1. A computer implemented method comprising the steps of: displaying a map including at least one river bank on a display unit; specifying an inflow section of one or more portions of the river bank on the map; deriving at least one line segment corresponding to the one or more portions of the river bank based on the inflow section; calculating a flow quantity vector for each line segment based on a direction normal to the line segment and an angle defined by the direction normal to the line segment and a calculation grid axis; displaying the flow quantity vector for each corresponding line segment on the map on the display unit; and performing inundation simulation using the flow quantity vector.