Investigation of the need for discharge adjustments for unsteady flow at selected gaging stations on streams in Tennessee

U.S. Geological Survey, Water Resources Investigations Report 84-4144

by Braxtel L. Neely and Roy H. Bingham

This report is available as a pdf below


Measured discharges made during rising or falling stages generally require adjustments to refine constant-stage rating curves (stage-discharge relations) for gaged sites. Measurements at 42 gaging stations on Tennessee streams were selected for adjustment. Two methods, generally accepted by the U.S. Geological Survey, were tested for adjusting the measurements: (1) the slope method adjusts the discharge for changing slope during the measurement and (2) the storage method adjusts for change in channel storage. The initial results of the storage method of adjustments were unsatisfactory because of uncertainty in determining channel storage at each site, and a specific trend in the results could not be defined. Consequently, the storage method of adjustment was deleted from successive analyses. Both methods are related to the rate of change in stage. Maximum adjustments in measured discharge using the slope method were less than 5 percent at 28 of the 42 stations and less than 10 percent at 39 of the 42 stations used in the analyses. The adjustments were small because most of the measurements were made during nearly stable stage. Adjustments of measurements made during stage changes of several feet per hour could be considerably larger than 10 percent.

Stage records at 10 stations showed that maximum change in stage during rises were considerably higher than the change in stage observed during the measurements. Large adjustments are usually required for a short period of time on the rising side of a hydrograph and smaller adjustments are required for a longer period of time on the falling side. Maximum adjustments in discharge using the slope method were less than 5 percent at three of the 10 stations and less than 30 percent at eight of the 10 stations used in the analyses. The mean discharge for the adjusted and unadjusted hydrographs are about the same.

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