An urban storm drainage system can be considered as an integrated system of components of urban surface, gutters, inlets, sewers branches, junctions, manholes, and other facilities. In this study the flow equations that can be used to solve storm drainage problems are critically reviewed and the mathematical methods for solving the St. Venant equations are compared. A method for routing the unsteady flow due to rainfall and other inputs through land surface and gutter to produce the inlet hydrograph is proposed. A mathematical simulation model for tree-type sewer networks is developed for flow prediction and design by using the St. Venant equations to route the inlet hydrographs through the network. An overlapping segment scheme is used to account for the backwater effects and mutual influences of the unsteady flow in the sewers. (Modified author abstract)