An unsteady flow calculation system in the Tonegawa River Basin including 6 tributaries, a distrib-utary, 6 flood retarding basins was constructed based on the Flow Assumption Type Unsteady Flow Calculation Method. The system was applied to a record-setting flood of September, 2015. A stable unsteady flow calculation was successfully achieved. Good reproductions were made in upper reaches of the main channel and branch reaches by set boundary conditions. As for the lower reach-es of the main river channel, however, some discrepancies between observed water stage values and calculated ones took place after the flood struck the peak. This system may be used for real-time forecast of flood flow to make flood fighting and residents' evacuation after making more detailed study of the roughness coefficients of the actual channels. An unsteady flow calculation combines observation of discharge and water level to better understand the hydraulical status of the river, hence make the basis of disaster prevention and mitigation.
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