The flow over gabion weirs presents distinctly different characteristics compared to regular non-porous weir structures. Because of the permeability of the gabion weir, a portion of the flow moves through the structure which results in a lower upstream hydraulic head for a given discharge as compared to the solid concrete weir. Mohamed (2009) experimentally determined the discharge and free surface profile characteristics for a gabion broad crested weir over a range of flow and gabion wall porosity configurations. It was demonstrated that standard weir equations and discharge coefficients for solid weirs cannot be used for gabion structures.
It is known that computational fluid dynamics tools (CFD) can offer excellent representations of complex free surface flows over weirs, what is less known is that CFD tools can also model porous structures and their effect on free surface hydraulics. Drawing on Mohamed’s experimental results, a CFD model which incorporates the weir’s porosity and upstream conditions was run to replicate the experiment. Excellent validation was obtained, indicating that CFD methods may be a valuable consideration in the design of fishway passages that include gabion weirs in their design.