PNNL

Abstract

A site near the downstream tip of Cascades Island with a mid-level chute outfall type was selected for the high flow (> 28.3 m3/s) outfall of the new surface flow outlet for juvenile salmonids at Bonneville Dam’s Second Powerhouse (B2). The new passage route and outfall are a result of modifications to the original ice and trash sluice chute to increase discharge capacity and improve passage conditions, including a new outfall type and site. Technical guidelines on high flow outfall location and design were established concurrently with the outfall development process. 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. For outfall siting, the selection process began with identification of nine initial alternatives. Screening, evaluation, and selection stages narrowed the list to two outfall sites – “Range D” 122 m directly downstream from the existing sluice chute outfall and “Range F” 760 m downstream near the end of Cascades Island. For outfall type, the selection process was initiated with conceptualization of 13 alternatives. Following successive screening, evaluation, consolidation, and selection stages, two outfall types became 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 and a Mid-Level Cantilever at the tip of Cascades Island in Range F was selected. During further engineering after our study, the cantilever was replaced with a monolith structure to reduce construction costs, resulting in a mid-level chute outfall that was installed in 2004. Post-construction evaluations indicated survival rates around 100% through the B2CC were the highest of all passage routes at Bonneville Dam. The B2CC surface flow outlet with its high flow outfall provided a major improvement to juvenile salmonid passage at Bonneville Dam.