The model MACH3 is the indispensable tool for planning and assessment of the effects of hydraulic structures. With the help of scenario-calculations, the height and location of dykes and weirs for storm surge safety precautions, embankments and dams for optimizing the currents, harbour moles for reducing currents and wave energy, effects of daming by weirs and dams, and effects of river restorations can be determined and can be opitmized. MACH3 captures not only the desired effects but also the unwelcome, such as the unwelcome increase of sedimentation caused by the desired decrease of current velocity. On the basis of these calculations, an efficient cost-benefit analysis can be made. Of great importance is the determination of the effects of hydraulic structures onto the environment and ecology. Potential changes in water exchange rates and in the transport of nutrients and pollutants can be determined by calculating the distribution of matters concentrations. Also, potential secondary effects, such as an increase of the thermal or haline stratification resulting in reduced ventilation of bottom waters and thus resulting in oxygen deficiencies, can be detected very exactly by scenario-calculations with MACH3.