USGS Continues to Analyze Flooding Effects from Hurricanes Frances and Ivan in Western North Carolina, 2004

The U.S. Geological Survey (USGS) operates more than 200 streamgages across the State of North Carolina to monitor river stage and streamflow. During flooding from Hurricanes Frances and Ivan in September 2004, period-of-record peak river stages were recorded at more than 20 sites in western North Carolina.

Many of the USGS streamgages in the Catawba, French Broad, and Pigeon River basins recorded period-of-record high stages after the passage of Frances. Several of these peaks were then exceeded following Hurricane Ivan at locations in the Pigeon River basin in North Carolina. Some of these streamgages were active during the floods of 1916 and 1940, yet flooding in September 2004 eclipsed many of the earlier events, which were considered the benchmarks for all floods measured since then in the mountains of North Carolina.

USGS streamgaging stations continually monitor river stage, but USGS staff must visit the sites during flood events and directly measure the streamflow with specialized measuring equipment. Sometimes peak streamflow cannot be measured directly because of the nature of the storm. Flashy peaks, which are more typical in mountain streams, can rise and fall before staff can safely travel to the site. In addition, extremely high stream velocities or large volumes of debris in the flow can create unsafe measuring conditions at the site. When these situations occur, peak streamflow is indirectly measured from high-water mark data collected after the event and subsequent surveys of stream channels and estimation of hydraulic conditions during the event. Since the passage of Hurricanes Frances and Ivan, USGS staff have been working on the computations of these indirect measurements of streamflow. Preliminary results of these analyses for selected sites in western North Carolina are listed below (table 1). The peak river stages recorded by the gage and peak streamflows determined from analysis of direct or indirect measurements are provided.

Peak river stage information is helpful in putting storms in historical context and determining the extent of flooding across the landscape, but peak streamflow (the volume of water passing a given location at the peak of an event) is critical in determining the recurrence interval or relative frequency of such an event. This information also is provided in table 1. Flood-frequency statistics, which report the streamflow values associated with the 100-year flood and the 2-, 5-, 10-, 25-, 50-, 200-, and 500-year events, generally are updated for USGS streamgaging stations about every 10 years. The most recent compilation of these statistics is available online at http://nc.water.usgs.gov/floodstats/ and includes information on the annual peaks for the period of record at each streamgage through 1996. (More information on flood-frequency analysis and flood forecasting is available online at http://water.usgs.gov/pubs/fs/FS-229-96/ and http://water.usgs.gov/wid/FS_209-95/mason-weiger.html )

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U.S. Department of the Interior | U.S. Geological Survey URL: http://nc.water.usgs.gov/info/news_release/floods04.html Page Contact Information: North Carolina Web Development Team Page Last Modified: Thursday, December 16 2004, 04:52:40 PM

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