Hydraulic Loss Coefficients for Culverts

Culverts are designed and constructed to be hydraulically efficient, such that they are able to pass flood flows without overtopping the road embankment. Flow passing through a culvert typically experiences an increase in velocity, relative to the approach channel flow, due to reductions in cross-sectional flow area. Increased flow velocity can cause additional outlet erosion as well as be a problem for many types of migratory species. In addition to migratory species, resident fish such as juvenile salmon can also be affected by culverts. Juvenile salmon move up and down streams as population pressures and food sources change. If high velocities in culverts provide barriers to this movement, food sources and population may be limited. Other fish species may have requirements similar to those of juvenile salmon or may require upstream movement for spawning. Research in the area of culvert hydraulics has centered on concrete box culverts and circular corrugated metal pipe culverts. The hydraulic analyses of these culvert types have been well defined for conventional installations, but not for environmentally sensitive and nontraditional culverts. It is desirable to design and construct some culvert crossings to minimize their impact on the natural environment. Culverts are now being designed to maintain natural velocities and minimize turbulence to allow migratory species to pass through the culvert barrel. Such designs may add baffles on the invert, bury the culvert invert, or use bottomless culverts to provide for a natural stream invert. Other designs use larger and wider culverts to reduce the amount of contraction and acceleration. In order to design these culverts that minimize impacts to the natural stream environment, designers need the associated hydraulic equations and loss coefficients to be evaluated and made more accurate. In NCHRP Project 15-24, Utah State University conducted physical, numerical, and computer modeling to refine existing hydraulic relationships and develop new ones for analysis and design of culverts for conventional and nontraditional, environmentally sensitive installations.