Scientific Investigations Report 2006-5023
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006-5023
Parks Highway Bridge at Nenana, Alaska, crosses a hydraulically complex reach of the Tanana River. The Tanana River is divided into two channels, the main channel and the slough channel, and both are crossed by bridges. The Nenana River enters the main channel several hundred feet downstream of the bridge and affects flow at the bridge crossing. A large pier at the bridge over the main channel is at an angle to the flow.
This site was selected for a comprehensive scour assessment that included a test case for a two-dimensional hydraulic model because of the site’s complexity and preliminary study results. The site was fully surveyed and modeled with a one-dimensional hydraulic model, HEC-RAS. The geometry data then were input into a two-dimensional model, RMA2. Output from both models was used to calculate pier-scour estimates. Contraction scour was calculated using only output from the one-dimensional model.
A high discharge measurement made on August 17, 1967, at the Tanana River, was used to calibrate both models. The models were run with four discharge scenarios using discharge in the Tanana River for 100- and 500-year recurrence intervals, and, because discharge in the Nenana River has a considerable effect on the flow conditions at the bridge, using low and high discharges in the Nenana River. Discharge measured in the Nenana River during the study survey was used for low flow and the discharge for a 100-year recurrence interval in the Nenana River was used as high flow.
The models also were used to simulate the discharge at the time of the survey. The one-dimensional model did relatively well using the same roughness values as used for the higher flows. The simulated water surface was 0.5 to 1 foot higher than that surveyed in the field. The two-dimensional model required a reduction in roughness to properly simulate the lower flow. Surveyed data and computational mesh were too coarse, creating an artificial roughness from the jagged edges. This shortcoming was offset by reducing the roughness factor. Additional surveying and a refinement of the mesh would be required to use the two-dimensional model for reliable low-flow simulations.
Water-surface profiles computed by each model were similarly shaped. The greatest difference in water surface elevation was less than 1.5 feet, near the mouth of the Nenana River, the area with the greatest horizontal two-dimensional flow variability. In reaches where flow directions are not as variable, the shape and slope of the profiles match closely. In scenarios with high discharge in the Nenana River, water surfaces upstream of the bridge were higher in the two-dimensional model than those in the one-dimensional model. This is due to the increase in cross-channel flow, which was simulated by the two-dimensional model but not by the one-dimensional model.
The models also determined the division of flow between the Tanana River main channel and the slough channel. With low flow in the Nenana River, the flows in the slough were similar between the models. For high flows in the Nenana River, the two-dimensional model simulated significantly higher flows in the slough because of the higher computed water surface upstream of the bridge. Pier-scour depth was calculated for all flood scenarios using output from both models. Estimates of pier-scour depth from the two-dimensional model output were within 6 percent of those from the one-dimensional model output.
Flow characteristics near the mouth of the Nenana River and upstream of the bridge over the main channel varied considerably between the two models. However, results from the scour calculations for the two-dimensional model were slightly higher, but within 5 percent of those from the one-dimensional model. The ability to compute an angle of attack from the two-dimensional model output rather than estimating an angle for the one-dimensional output greatly improved reliability of the scour estimate. Although the final results were similar, the two-dimensional model provided additional information about flow characteristics throughout the study reach and would be more useful for design or analysis than the one-dimensional model.
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
Persistent URL: https://pubs.water.usgs.gov/sir200695023
Page Contact Information: Publications Team
Page Last Modified: Thursday, 01-Dec-2016 18:59:24 EST
