Abstract:This paper evaluates the optimal spatial flood protection for people living with the threat of floods in the lower Mekong River region with respect to hydrology and economics. A hydrological numerical simulation contributes to an understanding of the relationship between flood control policy and inundation areas. The numerical simulation consists of a dynamic wave model in channels and a non-uniform flow model in inundation areas, which estimate the temporal and spatial dynamic distributions of water depth. In economics, an overlapping generations model expresses the economy's growth process considering flood magnitude and flood protection. This economic model is composed of industrial and agricultural production functions which are affected by flood control policies. The hydrological and economic models are verified by remote sensing images and data from the economic development of Thailand, respectively. Combining these two models, we can identify the area to protect and accept floods for regional development. Wide areas along the rivers should be flooded due to the benefits from fertilization for inundation agriculture in Cambodia. The conservation area should be about 2000 km 2 , and the spatial distribution of flooding should be identified. In the beginning of an economic growth period, there is little difference in the production of an inundation area in the range of 0-2000 km 2 . As the inundation area becomes larger, the production rate is larger in the stable growth period. This study explains that inundation agriculture should be maintained in riverside areas which supply much of the agricultural production in Cambodia.

This paper shows spatial distribution of merits and demerits of flood in the Mekong Basin.Firstly, the flood and inundation surrounding Phnom Penh, Cambodia in 2000 (the large flood) and 1994 (the normal flood), was simulated.Secondly, the benefits of the flood were calculated, combining the unit price of primary industry (agriculture and fishery) and the result of the flood simulation. As a result, the large flood gives high productivity of rice along rivers because it flowed out nutrition a lot It also expanded fish product area in the flood plain. Therefore, agriculture and fishery obtained higher benefits in 2000 than 1994. The spatio-temporal benefits and damages were constructed.