C-40

Regional Flooding and Rainfall Frequency Analysis in

Student Member †Md Ashraful ALAM (Osaka City University) SHASE Technical Fellow Kazuo EMURA (Osaka City University) Member Craig FARNHAM (Osaka City University) Member Jihui YUAN (HUST University)

In Bangladesh flooding is the most common natural disaster. During the period, most of the lands are inundated by water because of heavy rainfall. Also it’s the outlet of the -Brahmaputra-Meghna (GBM) basin. Because of the strong relationship with rainfall this paper discusses the regional rainfall frequency of different hydrological regions of Bangladesh and the relationship with floods in this region.

Introduction the floodplains area is originated by the sediments of the Bangladesh is one of the most flood prone countries in Ganges (padma), Brahmaputra (jamuna) and Meghna the world because of its geographical location. The rivers and their numerous , cover about country is located at the outlet of the three-quarters of Bangladesh. Land elevation ranges from Ganges-Brahmaputra-Meghna (GBM) basin, which is the -3m to 60m in the plains and the hilly areas vary from 100 third largest freshwater outlet to the world’s oceans. So, to 1000m. this area is more susceptible to regular flooding. During The climate of Bangladesh is tropical and humid with the last half century, at least 8 extreme flood events wide seasonal variations of rainfall, high temperatures and occurred affecting 50% or more of the land area of humidity. The seasons are mainly divided into four, these Bangladesh 1). are: Pre-monsoon (March to May), Monsoon (June to The main factors that contribute to this flooding are September), Post-monsoon (October to November), and excessive rainfall during the monsoon period, melting Dry (December to February). snows in the , geophysical instabilities and some manmade causes like deforestation and unplanned 2. Hydrology in Bangladesh development works. A large portion of the country Bangladesh is a riverrine country where 230 rivers flow. becomes flooded. Normally, 20-25% of the country is Among them, 54 are international rivers towards the Bay inundated during every monsoon from June to September. of Bengal. Most of the rivers are originated from the In the case of extreme flood events, 40-70% can be Himalayas, flow a long path throughout India or China inundated, which was amply proved during the extreme and enters Bangladesh. flood events of 1994-55, 1974, 1987-1988 and 1998 1). According to the WARPO (Water Resources Planning The aim of this paper is analysis the recurrence interval of Organization) the eight hydrological regions are: rainfall by using a Weibull distribution which is associated Northwest (NW), North Central (NC), Northeast (NE), with flood of Bangladesh. Southeast (SE), South Central (SC), Southwest (SW), Eastern Hills (EH), plus the active floodplains and 1. Physical Features and climatic condition charlands of the Main Rivers and Estuaries (RE) 2) as an ladesh is located etween to shown in Figure-1. latitude and to E longitude. Among the three major terrain types (hills, terraces and floodplains)

空気調和・衛生工学会大会学術講演論文集｛2015.9.16〜 18（大阪）｝ -1- 第10巻 the cross boundary rivers is around 1200 cubic kilometres 1). During the monsoon time, all the area of GBM catchment are prone to regular flooding.

3.3 Rainfall Induced Floods The main cause of this type of flood is high intensity local precipitation of long duration during the monsoon period. This type of flood occurs when drainage capacity into basins exceeds their carrying capacity. The south-western part of Bangladesh is facing this kind of flood. Also Dhaka and Chittagong city are facing urban flooding due to the rain-fed situation.

Figure-1 Hydrological Regions of Bangladesh 2) 3.4 Coastal Floods This type of flood occurs along the coastal region of 3. Flood types and causes Bangladesh. There are about 800 km of coast along the A flood is an unusual high stage of a river due to runoff southern part. Tropical cyclones are, most likely to occur from rainfall and / or melting of snow in quantities too during pre and post-monsoon periods (April-May and great to be confined in the normal water surface elevations October-November, respectively). They originate from the of the river or stream, as the result of unusual and cause a widespread damage to the lives meteorological combination 3). In Bangladesh there are and property of coastal people due to storm surges. four types of floods: flash floods, riverrine floods, rainfall induced floods and coastal floods. 4. Statistics and major sources of Flooding Bangladesh experiences floods almost every year with 3.1 Flash Floods considerable damage. The floods of 1954, 1955, 1974, In Bangladesh, flash floods occur due to heavy 1987, 1988, 1998, 2004 and 2007 all caused enormous downpours in the neighbouring hills and mountains in the damage, especially the floods of 1987, 1988, 1998, 2004 northern and eastern part of Bangladesh. This type of and 2007 flood caused heavy damages. The figure of flood is characterized by a very quick rise of water level of Year-wise flood area is shown in Figure-2. rivers and subsequent overbank spillage with high velocity.

