CHARACTERISTICS OF FLOODING IN MONSOON ASIA

Leonardo Q. Liongson

“Day of Asia and the Pacific” Session(18 March 2003)

3rd World Water Forum Kyoto, Japan

16-23 March 2003

Aim of the paper: to give a review and description of the characteristics of flooding in the Monsoon Asia region.

Outline:

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.

Monsoon Asia

• 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 deficitwith 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: Qmax = C A0.5

Creager’s type Formula: Qmax = C A ^ (b A-n) where Qmax = maximum recorded flood (m3/s); A = drainage area (km2); ^ = 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 lowerbasins of Mekong River: The 4,800 km-long Mekong River (A = 795,000 km2) originates in China, and flows through Myanmar, Lao PDR, Thailand and Cambodia before ending in the Mekong Delta of Vietnam.

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 seasonal rainfall.• 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 km2) 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 isthe 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 onnumber of deaths due tofloods & droughts(1990-2001)

versus

Year-2000population.

ADB data oneconomic lossesdue tofloods & droughts(1990-2001)(million USD)

versus

Year-2000population.

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 QT, 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:

QT / Qmean = f (T), or alternatively, the flood frequency factor formula, involving the frequency factor, K(T):

QT = Qmean + K(T) SQ or QT = Qmean [1 + K(T) CV ] where the parameters Qmean, SQ, and CV are derived as regression functions of the set of measurable basin properties, X: Qmean = Qmean (X) = mean of the annual maximum flood SQ = SQ (X) = standard deviation CV = CV (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 129,500China (9 rivers) 2,050 to 80,948Indonesia (8 rivers) 948 to 12,804Japan (9 rivers) 472 to 12,697Korea (9 rivers) 612 to 11,104Laos (6 rivers) 3,370 to 19,400New Zealand (4 rivers) 556 to 6,350Thailand (5 rivers) 4,609 to 38,862

For Qmean = C Ab

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 0.74109.81 0.82 0.82 70.97 0.26 0.65

Countries For SQ = C Ab

C b RAustralia (6 rivers) 1.21 0.83 0.93China (9 rivers) 4.40 0.61 0.68Indonesia (8 rivers) 205.38 -0.04 0.04Japan (9 rivers) 89.26 0.36 0.53Korea (9 rivers) 1.97 0.81 0.74Laos (6 rivers) 13.05 0.46 0.72New Zealand (4 rivers) 16.28 0.47 0.81Thailand (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.

THANK YOU.