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Adjustments to U.S. Geological Survey Peak-Flow Magnitude-Frequency Relations in Delaware and Maryland Following Hurricane Floyd, September 1999

By Edward J. Doheny and Jonathan J.A. Dillow

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On September 16, 1999, Hurricane Floyd entered the lower half of Maryland’s Eastern Shore and followed a northeasterly track over the lower part of the Delmarva Peninsula. Total rainfall amounts varied across the region. Some areas received more than 10 inches of rain, while other areas received less than 5 inches. The heaviest recorded rainfall occurred in the central part of Maryland’s Eastern Shore, where a maximum of 12.59 inches was reported at Chestertown, Maryland (Tallman and Fisher, 2001).

Record-high peak discharges were recorded at several active streamflow-gaging stations (referred to as "gages" in this report) in northern Delaware and on the upper Eastern Shore of Maryland. U.S. Geological Survey (USGS) personnel also surveyed high-water marks to estimate peak-flow magnitudes for the storm at selected discontinued gages in the region (Tallman and Fisher, 2001).

When severe flooding occurs, information about peak-flow magnitude and frequency of recurrence must be disseminated quickly to Federal, State, and local agencies concerned with public safety and emergency response, as well as to the public. The initial estimates of peak-flow frequency associated with a particular flood are based on peak-flow magnitude-frequency relations established prior to the flood. These relations have not incorporated the effect of the peak flows associated with the flood because peak-flow data must be reviewed for accuracy before being published or used in any analyses. As a result, it may take several months to incorporate new peak-flow data and revise the peak-flow magnitude-frequency relation for a given stream location. If the new peak flows associated with a flood are significantly greater than previously recorded or historical peak flows at a specific gage, adjustments reflecting the inclusion of the peak flows can cause significant changes to the magnitude-frequency relations.

This report summarizes the basic principles that are used to derive peak-flow magnitude-frequency relations, and discusses the adjustment techniques that may be applied to refine these relations. A comparison of the initial peak-flow recurrence-interval estimates and subsequent, adjusted estimates resulting from peak flows associated with Hurricane Floyd in 1999 for 28 gages in Delaware and Maryland are also presented.

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