Suphat Vongvisessomjai Flood Mitigation Master Plan for Chao Phraya Delta INWEPF 2007 Suphat Vongvisessomjai Professor, Water and Environment Expert TEAM Consulting Engineering and Management Co.,Ltd. e-mail : suphat@team.co.th

Abstract Master plans of flood mitigation for Chao Phraya Delta, rice bowl of Thailand, are as follows : First master plan of flood mitigation for Bangkok developed after 1983 flood from the King’s initiation by King’s Dike to protect rainwater from northern and eastern boundaries and the Chao Phraya river water from the west Second master plan of flood mitigation for Chao Phraya Delta after 1995 flood, Initially developed by AIT,DHI and ACRES for World Bank, further developed in details by JICA, and finally developed in Thai by Crown Property Bureau.  The proposed master plan of flood mitigation for Chao Phraya Delta after 2006 flood, with new diversion using AIT river network model.

1.Introduction The Chao Phraya Delta,rice bowl of Thailand FAO study in 1948 recommended that Thailand’s economic strength lay in exporting rice to alleviate world-wide food shortages due the war. 1950,Thailand secured a World Bank loan 1952,commenced work on the Greater Chao Phraya 1957,first phase completion of Asia’s largest irrigation project. 1961,additional component : Bhumibol dam and irrigation canal. 1977,Sirikit dam finished for 25 year irrigation program and Thailand is ranked first in rice export.  AIT river network model was developed by Vongvisessomjai and Suppataratarn 1998 and used as tool for Chao Phraya flood management review for World Bank and forecast future floods especially 2006 flood

Eastern Bangkok Polder 2. Literature Review 2.1 The First Master Plan for Bangkok The King recommended King’s Dike to protect Bangkok after 4 months flood in 1983 due to heavy rainfall Figure 1 Eastern Bangkok Polder (JICA 1986)

2.2 The Second Master Plan for Chao Phraya Delta  After 1995 flood with extensive damage, World Bank requested AIT,DHI, and ACRES to conduct a Chao Phraya flood management review  JICA provided assistance to develop an Integrated Plan for Flood Mitigation based on recommended work plans of World Bank report  Crown Property Bureau reported in Thai the framework of water resources management which proposed 3 mitigation measures for water shortages, floods and pollutions

3. Methodology 3.1 Important Data of 2006 Flood The proposed master plan is developed from 2006 flood data 3.1 Important Data of 2006 Flood (1) Severe floods occurred more frequently, i.e. in 1995,2002 and 2006 due to invasion of upper catchments, and insufficient drainage of flood flow resulted in extensive damages at Singhburi,Angthong,Ayutthaya and Bangsai which is a bottle-neck that limit the flow less than 3,500 m3/s resulted In flooding upstream in all areas of Ayutthaya (2) Flood waves in 2006 from Chao Phraya Dam to Bangsai : At Chainat, 7 m. and peak at 17.50 m. At Singburi, 6 m. and peak at 13.14 m. At Angthong, 5 m. and peak at 8.19 m. At Ayutthaya, 2 m. and peak at 4.70 m. At Bangsai, 1.5 m. and peak at 3.60 m.

Figure 3 2006 Flood Hydrographs of the Daily Highest Water Level (m.MSL) Date from 16 September to 15 November 16 Sep. 21 Sep. 26 Sep. 1 Oct. 6 Oct. 11 Oct. 16 Oct. 21 Oct. 26 Oct. 31 Oct. 5 Nov. 10 Nov. 15 Nov. Chao Phraya Dam (Present) Singhburi (Present) Angthong (Present) Ayutthaya (Present) Bangsai (Present) Chao Phraya Dam (-500 cms) Singhburi (-500 cms) Angthong (-500 cms) Ayutthaya (-500 cms) Bangsai (-500 cms) Chao Phraya Dam (-1,000 cms) Singhburi (-1,000 cms) Angthong (-1,000 cms) Ayutthaya (-1,000 cms) Bangsai (-1,000 cms) Figure 3 2006 Flood Hydrographs of the Chao Phraya River from Chao Phraya Dam to Bangsai and the Decreased Water Levels due to Diversion of 500 and 1,000 m3/s at Bangsai

(3) Satellite images of 1995 and 2006 in Figs 5&6 showed severe floodings in Chainat,Singhburi, Angthong and Ayutthaya  Undrained water’s at Bangsai inundated Chao-chet,Pakhai and Sena,then flowed to the Tha Chin and flooded Banglen to the mouth of the Tha Chin at Krathumban and Muang of Samut Sakhon.

Flooded Area and Maximum Figure 5 Flooded Area and Maximum Water Level in 1995

Figure 6 Satellite Image of 2006 Flooded Area in the Chao Phraya River (Contributed by GISTDA from Summary Report on Satellite Imagery of Flooding in 2006)

3.2 The Proposed Master Plan (1) The leftover mitigation, the diversion, is very costly and not so effective as shown in Fig.3 (2) The river diversion is the major factor to alleviate flooding (3) It can be seen in Fig.4 that the Tha Chin river flows from Chao Phraya Dam parallel to the Chao Phraya river. Its capacity is only 10 percent (350 m3/s) of the Chao Phraya river (3,500 m3/s) because of 4 regulators (Pholathep,Thabote,Samchuk&Phophraya) but downstream of Phophraya to the river month, the river sections are large enough for 1,500 m3/s (4) Therefore, a diversion canal in the upper Tha Chin is best mitigation

Figure 7 : Three-dimensional plot of the analytical model of water surface fluctuation with respect to distance and time. (Vongvisessomjai,S. and Chatanantavet,P. 2006)

with respect to distance and time. Water Level (m) Maha Sawat (89) Phra Pimon (121) River Mouth (0) Distance (km) Phraya Banlu (142) Date in January 2002 Figure 7b. Three-dimensional plot of water surface fluctuation in the Tha Chin River with respect to distance and time.

Figure 8a : Physical layout of the River (Vongvisessomjai,S. and Network Model (Vongvisessomjai,S. and Suppataratarn,P.1998)

Figure 8b : Schematization of the (Vongvisessomjai,S. and River Network Model (Vongvisessomjai,S. and Suppataratarn,P.1998)

Figure 9: 2006 Flood Hydrographs of the Daily Highest Water Level (m.MSL) Figure 9: 2006 Flood Hydrographs of the Chao Phraya River from Chao Phraya Dam to Bangsai and the Decreased Water Levels due to Diversion of 500 and 1,000 m3/s to the Tha Chin River

AIT river network model reveals that At Chainat and Singhburi, flood depths drop 2 m. At Angthong, drop 1.5 m. At Ayutthaya, drop 1 m. At Bangsai, drop 0.5 m. No more floodings in the Chao Phraya Delta. The reduced flow of 3,000 m3/s in the river will not have overspillage. This diversion from the Chao Phraya Dam can flow to the river mouth without pumps due to its high head. No more floodings from Banglen to the river mouth.

4. Conclusion (1) Floods in the Chao Phraya Delta were caused by the in sufficient drainages to the sea which inundated the rice bowl of Thailand  The eastern diversion could not be built due to its expensive land cost and high pumping cost (2) The new diversion was found most effective to mitigate flood in the Chao Phraya Delta.  It helped minimize floodings at Chainat,Singhburi,Angthong Ayutthaya and Bangsai  It also helped minimize floodings downstream of the Tha Chin from Banglen to the river mouth. (3) The new diversion costed less than the eastern diversion and had less impact to the people since its course was along the existing river which required less dredging and its merit of high head near to the Chao Phraya Dam which required no pumping.