PNNL

Abstract

A site near the downstream tip of Cascades Island and a mid-level cantilever outfall type were selected for the high flow outfall of the new surface flow juvenile fish bypass at Bonneville Dam’s Second Powerhouse. The new bypass will be a modification of the existing ice and trash sluice chute, which discharges into the tailrace with jet impact on the bottom near a shoreline that predators inhabit. Thus, a new site and type are necessary for this high flow (> 28.3 m3/s) outfall. Technical guidelines on high flow outfall location and design were established and applied during the outfall development process. Critical design parameters included discharge at 150 m3/s, entry velocities approaching 15.2 m/s, and tailwater elevation range of 6.1 m. For outfall siting, the selection process began with identification of nine initial alternatives. Screening, evaluation, and selection stages narrowed the list to two sites – “Range D” 121.9 m straight downstream from the existing outfall and “Range F” 760 m downstream near the tip of Cascades Island. For outfall type, the selection process was initiated with conceptualization of 13 alternatives. During successive screening, evaluation, consolidation, and selection stages, professional judgment and quantitative comparisons were used to select two finalists – “Adjustable Cantilever” and “Mid-Level Cantilever.” The four combinations of outfall site/type were evaluated in 1:30 and 1:100 scale physical hydraulic models. The process resulted in selection of a mid-level cantilever with plunge pool at the tip of Cascades Island. The system is scheduled for completion in March 2004.