Abstract

The Parks Highway crossing of the Tanana River at Nenana, Alaska, is a hydraulically complex site. Both the confluence of the Nenana River and the division of the Tanana River by an island into a main channel and a side channel, or slough, affect flow. A recent study determined that the large pier on the bridge over the main channel was susceptible to damage by scour. Further investigations were required to estimate the risk of pier scour and contraction scour at high flows.

The reach of the Tanana River that includes the confluence of the Nenana River and the Tanana Slough was surveyed on August 17-20, 1998. Channel geometry from this survey was used as input for two hydraulic models: a one-dimensional model and a two-dimensional model. Each model was calibrated to a high discharge measurement taken on August 17, 1967. Discharge scenarios were simulated for the discharge of August 17, 1967, and for floods in the Tanana River with 100- and 500-year recurrence intervals, each run with a low and a high discharge in the Nenana River. Low discharge simulated for the Nenana River was the discharge at the time of the survey. The high discharge was the discharge in the Nenana River with a 100-year return interval. Output from each model was used to calculate pier scour. Contraction scour was calculated using only output from the one-dimensional model.

Water-surface profiles, pier- and contraction-scour estimates, and slough discharge simulated by the models were compared to determine the effectiveness of using a one-dimensional model to simulate two-dimensional flow. The models yielded similar pier scour estimates, but results in areas of two-dimensional flow, such as at the mouth of the Nenana River, varied considerably.

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