Using small triangular baffles to facilitate upstream fish passage in standard box culverts

Abstract

A culvert is a covered channel to pass streams and floodwaters through an embankment. The ecological impact of culverts has been recognised, in particular in terms of stream connectivity, but existing guidelines lead often to un-economical culvert design. Herein, a small triangular corner baffle system was tested physically in a near-full-scale fish-friendly facility of a box culvert barrel. Experiments were repeated with several configurations to characterise the flow properties for a range of less-than-design flows, baffle sizes and spacings. In presence of triangular corner baffles, the flow was asymmetrical, owing to the wake behind each baffle. The presence of triangular corner baffles had a moderate effect on the flow resistance and discharge capacity, albeit the data indicated the combined effect of relative baffle height and spacing on the friction factor. With triangular baffles, the surface area of slow velocity regions increased by a factor of two to three. Such low velocity regions are preferential swimming zones for fish, beneficial to small-bodied fish passage. Testing with small-bodied fish showed that fish preferred to swim upstream in slow-velocity regions, typically next to the sidewalls and in the left corner where the triangular baffles were located. The presence of small triangular baffles facilitated substantially the upstream passage of small fish, including in terms of endurance, compared to a smooth un-baffled box culvert barrel, when the baffle size was comparable to the fish length. The present findings highlighted the importance of physical modelling at near full-scale for the development of fish-friendly culvert designs.

Appendix: Theoretical calibration of Prandtl–Pitot tube

A Prandtl–Pitot tube may be used to determine the shear stress at a wall in a turbulent boundary layer [35, 36]. The (skin friction) boundary shear stress is deduced from a calibration curve between the velocity head and the shear stress, when the tube is in contact with the wall. On the basis of the velocity distribution shape, a theoretical calibration may be derived. Herein, V_b is the velocity measured with the Prandtl–Pitot tube lying on the boundary and z_b equals half of the Prandtl–Pitot tube outer diameter. For a turbulent flow, the velocity distribution in the whole boundary layer may be approximated by a power law [2, 17]:

$$\frac{{V_{x} }}{{V_{\hbox{max} } }} = \left( {\frac{z}{\delta }} \right)^{1/N}$$

(5)

where V_max is the free-stream velocity at the outer edge of the boundary layer: V_max = V_x(z = δ), z is the vertical elevation and N = 7 for a smooth turbulent boundary layer [25, 38].

In the wall region of a turbulent boundary layer, the Prandtl mixing length may be: l_m = κ  × z where κ is the von Karman constant (κ = 0.4) [9, 38]. At the wall, the boundary shear stress equals:

$$\tau_{o} = \rho \times \nu_{T} \times \left( {\frac{{\partial V_{x} }}{\partial z}} \right)_{z = 0} = \rho \times l_{m}^{2} \times \left( {\frac{{\partial V_{x} }}{\partial z}} \right)_{z = 0}^{2}$$

(6)

where ν_T is the momentum exchange coefficient or “eddy viscosity”. If the velocity distribution follows Eq. (5), the velocity gradient equals:

$$\frac{{\partial V_{x} }}{\partial z} = \frac{{V_{x} }}{N \times z}$$

(7)

and the boundary shear stress becomes:

$$\tau_{o} = \rho \times \kappa^{2} \times \frac{{V_{b}^{2} }}{{N^{2} }}$$

(8)

Equation (8) gives an expression of the boundary shear stress as a function of the velocity V_b measured with the Prandtl–Pitot tube lying on the boundary. Note that the result is independent of the tube diameter, contrarily to the findings of Patel [35] and Macintosh [27], although Eq. (8) implies that z_b is higher than viscous sub-layer and within the wall region.