The Department of the Army, Army Corps of Engineers, Northwestern Division, Walla Walla District, has a requirement for Construction of a new fish passage facility and modification the existing levee, diversion dike and stilling basing at the Mill Creek Diversion Dam in Walla Walla, WA.

N/A

• This project involves the Construction of a new fish passage facility and modification the existing levee, diversion dike and stilling basing at the Mill Creek Diversion Dam in Walla Walla, WA to meet agency fish passage and flood control criteria.
• Prior to construction of the fish ladder, the contractor will be responsible for diversion and care of water in Mill Creek. Flows will need to be diverted around the worksite. Flows during the in-water work window (July­September) are expected to be around 25 to 40 cfs. Flows during flood season can exceed 6,000 cfs. The exact composition of the Mill Creek bed is unknown but it is expected to be highly porous sands and cobbles with a mixture of fine sediments. There are no known sources of groundwater inflow within the reach where dewatering is required. However, the stream bed composition passes groundwater flow readily so that even with no leakage on a dewatering structure pumping will be necessary to maintain a dry work site.
• The contractor will construct a new right-bank vertical-slot fish ladder, to include juvenile and low-flow bypass channels. Vertical slide gates operated by VFD’s, with manual overrides, will control water flow through the facility. Cameras will provide fish counting capabilities. Perimeter fencing will provide security for facility operations and public safety. Walkways, guardrails, and stairs will provide maintenance access throughout the facility.
• Modifications to the existing diversion dike, levee, and stilling basin on the right bank of the project are required to accommodate these improvements. This work also requires partial concrete demolition of the existing diversion dam wing walls to incorporate the new fish passage structure. Reinforced concrete will be formed and placed. Excavated portions of the levee and diversion dike will need to be rebuilt and riprap restored on both sides of the new fish ladder. Bicycle path will be replaced along the new levee alignment. Accommodations will be made for future installation of PIT tag antennas.
• On-site work is expected to cross one fish passage season and one flood season. The contractor shall allow for the continued fish passage as well as flood protection during these seasons. The existing fish ladder and low flow outlet on the left bank can be used to divert water and provide fish passage during construction and will remain in place as a backup for the primary fishway and for use during maintenance activities.