Project for the Comprehensive Flood Management Plan Main Report for the Chao Phraya River Basin in the Kingdom of Thailand Chapter 9 Strategy of Master Plan Formulation CHAPTER 9 STRATEGY OF MASTER PLAN FORMULATION 9.1 Basic Approach to the Master Plan 9.1.1 Concept of Master Plan The Chao Phraya River Basin is composed of three (3) areas; namely, the Highlands, the Upper Central Plain and the Lower Central Plain. The characteristics of each area and its required measures have been examined to formulate the Master Plan of Flood Disaster Management for the Chao Phraya River Basin. The study area is outlined from flood disaster management aspects as follows: 1) The Highlands are the watersheds of the major tributaries of the Chao Phraya River Basin, which are the Ping, Wang, Yom and Nan rivers. The areas are covered by forest, but the forest area has been devastated and the degraded forest areas have been identified by the Royal Forest Department. For flood disaster management, restoration of the degraded forest areas and the improvement of forest management are required. 2) The Upper Central Plain is located at the Upper Nakhon Sawan and composed of the river basins of the Ping, Wang, Yom, Nan and Chao Phraya. During the 2011 flood in the Upper Central Plain the inundation started along the Yom River in late July and at Nakhon Sawan in early September. The areas are flat and have wide low-lying areas along the rivers, which have a functional role in natural flood retarding basin and partly habitual inundation areas in rainy season, but partly used as agricultural lands in dry season. For flood disaster management purposes, optimum measures are required for local core cities and the inundation area to promote controlled inundation. 3) The Lower Central Plain is located in Lower Nakhon Sawan and also has low-lying areas which are flood retarding areas in rainy season, but mostly used as irrigation area in dry season. During the 2011 flood, dyke breaches and overflows occurred mainly in the middle of September, and huge volumes of floodwater inundated Ayutthaya and Bangkok from early October to late October. The major protection areas are Bangkok and its vicinity and the local core cities along the Chao Phraya River. For flood disaster management purposes, optimum combination of structural and non-structural measures are required for the protection area of Bangkok and its vicinity as well as the local core cities, and for the inundation area to promote controlled inundation. Basic conditions for structural and non-structural measures are as discussed below. CTI Engineering International Co., Ltd. 9-1 Oriental Consultants Co., Ltd. Nippon Koei Co., Ltd. CTI Engineering Co., Ltd. Main Report Project for the Comprehensive Flood Management Plan Chapter 9 Strategy of Master Plan Formulation for the Chao Phraya River Basin in the Kingdom of Thailand 9.1.2 Setting of Basic Conditions (“A” to “F”) The Master Plan is formulated by setting the basic conditions (A to F) and action countermeasures (a to h) as shown in Figure 9.1.1. A. Setting Design Flood B. Setting DHWL C. Evaluation of Flow capacity D. Setting Protection Area E. Habitual Flood Area F. Setting Flow Distribution ｆ. Landuse Control in Inundation Area a. Improvement of Dykes and e. Possible Flood Control Volume in River Channel Retarding and Retention Area b. Effective Operation of g. Stormwater Drainage Existing Dams d. Flood Control with ｃ. Flood Control Volume with Diversion Channels New dams h. Forest Restoration Confirmation of proposed plans within DHWL Figure 9.1.1 Basic Procedure of Master Plan Formulation A: Design Flood The scale of the 2011 flood was proposed as the design flood for the Master Plan. The Government of Thailand has applied a 100-year probable flood for the flood control plan of urban areas and a 25-year probable flood for agricultural/rural areas to all accros the country river basins. However, after the 2011 flood the Government decided that urgent measures of flood control like restoring the damaged flood dykes and heightening of road elevation must be undertaken to protect the urban areas against the flood crest similar to the 2011 flood. According to the result of rainfall analysis by JST, the scale of the 2011 flood was assessed to be an approx. 100-year probable flood, which meets the above Government policy on design flood level. In additon, for considering different rainfall patterns, the study on structural measures was conducted by enlargement of the other rainfall patterns to the scale of the 100-year return period. B: Design High Water Level (DHWL) The setup of the DHWL is an important procedure to implement an effective river management including flood prevention, control for deliberate inundation, river improvement works and so on. The optimum DHWL of target areas was based on (i) the height of existing dykes and river banks; and (ii) the past flood water levels and design high water levels of restoration of dykes as well as the land use situation of protected areas. 