Recently the numerical simulation plays an important part in the emergency management for sediment-related disaster. Water and sediment go down streams that have conjunctions, curves, narrow and wide parts, so that the numerical simulation has to treat the complex geometry. Moreover, it is necessary for the numerical simulation to be applied for not only very fast flows such as debris flows and floods but also very slow flows such as still water. The numerical simulation needs to be based on unstructured mesh and to be improved in order to handle very slow flow such as still water. This study aims to develop a numerical model for still water and to verify the numerical model by reproducing the experimental results. In the developed numerical model, it is assumed that the averaged gradient of flow surface is equal to the resultant force of the hydrostatic force acting on the boundaries of element. It is found that the numerical simulation with the developed numerical model can reproduce the still water and the surges triggered by the flow going into the reservoir.