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 150-8925, 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
FLOOD DISASTER TRENDS IN ASIA
The main source of the datasets used in this study is from CRED for the past 30 years
from 1973-2002, 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.
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].
Figure 2. Flood disaster trend in Asia in the last 28 years compared to other natural
disasters
Figure 3. Flood events in different Asian countries in last 28 years
Figure 4. Flood trend in most frequently flood affected 10 countries in Asia
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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 CentralNorthern part of Thailand between longitude 99.5E to 100.5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 km2 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].
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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 1995. 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
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.
a
b
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
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Table 1. Economic damage to buildings due to the simulated floods
for three different conditions
Damage
Categories
Damage to
Damage (in million Baht)
with the
After extension of
If upstream
existing dyke
the existing dyke
storage is allowed
12.06
5.57
1.1
49.05
22.38
13.5
61.11
27.95
14.6
building structure
Damage to
building
content/stock
Total damage
(in million Baht)
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.
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precipitation over the Indochina peninsula, Journal of Hydrometeorology, Vol. 2,
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Risk of Great Floods in a Changing Climate, Nature, Vol. 415, pp. 514-517.
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Urban Flood Risk Analysis System, Proceedings of the First International
Conference of Asia-Pacific Hydrology and Water Resources Association, Kyoto,
Japan, pp. 821-826, March.
[7] INCEDE Newsletter, 1996. 1995 Floods in Thailand, Vol. 4, No. 4, January- March
1996, pp.
8
[8] Hungspreug, S., Khao-uppatum, W. and Thanopanuwat, S., 2000. Operational flood
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