The flood management in dam reservoirs is important for the improvement of the safety level control in downstream rivers of the dam. Figure 1 presents the dam reservoir flow during a large flood. The inflow discharges to dam reservoirs have been evaluated by using rate of storage volume dS/dt from the H-V relation assuming the horizontal rise of reservoir water level and outflow discharges estimated by reservoir water elevation near the dam body. This provides a simple estimation method for the outflow and inflow discharges in dam reservoirs. Assuming the horizontal rise of reservoir water level means that the flood flow propagates with very large velocity within the reservoir. However, floods in dam reservoirs flow down transforming a water-level and discharge due to three dimensional reservoir flows. But, the present reservoir management conducted by assuming the horizontal rise of water levels has not considered flood propagation mechanism and flow dynamics of flood water in reservoirs.