Aim of the paper : to give a review and description of the
characteristics of flooding in the Monsoon Asia region.
1. Hydrological regionalization of the countries and their river basins,
based on a climatological water budget (by Kondoh & Kojiri).
2. The maximum flood envelope curves which were applied to the region.
3. Flood risks: an example of flood risk map for the Mekong River
(with gradually changing or ”gentle” floods due to the monsoons).
4. Flood risks: an example of Marikina River hydrograph near Manila,
(with flashy nature of floods caused by typhoon-intensified monsoons).
5. Flood risks: relationship between ADB country data for the number of
deaths and economic losses due to floods and droughts (1990-2001)
and the population in year 2000 in the Monsoon Asia region.
6. Initial results of regional flood frequency analysis, using annual flood
data in the UNESCO-IHP Catalogue of Rivers for SE Asia and the Pacific.
is an extensive region in South and East Asia which face the
major sea bodies of the Indian Ocean and the western Pacific Ocean.
receives most of its precipitation from the seasonal exchange
of moist air mass or wind, called the monsoon, which is driven
by the differential heating between the Asian continent and the oceans.
is further divided into sub-regions:
South Asia - India, Sri Lanka (former Ceylon) and Bangladesh;
Southeast Asia or ASEAN - Myanmar, Thailand, Lao PDR,
Cambodia, Vietnam, Malaysia, Singapore, Brunei Darussalam,
the Philippines, Indonesia and Papua New Guinea;
East Asia – significant parts of China, DPR Korea, R. of Korea, and Japan.
Note: Asia Pacific (AP) region also includes Australia, New Zealand and
the western Pacific island nations (considered outside Monsoon Asia).
Monsoon Asia is hardly uniform in characteristics.
It is characterized by “the variability of natural and social environments.
It ranges from humid tropics to arid regions and there are various
hydrological phenomena occurring in such diverse regions”
(Kondoh & Kojiri, 2002).
Kayane’s method of climatological water budget
was utilized as a way of hydrological regionalization.
used global datasets (precipitation, air temperature, sunshine duration,
albedo, elevation, soil moisture, total runoff integrating pathways)
open in the Internet or available CD-ROMS, and
made a GIS-grid-based water budget map, with 0.5 degree resolution.
The regions are defined in a grid map, using the parameters:
ATS = annual total surplus (mm)
ATD = annual total deficit (mm):
Region A: water surplus all year round (ATS > 200 mm) – Japan, eastern China & Taiwan, Malaysia, Borneo/Kalimantan, parts of Sumatra & Irian Jaya in Indonesia, Papua New Guinea, southeastern Philippines (eastern Mindanao). Region B1: water surplus with some months deficit (ATD < 200 mm) – west of Ghats Mountains & Assam in India, central & southern China, Java & Sulawesi in Indonesia, eastern, central & southwestern Philippines (eastern Luzon, Visayas, western Mindanao).
Region B2: water surplus with some months deficit (ATD > 200 mm) – east part of India, parts of Myanmar & Thailand along the Bay of Bengal, Lao PDR, Cambodia., Vietnam, northwestern Philippines (western Luzon). Region C1: water deficit with some months deficit (ATD < 200 mm) – Mongolia, part of Tibetan Plateau. Region C2: water deficit with some months deficit (ATD > 200 mm) – Deccan Plateau in India, leeward area of the southwest monsoon in Myanmar and Thailand, northern Australia.
The WMO (1967) developed the first set of envelope curves for maximum floods in the monsoon areas of the ECAFE (now ESCAP) region Modified Myer’s Formula: Q max = C A 0.5 Creager’s type Formula: Q max = C A ^ (b A -n ) where Q max = maximum recorded flood (m 3 /s); A = drainage area (km 2 ); ^ = exponential operator; C, b & n = empirical coefficients. Sub-region A (Burma, Ceylon, Pakistan, India): C = 206 (Modified Myer’s) C = 5.37, b = 1.44, n = 0.05 (Creager’s) Sub-region B (Cambodia, China, Indonesia, Laos, Malaysia, Thailand): C = 97 (Modified Myer’s) C = 0.35, b = 1.80, n = 0.05 (Creager’s) Sub-region C (Taiwan, Japan, Korea, Philippines, Vietnam) – typhoon subregion C = 364 (Modified Myer’s) C = 87, b = 1.00, n = 0.04 (Creager’s)
A flood risk map of the lower
basins of Mekong River:
The 4,800 km-long Mekong River
(A = 795,000 km 2 ) originates in China,
and flows through Myanmar,
Lao PDR, Thailand and Cambodia
before ending in the Mekong Delta
The map shows the Tonle Sap Lake
north of Phnom Penh, Cambodia
which rises and inundates due to the
backflow from the swelled Mekong River.
