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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5159

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Recalculating the 1921 Peak Discharge

The final n value of 0.033 for the main channel areas that was calculated for the 2003 flood and the slope for the 2006 flood was used to recalculate the 1921 peak discharge using cross-section geometry, reach length, and fall from Stewart’s 1923 survey (James Stewart, U.S. Geological survey, written commun., November 1923). For this recalculation, XS2 from the 1923 survey was subdivided into two areas at station 256 (see fig. 3) based on the guidelines discussed previously. Even though XS2 was subdivided, an n value of 0.033 was used for both subareas because the island/bar was assumed to be bare of vegetation at the time of the 1921 peak with similar roughness elements as the main channel. The resulting recalculated peak discharge is 219,000 ft³/s.

A re-examination of the n-verification done by Flynn and Benson (U.S. Geological Survey, written commun., August 1952) using the 1949 flood data also computed an n value that could be used to recalculate the 1921 peak discharge. In both recalculations, Stewart’s surveyed data for the reach between XS2 and XS3 were used with the different n values to calculate different peak discharges for the 1921 flood.

Recalculation of the 1921 peak using an n value calculated from 2003 and 2006 data assumes that the channel roughness conditions and hydraulics of the reach have not changed since 1921. However, this assumption may not be valid because the island/bar that probably was bare of vegetation in 1921 has built up slightly from deposition (fig. 3) and is now densely forested (Stewart’s survey notes for his indirect measurements in 1923 do not mention anything about the vegetative character of the island). This alteration of the island/bar probably has complicated the flow hydraulics, which makes the recalculation of the 1921 flood using 2003 and 2006 flood data less reliable.

A more reliable recalculation of the 1921 flood peak may be obtained using the analysis of the 1949 flood because the conditions in the reach in 1949 may have been more similar to the conditions in 1921. In fact, a 1932 and a 1948 photograph show the island/bar mostly bare of vegetation (fig. 4). The 1948 photograph indicates that the influence of vegetation on the island/bar was minimal prior to the November 1949 flood that was used for the n-verification study by Flynn and Benson (U.S. Geological Survey, written commun., August 1952).

Flynn and Benson (F.J. Flynn and M.A. Benson, U.S. Geological Survey, written commun., August 1952) computed an n value of 0.0305 for the 1949 flood data, and using that n value, Flynn and Benson recalculated the 1921 peak discharge to be 225,000 ft³/s. They surveyed four cross sections labeled A through D in downstream order beginning with A at the upstream end about 300 ft downstream of Stewart’s cross section 1. Cross sections B and C fell on either side of Stewart’s cross section 2 and cross section D was about 700 ft upstream of Stewart’s cross section 3. Reach A-B was not used because of the large expansion in the reach. The surveyed HWMs provided a reasonably accurate water-surface profile for the 1949 flood (fig. 5); however, the analysis did not subdivide any of the cross sections.

Looking at the shape of cross sections B and C (fig. 6), and using the guidelines that are now available for subdividing (Davidian, 1984), the cross sections clearly needed to be subdivided. Based on these guidelines, cross sections B and C were subdivided and the 1949 analysis was recomputed in order to come up with a revised n value for the 1949 study. Through trial and error with the SAC program using different n values, an n value of 0.0315 provided a calculated discharge of 154,000 ft³/s for the reach from B to D, which is very near the 1949 flood discharge of 153,000 ft³/s. Using this n value in the recalculation of the 1921 flood using Stewart’s data with a subdivided cross section 2 as described above, resulted in a discharge of 228,000 ft³/s.

In summary, two recalculations of the 1921 peak discharge based on two n-verification analyses, one using 1949 peak-flow data and one using the 2003 and 2006 peak-flow data, both resulted in discharges less than Stewart’s original calculated discharge. The recalculated value of 228,000 ft³/s for the 1921 peak discharge that used 1949 peak-flow data and subdividing techniques is 5.0 percent less than Stewart’s original value of 240,000 ft³/s and the recalculated value of 219,000 ft³/s that used 2003 and 2006 peak-flow data and subdividing techniques is 8.8 percent less than Stewart’s original value. The slope-area recalculation of the 1921 peak flow using the n value from1949 peak n-verification analysis and subdivision is considered a fair measurement within 15 percent of the actual value (Benson and Dalrymple, 1967). Note that Stewart’s calculation falls well within the error bounds of this recalculated peak discharge. The roughness due to vegetation on the island/bar in 1949 is likely to have been minimal as it was in 1921, but much different than the 2003 or 2006 peak flows. Due to the complication of the hydraulics of flow through and around the forested island/bar, more confidence can be given to the recalculation of the 1921 peak based on the 1949 peak when the reach conditions were more similar to 1921 than in 2003 or 2006. The recalculation of the 1921 peak discharge using the 2003 and 2006 peak discharge information is only 2.7 percent less than the recalculation based on the 1949 peak discharge information. Despite some doubt in the accuracy due to the complications of accounting for the hydraulics around and through the forested island/bar, the recalculation of the 1921 peak using 2003 and 2006 information supports the assertion that Stewart’s original value for the 1921 peak is too high.

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