Evaluation of the Flood Mitigation Function of Agricultural Infrastructures in Low-lying Rural Areas

In late years, the flood damage caused by the heavy rain occurs frequently in low-lying rural areas of all over the world. Dams for flood control have been constructed and played an important role for flood disaster prevention. However, the torrential rain which exceeds the flood storage capacity of dams occurs frequently and flood risk at the downstream area of dam has been increased. On the other hand, many agricultural infrastructures have the flood mitigation function in rural areas. Agricultural irrigation canals are usually aimed for water supply to crops, but can temporarily store excess rain water and mitigate drastic outflow. The diversion weir and rice paddy fields can produce the time delay for flood arrival to the downstream. The aims of this study are evaluating the flood mitigation function of irrigation and drainage facilities which are used for farming and optimizing their operation for disaster prevention under the flood condition. To develop the analytical model of flood, the land use condition and the hydrological situation were investigated in a part of Chikugo basin, and factors and influences of the flood were analyzed using GIS. The results indicated that there were the several cases that a disaster was avoided because the flood water level was kept low by agricultural infrastructures and rainfall at the small rang of the basin shows spatial and temporal distribution. A simulation model describing the one dimensional unsteady flow was introduced to evaluate the flood mitigation effect of dams, canals, water gates and pump stations on the downstream. In addition, the two dimensional surface flow model was developed in the basin to visualize influence of the flood. Using these models, scenario analyses were performed to evaluate the flood mitigation function of agricultural infrastructures using the past heavy rainfall data. The simulated result indicated that the flood damages were reduced by agricultural infrastructures and operating agricultural facilities appropriately mitigated flood in the heavy rain more than the tolerance of disaster prevention facilities. The method introduced here is effective for planning flood disaster prevention in rural areas.