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Site-Specific Probable Maximum Precipitation & Probable Maximum Flood

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Gomez and Sullivan is developing site-specific Probable Maximum Precipitation (PMP) values and Probable Maximum Floods (PMFs) to update the PMFs for three sites on the Connecticut River; the longest river in New England. The projects are located in the upper portion of the Connecticut River Basin, with a total project drainage area of approximately 3,400 square miles situated between the White Mountains and the Green Mountains. Site-specific PMP and PMF studies can be important in assessing dam-safety concerns.

Due to the mountainous location of the study basin, a site-specific approach to determining the Probable Maximum Precipitation (PMP) is ideal for these projects. Standard methods for determining PMP such as Hydrometeorological Report 51 (HMR-51) do not fully address the effects of mountains in blocking moisture inflow. A site-specific study also utilizes actual extreme storms which could occur in the study basin in determining the PMP. For other site-specific studies that Gomez and Sullivan has worked on, the site-specific determination of the PMP has been significantly different from that determined by standard methods.

PMP tasks include evaluation of historic storms, storm maximization and transposition, adjustments for topographic barriers and within basin orographic effects, and development of site-specific PMP envelope curves. GIS was used to evaluate the impact of topographic barriers in diminishing the moisture carried by the studied historic extreme storms transposed to the basin. GIS was also used as an aid in determining factors such as the maximum dewpoint at the storm source which affect the development of the storm.

In order to estimate the PMF for the study basin, a hydrologic model was developed. PMF tasks include calibration of the hydrologic model to large storm events, evaluation of loss rate parameters, unit hydrograph and routing parameters, initial reservoir conditions, gate operating conditions and evaluation/development of updated discharge rating curves. GIS was used in developing the hydrologic model for delineating sub-basins as well as making initial estimates of basin unit hydrograph parameters.

Since cold-season events have produced many of the high flow events in the basin, Gomez and Sullivan is evaluating both warm season and rain-on-snow PMF type events. Calibration of historic rain-on-snow events has included evaluation of temperature, wind, rainfall, solar radiation, and basin cover parameters in calibrating snowmelt parameters. To determine the PMF for the site, the higher of the rain on snow and warm-season PMF is utilized.