The Yakima County Flood Control Zone District was replacing the Nelson Dam irrigation diversion structure located on the Naches River, near the City of Yakima, WA. The original 8-foot tall inline weir structure, nearing the end of its design life, had been observed to adversely affect upstream flood levels, fish passage, and sediment transport continuity. The new design proposes to mitigate, or reverse, these historic adverse effects while still providing a reliable location for future diversion. The design includes a “nature-like” fish channel and flood bypass constructed immediately adjacent to a new gated diversion sluiceway.
NHC was retained to first develop conceptual designs for a bypass that specifically address concerns with regard to fish passage, flood levels, channel stability, and sediment transport. Both upstream and downstream passage of Chinook, coho, and sockeye salmon, as well as steelhead, bull trout, and lamprey were considered.
NHC constructed a 1:24 scale mobile bed physical model of the 1,500-foot long Nelson Dam reach and used the model to assess hydraulic and sediment transport patterns, sediment flushing capabilities in front of the diversion fish screens, and depth/velocity conditions for fish passage over a range of flows. Model observations were used to modify the design to improve performance of the bypass and diversion. The final design, consisting of a 2.5% slope bypass structure with inset fish channels, successfully demonstrated compliance with fish passage regulations, as well as performance with regards to flood level reduction and diversion operation. NHC is currently using physical model results and new SRH-2D modeling to finalize the design.