The “gentle floods” of rivers such as
the Irrawaddy and the Mekong:
with the gradual rise (and fall) of the water
levels & the consequent overbank flows
and inundation, as a result of long-term
high tides affecting the lower reaches
add to the maximum flood levels.
the hydrograph peaks are not very sharp,
these floods can contribute most of
the annual flood volumes of the basin .
The Marikina River (A = 534.8 km 2 ) near Manila, draining to Manila Bay. Example of the annual daily rainfall hyetograph & streamflow hydrograph which are marked by the sharp peaks caused by the typhoon-intensified monsoon in 1990 – Also evident in the figure is the distinct dry season from January to May and the distinct wet season from June to December, … . consistent with the region B2 classification of western Luzon, Philippines (also called Type I climate in the Philippine Coronas classification system).
ADB data on number of deaths due to floods & droughts (1990-2001) versus Year-2000 population.
ADB data on economic losses due to floods & droughts (1990-2001) (million USD) versus Year-2000 population.
Regional flood frequency analysis
performed on available annual maximum flood data -
within a group of climatologically and hydrologically “similar” basins –
by regression of derived moments or parameters
(such as the mean, standard deviation or coefficient of variation of annual floods)
against the measurable basin properties such as drainage area,
drainage network density, slope, and extents of soil types and land-use/cover types.
Estimates of Q T ,
the annual maximum flood of specified return period T, or the T-year flood,
in terms of the “regional growth curve” function, f(T), expressed as:
Q T / Q mean = f (T),
or alternatively, the flood frequency factor formula, involving the frequency factor, K(T):
Q T = Q mean + K(T) S Q or
Q T = Q mean [1 + K(T) C V ]
where the parameters Q mean , S Q , and C V are derived as regression functions
of the set of measurable basin properties, X:
Q mean = Q mean (X) = mean of the annual maximum flood
S Q = S Q (X) = standard deviation
C V = C V (X) = coefficient of variation
Countries in the UNESCO-IHP Catalogue of Rivers with analyzed annual maximum flood data (from Loebis, 2002) Countries Drainage Area (sq.km.) Australia (6 rivers) 25.9 to 129,500 China (9 rivers) 2,050 to 80,948 Indonesia (8 rivers) 948 to 12,804 Japan (9 rivers) 472 to 12,697 Korea (9 rivers) 612 to 11,104 Laos (6 rivers) 3,370 to 19,400 New Zealand (4 rivers) 556 to 6,350 Thailand (5 rivers) 4,609 to 38,862 For Q mean = C A b C b R 1.58 0.81 0.94 0.92 0.85 0.75 30.12 0.40 0.52 50.05 0.50 0.53 2.50 0.80 0.80 10.53 0.56 0.74 109.81 0.82 0.82 70.97 0.26 0.65 Countries For S Q = C A b C b R Australia (6 rivers) 1.21 0.83 0.93 China (9 rivers) 4.40 0.61 0.68 Indonesia (8 rivers) 205.38 -0.04 0.04 Japan (9 rivers) 89.26 0.36 0.53 Korea (9 rivers) 1.97 0.81 0.74 Laos (6 rivers) 13.05 0.46 0.72 New Zealand (4 rivers) 16.28 0.47 0.81 Thailand (5 rivers) 0.57 0.68 0.62
Conclusion: This paper has given a review and description of the characteristics of flooding in the Monsoon Asia region. A hydrological regionalization of the countries, based on a climatological water budget, is cited. Several maximum flood envelope curves are also reviewed. An example of a flood risk map for the Mekong River is provided. Likewise, an annual hydrograph of Marikina River near Manila is given as an illustration of the flooding effects of typhoon-intensified monsoons. The ADB data for the number of deaths and economic losses due to floods and droughts in the period 1990-2001 are plotted against the population in year 2000 of the various countries. Lastly, initial published results of regional flood frequency analysis, using some annual maximum flood data in Southeast Asia and the Pacific, are presented.