Countersunk culvert design for stream maintenance

Roadway culverts may be the most common and most prevalent barrier to aquatic organism passage. These culvert barriers become more critical in the presence of stream restoration projects, as restoration project goals may be jeopardized by the continuing habitat connectivity challenge or project finances could be hampered by oversized crossing infrastructure to completely avoid any conflict. One restoration-related roadway crossing method used in recent applications-"stream simulation" culverts-may initially meet project goals but have the potential to propagate any stream instabilities in an upstream direction. As additional techniques for design and construction of roadway culverts in the vicinity of restored or stable stream sections continue to be needed, a countersunk culvert design was developed. A laboratory model, constructed at 1:18 scale, was utilized to generate data on the flow conditions of countersunk culverts, considering a range of countersinking and several longitudinal culvert slopes. This model was also used to develop preliminary results on sediment transport from and debris passage through countersunk culverts, as well as the effects on downstream scour hole geometry. As part of a larger stream restoration effort, a 9-ft diameter prototype culvert was constructed on a small headwater stream to provide field-scale data for a particular countersinking configuration. Further modeling was completed using HY-8 culvert analysis software to relate the laboratory and prototype data to typical current design methodologies. Countersunk culvert hydraulic data were compiled into performance curves and further refined into dimensionless curves useful for determining headloss through countersunk culverts of any diameter or degree of countersinking; the data were based on non-overtopping, clear water discharges with a range of tailwater variation specific to the prototype installation. The model-scale sediment experiments indicated that a wide range of sediment grain sizes can be transported through any of the countersunk configurations examined. The arch-like inlet face geometric configuration appears to increase debris passage capacity relative to that of comparable on-grade culverts. Energy dissipated within a countersunk culvert section may reduce the required length, width, and depth of scour pools from previously-reported values. This study found that countersunk culvert designs could provide flow depths and velocities compatible with aquatic organism passage, while improving debris passage, reducing downstream scour dimensions, and not impeding sediment transport.