Site-Specific Probable Maximum Precipitation for the Murphy Dam Project

The Murphy Dam Project is located on the Connecticut River in northern New Hampshire. The total drainage area upstream of the dam is 170 mile
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. The project is owned and operated by the State of New Hampshire Department of Environmental Services-Water Division. A complete dam safety review was recently completed that included an evaluation of the Probable Maximum Flood (PMF). One aspect of performing a PMF analysis for a dam is associated with the determination of the appropriate Probable Maximum Precipitation (PMP) over the drainage basin.

National Weather Service (NWS) Hydrometeorological Report No. 51 (HMR 51) provides generalized PMP estimates for the United States east of the 105 meridian. The generalized PMP maps are stippled (shaded) in certain mountainous areas to indicate that the generalized PMP estimates might be deficient because detailed terrain effects were not evaluated. HMR 51 recommends that projects within the stippled regions be considered on a case-by-case basis. Furthermore, given the generalized nature of the HMR 51 PMP estimates, development of a site-specific PMP may be warranted for projects that are outside of the stippled areas. This is demonstrated by the influence of specific storms on PMP values in areas outside of their transposition limits due to the smoothing processes employed in HMR 51. Therefore, for the Murphy Dam PMF analysis, it was decided to develop a basin site-specific PMP.

Site-specific PMP values were derived for the Murphy Dam drainage area using standard accepted methods for site-specific PMP studies. The study built upon previous site-specific PMP evaluations performed in New England. The study included PMP candidate storm selection, storm maximization, transposition, envelopment and smoothing.

The results of the site-specific analysis provide an array of PMP rainfall depths for various area and duration combinations. The site-specific PMP was then used to develop the PMF for the Murphy Dam Project. For comparison to the generalized NWS PMP estimates, the 24-hour, 200-mile
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value is used since it is close to the project drainage area and bounds the basin response time. The Murphy Dam Project site-specific PMP was 13.7 inches as compared to 18.5 inches from HMR 51. This reduction to the generalized PMP is similar in magnitude to that of a FERC-approved site-specific PMP study for a similar sized basin located in southern Vermont.

The results of this study along with those of several other site-specific PMP studies in New England support consideration of similar studies for other projects. 8 pp., 12 references.