Hydraulic Analysis at the Interface of the Yellowstone River and the Huntley, Montana Irrigation Diversion Fish Bypass

Abstract

The nature-like bypass channel built to allow fish to circumvent the Huntley Diversion Dam on the Yellowstone River was constructed in 2015. A project was commissioned in 2019 to determine the effectiveness of the bypass using hydraulic modeling and fish detection techniques. During the course of the study it was observed that there may be a localized zone of high water velocity at the interface between the upstream end of the bypass and the main channel of the river -an area just upstream of the low-head dam. The concern this raises is that some fish that successfully negotiate the bypass channel may be returned directly over the dam due a difficult hydraulic condition at the interface. That observation prompted a more focused hydraulic modeling exercise as reported herein. A detailed 2-D HEC-RAS model was developed to investigate the hydraulic conditions. The model predicts localized water velocities of up to 15 ft/sec. At low river flows there appears to be adequate pathways for fish to avoid this high velocity region, but as river flow increases so does the area in which the velocity is high. From these results it is likely that, during higher river flow periods, the bypass channel may be passable, but fish may struggle to re-enter the river channel successfully. Suggested physical alterations to the site to help overcome this range from the addition of large rip rap to rerouting the upstream end of the bypass channel. Acknowledgement