A computer model called STORM.HYD was created to analyze and design storm sewer systems using hydrographs. The model calculates inflow hydrographs at each structure in a sewer system using the Santa Barbara Urban Hydrograph method and the Soil Conservation Service's Runoff Curve Number procedure. These hydrographs are then combined and directed through the sewer system to produce an outflow hydrograph. The model is interactive and can analyze or design a complete tree-like system in one go. The model was tested on real rainfall events in a complex urban area, showing promising results.