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Impact of Climate Change on Floods of Bangladesh and Introducing Flood Intensity Index to Characterize the Flooding Scenario

Md. Shahjahan Ali*
Department of Civil Engineering, Khulna University of Engineering & Technology, Khulna-9203, Bangladesh

Tanziha Mahjabin
Department of Civil Engineering, Khulna University of Engineering & Technology, Khulna-9203, Bangladesh

Takashi Hosoda
Department of Urban Management, Kyoto University, Kyoto 615-8540, Japan

Abstract/ Executive Summary:

Bangladesh is a disaster-prone country, and the effect of climate change magnifies the governing factors of the disasters. In this paper, firstly the temporal changes of the pattern of different disasters in Bangladesh are discussed based on their occurrence rate, number of people killed, and exposure to human and economic loss. The paper is mainly concentrated on the temporal variation of floods in Bangladesh. The decadal change in return period and probability of low, moderate and high flood events are presented. The term ‘Flood Intensity Index (FII)’ is introduced to characterize the flood, which is considered as a function of inundation depth and duration. FII is calculated for two megafloods of 1988 and 1998. It is observed that although the average depth of inundation among two megafloods does not differ too much, the FII in 1998 was much higher (about double) than 1988 flood, and that is why the economic loss in 1998 flood was much higher (about 2.33 times) than that of 1988 flood. Previous data and various future predictions showed that both the parameters in FII are in increasing trend due to climate change impact. The climate change-induced increment in river discharge during monsoon will increase the inundation depth, and the increasing trend of Sea Level Rise will increase the duration of flood due to the backwater effect of the sea. Therefore, the Flood Intensity Index will significantly be increased due to climate change. Since the ‘Flood Intensity Index (FII)’ is a measurable index to characterize a flood accounting for all the impacts of climate change, this index can be used in expressing the change in flooding scenarios due to climate change.

Keywords: Climate Change, Floods, Flood Intensity Index, Inundation, Sea Level Rise.

Location: Department of Civil Engineering, Khulna University of Engineering & Technology, Khulna-9203, Bangladesh

Start Date:

End Date:

Program Area: Risk Management in Agriculture

Commodity: Climate change, Flood

Objectives:

In this study, the term ‘Flood Intensity Index (FII)’ is introduced to characterize the flood, which is considered as a function of inundation depth and duration. A heuristic approach is described to explain how climate change will magnify the flood intensity index of this country. In this paper, the temporal changes of a pattern of different disasters in Bangladesh are also discussed based on their occurrence rate, number of people killed, and exposure to human and economic loss. The paper is mainly concentrated on the temporal variation of floods in Bangladesh. The decadal change in return period and probability of low, moderate, and high flood events are presented.

Methodology:

2. DATA Three types of data are used in this study: i) the rainfall data, ii) the river discharge and flood hydrograph data and iii) the time series of disasters in Bangladesh. For the first one monthly rainfall records of 17 stations (out of which, complete set of data was available in 12 stations) of Bangladesh for fifty years (1958–2007) are collected from Bangladesh Meteorological Department. For this data, homogeneity of rainfall records are analyzed by Shahid (2009), and reported that the data of all the stations are homogeneous. The data on the time series of historical floods in Bangladesh with inundation areas are collected from the Bangladesh flood forecasting and warning center (FFWC). The disaster-related data were collected from OFDA/CRED International Disaster Database EM-DAT (2011), Université Catholique de Louvain, Brussels, Belgium.

Source of Information: Journal of Engineering Science 04(1), 2013, 23-34

Article Link (Online Journal):

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

Although the paper is mainly concentrated on the temporal variation of floods in Bangladesh, the temporal changes of the pattern of other disasters in Bangladesh are also discussed and compared with flood based on their occurrence rate, number of people killed, exposure to human and economic loss. Based on the analysis, the following conclusions are made: 1. Comparing the disaster parameters for two-time spans, it is found that the disaster numbers, the number of affected people and economic loss increase with time in an alarming rate. It is observed that the flood is the most devastating disaster in Bangladesh compared to other disasters as it ranked 1st in the world for the case of the population exposed and 3rd in the case of economic exposure. 2. Since the discharge in the river is highly influenced by the huge upstream flow outside the country, it is not expected to show a definite relationship between rainfall and river discharge patterns. But the 30 years average discharge curve of the Ganges river is found to be a similar pattern of rainfall (in the Ganges Basin within Bangladesh) lagged by months. Comparing 50 years of data of rainfall anomaly with the flood, it is observed that for more than 85% of cases, the flood anomaly of Bangladesh is in phase with that of territorial rainfall anomaly. 3. In the wet season (June to October) the yearly increasing rate of Brahmaputra river discharge is about 54 cumec and that of the dry season (November to May) is about 114 cumec. The Brahmaputra discharge at Bahaduarabad point has been increased about 7.5% in the last 50 years. 4. The historical time series of floods in Bangladesh shows that the top 5 floods in terms of inundation area have occurred in the last 20 years in 60 years of history. It is observed that the number of moderate floods is decreased highly and they are converted to either extreme high floods or extreme low floods. From the trend line for 50 years of rainfall data, it is found that the monsoon rainfall increases very gently as 2.65 mm/year. These changing phenomena in rainfall and flooding scenarios in Bangladesh can be explained as the probable impact of climate change. 5. The term ‘Flood Intensity Index (FII)’ is introduced to characterize the flood, which is considered as a function of inundation depth and duration. Flood Intensity Index is calculated for two megafloods of 1988 and 1998. It is observed that although the average depth of inundation among two megafloods does not differ too much, the Flood Intensity Index in 1998 was much higher (about double) than 1988 flood, and that is why the economic loss in 1998 flood was much higher (about 2.33 times) than that of 1988 flood. Previous data and various future predictions showed that both the parameters in Food Intensity Index are in an increasing trend due to climate change impact. The climate change-induced increment in river discharge during monsoon will increase the inundation depth, and the increasing trend of Sea Level Rise will increase the duration of flood due to the backwater effect of the sea. Therefore, the Flood Intensity Index will significantly be increased due to climate change. Since the ‘Flood Intensity Index (FII)’ is a measurable index to characterize a flood accounting for all the impacts of climate change, this index can be used in expressing the change in flooding scenarios due to climate change.

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