Author(s): Phatcharasak Arlai; Manfred Koch; Monthon Yongprawat; Sudarat Pirom; Tuantan Kitpaisalsakul
Keywords: Flood mitigation; Retention reservoirs; Groundwater recharge; HEC-RAS; MODFLOW
Abstract: Gravitational recharge from flood retention reservoirs in the upper Chao Phraya river basin into the underlying Bangkok aquifers is numerically investigated by coupling the HEC-RAS river model to the MODFLOW groundwater model over the reservoir module RES1. As the Chao Phraya river basin often encounters flood inundation during the long Thai rainy season while, at the same time, the groundwater aquifer is increasingly being depleted from excessive pumping, the present study simultaneously serves two purposes. Thus the fundamental idea is to divert excess flood flow from the Chao Phraya river into the flood retention reservoirs to mitigate flooding in the lower parts of the basin. This retained water will gravitationally recharge into the Bangkok subsurface system, herewith increasing the amount of fresh groundwater. The feasibility of such a two-prong approach is investigated, whereby a flood retention reservoir in a seriously flood-inundated area in the basin, upstream of the Chao Phraya dam is selected. Using data from the heavy flood of 2006, the water stages in the retention reservoir are calculated for the flood-year 2006 with HEC-RAS. These serve as input in the reservoir module RES1 of the MODFLOW groundwater model, whereby groundwater recharge and heads are computed. The results disclose the positive potential of retention reservoirs for mitigating river flood while at the same time increasing recharge into the aquifers.