Helix Design for Downstream Fish Passage

The U.S. Bureau of Reclamation has developed a helix design for downstream fish passage at Cle Elum Dam that will allow fish to self-guide into a structure that carries them around the dam and into the downstream river channel. One of the major challenges in designing the structure was that passage must be provided, when reservoir fluctuations vary more than 60 feet.  In addition, flow depths and velocities within the structure must promote continuous downstream movement, and any transitions within the system must be extremely smooth hydraulically, while dropping flow and fish more than 70 feet in elevation. A mathematical model was used to determine combinations for overall diameter, slope, and geometry that would prevent flow rollover and would minimize secondary rotational flows and sweeping currents, to produce a significant “sweet spot” within the flow regime where fish can ride out relatively high velocities without injury. Once a geometry and diameter were determined from the numerical model, a physical model of the structure was constructed and tested in Reclamation’s Denver laboratory.  The physical model was used to verify and refine the final design for the helix structure. This design could potentially be adapted at many high head dam sites.