods may suggest efficient ways of reducing flood damages. Indigenous solutions such as the building of suitable housing, shelters and infrastructures also deserve serious consideration.
INTER-BASIN COOPERATION: Without regional cooperation between the co-riparian nations, any major inter-basin development activity is almost impossible. A flood control project should be designed to serve the common interests of the peoples of the countries concerned in order to be successful. For example, the construction of reservoirs in the upstream of the Brahmaputra River - to hold surplus water during rainy seasons - can reduce flood propensity in Bangladesh. The trapped water can be used to produce electricity at summer in order to meet some of the irrigation demands for Bangladesh and energy demands for Nepal and India.
The writer is a columnist, environment activist, wildlife conservator, and correspondent of American International News Service.
munna­[email protected]