How do simple wave models perform compared with sophisticated models and measurements in the Gulf of Riga, eastern Baltic Sea?

Design and coastal management are based on site specific wave information. As measurements are scarce and short, modelling is used to produce wave data. Depending on the problem and funding, simple methods are frequently used instead of complicated wave models with space and time varying forcing. This approach is apparently sufficient in open ocean conditions where spatial variations in wave properties are normally limited. The situation is different in nearshore areas of complicated shapes, where wave properties can be highly variable. The use of default settings of wave models means that possible errors remain unknown, and employing data with substantial uncertainties could lead to structural failures or too expensive structures. We study the magnitude of possible errors by comparing the output of simple wave models (such as the stationary/non-stationary fetch-based SPM model or the SWAN model forced with one-point homogenous wind) and the sophisticated multi-nested SWAN wave model forced with ERA5 winds with wave measurements in various nearshore locations in the Gulf of Riga, eastern Baltic Sea. The modelled results are compared with records of different length spanning over more than fifteen years. It is shown that in many locations simple models or models forced with homogenous wind yield good results, while sophisticated models are dependent on site-specific tuning of parameters. Surprisingly, stationary models yield better results in selected locations. The outcomes of our analysis provide several site-specific hints for practical coastal engineering.