Scientific Investigations Report 2006-5023
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006-5023
Water-surface profiles from each model are shown in figure 6. The water-surface elevations from RMA2 are from the approximate center of the main channel. Those from HEC-RAS are the average water surface from each cross section. The largest difference in the profiles was less than 1.5 ft, just downstream of the main bridge (at 1.18 mi upstream of the downstream boundary), where the models are affected most by the two-dimensional nature of the flow. The difference between the models is much greater when the discharge in the Nenana River is high (fig. 6B).
When the Nenana River has relatively low discharge and the Tanana River discharge is high, the flow in the main channel of the Tanana River appears as parallel velocity vectors in the two-dimensional model (figs. 7A, 7C). When the Nenana River discharge is high, the velocity vectors indicate that it forces flow in the Tanana River to cross the channel from left to right (figs. 7B, 7D). Once the flow vectors return to mainly parallel at about 0.75 mi upstream of the downstream boundary, the two models match reasonably well. The difference in flow patterns at the convergence of the channels between the models with low and high flow in the Nenana River is shown in figure 7.
The profiles show some variation in the reach upstream of the bridge (fig. 6B). RMA2 computed a larger backwater effect from the increased discharge in the Nenana River. When the Nenana River is low and the flow is mostly one-dimensional, then the RMA2 and HEC-RAS profiles are quite similar (fig. 6B).
Comparing the flows in Tanana Slough also demonstrates the difference in backwater simulated by each model (fig. 8). Simulations were similar for scenarios with low discharge in the Nenana River, but RMA2 simulated considerably more flow in the slough for scenarios with high discharge in the Nenana River.
Pier-scour depth calculated using output from the two-dimensional model was slightly greater than that using the output from the one-dimensional model, because the two-dimensional model computed slightly different angles of attack than were used in the one-dimensional model calculations (fig. 9). Differences in values were less than 6 percent and were negligible.
Streamflow velocities along the cross section at river mile 1.377, measured on August 19, 1998, and calculated from each model type also were used to compare the models (fig. 10). Velocities for 1998 were measured with a Price AA current meter using standard USGS methods (Buchanan and Somers, 1969). Velocities calculated from the two-dimensional model are the total velocity magnitude. Flow direction at this cross section is perpendicular to the cross section, so neither velocity data set was adjusted for angles. Velocity calculated from the one-dimensional model is the average velocity for the cross section. Velocities from all three sources match well.
For more information about USGS activities in Alaska, visit the USGS Alaska Water Science Center home page.
U.S.
Department of the Interior | U.S. Geological
Survey
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