This paper will discuss the wet-weather facilities planning for the East Bay Municipal Utility District (EBMUD), Special District 1. Studies have been conducted since the early 1970s culminating in a recommended first-stage plan. An Environmental Impact Hearing was held in July 1980. The major focus of this paper will be on the computer modeling used n the studies to define the scope of the problem and to analyze alternatives. The paper first provides a discussion of the setting and background for the study. Then, details of hydrograph and pollutograph development, detailed hydrodynamic modeling in the interceptor systems, and the two-dimensional finite element modeling of pollutant dispersion in San Francisco Bay are briefly discussed. Finally, the paper focuses in more detail on a model which was developed to define the requirements for treatment expansion at the main water pollution control plant (WPCP) to handle wet-weather flows. This simplified model was used to analyze treatment plant effluent quality after construction of the recommended wet-weather facilities. The recommended facilities include relief interceptors, pumping stations, and approximately 40 million gallons of storage capacity located at the extremities of the existing interceptor system. The simplified model was operated with 28-1/2 years of hourly rainfall data. Hourly, daily average, 7-day average, and 30-day average effluent qualities were predicted using the model.