(53t) Going Beyond Design Basis Floods

The design of all facilities, implicitly or explicitly, is based on a design-basis flooding event. The U.S. government has conducted a thorough analysis and provides the possibility of flooding events at various parts of the country in terms of return periods (e.g., 100 year flooding). Recent devastating events in Japan and Houston have shown the potential for rainfall and flooding events that go well beyond design bases of the facilities or a degradation in the design bases making the facility more vulnerable to such events. In this article, the following topics are addressed:

(a) The proper interpretation of return periods is discussed to help the analysts use such frequencies in Process Hazard Analysis (PHA) and Layer of Protection Analysis (LOPA).

(b) A method is presented for estimating site-specific flooding event frequencies including very rare, severe events.

(c) An overall method for analyzing a plant for the impact of severe flooding event Specific examples will be provided for each topic discussed.