Authors: C. Mannheim, C. Sweeney, K. Hansen
Waterpower XV in Chattanooga, Tennessee, July 23-26, 2007
Abstract
Following the successes of the removable spillway weirs (RSWs) at Lower Granite
and Ice Harbor Dams on the Columbia River in safely passing downstream
migrating fish, the U.S. Army Corps of Engineers’ Walla Walla District targeted
Lower Monumental, Little Goose, and McNary Dams for design and installation of
additional surface bypasses. ENSR completed physical model tests of an RSW for
Lower Monumental Dam in 2005 and a prototype structure is scheduled for
installation in 2007. The USACE proposed a temporary spillway weir (TSW) concept
as an alternative for prototype testing at McNary Dam. The TSW concept uses a
weir structure upstream of the spillway gates to create top spill into a spillway bay.
The primary goal is to create a flow pattern in the forebay that will attract smolts
and pass them safely onto the spillway. If successful, the surface oriented flow will
pass smolts more efficiently than normal spill flow released at depth under the
gates. The result will be development of a permanent installation to improve
downstream migration with less impact on power generation and lower potential for
spill-related increase in downstream total dissolved gas levels.
This paper presents the development of a TSW concept for McNary Dam using a
1:25 scale sectional physical model of three spillway bays and one turbine unit. The
design alternatives were evaluated based on measured hydraulic parameters and
resulting fish injury potential, such as: approach flow velocity gradient, nappe
trajectory, nappe angle, spillway impact pressure, and estimated nappe back roller
pool size. The paper discusses the relative fish injury potential of the different TSWs
nappe spillway impact hydraulic conditions.
