Aftermath of Hurricane Fran in North Carolina --
Determination of Peak Flows
River stage and flow (or discharge) are measured at locations called stream-gaging stations. Stage is generally measured continuously and reported to an accuracy of 0.01 foot. Stage information is transmitted by satellite radio to USGS computers, or is stored on site and retrieved periodically by USGS staff.
Flow is more difficult to measure accurately and continuously than is stage. Discharge for a gaging station is typically determined from a pre-established stage-discharge relation, or rating curve. Individual discharge measurements are made at the gaging station using standard procedures, ideally during a time when the river stage is not changing. A series of these measurements made over a range of flow conditions defines the rating curve, which is used to convert continuous measurements of stage to a continuous record of flow. Channel changes, resulting from scour, deposition, or other processes, alter the stage-discharge relation, so that discharge measurements must be made routinely to ensure that the rating curve remains accurate.
The rating curve is considered accurate only over the range for which discharge measurements have been made. Discharge measurements generally are not available for the full range of flows at gaging stations which have been in operation for only a few years. Even at gaging stations which have been in continuous operation for 30 years or more, discharge measurements for extremely high flows, such as those that occurred following Hurricane Fran, are difficult to obtain because (1) these events are rare, (2) peak flows may persist for only a short time making measurement difficult, and (3) measurement sites often are inaccessible during extremely high flows.
Estimates of peak flows, which are outside the range of the established rating curve, may be made by an extrapolation of the rating curve to the peak stage. Many of the peak flows reported in table 1 and used in the loading calculations (table 2) were estimated in this manner. At some gaging stations having large rating curve extensions, indirect methods of discharge determination based on channel properties and hydraulic principles may be used to obtain an independent estimate of the peak flow. These indirect methods generally require accurate determination of channel morphology through surveying and application of computer programs, and may require several months to complete for all of the affected gaging stations. Consequently, continuing evaluation of Hurricane Fran flood records may result in some slight revision of the peak flows presented in this report.
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