Integrated Flood Risk Management Plan for the West Bassac Area in Cambodia
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THE MEKONG RIVER COMMISSION SECRETARIAT Integrated Flood Risk Management Plan for the West Bassac area in Cambodia The Flood Management and Mitigation Programme, Component 2: Structural Measures & Flood Proofing in the Lower Mekong Basin December 2009 Draft Final Report, Volume 6C HASKONING NEDERLAND B.V. UNESCO IHE Guide to the reporting structure of the Flood Management and Mitigation Programme - Component 2, Structural Measures and Flood Proofing Component 2 on Structural Measures and Flood Proofing of the Mekong River Commission's Flood Management and Mitigation Programme was implemented from September 2007 till January 2010 under a consultancy services contract between MRCS and Royal Haskoning in association with Deltares and Unesco-IHE. The Implementation was in three Stages, an Inception Phase, and two implementation Stages. During each stage a series of outputs were delivered and discussed with the MRC, the National Mekong Committees and line agencies of the four MRC member countries. A part of Component 2 - on 'Roads and Floods' - was implemented by the Delft Cluster under a separate contract with MRC. The consultancy services contract for Component 2 specifies in general terms that, in addition to a Final Report, four main products are to be delivered. Hence, the reports produced at the end of Component 2 are structured as follows: Volume 1 Final Report Volume 2 Characteristics of Flooding in the Lower Mekong Basin: Volume 2A Hydrological and Flood Hazard in the Lower Mekong Basin; Volume 2B Hydrological and Flood Hazard in Focal Areas; Volume 2C Flood Damages, Benefits and Flood Risk in Focal Areas, and Volume 2D Strategic Directions for Integrated Flood Risk management in Focal Areas. Volume 3 Best Practice Guidelines for Integrated Flood Risk Management Volume 3A Best Practice Guidelines for Flood Risk Assessment; Volume 3B Best Practice Guidelines for Integrated Flood Risk Management Planning and Impact Evaluation; Volume 3C Best Practice Guidelines for Structural Measures and Flood Proofing; Volume 3D Best Practice Guidelines for Integrated Flood Risk Management in Basin Development Planning, and Volume 3E Best Practice Guidelines for the Integrated Planning and Design of Economically Sound and Environmentally Friendly Roads in the Mekong Floodplains of Cambodia and Vietnam1 Volume 4 Project development and Implementation Plan Volume 5 Capacity Building and Training Plan Demonstration Projects Component 2 prepared five Demonstration Projects which have been reported separate from the main products: Volume 6A Flood Risk Assessment in the Nam Mae Kok basin, Thailand; Volume 6B Integrated Flood Risk Management Plan for the Lower Xe Bangfai basin, Lao PDR; Volume 6C Integrated Flood Risk Management Plan for the West Bassac area, Cambodia; Volume 6D Flood Protection Criteria for the Mekong Delta, Vietnam Volume 6E Flood Risk Management in the Border Zone between Cambodia and Vietnam The underlying report is Volume 6C of the above series. 1 Developed by the Delft Cluster MRC Flood Management and Mitigation Programme Component 2: Structural Measures and Flood Proofing Summary This report presents the findings of the FMMP-C2 Demonstration Project that aims to assist Cambodia in formulating an Integrated Flood Risk Management Plan for the West Bassac area in the south-western part of the country; the area is located in Kandal and Takeo provinces. The West Bassac IFRM Plan area. Integrated Flood Risk Management Strategy Three flood risk management zones were identified based on present flood conditions, existing road and flood embankments, human settlements and land use. Subsequently, the type of structural components required for each area has been identified and preliminarily designed. The formulated scenario is based on the concept of living with flood and to take maximum advantage from flood and to reduce risk by avoiding the maximum flood period for agricultural activities in deep flooded areas. Due to the low level of infrastructure development in the area, flood risk reduction alone could not justify large investment for flood protection. On the other hand it would lead to an optimum use of the land use potential of the area by increasing the crop intensification and diversification. The West Bassac area can be sub-divided into three management zones Stage 2 ‐ S i ‐ December 2009 West Bassac IFRM Plant MRC Flood Management and Mitigation Programme Component 2: Structural Measures and Flood Proofing Zone 1 This zone is delimited to the east by the RN21, a flood free road built on existing river levees along the Bassac River. The RN21 runs from Phnom Penh to the Cambodian- Vietnamese border at Chhrey Thom. To the west, zone 1 is partly delimited by the existing left embankment and the planned embankment of the flood plain main drain. Currently some 90 colmatage canals divert a large amount of the Bassac flood water across this zones extending 2 to 5 km into the flood plain conveying and