Realistic Rock Weir Hydraulics Through Computational Fluid Dynamics

Typical engineering designs of rock weirs rely on simplified, one-dimensional empirical equations. For example, nature-like fish passage design guidelines recommend designing the rock weir structures based on one-dimensional submerged broad-crested weir equations developed for solid, nonporous weirs. Not surprisingly, this traditional method fails to predict the real hydraulic conditions through rock weir structures because it does not consider flow through interstitial spaces between rocks and the way interspatial flow alters the head-discharge relationship. Furthermore, no consideration is given in the weir coefficients to the irregularity of the boulders nor the weirs cross-channel shape. To improve the design methodology and to better capture the complex hydraulics past rock weirs, a three-dimensional, high-resolution computational fluid dynamics model is used. Due to the spatially variable water surface elevation and the turbulent nature of the flow, the 3D model captures the free surface with a volume-of-fluid method and turbulence by large eddy simulation. The simulation results show that the flow phenomena and head-discharge relationship are significantly different between broad-crested weirs and rock weirs. Based on the results, we propose a linear decomposition approach to quantify the flow rate through a rock weir structure. The decomposition includes contributing flows from (1) weir flow over the individual rocks, (2) flow through the weir’s notch, and (3) interstitial flow between rocks. In this paper we demonstrate the applicability of the proposed decomposition. More cases will be tested to parameterize the discharge coefficients for different flow conditions and weir geometries.