Labyrinth weirs increase the discharge rate by increasing the effective length of the weir within a confined width. In this paper, an attempt was made to improve the hydraulic performance of labyrinth weirs by both notching the weir wall and inclining the weir crest edge. To simulate the free flow surface, the volume of fluid (VOF) method, and the turbulence, the RNG k-ε model was adopted in the FLOW-3D software. To validate the numerical model with the experimental data, the results indicate that the maximum relative error is 4.8%, which confirms that the numerical model is fairly well to predict the specifications of flow over on the labyrinth weir. Numerical simulation results showed that for low HT/P ratios (HT: approach flow head, P: weir height), modifying the geometry using either methods improved the discharge coefficient and discharge capacity over the labyrinth weir compared to a conventional weir. By increasing lc/wc (lc: the weir cycle length and wc: the weir cycle width) for low ratios of HT/P, the flow magnification ratio Q/Qn (Q: weir discharge, Qn: conventional weir discharge) increases. By increasing the HT/P ratio, the flow magnification ratio and the flow rate both decrease significantly, that is because of the vortex flow formation, the rejection of the flow upstream, and local submergence of entry cycles with a high HT/P ratio. Therefore, modifying the labyrinth weir geometry at low HT/P ratios (HT/P < 0.2) results in a better hydraulic performance, a higher discharge coefficient, and larger discharge values.
