Probable Maximum Flood (PMF) Analyses and Its Impacts on Design and Planning of Dam Structures

In recent years, many states have implemented new dam safety regulations requiring re-evaluation of high hazard dams infrastructure using PMF. In this paper an effort will be made to explain the concept of (PMF) from dam owner perspective. Methods to calculate key parameters in analyses of PMF e.g. probable maximum precipitation (PMP), rain fall distribution, watershed time of concentration and curve number will be presented. One (1) case study will be presented to explain the sensitivity of results to the time of concentration and rainfall distribution. Elliot Dam located in western Prince William County, Virginia and owned by the City of Manassas is required to be in compliance with the September 2008 DCR, Division of Dam Safety and Floodplain Management, Virginia Impounding Structures Regulations (rules). Based on the rules, the dam should safely pass the flood resulting from 100-percent of the Probable Maximum Precipitation (PMP). The calculation of the time of concentration and lag time is challenging for events as extreme as the PMF as standard methodology for calculation was developed from observations of rainfall and runoff for more frequent (2 to 100-year) recurrence interval events. Four methods were used to calculate the watershed time of concentration and lag time including the TR-55 segmental approach, NRCS lag time equation, Watt & Chow equation, and the Anderson Method. PMF generated using these four methods will be presented and compared. 10 pp., 23 references.