Culverts are designed and constructed to be hydraulically efficient, such that they are able to pass flood flows without overtopping the road embankment. In general, culverts designed for hydraulic efficiency cause the flow to contract and accelerate inside of the relatively smooth culvert barrel. Increased velocities can cause increased 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 downstream 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 also be resident and have requirements similar to juvenile salmon or may require upstream movement for spawning (e.g., Arctic Grayling).
It is desirable to design and construct some culvert crossings to minimize their impact to the natural environment. Culverts are 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 (i.e., arch, pipe arch, and bottomless shapes) to reduce the amount of contraction and acceleration of the flow.
The principles and the methods for hydraulic assessments and design of culvert crossings are inexact and still evolving. Hydraulic loss coefficients and hydraulic equations range from semi-rational to empirical; they are based on laboratory experimentation and are supported by limited prototype testing. Consequently, it is understood that hydraulic analyses and predictions for culverts are approximate and subject to relatively high factors of safety. In order to design culverts that maintain natural velocities and, therefore, minimize impacts to the natural stream environment, designers need hydraulic loss coefficients to be evaluated and made more accurate.
Research in the area of culvert hydraulics has centered around concrete box culverts and circular corrugated metal pipe culverts. The hydraulic analyses of these types of culverts is well defined for conventional installations but not for environmentally sensitive and nontraditional culverts. Basic understanding of the hydraulics of culverts and their influences on analysis and predictions is critical in high-risk situations and needs to be enhanced.
The objectives of this research are to refine existing hydraulic loss coefficients and to develop new hydraulic loss coefficients for analysis and design of culverts for conventional and nontraditional, environmentally sensitive installations. Inlet control design curves and outlet control entrance- and exit-loss coefficients for various culvert shapes and end treatments. Each shape should be evaluated for submerged and unsubmerged conditions for the following: (1) lowered culvert inverts (e.g., the invert of a culvert may be lowered by a certain depth below the natural stream flowline to allow for fish passage), multiple circular culverts, resulting in flow split, and rehabilitated circular culverts (i.e., relined culverts) and (2) Composite hydraulic loss roughness coefficients for bottomless (i.e., open footing) culverts or culverts with buried bottoms. Coefficients shall be developed for both full and partially-full conditions and shall include various streambed materials for both concrete and metal culverts.
NCHRP Report 734 Hydraulic Loss Coefficients for Culverts is here.