Advances in Fisheries Bioengineering

Outfall Site and Type Selection for a New Surface Flow Outlet to Pass Juvenile Salmonids at Bonneville Dam’s Second Powerhouse, Columbia River

Gary E. Johnson, Blaine D. Ebberts, Albert E. Giorgi, Karen A. Kuhn, Randall T. Lee, John H. Plump, David A. Stensby, and Charles E. Sweeney

doi: https://doi.org/10.47886/9781934874028.ch13

Abstract.—At Bonneville Dam’s second powerhouse (B2) on the Columbia River, a site near the downstream tip of Cascades Island was selected for the high-flow (>28.3 m³/s) outfall of the new surface-flow outlet (B2 corner collector or B2CC) for passing juvenile salmonids. The new passage route and outfall are a result of modifications to the original ice and trash sluice chute, which increased discharge capacity and improved passage conditions. Technical guidelines specifying the site and design for high-flow outfalls were established concurrently during this study. Critical design parameters for the new B2 outfall included discharge of 150 m3/s, jet entry velocities approaching 15.2 m/s, and a tailwater elevation range of 6.1 m. The selection process for siting the outfall began by identifying nine initial alternatives. The screening, evaluation, and selection process narrowed the list to two outfall sites—“D” was 122 m directly downstream from the existing sluice chute outfall, and “F” was 760 m downstream near the end of Cascades Island. We started the selection process for outfall type by identifying 13 alternatives. In the end, two outfall types were selected as viable solutions: the adjustable cantilever and the mid-level cantilever. The four combinations of outfall site/type were then evaluated using two physical hydraulic models (1:30 and 1:100 scale). Based on those hydraulic studies a mid-level cantilever at the tip of Cascades Island at site F was selected. Subsequent to our alternatives study, the cantilever design was refined, resulting in a monolith structure that reduced construction costs without compromising performance. Construction was completed in 2004, and subsequent biological evaluations indicated that smolt survival rates through the B2CC were near 100%, the highest of all passage routes at Bonneville Dam. The B2CC surface flow outlet with its high-flow outfall provided a major improvement for juvenile salmonid passage at Bonneville Dam.