Introduction

The design of high-hazard structures, such as spillways on major dams or nuclear power plants located on floodplains, requires assessment of the largest flood-induced forces that a structure may need to withstand during its lifetime. Under standard engineering practices, determination of maximum forces is made in two basic steps. First, estimates are made of the maximum rainfall anticipated for the drainage basin under consideration for time scales appropriate to flood production in the basin. Second, the maximum rainfall is converted to river discharge, velocities, and shear stresses using numerical models. Estimates of maximum rainfall for drainage basins are thus a direct input to engineering design for high-hazard structures. Procedures for determining rainfall extremes remain a subject of debate. This report provides the results of a brief assessment of estimated bounds on extreme precipitation events.

Maximum observed rainfalls worldwide (see Figure 1) provide some guidance for upper bounds on rainfall. Truly remarkable raingauge accumulations have been recorded (WMO 1986), including:

  • 31 millimeters of rainfall in 1 minute at Unionville, Maryland, on 4 July 1956;

  • 38 millimeters in 1 minute at Barot, Guadeloupe, on 26 November 1970;

  • 800 millimeters in 4.5 hours at Smethport, Pennsylvania, on 18 July 1942; and



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Estimating Bounds on Extreme Precipitation Events: A Brief Assessment Introduction The design of high-hazard structures, such as spillways on major dams or nuclear power plants located on floodplains, requires assessment of the largest flood-induced forces that a structure may need to withstand during its lifetime. Under standard engineering practices, determination of maximum forces is made in two basic steps. First, estimates are made of the maximum rainfall anticipated for the drainage basin under consideration for time scales appropriate to flood production in the basin. Second, the maximum rainfall is converted to river discharge, velocities, and shear stresses using numerical models. Estimates of maximum rainfall for drainage basins are thus a direct input to engineering design for high-hazard structures. Procedures for determining rainfall extremes remain a subject of debate. This report provides the results of a brief assessment of estimated bounds on extreme precipitation events. Maximum observed rainfalls worldwide (see Figure 1) provide some guidance for upper bounds on rainfall. Truly remarkable raingauge accumulations have been recorded (WMO 1986), including: 31 millimeters of rainfall in 1 minute at Unionville, Maryland, on 4 July 1956; 38 millimeters in 1 minute at Barot, Guadeloupe, on 26 November 1970; 800 millimeters in 4.5 hours at Smethport, Pennsylvania, on 18 July 1942; and

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Estimating Bounds on Extreme Precipitation Events: A Brief Assessment 1800 millimeters (about the height of an adult) in 24 hours at Foc Foc, La Réunion (a small island in the Indian Ocean, east of Madagascar), on 15–16 March 1952. However, not all places on our planet are likely to experience such extremes. If we examine the hypothetical problem of estimating bounds on rainfall in Washington, D.C., these observations would have different degrees of relevance. The 1-minute record accumulation from Unionville, Maryland, which is within commuting distance of Washington, D.C., is probably a minimum value for the upper bound on 1-minute rainfall accumulation for the D.C. area. The 1800-millimeter accumulation during 24 hours from the Indian Ocean site is less applicable. Beyond the simple criterion of proximity, there are physical arguments that can be made to discount the possibility that observations from the island of La Réunion represent processes that could occur in the Washington, D.C., area. These arguments concern both the meteorology of storms in the two areas (La Réunion is subject to numerous tropical cyclones) and the contrasting physical settings (the island of La Réunion has 3000-meter mountains in close proximity to open ocean). While it is easy to dismiss the La Réunion observation for applications to Washington, D.C., the Smethport, Pennsylvania, observation is more problematic. Questions relating to proximity, physical setting, and storm type arise in deciding how to use the Smethport observation in assessing rainfall bounds in Washington, D.C. There are currently over 10,000 high-hazard dams in the United States (FEMA 1993). The vast majority of these dams are less than 15 meters in height and are located in watersheds with less than 100 square kilometers of tributary drainage area. A much smaller group of very large dams represent a very large infrastructure investment. These very large dams are usually located on major rivers with tributary drainage areas from 1000 to over 40,000 square kilometers. Thus, estimates of extreme precipitation are needed for a wide range of storm areal coverages. The technology and procedures used for assessing bounds on precipitation have remained relatively unchanged during the past 30 years. The most widely used methodology is based on the Probable Maximum Precipitation (PMP), which is an estimate of the greatest amount of precipitation possible for a given place within a given amount of time. During the past three decades there have been marked advances in the science of meteorology and in observations of precipitation, especially through remote sensing technologies. This report examines whether these advances can be translated into advances in technology and proce-

