Turbulent circular culvert flow: Implications to fish passage design

Culverts are popular conveyance structures in America and Canada to be used as a fish passage or in sewage collection and disposal systems. Fish passage design criteria is based on biological capabilities of fish whereas it should satisfy hydraulic and hydrological constraints as well. Failing to provide enough low velocity regions for aquatic organisms may result in their mass extinctions. Currently, proper road crossing design depends on either model studies or numerical simulations via Computational Fluid Dynamic (CFD) packages, both of which are expensive and time consuming. Further, although the design procedures released by FHWA recently ensures safe fish migration from downstream toward upstream, it results in an unnecessarily large cross-sections increasing the overall project cost. In addition, the extensively used Manning model is not capable of providing local velocities or modeling the maximum velocity position (dip phenomenon). This study describes an attempt to solve Reynolds-Averaged Navier-Stokes Equations (RANS) for a steady state and uniform circular open channel flow at mild slopes, resulting in an analytical solution for a partially-filled pipe velocity distribution. Moreover, an empirical model based on the modified log-wake (MLW) law from previous knowledge of rectangular open channel flow was developed to further shed light on the topic. Both models were verified with experimental data collected with Stereoscopic Particle Image Velocimetry (S-PIV) through the work of the writer in the Turner-Fairbank Highway Research Center of the Federal Highway Administration and various other benchmark data from the literature. These models can be used as a novel and powerful design tool resulting in an optimum cross-section along with an enough low velocity region to facilitate safe fish passage