The Piano Key Weir (PKW) is the new shape of labyrinth, is an interesting alternative. The structure of PKW provides a longer total effective crest length for a given spillway width and it has been used to increase the discharge capacity of the related spillways. PKW has been applied in many projects in several countries in the world.  However, the hydraulic behavior of PKWs is still not complete understood. All the current projects under development to assess the hydraulic capacity are based on physical modelling.

Numerical modelling is nowadays another powerful tool to solve many of hydraulics engineering problems and to predict with accuracy flow patterns on a large set of hydraulic structures. Such an approach is currently widely used by engineers all over the world. Indeed, numerical models have low application costs compared to physical models, are flexible and enable easy results analysis.

In this study, combined application of physical and numerical modelling techniques, called composite modeling has been implemented for investigation of flow over Piano Key Weirs (PKW). It enables combining the inherent advantages of both approaches, which are complementary, while being beneficial to the delays as well as the quality of the analysis.

Experimental investigations are performed on several PKW models. The effects of the variation of each of its geometric parameters on the discharge coefficient were assessed using sensitivity analysis. 3D numerical model of the flow over a PKW has been performed by using the commercial software Flow3Dđ and 2 cases of model are applied. One is obtained from 2D physical model with constant width W, the inlet widths (W_i) and outlet widths (W_o) are changed. The other is 3D model of  Dakmi 2 PKW project. The experiment results are used for calibration of numerical model. Based on results, geometric parameters of Dakmi 2 PKW and operating procedures of weir have been proposed.