The mathematical modelling of almost any natural process is highly sensitive with respect to the model input data. The issue of the input data series for the prognostic morphodynamic nearshore calculations is addressed in the paper. Non-linear morphodynamic response to the windwave-waterlevel forcing requires providing nearly-natural input data series for performing forecast simulations. Further, the quantifying of the severeness of the provided data sets is essential to distinguish typical and critical prognostic scenarios.A closed approach is proposed aiming to replace widespread praxis of using "representative"