Flood embankments have played an important role in flood protection systems for centuries. Tightness evaluation of water structures should result in determining whether floods may cause seepage incidents, consequently leading to their damage or even destruction. It is assumed that the time of water passage from the river to the protected area under steady-state conditions can be a good indicator of the embankment resistance to long-term water rise. The curve of water passage times shows the values obtained at different ordinates of the dammed water and can be regarded as a characteristic parameter of flood embankment tightness. Determination of the water passage time ( T_p) additionally requires finding the trajectory along which this time is the shortest possible. However, there is no need to analyse the unsteady filtration, which requires the determination of an initial condition and the course of time-dependent boundary conditions. Engineers in practice, often use the time T_pp which elapses from the beginning of flooding to the occurrence of seepage in the protected area. The relationship between the passage time ( T_p) and the seepage onset time ( T_pp) was analysed on a model example. Practical use of the curve of passage times is showed on the example of the reconstruction of the left-bank Warta embankment in the area of Konin.