TREND OF FLOODS IN ASIA AND FLOOD RISK MANAGEMENT WITH INTEGRATED RIVER BASIN APPROACH

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1 TREND OF FLOODS IN ASIA AND FLOOD RISK MANAGEMENT WITH INTEGRATED RIVER BASIN APPROACH DUSHMANTA DUTTA RNUS/WEM, School of Civil Engineering, Asian Institute of Technology P.O Box 4, Klong Luang, Pathumthani 12120, Thailand SRIKANTHA HERATH ESD, United Nations University, 53-70, Jingumae 5-chome, Shibuya-ku, Tokyo , Japan This paper presents the findings of an analysis of flood disaster trends in Asian countries during the last 30 years. The analysis was carried out to understand the changing characteristics of flood disasters in Asian countries in the past three decades with the rapid changes in socio-economic conditions. The second half of the paper presents the results of a modeling study of flood risk management with the basin wide integrated approach. A case study application of a GIS and hydrologic model based distributed flood risk analysis system is demonstrated. Based on the analyses, conclusions are for flood risk management in the basin. INTRODUCTION In Asia, floods are by far the most frequent and devastating natural disasters. An analysis has been carried out to understand the trend of floods in Asia in the past few decades. It shows a clear and significant increasing trend of floods. The Intergovernmental Panel on Climate Change (IPCC) has predicted significant increase of extreme rainfalls in the next few decades due to climate changes in monsoon Asia region and that is definitely going to worsen the flooding situation in Asia [1]. Traditionally, flood control measures in most of the Asian countries are confined to major rivers. It has been witnessed that this approach is not effective to control large magnitude floods. To cope with increasing frequency of floods with high magnitudes, new and holistic approaches have to be adopted. Basin wide integrated flood management is considered to most suitable for coping with new challenges in flood disaster mitigation. In this paper, we present the outcomes of an analysis of flood disaster trends in Asia based on the available records of flood events from different sources during the past 30 years. A case study has been conducted using a distributed flood risk analysis system to understand the importance of basin wide flood risk management approach in a river basin in Thailand. Two approaches of flood controls - i) traditional measure of dyke along the main river, ii) measure in upstream of the basin - have been analyzed for understanding their impacts on flood reduction. The second half of the paper presents the results of this case study, which can be useful towards adopting new flood control policy. 1

2 2 FLOOD DISASTER TRENDS IN ASIA The main source of the datasets used in this study is from CRED for the past 30 years from , which is supplemented with data from some other sources [2]. Although there are no standard criteria for selecting the flood events and defining their magnitudes for analysis, all the events considered here are only those that caused socio-economic damage and were reported. The flood records included here Floods account for about 33% of all the natural disasters. Asia is the most frequently affected continent by floods. During the past 30 years, total flood disasters occurred in Asia is the largest, 40% of the total events, compared to any other continents: America (25%), Africa (17%), Europe (14%) and Oceania (4%). Most of the Asian countries are affected by floods every year. Figure 1a shows the regional distribution major flood events in Asia in the last 30 years. The regional distribution shows that South Asia is the most frequently affected region by floods (about 39%), followed by South-east Asia (about 30%) and East Asia (about 25%). The West Asia region including Russian Federation is the least affected region (6%) (Fig. 1b). Annual statistics of natural disasters of the past 30 years show that number of floods and windstorms are rapidly increasing in Asia compared to all other disasters. In the last 30 years annual flood frequency has doubled. As shown in Figure 2, there were on average about 150 flood events annually in 1970s, it became 30 in 1990s, and in the last three years it has increased to about 50 events per year. The rate of increase of flood frequency is more prominent in the last 10 years, especially, the recent three years statistics show rapid increase of floods in Asia. a) b) East Asia (25%) Russian Fed. (3%) South Asia (39%) South-east Asia (30%) West Asia (3%) Figure 1. a) Regional distribution of flood events in Asia in the last 30 years; (b) spatial distribution of flood event, b) regional distribution of floods in Asia in percentage As shown in Fig. 3, among the Asian countries China is the most frequently affected country by floods followed by India. The other eight of the top ten flood affected countries are Indonesia, Philippines, Bangladesh, Iran, Thailand, Sri Lanka, Vietnam and Pakistan in descending order. Except China and Iran, all these countries are located in South and South-east Asian Regions. The 5-year average flood statistics of last 30 years show that flood frequency is increasing in all these countries (Fig. 4). China shows higher

