In the present study, the discharge capacity and the hydrodynamic behaviour of the piano key weirs with simple lateral walls as well as modified lateral walls with zigzag crests with various geometries including triangular, rectangular, trapezoidal and semicircular indentated crest were investigated. First, by using laboratory data and results of numerical simulation with Flow-3D software trhe discharge capacity and the discharge coefficient of the weirs were determined and compared. Then the hydraulic formation of the flow over the weirs in different submergence was studied. For this purpose, the formation of streamlines passing through the weirs were extracted from numerical results and analyzed after compliance with laboratory observations. In addition, the energy dissipation of the weirs was calculated using numerical results and the effect of the geometry of the crests on the weirs energy loss was investigated. On conclusion, the main results of this study can be summarized as follows.In Ht/P<0.5, the zigzag indentation on the lateral crests of the piano key weir significantly increases the discharge capacity. The weir with circular shape zigzag indentation (CCM) has highest discharge capacity. By increasing the upstream head of the weirs, the effect of the zigzags indentation for the lateral crest on increasing the weir discharge capacity, decreases. In other words, all weirs have the same discharge capacity at high upstream head. The presence of zigzag indentation on the lateral crests of the weirs causes these overflows to reach the submergence threshold by higher water head. Meanwhile, the CCM weir has the highest relative water head for submergence threshold. The presence of zigzag indentation on the lateral crests of the weirs increases the energy dissipation of the piano key weir at low water head. As the upstream water head of the weirs increases, the submergence of the weirs increases and the energy dissipation of all overflows becomes equal.