AbstractTypical engineering design of rock weirs rely on simplified one-dimensional equations dependent on empirical coefficients. However, most simplified methods fail to accurately predict the hydraulics through rock weirs because they do not consider flow through interstitial spaces between rocks and the way interstitial flow alters the head-discharge relationship. To improve the design methodology and better capture the complex hydraulics past rock weirs, a three-dimensional high-resolution computational fluid dynamics model was utilized to study the problem. The simulation results demonstrate that the flow phenomena and head-discharge relationship are significantly different between broad-crested weirs and rock weirs. The interstitial spaces between rocks not only drain a portion of total discharge, but also accelerate the weir overflow. Based on the results, a flow decomposition approach is proposed to quantify the discharge through a rock weir. The decomposition includes contributing flows from (1) weir flow over the individual rocks, and (2) interstitial flow between rocks. Discharge coefficients for both contributing flows were found to be approximately linearly proportional to the porosity. The applicability of the proposed decomposition was demonstrated with an independent case. Despite the success, future improvement is needed with more rock weir variations.