A. R. Bowman Dam Outlet Modifications — Lessons Learned

Ochoco Irrigation District (District) operates and maintains Arthur R. Bowman Dam, an earth fill dam constructed by the Bureau of Reclamation between 1958 and 1961. Bowman Dam is located on the Crooked River near Prineville, Oregon and forms Prineville Reservoir. The District operates Bowman to fill during spring snowmelt runoff and for release of stored water during summer irrigation season. The outlet works tunnel is located in the right abutment and bifurcates into a gate chamber consisting of two 1.22 m (4 ft) by 1.82 m (6 ft) emergency gates and two 1.22 m (4 ft) by 1.82 m (6 ft) regulating gates. A training wall extends 9.14 m (30 ft) from the gate chamber and separates the flow exiting the two gates. The high-velocity flow is released into a 121.92 m (400 ft) long 3.35 m (11 ft) wide horseshoe shaped outlet tunnel. Historically, OID has shut off all of the water to the Crooked River to perform routine maintenance on the gates and outlet tunnel. As part of the Settlement Agreement for the Pelton Round Butte Hydroelectric project, Mid-Columbia Steelhead are scheduled to be released in the Crooked River in the spring of 2008. With increased concern over wildlife habitat in the Crooked River, the Bureau of Reclamation and the District needed a solution to allow minimum stream flow to pass through the outlet works during routine maintenance. The Bureau of Reclamation recommended to the District the construction of a 1.22 m (4 ft) high "splitter wall" to run the length of the outlet tunnel downstream from the gates. As part of the Districts' maintenance agreement, the District contracted with David Evans & Associates, Inc. (Engineer) to design the slitter wall. In addition to the splitter wall, DEA was challenged to design a safe and functional method to by-pass a minimum of 1.13 m³/s (40 cfs) through the tunnel during construction on the splitter wall. Construction challenges included working within agreed upon 2 hour shut down periods, dissipating the energy exiting the control gate and redirecting the flow into a temporary by-pass pipe thus creating an acceptable work environment within the outlet tunnel. The completion of the tunnel splitter wall project provides safe access to either side of the downstream outlet tunnel while flows up to 5.66 m³/s (200 cfs) pass by on the other side of the wall and on to the Crooked River. This paper will review the project from conceptual design through construction and evaluation. The author will share his experience in working in conjunction with the irrigation district highlighting successes, failures and lessons learned.