Evaluation of Bar Rack Spacing and Approach Velocity for Preventing Entrainment of Silver American Eels at Hydropower Projects

State and federal resource agencies have been prescribing restrictive intake bar spacing and approach velocities for downstream passage of silver American eels at hydropower projects in the U.S.  These criteria could result in significant costs and lost generation at many projects.  To determine the applicability of eel-specific criteria versus those used for other species, we conducted a laboratory evaluation of bypass efficiency for silver eels exposed to a bar rack oriented 90^o to the flow with clear spacings of 19 and 25 mm, approach velocities of 0.46 and 0.61 m/s, and a full-depth bypass.  Bypass efficiencies were higher for the smaller bar spacing and decreased slightly with increasing approach velocity for both spacings.  An analysis of morphometric measurements indicated that eels with lengths greater than 930 mm would be physically excluded from entrainment through 19-mm clear bar openings and eels greater than 1,135 mm would be excluded by 25-mm spacing.  The results of this study can be used to assess the relative effectiveness of various bar rack spacing and approach velocity criteria to develop site-specific designs for silver eels that balance biological effectiveness and impacts to project operation.