Data-driven modeling of multiple longshore bars and their interactions

Long-term bathymetric surveys at a coastal segment of the southern Baltic coast were investigated with empirical orthogonal functions (EOF) to determine the characteristic evolution patterns of multiple longshore bars and with canonical correlation analysis (CCA) to evaluate the importance of interactions among them. The investigated beach is located at Lubiatowo on the Polish Coast and is mildly sloping with multiple, usually four bars, and a median grain size of 0.22 mm, being typical for the coast in the southern Baltic Sea. Data on bed topography have been collected at Lubiatowo since 1987, predominantly twice a year, to record changes in the seabed along 27 lines covering a longshore×cross-shore area of 2600 m×1000 m. The EOF analysis revealed three persistent long-term cross-shore patterns accounting for about 2/3 of the overall signal variance and describing key elements of the evolution of multiple bars. Moreover, it detected subsystems of two inner and outer bars. The CCA study demonstrated that more that 60% of the evolution of inner bars can be explained by the variability of outer bars. The two subsystems thus display significant interactions; they can be largely attributed to vertical variations of the whole nearshore zone, expressed through changes in the equilibrium, Dean-type profiles. Such changes can only be produced by large scale phenomena, e.g. sequences of extreme events, affecting the whole nearshore topography. Hence, part of the variability of inner bars not related to the evolution of outer bars should stem from changes occurring during calm periods, when small waves pass over outer bars unaffected. The paper demonstrates usefulness of the CCA method in coastal engineering and a more general strategy of data-driven investigations with more than one method in order to make use of synergy generated with such a complex approach. The benefit of such a synergy is that results from both investigations can be critically intercompared, so more accurate joint interpretations become feasible.