9-2 CTI Engineering International Co., Ltd. Oriental Consultants Co., Ltd. Nippon Koei Co., Ltd. CTI Engineering Co., Ltd. Project for the Comprehensive Flood Management Plan Main Report for the Chao Phraya River Basin in the Kingdom of Thailand Chapter 9 Strategy of Master Plan Formulation C: Evaluation of Flow Capacity The flow capacity is imporatnt information to identify the weakness sections of the river and determine which countermeasurs to be implimented preferentially. The flow capacity of the existing rivers/canals was calculated at bankful level with one-dimensional unsteady flow analysis, considering the tide-affected and tide-unaffected compartment. D: Protection Areas To establish an effective comprehensive flood control plan, it is indispensable to define the area should be protected against huge flood like the 2011 flood. The economically important area (approx. 5,600 km²) which covers Bangkok and its vicinity up to the southern part of Ayutthaya and major regional core cities in the basin, were been defined as key protection areas by the Government of Thailand. Accoriding to this policy, the M/P aimed to mainly protect these areas from flooding. E: Habitual Flood Area Accroding to the past inundation records, some areas of the basin are consistently inundated. Consideirng the meteorological, hydrological and topograhpical featrues in the Chao Phraya River, it is much difficult to protect all areas from inundation with flood control facilities inclding construction of dams and river improvement works. Therefore, the flood-prone areas was delineated as controled inundation areas based on the past inundation areas and the land use map. And then, some management measures, mainly software measures such as land use regulation and disaster insurances were recommended. F: Flow Distribution The flow distribtuion of Chao Phraya River system will be changed by structural measures as followings, 1) Improvement of dyke and river channel 2) Effective operation of the existing dams 3) Effectiveness of new dams 4) Effective flow discharge capacities of diversion channels 5) Effective flood control by retarding and retention areas Basically, a flood water after taking the countermeasures should not be larger than existing condition. Therefore, not only water level and also discharge are checked not to reduce the flood safety level due to the countermeasures. 9.1.3 Study on Countermeasures (items “a” to “h”) (a) Improvement of Dykes and River Channels To improve the flow capacity of the rivers/canals in the Chao Phraya River Basin, proper river improvement works were examined. In the Chao Phraya River basin, it is much difficult to prevent the flooding with river improvement works only, since the flow area of the river is absolutely not enough against inflow from river basins. In addition, if high and long dyke is constructed upstream, discharge from the upstream will increase and could cause floods inundation downstream. Therefore, the dyke elevation works should be conducted precisely considering flow capacity, land use condition along the river, the gap of flood safety level between the upstream and downstream, etc. (b) Effective Operation of Existing Dams Among the existing dams in the Chao Phraya River Basin, four (4) large dams, Bhumibol Dam, Sirikit Dam, Pasak Dam and Kwae Noi Dam, have high effectiveness for flood control during flood events. However, existing dam operation controlled by RID and EGAT gives priority to water use for irrigation mainly. In this study, considering existing water use condition such as irrigation, drinking and environmental flow downstream at C.2, existing dam operation rule was modified to release the water from dams adequately for both water use and flood control. CTI Engineering International Co., Ltd. 9-3 Oriental Consultants Co., Ltd. Nippon Koei Co., Ltd. CTI Engineering Co., Ltd. Main Report Project for the Comprehensive Flood Management Plan Chapter 9 Strategy of Master Plan Formulation for the Chao Phraya River Basin in the Kingdom of Thailand (c) New Dams Over a hundred new dams are planned to be constructed by RID after the 2011 flood. Among the these dams, the effects of flood control of the Kaeng Sua Ten Dam in the Yom River Basin where no dam and Mae Wong Dam located in the Sakae Krung which seems to have a high effectiveness for flood control were examined. Regarding the operation of new dams, revised dam operation rule (refer to “countermeasures “b”) was applied. (d) Flood Control with Diversion Channels The three flood diversion channels is proposed by the Government of Thailand; namely, East Diversion Channel, West Diversion Channels and Outer Ring Road Diversion Channel. Their total flow capacity is 2,000m3/s or more. These floodways are so long and large that they seem to impact on economic and socio-environmental.