Culverts have been traditionally designed and constructed to be hydraulically efficient for conveying flood flows. This efficiency is achieved by causing the flow to contract and accelerate inside of the relatively smooth culvert barrel. In some cases, achieving the objective of culvert efficiency also results in constructing barriers to fish passage. National attention is now focused on modifying traditional design methods (emphasizing culvert efficiency) in such a way that the objective of fish passage is also achieved. To prepare successful designs for fish passage, there is an immediate need to develop more information about the hydraulics of low flows in large culverts. Current information on culvert hydraulics available from FHWA and culvert manufacturers typically addresses flow magnitudes on the order of the bankfull stage or greater. This information is being extrapolated to estimate velocities during low flow conditions, and the results are unreliable. Previous studies of fish movements indicate that the fish tend to move upstream along the sides of the culverts where flow velocities are lowest. Therefore, there is a need for a more accurate method to determine the extent of the variation of local velocities within the cross-sectional area of the culvert under low flow conditions. Additional information is also desired in regard to how entrance and exit flow conditions affect fish movement. Urgency, Payoff Potential, and Implementation The immediate need for this study was emphasized at a February 15-16, 2006, conference on fish passage held in Denver, Colorado, that was organized by the FHWA and attended by representatives of FHWA and other Federal agencies, various State DOT¿s, academia and private industry. At present there is still a lack of basic knowledge concerning the hydraulics of low flows in culverts. Because of the availability of advanced instrumentation, FHWA is now able to obtain more precise measurements than were possible in the past to address the concerns about low flow hydraulics. Considerable interest in this proposed study was voiced by the conference participants. The relatively modest cost of this research and the great value of the basic research results make this a high priority study with benefits to all Federal and State agencies involved in solving fish passage problems at highway culverts. The study results can be incorporated in FHWA Hydraulic Engineering Circulars (Future HEC-26, etc.) for nationwide distribution and implementation.

The objective of this project is to develop a report to document the study approach and the research results. The report will include a practical design method for estimating average and local velocities in culverts and will describe how the results can be used to develop improved methods for facilitating fish passage. The proposed is to study low flow hydraulics in large culverts. The study will be conducted by the FHWA at the Turner-Fairbank Highway Research Center's J. Sterling Jones Hydraulic Research Laboratory in McLean, Virginia. Hydraulic variables will include flow depth, flow condition (laminar, transition and turbulent), Froude number and the variation of flow velocities within the culvert cross-section. Culvert variables will include size and shape, slope, sediment deposits and culvert material. The experiments will be conducted in a new designed culvert test facility where a 15- to 20-foot length of specified culvert geometries will be installed between the headbox and tail box. Only the symmetrical half of the culverts will be modeled allowing larger scale models (1:5). Current FHWA laboratory instrumentation can now achieve much more precise measurements than was possible a few years ago to evaluate these hydraulic characteristics under low flow conditions. An advanced particle image velocimetry (PIV) technology will be used for measuring instantaneous flow fields in the culvert. The particle image velocimetry technique (PIV) is an optical flow diagnostic based on the interaction of light refraction and scattering with inhomogeneous media.

The scope of work consists of researching entrance, friction and exit losses for low flows in culverts, to address the effects of low flow conditions on "n" values and to study boundary layer flow fields. The project will consist of the following tasks: Task 1. Assemble a technical advisory committee that will provide oversight and guidance on all aspects of the project. Task 2. Solicit information from participating States on current methods being used to evaluate/design culverts for fish passage. Solicit information on the range of variables (culvert shapes, sizes, slopes, lengths, materials, entrance conditions, outlet conditions and bed load material), which need to be considered in their fish passage designs. Task 3. Based on the input received from the participating states, develop a detailed study matrix for the hydraulic tests. Task 4. Fabricate transparent models of the culverts to be studied on a scale of 1:5. Conduct various hydraulic flow test runs; varying spans widths, culvert materials, flow depths, slopes and bed load materials (sand, gravel, and cobbles). Task 5. Develop a procedure for computing average velocities and local velocities (using "iso-velocity lines") for the range of selected culverts and study conditions. If necessary, numerical modeling will be used to augment the hydraulic laboratory study results. Task 6. Run full-scale flume tests of commonly used corrugation patterns on flat plates; measure average velocity friction factors for various flow depths above the corrugations and local velocity variations in the vicinity of and between corrugations as a possible explanation for the observation that small fish seem to navigate through culverts at much higher average velocities than would be expected. These full-scale tests will serve to further validate the results of the scale model tests performed in Task 3. Task 7. Prepare a report to document the study approach and results. The report will include a practical design method for estimating average and local velocities in culverts and will describe how the results can be used to develop improved methods for designing culverts to facilitate fish passage.

Requested contribution from partners is $15,000 per year. Co-developer of proposal: Andy Kosicki, P.E. Chief, Structures Hydrology and Hydraulics, Office of Bridge Development, Maryland State Highway Administration akosicki@sha.state.md.us The Federal Highway Administration will serve as the coordinator for this pooled-fund project. State DOT's will be solicited for their interest and participation in this study. FHWA will issue a task order contract to the support services contractor to conduct the study. Periodic reviews will be arranged to keep participating states and agencies up-to-date on current developments. These reviews may include meetings in Washington D. C. during the annual TRB Session, e-mail submittals and conference calls.