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<br />FINAL REPORT, November 2003 <br />High-jlow Requirements for the Duchesne River <br /> <br />calibration data impossible that year. Peak runoff in 2001 was also below normal, but peak <br />water surface measurements at discharges up to 2,450 fe/s were obtained in May. Cross section <br />geometry was not re-surveyed at high flow. <br />The 24-hour Camp site consists of 10 monumented cross sections and five supplementary <br />cross sections (Figure 9). Stage observations most suitable for high-flow calibration at 24-hr <br />Camp were made May 26,2001 when discharge was 2,450 ft3/s and May 22,2001 when <br />discharge was 1,050 ft3/s. Five of the 24-hour Camp cross sections traverse a chute channel <br />active at moderate discharges. The chute portions of these cross sections were separated from <br />the main channel portions and used to construct an independent stream segment connected to the <br />main channel at upstream and downstream junctions. Flow in the chute began when main <br />channel discharge was about 1,050 ft3/s, and discharge in the chute was measured at 58 fe/s <br />when main channel discharge was 2,600 ft3 Is. <br />Five monumented cross sections were installed at Above Pipeline (Figure 10). A single <br />stage observation above base flow was obtained for the cross sections at the Above Pipeline site <br />on May 23, 2001 when discharge was 770 ft3/s. In spite of the small number of cross sections at <br />this site and the small discharge used in calibration, we believe that HEC-RAS modeling <br />provides a more robust method of estimating stage-discharge relationships at cross sections than <br />considering single cross sections in isolation. <br />At Wissiup Return, we installed seven monumented cross sections and three <br />supplementary cross sections (Figure 11). Useful stage observations were made at this site on <br />May 28,2001 when discharge was 1,840 ft3/s and May 16,2001 when discharge was 1,040 fe/s. <br />The Wissiup Return model also includes an independent chute channel segment consisting of <br />portions of six cross sections. This chute becomes active at higher flows than the chute modeled <br />at 24-hour Camp. The chute was conveying little flow when main channel discharge was 1,840 <br />ft3 Is, which is the highest discharge observed at the site. <br />Extrapolating observed stages to larger discharges at a single cross section, either by <br />applying a uniform flow equation such as the Manning equation or by fitting observed data to a <br />curve, does not take into account the effects of changes in downstream hydraulic controls that <br />may occur with changing discharge. Use of the HEC-RAS model allowed us to include the <br />effects of downstream controls on upstream cros-s sections. We developed one stage-discharge <br />relationship to serve as a downstream boundary condition for each modeled reach. Assuming <br /> <br />25 <br />