Scientific Investigations Report 2006-5019
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006-5019
The HEC-RAS computer model (Brunner, 2002a and 2002b; and Warner and others, 2002) was used to construct a surface-water, hydraulic-flow model of the Kootenai River. HEC-RAS is a computer program that simulates 1-D, gradually varied, steady flow in open channels with fixed boundaries. The HEC-RAS model uses the standard step method (Chow, 1959, p. 265) to determine changes in water-surface elevations from one cross section to the next by balancing total energy head at the sections. This 1-D model assumes that energy is uniform in a cross section. This assumption is not valid at locations where flow is not parallel to the main channel or where vertical velocities are significant. The model also assumes that flow is unobstructed within the channel and free of debris. The modeled reach extends from Leonia to Porthill (fig. 1), a distance of 105.6 km.
A total of 164 cross sections were used in HEC-RAS: 131-field-surveyed, 8 Triangulated Irregular Network (TIN) generated, 2 bridges, and 23 interpolated. The field-surveyed cross sections by Barton and others (2004) were used to define channel geometry characteristics. The eight TIN-generated cross sections were developed for the Shorty Island side channel, and the two bridge cross sections were generated from bridge design plans. Distances between cross sections ranged from about 180 m in the valley flat near Deep Creek and Shorty Island to as much as 2.4 km in other areas. Cross sections in the white sturgeon spawning habitat reach are about 300 m apart.
The model was calibrated to surface-water elevation at specific discharges at five gaging stations in the study reach. Nine calibration points were identified where discharge and water-surface elevation remained steady throughout the study reach for at least several days. Calibrated water-surface elevations ranged from about 531.3 to about 536.2 m, and discharge used in calibration ranged from 5,000 to 47,500 ft3/s. Model calibration was considered acceptable because differences between measured and simulated water-surface elevations was ±0.031 m or less. Measured and simulated average velocities also were compared at selected cross sections. These comparisons were acceptable because differences between measured and simulated average velocities were 0.076 m/s or less.
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U.S. Department of the Interior |
U.S. Geological Survey
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