Effect of Fish Baffles on the Hydraulic Roughness of Slip-Lined Culverts

The use of fish baffles in HDPE slipliners is growing in popularity to improve hydraulic conditions for fish passage, yet little is known on how baffles affect the outlet-controlled discharge capacity of these types of culverts. To fill this gap in knowledge, roughness coefficients (Manning’s $n$ and friction factor $f$ values) were experimentally determined for weir baffle, slotted weir baffle, and spoiler baffle configurations at four relative spacings (${\lambda }^{+}=0.6$, 1.2, 1.8, 2.4) and three values of relative roughness height ($h^{+}=0.15$, 0.10, 0.05). Relative roughness height ($h^{+}=h/D$ where $D$ is the pipe diameter) was found to be the determinant geometric parameter affecting energy losses. Relative spacing (${\lambda }^{+}=\lambda /D$) was found to play an important secondary role. An analytical model was developed and analyzed to determine the effects of the following: roughness reduction ($\alpha =n/n_o$) between the Manning’s coefficients of the host and baffle equipped slipline culvert; diameter reduction ($\beta =D/D_o$) between the diameters of the host and slipline culverts; relative length ($L^{+}=L/D$); and inlet treatments ($k_e$) on the hydraulic capacity of corrugated steel culverts after being sliplined with baffled high-density polyethylene (HDPE) culverts. Results demonstrate that many HDPE slipliner culverts can house baffles with $\alpha$ values in the range of 0.5 to 0.9. Design recommendations for the use of baffles in slipline culverts are discussed.