3 rate of increase of frequency than India in the last 10 years. Thailand also shows a higher rate of increase of flood frequency compared to its neighboring countries in the last 15 years. Among these 10 countries, the rate of increase of flood frequency was the lowest in Sri Lanka and the Philippines in the last 15 years. According to several research studies and observations, the causes of growing trends of floods in Asia can be attributed to mainly two factors: i) climate change and ii) landuse change and surface degradation [3],[4],[5]. 3 Number of Events Drought Floods Earthquake Extreme-temperature Famine Slide Volcano Wild-fire windstorm Year Figure 2. Flood disaster trend in Asia in the last 28 years compared to other natural 160 disasters Number of flood events Number of flood events Kazakhstan Mongolia Azerbaijan Taiwan (China) Cambodia Korea, Dem P Rep Myanmar Lao, P Dem Rep Tajikistan Malaysia Yemen Japan Nepal Korea, Rep Country Afghanistan Pakistan Viet Nam Sri Lanka Thailand Iran Bangladesh Philippines Indonesia India China Figure 3. Flood events in different Asian countries in last 28 years Pakistan Viet Nam Sri Lanka Thailand Iran Bangladesh Philippines Indonesia China India Year Figure 4. Flood trend in most frequently flood affected 10 countries in Asia

4 4 CASE STUDY To cope with increasing frequency of high magnitude floods, water managers all over the world have become conscious that a new approach towards flood risk prevention is needed. After centuries of battling against the rivers and raising the dikes the new philosophy is based on live with the rivers and give the rivers the room they need. Most of the Asian countries have been taking structural measures along major rivers as main flood control strategy. Most of the Asian countries have witnessed the unsuccessfulness of the structural measures a long the major rivers for flood control. With the increasing intensity and magnitude of the flood hazards, time has come to think of alternative approach for effective mitigating measures. After years of experiences various flood control approaches, integrated basin wide approach are considered to be more suitable for flood risk reduction. It is important to analyze and understand the flood mechanism in a river basin for adopting integrated approach for planning and managements. A case study is carried out in an urban area of the Yom River Basin in Thailand to analyze the integrated flood control measure in risk reduction. An Urban Flood Risk Analysis System, developed at the University of Tokyo, Japan was used for conducting this study [6]. The system consists of a physically based distributed hydrologic model, which can effectively simulate flood inundation parameters and a economic loss assessment model, which can estimate economic losses due a flood based on simulated flood inundation parameters by the distributed hydrological model. Study Area The river basin selected for this study is the Yom River basin, located in the Central- Northern part of Thailand between longitude 99.5 E to E and latitude 15.6 N to 19.4 N. It is a sub-basin of the Chao Phraya River Basin with an area of 19,516 km 2 and annual average runoff of 3,684 cumec. The Phrae city, located by the side of the river Yom River in the central part of the catchment, is one of the most frequently flooded cities in Thailand. The municipal area of Phrae city covers approximately 9 km 2 with a population of nearly 20,000 (Figure 5). Flooding is a frequent occurrence in this city and in general floods occur 3-5 times a year. Topography of the city is rather flat, and gently sloping towards the river except for an elevated strip on the Northern side which appear as a remnant of an earth bund that was in place to protect part of the city. At present this feature does not indicate continuity and it is said that though it protects the inner city from most of the floods, the people experience significant problems in removing water once large floods flow over some of the lower elevated sections of this strip. During the last ten years, the city suffered from several severe flood disasters, the most severe one was in 1995, which was experienced through out Thailand as one of the most severe floods in history. It caused widespread damage and causalities affecting over 85% of the total population of Thailand [7], [8].

5 Analyses of flood Management Options Due to frequent and devastating floods in the Yom River basin, the Thai Government has been seriously thinking of taking flood control measures for flood disaster reduction in the basin. Several flood control proposals are under consideration of a governmental agency responsible for flood control in the basin. One of such proposals for protecting the Phrae city from flooding is the extension of the existing dyke. In this study, two flood control plans were analyzed using the model to find out their effectiveness in flood risk reduction. The first plan is the proposal of the government agency, i.e., the extension of the existing dyke. Other plan is the storage of flood water in upstream by means of retarding basin or allowing floodwater to flow into less vulnerable areas. Such approach has been adopted by several countries including Japan as a part of the basin wide integrated flood management strategy. Considering the severity of August 1995 flood event, it was considered for the present analysis and thus, boundary conditions and rainfall patterns for the analyses were kept same as the flood event of For the comparison of the two plans for effectiveness in flood reduction, flood damage to residential and non-residential buildings was considered as the main factor. Urban flood loss functions were used for estimation of losses for both the cases [9]. To incorporate the first plan into the model, DEM of the study was modified by inserting the added elevation for the grids covering the proposed extension of the existing dyke. Figure 6 shows the DEM at the present condition and after proposed extension of the dyke. Other input data were kept same. For the second plan, the upstream boundary condition was modified to take into account of upstream storage. A storage volume of 2.5 million m3 was considered based on the feasibility of such upstream storage basins. Other input datasets remain same as 1995 event simulation. Figure 7 shows the maximum simulated flood inundations after incorporation of these two flood control plans together with the present condition. These show that flood inundation can be reduced substantially by implementing any of these two plans. The second plan shows higher reduction in surface inundation. In term of economic damage to buildings, the first plan can reduce damage by about 54%, whereas the second plan reduces damage by about 76% (Table 1). Although there is no cost-benefit analysis performed here for these plans due to limitation of data, it can be easily seen that the second plan can reduce the economic risk to higher degree. Also, the first plan has its own limitation such as, extension of the dyke will increase the vulnerability of the Phrae city for extreme events. CONCLUSIONS The analysis of flood trends in Asia shows that flood disasters in most of the Asian countries are increasing significantly, especially in the last few years Asia witnessed a rapid increase of flood disasters. The case study has shown two important aspects of flood risk management. First aspect is the need of integrated basin wide flood management approach: through the case study, the importance of basin wide flood 5