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Estimating Bounds on Extreme Precipitation Events: A Brief Assessment dures used for engineering designs that rely on estimates of the magnitude and likelihood of extreme precipitation events. Major studies of PMP methodology have been carried out in recent years. The Electric Power Research Institute (EPRI) commissioned a series of research projects (EPRI 1993a, b, c, 1994) that examined the potential of new procedures and databases for PMP analyses. Particular attention was given to radar, satellite, and paleohydrologic data. The Federal Emergency Management Agency (FEMA) held a workshop on PMP and the related measure of Probable Maximum Flood (PMF) in 1990. Workshop participants recommended the continued use of PMP but suggested areas for review and research (FEMA 1990). Previous National Research Council (NRC) efforts have examined similar issues. A 1985 NRC report, Safety of Dams, provides a good general context for the use of PMP as it pertains to dam design and an extended summary of how PMP is calculated (NRC 1985, Appendix C). That report recommends continued use of PMF based on PMP. However, it also recommends research into estimates of probabilities of extreme rainfalls in specific drainage basins. Techniques for a probabilistic approach were examined in Estimating the Probabilities of Extreme Floods (NRC 1988). The program suggested by the recommendations in that report has largely not yet been executed. The NRC Committee on Meteorological Analysis, Prediction, and Research (CMAPR) has examined related topics; in particular, research suggestions made in Advancing theUnderstanding and Forecasting of Mesoscale Weather in the United States (NRC 1990) and Coastal Meteorology—A Review of the State of the Science (NRC 1992) would greatly contribute to our understanding of extreme precipitation events. CHARGE TO THE COMMITTEE The CMAPR was asked by the Federal Energy Regulatory Commission to perform a preliminary assessment of probabilities and bounds on extreme precipitation events. Specifically, the committee was requested to: evaluate current scientific understanding of extreme precipitation events; evaluate the status of the measure PMP; and examine alternatives to PMP for characterizing extreme precipitation events, as well as new technologies that might provide a stronger basis for determining PMP or its alternatives. Since there were only sufficient resources for a limited assessment, the

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Estimating Bounds on Extreme Precipitation Events: A Brief Assessment Committee was asked to determine whether further study by the NRC of issues involved in estimating the probabilities and bounds on extreme precipitation is warranted and, if so, to make recommendations for the extent and goals of such a study. PMP OVERVIEW Probable maximum precipitation is defined by the World Meteorological Organization as “theoretically the greatest depth of precipitation for a given duration that is physically possible over a given size storm area at a particular geographical location at a certain time of year” (WMO 1986). This definition represents PMP as a physical upper bound on precipitation accumulation, with the magnitude of PMP depending on location, duration, and storm area. The procedures for estimation of PMP were developed in the United States largely as meteorological analyses (see Myers, 1967, for a historical overview of PMP). Showalter and Solot (1942) classified the physical limitations on rainfall rate in terms of the following elements: a limit on the precipitable water (that is, the vertically integrated water vapor density) in the air that flows above a drainage basin, a limit on the rate at which wind can carry water vapor over the basin, and a limit on the fraction of water vapor that can be converted to surface precipitation. Attempts to directly determine physical limits on each of these elements and combine them to produce a first-principles assessment of PMP have not proven successful. The last two elements, involving moisture convergence and precipitation efficiency, have proven especially intractable. In practice, PMP is often defined by the procedures used to calculate it. The WMO definition of PMP is supplemented by the following (WMO 1986): An operational definition [of PMP] may be considered as consisting of the steps followed by hydrometeorologists in arriving at the answers supplied to engineers or hydrologists for hydrological design purposes. Whatever the philosophical objections to the concept, the operational definition leads to answers that have been examined thoroughly by competent meteorologists, engineers, and hydrologists and judged as meeting the requirements of a design criterion with virtually no risk of being exceeded. The result of applying the operational definition over an entire region is to approach uniformity in design, safety and cost.

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Estimating Bounds on Extreme Precipitation Events: A Brief Assessment The definition of PMP that appears in the Glossary of Meteorology of the American Meteorological Society (1959) incorporates both WMO definitions. PMP is defined as “the theoretical greatest depth of precipitation for a given duration that is physically possible over a particular drainage area at a certain time of year. In practice this is derived over flat terrain by storm transposition and moisture adjustment to observed storm patterns.” The dual definition of PMP as a physical upper limit of precipitation and as the collection of procedures used to compute an upper limit has created confusion and hindered procedural developments. Furthermore, there is some debate over the existence or usefulness of an absolute upper limit. The primary use of PMP is as input to the calculation of PMF. This calculation requires further consideration of both hydrometeorological and hydrological processes, such as series of events, underlying conditions, and topography. For example, heavy spring rains may be accompanied by rapid snow melt. Variations in topography can sometimes lead to great variations in runoff rates. Accordingly, for some basins, more accurate determinations of PMP might not be as important for improving engineering practices as more complete understanding of runoff processes. As defined and determined, PMP and PMF are conservative measures designed to provide a high degree of safety when used to set engineering standards. Recognizing this, current engineering practices allow use of specified fractions of PMF when designing structures that lead to only low hazards. However, these ad hoc adjustments and the applications of PMP and PMF are beyond the scope of this study. In concept, PMP is limited to examinations of single storm events. We recently witnessed dramatic events in the Midwest during 1993 when the Mississippi River and Missouri River basins experienced many closely timed, large-scale rainstorms that fell on saturated soils with already swollen rivers, leading to widespread flooding. In this brief report, discussion is limited to the hydrometeorology of extreme precipitation and, more specifically, what might be considered single meteorological events. It must be remembered that those who make use of information on extreme precipitation events still need to allow for a wide range of additional conditions and possibilities.

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