3.2 Riverrine Floods During monsoon, most of the floodplains areas are subject to riverrine floods. Also it is thought that snow melt in the Himalayas has a significant role in riverrine floods. The rivers of Bangladesh drain about 1.76 million 4) sq km area of which 93% lies outsides its territory in India, Figure-2 Year wise inundated area by floods in Bangladesh

Nepal, Bhutan and China. The annual average runoff of

空気調和・衛生工学会大会学術講演論文集｛2015.9.16〜 18（大阪）｝ -2- The major flood sources of different hydrological regions 5.2 Discussion and Analysis are given in Table-1. Return periods of maximum monthly rainfall are Table-1 Major Sources of Flooding analyzed for different hydrological regions which are

Region Major Sources of Flooding shown in Figure-3 to Figure-9. Southwest Tides and cyclonic rainfall Northwest regions lie on the Ganges and Brahmaputra South Tides, cyclonic rainfall and surges, and Basin. By excessive rainfall the adjacent area of two major Central overspill of Lower Meghna rivers, Ganges and Brahmaputra, are flooded. In Rangpur Northwest Local intense rainfall, impeded drainage, breaches in the Teata and Brahmaputra the maximum monthly rainfall is 1314 mm which comes Right Embankments and breaches in every 31 years is shown in Figure-3. internal polder embankments and drainage congestion preceded by high flows in the major rivers North Local intense rainfall, impeded drainage, Central spilling from Brahmaputra and congested drainage on the Meghna Northeast Flash floods on transboundary rivers, local intense rainfall, impeded drainage and drainage congestion on the Meghna Southeast Flash floods on transboundary rivers, local intense rainfall, impeded drainage and drainage congestion on the major river Eastern High inflows through the Ganges and the Hills Rivers Figure-3 Rainfall frequency curve of Northwest region Brahmaputra and surges & Estuaries 2) Source: National Water Management Plan, 2001 Most of the parts of North Central region lie on the Brahmaputra basin. Due to excessive rainfall and lack of 5. Rainfall frequency analysis proper drainage system, Dhaka city and other regions face Although there are many factors contributing to floods in flood during the monsoon period. Bangladesh, rainfall is one of the vital determinants of flood severity. The 1987 flood was predominantly caused by local rainfall 5).

5.1 Method of analysis For our flood frequency analysis, we use the maximum monthly rainfall of 35 different locations with maximum 30 years of each. Data were collected from Bangladesh

Meteorological Department (BMD). We use the Weibull Figure-4 Rainfall frequency curve of North Central region formula for our return period analysis. Northeast regions lie on the Meghna basin. Due to excessive rainfall this region faces floods especially the Where, low land area. Both Sylhet and Srimangal stations show Tp : return period or recurrence interval, n : number of excessive rainfall. In Sylhet the monthly maximum years of record, m : order number of ranked data. rainfall is 1394 mm. The result is shown in Figure-5.

空気調和・衛生工学会大会学術講演論文集｛2015.9.16〜 18（大阪）｝ -3- show over 2000 mm of rain. It is shown in Figure-9.

Figure-5 Rainfall frequency curve of Northeast region Figure-8 Rainfall frequency curve of Southeast region Under the Ganges basin the southwest part faces floods because of excessive rainfall during the monsoon period.

Figure-9 Rainfall frequency curve of Eastern Hills Rivers & Estuaries region

Figure-6 Rainfall frequency curve of Southwest region 6. Conclusions In the eastern hills rivers and estuaries region the intensity of rainfall is higher than other areas. Because of rainfall Northeast region faces flooding every year. Also some part of southwest region faces flooding because of proper drainage management system. Overall almost every regions faces flooding during the monsoon period.

References Figure-7 Rainfall frequency curve of South Central region 1) Department of Disaster Management, Ministry of Disaster Management and Relief, Bangladesh (2014). Flood Response Preparedness Plan of Bangladesh, June 2014, Most of the South Central and Southeast regions are on Dhaka, pp.3-9. 2) Water Resources Planning Organization (2001): National the Meghna basin and these regions are susceptible to Water Management Plan, Volume 2, Ministry of Water Resources, Government of Bangladesh, Dhaka, pp.14,27. flood. Also the maximum monthly rainfall of Southeast 3) Raghunath, H.M 2009 revised edition, Hydrology: principle analysis and design, New Delhi, New age international region is high. At Mongol Coart it is 1390 mm. It is shown publishers. in Figure-8. 4) Bangladesh Water Development Board (2014): Annual Flood Report 2014, FFWC, p.7. In the hilly and mountain areas flash floods occur. Also 5) Miah,M.M. (1988): Flood in Bangladesh: A Hydromorphological Study of the 1987 Flood, Academic the rainfall is high in these areas. Teknaf and Sandwip Publishers, Dhaka,p.9.

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