Free surface flows are gravity-driven flows that occur in open water systems such as canals, rivers, lakes and oceans. For the majority of practical applications, the study of free surface flows can be simplified by assuming that the curvature of streamlines is small, leading to negligible vertical velocities and accelerations, and consequently to hydrostatic pressure distribution (Figure 5.1). The assumption of this type of ‘quasi-horizontal’ flows facilitates the mathematical solution since the pressure can be estimated directly from the water depth.