Comparison of Temporal Rainfall Distributions for Near Probable Maximum Precipitation Storm Events for Dam Design

The Natural Resources Conservation Service (NRCS) has assisted local sponsors with analysis, design and construction of approximately 26,000 of the 80,000 dams in the National Inventory of Dams. Many of these dams are reaching their design life and require rehabilitation. Rehabilitation efforts come at a time when NRCS is in the process of evaluating a potential change from a minimum 6-hour storm duration design criteria east of the 105 principal meridian to either 24-hour duration criteria or a multiple duration criteria. The change in long standing NRCS criteria results from a number of both internal and external considerations, which will be enumerated in this paper. This paper will focus on the 24-hour duration but the same procedures and process presented may also be used for storm duration other than 24-hours. This paper will evaluate current and proposed NRCS precipitation distributions as well as other distributions that are considered appropriate for use with 24-hour Probable Maximum Precipitation (PMP) events. Potential 24-hour rainfall distributions were narrowed to four distributions through preliminary screening, which will be evaluated in greater detail. These four distributions are: 1) The 24-hour version of the NRCS standard emergency spillway and freeboard hydrograph (ESFB) rainfall distribution, 2) The five-point PMP distribution, 3) The critically stacked 24-hour PMP distribution and 4) The temporal rainfall distribution based on the maximum rainfalls recorded on world scale. The merits and deficiencies of each procedure will be evaluated. The importance of the period of maximum intensity (i.e. maximum 1-hour) and its placement within each distribution will be evaluated. A data set of seventy-two (72) NRCS Floodwater Retarding Structures will be evaluated using 24-hour duration PMP or near PMP rainfall. The spillway flow depths of the four distribution patterns are compared to the current NRCS 6-hour duration design for comparative purposes. The NRCS SITES computer model was used to determine the spillway flow depths for each scenario.