6 management approach in flood risk reduction in the view of increasing extreme events can be understood. Another important aspect is technological tool for flood risk management: the case study presents an application of an advanced hydrological modeling tool and through this application, we can see the importance and usefulness of such advanced modeling tool for effective flood risk management. 6 CHIANG MAE Yom River 20 o N Ping River a NAKHON SAWAN Chao Phraya River BANGKOK Wang River Y20 Phrae City Y1C Study Area Nan River Yom River N km Scale Legend Rivers Chao Phraya river basin upto Nakhon Sawan o 15 45'N 98 o E Chao Phraya River Basin 101 o E upto Nakhon Sawan (110,000km2) Changwat Phrae b Amphoe Muang Phrae Yom River Phrae City Major road Small road Scale km Figure 5. Location map of the study area Figure 6. 50m DEM of study area before (a) and after (b) proposed extension of dyke a b c Figure 7. Simulated maximum flood inundation for condition of 1995 flood event for: a) present situation, b) after implementation of the proposed dyke extension, and c) after implementation of storage of flood discharge in upstream

7 7 Damage Table 1. Economic damage to buildings due to the simulated floods for three different conditions Categories Damage to building structure Damage to building content/stock Total damage (in million Baht) with the existing dyke Damage (in million Baht) After extension of the existing dyke If upstream storage is allowed With the experience of this case study application, importance of basin wide flood management approach for reduction of risk of catastrophic floods is highlighted. Modeling tool is found to be very effective for proper understanding of flooding mechanism and comparative analyses of various flood control plans for selecting the best suitable plan for actual implementation. Such a tool is very effective in integrated flood control planning. REFERENCES [1] IPCC, (TAR) (2001). Third Assessment Report (TAR) of the Intergovernmental Panel on Climate Change, Parts 1, 2 and 3, Synthesis Report and Policy Makers Summaries, Cambridge University Press, Cambridge, UK. [2] CRED (2003). International Disaster Database, Centre for Research on the Epidemiology of Disasters, Brussels, Belgium. [3] Appleton, B. (Editor) (2003). Climate Changes the Water Rules: How water managers can cope with today s climate variability and tomorrow s climate change, Dialogue on Water and Climate, Netherlands. [4] Kanae, S., T. Oki and K. Musiake (2001). Impact of deforestation on regional precipitation over the Indochina peninsula, Journal of Hydrometeorology, Vol. 2, No. 1, pp [5] Milly, P.C.D., R.T. Wetherald, K.A. Dunne and T.L. Delworth (2002). Increasing Risk of Great Floods in a Changing Climate, Nature, Vol. 415, pp [6] Dutta, D., S. Herath and S. Wijesekera (2003). Development and Application of an Urban Flood Risk Analysis System, Proceedings of the First International Conference of Asia-Pacific Hydrology and Water Resources Association, Kyoto, Japan, pp , March. [7] INCEDE Newsletter, Floods in Thailand, Vol. 4, No. 4, January- March 1996, pp.

8 [8] Hungspreug, S., Khao-uppatum, W. and Thanopanuwat, S., Operational flood forecasting for Chao Phraya river basin, Proceedings of the International Conference on The Chao Phraya Delta: Historical Development, Dynamics and Challenges of Thailand s Rice Bowl, December 12-15, 2000, Kasetsart University, Bangkok, Thailand. [9] Dutta, D. and T. Tingsanchali (2003). Development of Loss Functions for Urban Flood Risk Analysis In Bangkok, Proceedings of the 2nd International Symposium on New Technologies for Urban Safety of Mega Cities in Asia, ICUS, The University of Tokyo, pp: