DEVELOPING A TYPICAL RAINFALL PERIOD FOR LONG TERM CSO ANALYSIS IN SAN FRANCISCO, CALIFORNIA

This paper describes the selection and development of a continuous long-term rainfall period lasting five years which is currently being used by the City of San Francisco to evaluate sewer flows in a collection system model. The analysis presented in this paper addresses issues such as global climate change as well as spatial variation during storm events.The long-term rainfall was developed by selecting a period that closely matched the average record from the National Weather Service (NWS) rain gage for the past 30 years. The period 1977–1981 was selected because it most closely matched the 30-year average in terms of number of storms in different “bins” of total depth and peak intensity. The rainfall from these years was modified by adding and removing storm events to further improve the match to average conditions.The City of San Francisco's dense rain gage network was used during typical period simulations to take into account spatial variation in storms. The five year period was necessary (as opposed to a shorter one year period) to account for year to year variations at the City rain gages. For example, although a single year may be “typical” at the NWS rain gage, it is not necessarily typical at a local rain gage. Comparisons of the available rainfall indicate that a five year period was sufficient to average out these year-to-year differences.The five year typical period was used to simulate the overflows using a calibrated InfoWorks CS model of the San Francisco collection system. The resulting overflows were compared to the numbers that have reported by the City for the past 20 years. The results of the model simulation indicate that the typical five year period closely reflects average overflow performance.