Earthen levees are designed for little wave overtopping during a design storm, but excessive overtopping and overflow can occur due to the combined effects of an extreme storm, sea level rise, and land subsidence. The transition from little wave overtopping to excessive wave overtopping and overflow on an impermeable smooth levee is examined in wave-flume experiments consisting of 107 tests. Existing empirical formulas are shown to be applicable to the cases of excessive wave overtopping and overflow in these tests. A numerical model based on time-averaged continuity, momentum, and wave action equations is connected to a new probabilistic model for the wet-and-dry zone, in order to predict the cross-shore variations of the mean and standard deviation of the free surface elevation and depth-averaged fluid velocity from outside the surf zone to the inner slope of the levee. The new model is calibrated to predict the measured overtopping and overflow rates within a factor of about two. The agreement is also shown to be similar for the water depths and velocities measured in the wet-and-dry zone on six different structures in 100 Dutch tests. The developed hydrodynamic model is coupled with new formulas for suspended-sand and bedload transport rates to predict dune erosion and overwash. The coupled model is compared with two small-scale tests on dune erosion with minor overwash, three large-scale tests on dune erosion, and field data on dune erosion and overwash due to severe storms lasting several days. The overwashed-dune profiles are predicted reasonably well, but the coupled model will need to be evaluated using measurements of wave overtopping and overwash rates.