Scour Pool Development at Countersunk Culverts

Extensive studies have been completed on stream profile and culvert interaction, particularly addressing downstream scour and upstream aggradation processes. However, geomorphic placement of culverts has not been studied in comparable detail. To meet dual objectives of aquatic organism passage and stream profile maintenance, a roadway culvert was constructed below the thalweg profile in a manner similar to a "stream simulation" culvert. The installation included an inlet headwall to maintain stream grade and a scour pool for downstream energy management, as culvert outlets are typically marked by large downstream scour pools associated with expansion of the exiting flow. This countersunk culvert was physically modeled at model and prototype scales to determine effects on scour pool development and downstream energy management as functions of countersinking and longitudinal pipe slope. These physical methods indicated that energy dissipated within a countersunk culvert section may reduce the required length, width, and depth of scour pools from previously reported values. The extent of scour with increasing culvert countersinking generally follows a similar pattern for all geometric measures—a reduction in the amount of scour compared with that of a lesser countersunk culvert. This is particularly true for the percent countersunk values greater than 20%. It is likely that the presence of the downstream knickpoint (forced by proper placement of a riffle crest) and the associated backwater pool provides increasing resistance to the turbulent jet that typically exits from a culvert, thereby forcing greater turbulence and greater energy dissipation within the culvert.