storing water through and in the flood plain. Behind the Bassac levees (RN 21), land elevation is about 3.5 to 4.0 m sloping towards the flood plain. Major population settlement is concentrated along the RN21 road embankment and parts of the Preik Ambel embankment and a few higher road embankments around Sa’ang town. Land use intensification and diversification potential of this area is amongst the highest in the part of the Mekong delta in Cambodia. Currently only a few of the existing colmatage canals are gated providing water control capability. A small portion of this zone has been developed to be a polder (pilot stage). Under existing conditions, the “Chamcar” occupies higher ground near to the RN21 and the area near to the flood plain main drain is generally used for rice flood recession crop. With steady improvement of the road embankment of the left bank of the Preik Ambel, the area tends to develop itself as a series of polders. This can only be successful with a comprehensive integrated plan combining integrated flood risk management and integrated water resources development taking the whole flood plain into consideration. It is proposed to conceive zone 1 as an area with full flood protection but with the benefits from flooding. This means that the infrastructural design and management should allow controlled flooding for re-supply of sediment and sanitation of farm conditions. Zone 1 will be sub-divided into five large polders using the flood free RN21 as a protection dike from the Bassac. On the western side the embankments of the flood plain main drainage channel function as flood protection dikes. The number of colmatage canals will be reduced to only one per polder connecting between the Bassac and the flood plain main drain. Remaining colmatage canals will be closed but will be maintained and improved for drainage and irrigation water supply to each part of each polder. Existing lateral irrigation/drainage canals of each polder will be improved. Each polder will be equipped with a pumping station equipped with a reversible pump installed at the lowest part of the polder serving both for drainage and irrigation water supply. The deep flooded area has limited settlement space, the right bank of the flood plain main drain will be raised to a flood free level with a series of flood proof causeways of 100 m long at an interval of every 2km for flood passage with design crest above 10 year return period of July maximum water level. The crest of the embankment of the flood plain main drain on the left bank is designed for a 100 years annual maximum return period flood. The flood plain main drain will be dredged and improved for navigation with rural port facilities at Angkor Borei, head of Preik Ambel and Sa’ang. The hydraulic simulation model will be used for determination of appropriate canal dimensions following the Best Practice Guideline for Structural Measures for Integrated Flood Risk Management. The Preik Ambel is affected by tide, the operation of the diversion/drainage canal and the related water control infrastructure shall be the main focus during the operation Stage 2 ‐ S ii ‐ December 2009 West Bassac IFRM Plant MRC Flood Management and Mitigation Programme Component 2: Structural Measures and Flood Proofing phase to optimize the operation cost. The water management shall be the task of a specialized qualified government line agency. It is expected that water use communities will be established for the management of each polder. Farmer will use their own pumps to irrigate their field from the existing network of colmatage canals. The design of Integrated Flood Risk Management and Integrated Water Resources Management in Zone 1 is based on existing and new infrastructure and local condition specific to each polder. The approach used is based on the maximum use of benefit from flooding which would rely mainly on the management capability and flexibility rather than on a full control by protecting against maximum design flood and assure timely drainage for different sets of crop calendars. The following infrastructures are proposed: 1. The RN21 which is currently flood free will be reviewed according to standards developed by proposed Best Practice Guideline for development and design of structural measures and flood proofing; 2. A main flood plain drainage channel connecting the Preik Ho- Priek -Ambel- Preik Moat Chhrook to the border with Vietnam. The final drainage capacity will be based on modelling exercise in such a way that it could carry sufficient flow capacity without increasing water level in the Bassac. The left embankment of the flood plain main drain will be raised to 100 year return period protection level. 3. Five principal diversion/colmatage canals linking the Bassac with each of the five polders and the main flood plain drain/diversion canal. Each of the canals will be fully controlled by sluice gates in such a way that they could be used not only for flood diversion but also for water supply for irrigation taking advantage of tidal effects and the need for drainage of local rainfall; 4.