Hydrologic Issues for Dams - Association of State Dam Safety Officials

Procedures and Analysis Technologies used in PMP StudiesThe initial task in performing PMP studies is the identification of extreme stormevents. During the first half of the last centu ry, there appears to have been considerableeffort expended on the identification of extreme storm events as soon as possible afterthey occurred. For the largest of these storms, teams were dispatched to the stormlocation to collect additional rainfall information to aid in storm analyses.During more recent years, identification and study of extreme rainfall storm events hasnot accomplished as extensively. Where there is particular interest, storm studies havebeen completed, e.g. Tropical Storm Alberto in Georgia 1994 and Fort Collins, Colorado1997.Once extreme rainfall producing storms have been identified, the rainfall amountsand patterns need to be evaluated. Isoheytal analyses of the rainfall amounts aredeveloped along with mass curves for rainfall at locations with the heaviest rainfall.Depth-area-duration analyses (D-A-Ds) are completed to quantify the amount of rainfallthat fell over various areas sizes for various durations. These analyses become the basisfor describing the rainfall that actually resulted from the historic storm as well asestimating the maximum rainfall that the storm could have produced.Standard procedures have been developed to increase observed extreme rainfallamounts to estimate the maximum rainfall potential for each historic extreme rainfallstorm. This procedure identifies dewpoint temperatures that are associated with the airmass that provided the atmospheric moisture to the storm that was converted to rain onthe ground. A climatology of maximum dewpoint temperatures is used to determine themaximum dewpoint temperature that could have potentially have been associated withthe storm and could have potentially contributed to an increase in the rainfall. Thecurrent climatology for maximum dewpoint temperatures for the US was published in1965 and provides maximum observed dewpoint temperatures. It was developed from alimited data base using procedures that are not well documented. The most recentHydrometeorological Reports (HMR 57 and HMR 59) as well as recent site-specific PMPstudies have developed updated climatologies to provide values for maximum dewpointtemperatures. These updates provide regional climatologies and use various methods tocompute the maximum dewpoint climatologies used in each study.Historically 12-hour persisting dewpoint temperature values are used in the PMPprocedure. This value provides the maximum dewpoint temperature that has persisted for12 hours or in other words, the minimum dewpoint temperature that occurred during the12 hour period. At least one regional PMP study has use average dewpoint temperaturesaveraged over time period consistent with the duration of the rainfall in an effort to moreeffectively represent the atmospheric moisture available to the storm for conversion torainfall.The procedure for identifying the location for where the dewpoint temperaturesare selected is based on an analysis of the winds fields associated with the stormenvironment. For older storms (pre-1950), surface wind observations have been theprimary data source for determining the inflow wind direction and magnitude. Morerecent storms have occurred when upper air wind analyses derived from weather balloonobservations have been available and those wind fields have been used together with thePaper 15 - Tomlinson 108