Floods in Bangladesh: Causes and possible

Publish Date : September 06, 2014
by Tarequl Islam Munna

Tarequl Islam Munna

Tarequl Islam Munna

Flooding propensity in an area can vary greatly with a change in the water carrying capacity of a drainage basin and/or with a change in land elevation with respect to the base level of rivers - the ocean. The flood problem and the solution can be analyzed in the context of two fundamental parametres: water carrying capacity of rivers and land elevation. Bangladesh is drained by 300 major rivers and channels most of which originate outside the country. Therefore, formulating solutions to the flooding problem requires an understanding of the regional geologic setting and the underlying cause of floods.
Causes of flood in Bangladesh: The factors responsible for the recent low frequency floods in Bangladesh can be analyzed in terms of short-term and long-term processes. Evident phenomena that take place prior to and during floods are termed short-term processes. On the other hand, the slowly occurring phenomena which cannot be tied to the flood problem directly are termed long-term processes. There are various other causes too.
Monsoon downpour: An increased amount of precipitation can cause flooding. An excessive monsoon downpour in the Ganges-Brahmaputra-Meghna drainage system is thought to be the primary cause of the 1988 flood in Bangladesh. It is not known, however, if the heavy precipitation is actually an effect of other processes such as the greenhouse effect or destruction of forests in the upstream region.
Synchronization of flood peaks: The synchronization of flood peaks for the major three rivers took place within a two week time, causing a sudden increase in water level in virtually all areas of the country. While the synchronization of flood peaks can explain the cause of the 1988 flood, it fails to explain the reason for an overall increased propensity for low frequency floods in recent years such as occurred in 1974, 1984, 1987, and 1991.
Local relative sea level rise: The land elevation is measured with respect to sea level in an area. Therefore, any change in sea level causes land elevation to change. The rate of local relative sea level rise is 7 mm/year around the coastal areas of Bangladesh. An increase in sea level raises the base level of rivers, which in turn reduces the gradient of river flow. As a result, 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 sea level rise. Limited data show that the average sediment accumulation rate for the last few hundred years in the coastal areas of Bangladesh is 5-6 mm/year, which is not enough to keep pace with the rising sea level. As a result, net land elevations must have been decreasing over time, resulting in more flood inundations.
Riverbed aggradation: Due to relatively higher settling 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 aggradation is so pronounced in Bangladesh that changes in riverbed level can be observed during one's lifetime. For example, the Old Brahmaputra was navigable for steamers only about 30 years ago, and is presently an abandoned channel. Riverbed aggradation reduces the water carrying capacity of rivers, causing them to overflow their banks. This recent increase in riverbed levels must have contributed to the increased flooding propensity in Bangladesh.
Deforestation in the upstream region: A rapid increase in population in the Indian Subcontinent over the course of the present century has resulted in an acceleration of deforestation in the hills of Nepal to meet the increasing demand for food and fuel wood. Deforestation of steep slopes is assumed to lead 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 of reduced velocity, the sediments carried by the river start to settle down faster on the riverbed, causing riverbed aggradation and in turn reducing the water carrying capacity of the river. The Farakka Barrage on the Ganges has already caused tremendous damage to 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 run-off can easily wash away the topsoil from cultivated land. This surface erosion reduces land elevation, which in turn increases flood intensity in an area. The land elevations in Bangladesh must have been reduced over time due cultivation. Aside from this, the tilling on the mountain slopes of the Himalayas is thought to be responsible for massive soil erosion in Nepal that eventually causes rapid riverbed aggradation in Bangladesh.
Seismic and neo-tectonic activities: Bangladesh lies on the Indian lithospheric plate which is pushing against the Asian plate, causing growth of the Himalayas and occasional earthquakes in the region. Earthquakes cause movement of the land, and this can change the topography of the region and alter river courses. A sudden change in a river course can cause substantial flooding. For example, the Old Brahmaputra changed its course to its present location following an earthquake in the mid eighteenth century. Neotectonic activities (recent movements in the Earth's crust) are affecting river courses in the northern region of Bangladesh. Most of the recent floods have been approximately simultaneous with earthquake activities. For example, the floods of 1988 and 1991 were coincident with earthquake activities in northern parts of Bangladesh.
Greenhouse effect: The world is about to enter a period of rapid warming. Bangladesh will be severely impacted by an increase in the sea level. Besides many other adverse environmental, economic, and climatic consequences and the base level of all rivers will change following any change in the sea level. The greenhouse effect will also increase the amount of rainfall and storminess, which will further aggravate the flood problem.
Possible ways of solutions: Solutions to the flood problem in 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. Structural solutions are in practice on a limited scale in Bangladesh as part of a flood control project. The basic requirement for formulating a solution to the flood problem is an understanding of underlying geologic processes that cause floods. The geologic solution is more farsighted in nature. It is to permit the delta to grow both vertically and horizontally at a rate that would keep pace with the relative sea level rise in the region.
Dredging and re-excavation of rivers: Continuous dredging of the rivers and channels and dispersion of the dredged sediments on the delta plain will not only increase elevation of the land, but will also increase the capacity of the rivers. The rivers in Bangladesh carry ample sediment for dredging and dispersion on the flood plains in spite of the fact that the average sediment load of the Ganges-Brahmaputra river system has declined from 2.4 billion tons/year to 1.6 billion tons/year since the diversion of the Ganges through the Farakka-Barrage damming project. The recent increase in sediment discharge has caused the river bed level to rise about 5-7m, thus decreasing bankfull stage and increasing its flooding propensity. If dredged and dispersed properly, this extra source of 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 remaining from their rice crops during harvesting before the rainy season. They should also be given more information about the problem of soil erosion.
Flood preparedness: An understanding of how individuals have adapted to and are affected by floods may suggest new and less costly ways of reducing flood damages. Indigenous solutions such as the building of suitable housing, shelters and infrastructures also deserve serious consideration.
Interbasinal cooperation: Without regional cooperation between the co-riparian nations, any major interbasin development activity is almost impossible. In order for any interbasinal flood control project to be successful, it will have to be designed to serve the common interests of the people of the countries concerned. For example, construction of reservoirs in the upstream regions of the Brahmaputra to hold excess water during rainy seasons can reduce flood propensity in Bangladesh. The trapped water can be used to produce electricity during the summer months, to meet some of the irrigation demands for Bangladesh and energy demands of Nepal and India.

Source link: http://www.observerbd.com/2014/09/06/41457.php

The writer is correspondent of American International News Service, Columnist and Conservator, wildlife and environment. E-mail: munna_tareq@yahoo.