Bonneville Dam

To sustain and enhance juvenile fish passage at Bonneville Dam, the U.S. Army Corps of Engineers - Portland District, conducted a feasibility study of creating large-scale dewatering or screening and outfall facilities at Powerhouse No. 1. Cornforth Consultants was part of a team of consultants retained by the Corps of Engineers to conduct the study.

Bypass elements that were evaluated included a dewatering facility, a bypass channel, and an outfall system. To minimize predator activity and sustain downstream fish movement, the collected juveniles were to be released by large outfall structures at some distance downstream from the dam. Due to the high-volume dewatering requirements (up to 15,000 cfs), major outfall structures would require support by large-diameter drilled piers. Fish bypass conduits and outfalls of this magnitude had never been developed for any project of this type.

Cornforth Consultants performed geotechnical studies for the following bypass elements:

Dewatering Facility at the existing powerhouse: open excavations, shoring, and construction dewatering.
Bypass Channels: several flume alignments were considered. Some alignments required the evaluation of cellular cofferdams and the driving of sheetpiles through bouldery terrain.
Outfall Structure: preliminary design recommendations for 6 and 10-foot diameter drilled shafts. Design parameters were provided for allowable axial loads, estimated settlements, lateral capacities, estimated deflections, seismic resistance, and group effects.
Alternative Bypass Channel: for a Porous Invert Flume design parameters were provided for granular filters, drains, and of open-graded gravel.