Wastewater agencies traditionally use a design storm approach to predict how wet weather influences flows and to define the system capacity required for avoiding CSOs and SSOs. When using the design storm approach, it is essential to understand the relationship between rainfall and
wastewater flows. There are two important concepts in developing this understanding. The first is the return frequency of a storm as opposed to the return frequency of a flow in a sewer collection system as a result of the storm. The return period or frequency of a storm event may have little
or no correlation to the return period of the peak flow in the wastewater system. The second concept is the distinction between a meteorological storm event and a storm event as it relates to generating peak flows in a sewer. Understanding these concepts enables the successful selection of
design storm parameters including storm duration, rainfall depth, and rainfall pattern, as illustrated by experiences in Bremerton and Everett, Washington. Recommendations for how to select a design storm include carefully reviewing historical storm events and flow responses to determine system
responses, verifying historical rainfall depths with a frequency analysis, and selecting storm patterns that have a higher intensity of rainfall toward the end of the storm.
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