The use of a flood index to characterise flooding in the north-eastern region of Bangladesh

Flooding in the Haor region in the north-east of Bangladesh is presented in this paper. A haor is a saucershaped depression, which is used during the dry period (Dec to mid-May) for agriculture and as a fishery during the wet period (Jun-Nov). Pre-monsoon flooding till mid-May causes agricultural loss. The area is bordering India, and is fed by some flashy Indian catchments. The area is drained mainly by the Surma-Kushiyara river system. The terrain generally is flat and the flashy characteristics die out within a short distance from the border. Limited studies on the region, particularly with the help of numerical models, have been carried out in the past. Therefore, an objective of the current research was to set up numerical models capable of reasonably emulating the physical system. Such models could, for example, associate different gauges to the spatio-temporal variation of hydrodynamic variables and help in carrying out a systemic study on the flood propagation. A 1D2D model, with one-dimensional model for the rivers (based on MIKE 11 from DHI) and a two-dimensional model for the haors (based on MIKE 21 from DHI) were developed. In order to characterize flooding in the large area a flood index is proposed, which is computed based on the hydrograph characteristics such as the rising curve gradient, flood magnitude ratio and time to peak. The index was used in characterising flooding in the Haor region. In general, two groups of rivers were identified. The study enabled identifying the hot-spots in the study area with risks from flooding.


Introduction
Bangladesh is one of the largest deltas in the world, formed by a dense network of the distributaries of the rivers Ganges, Brahmaputra and Meghna, and more than 230 major rivers and their tributaries (Mirza, 2001).The Ganges, Brahmaputra and Meghna basin covers a combined total catchment area of about 1.76x106 km 2  and extends over Bangladesh, Bhutan, China, India and Nepal.Only 7% of this huge catchment area is in Bangladesh (Guna, 2002).The average annual rainfall in the country varies between 1500 to 5000 mm with an average of 2300 mm.The geographical location, land characteristics, multiplicity of rivers and the monsoon climate render Bangladesh highly vulnerable to flood hazards.Floods are annual phenomena, with the most severe floods occurring during the months of July and August (Mirza, 2003).
Four main types of floods are observed in Bangladesh (Ahmed, 2000): flash flood, rainwater flood, riverine flood and coastal flood.Flash floods occur during the pre-monsoon period (March to May) and are observed in the north-eastern region due to intense rainfall events in the neighbouring hilly regions in India.Rain-fed flood is caused by drainage congestion and heavy rainfall.River floods result from snow-melt in the Himalaya region and heavy monsoon rainfall over the Himalayas, Assam Hills, Tripura Hills and the upper Brahmaputra and Ganges floodplains.Coastal flooding mostly occurs during the pre-monsoon and post-monsoon season.At this time, a number of depressions are formed in the Bay of Bengal, which often turn into cyclones that result in storm surges with a rapid onrush of water of high amplitudes in the coastal region.
Regular river floods affect 20% of the country, increasing up to 68% in extreme years (DMB, 2010).The floods of 1987, 1988, 1998 and 2004 were particularly catastrophic, resulting in large-scale destruction and loss of lives.Approximately 37%, 43%, 52% and 68% of the country is inundated with floods of return periods of 10, 20, 50 and 100 years respectively (DMB, 2010).The occurrence of water-induced disasters is a regular phenomenon in Bangladesh (Guna, 2002).The expression "living with floods" is used in a variety of contexts to illustrate the flooding situation in Bangladesh (Kundzewiez and Kaczmarek, 2000;Kundzewiez and Takeuehi, 1999).The flooding situation is anticipated to be exacerbated with the climate change.Most of the general circulation models (GCMs) predict an increase in temperature due to an enhanced greenhouse effect, which will increase monsoon precipitation in South Asia.This may lead to increase in peak discharges of the major rivers and may eventually exacerbate the problem in Bangladesh (Mirza, 2001).
This paper reports the research conducted in the framework of a project, which focused on the flooding issues in the Haor region in the north-east of Bangladesh.Section 2 introduces to the study area, Section 3 presents the set up of the simulation models, Section 4 introduces to flood index used subsequently in characterising flooding in the study area and Section 5 presents the results.The probable impact of climate change is discussed in Section 6.

Figure 3 .
Figure 3. Flood indexing parameters for floods in theHaor region as per the presented approach.

Figure 4 .
Figure 4. Temporal variation of flood index in different rivers of the Haor region for different return periods.