## 2 Materials and Methods
### Computational Levels Defined  
A multilevel, SWAN-based wave prediction system has been implemented and focused on the western side of the sea in a downscaling process [32; 33; 34; 35]. Various sensitivity tests and validations have been carried out for each computational level [36], taking into account also the computational strategies adopted in other coastal environments where the model results have been intensively validated against in situ measurements and remotely sensed data [37]. A special attention was paid to the coastal areas with more complex coastal dynamics, as for example the mouths of the Danube River where strong interactions occur between the waves and currents generated by the Danube River outflow [38; 39]. From this perspective, four SWAN computational levels have been defined in the present work, performing model simulations in eight different areas. The first three levels correspond to the spherical coordinates and their characteristics are provided in Table 1. In this table \(\Delta\lambda\) and \(\Delta\varphi\) represent the spatial resolution, \(\Delta t\) the time resolution, \(nf\) number of frequencies, \(n\theta\) number of directions, \(n\lambda\) number of grid points in longitude, \(n\varphi\) number of grid points in latitude, and \(np\) total number of grid points. Thus, the first computation level (L1) corresponds to the generation area, which comprises the entire basin of the Black Sea and the corresponding SWAN domain was denoted as Sph1 and has the resolution in the geographical space 0.08\({}^{\circ}\)\(\times\) 0.08\({}^{\circ}\). The second level (L2) reflects the SWAN domain (Sph2) defined to drive the coastal wave transformation in the Romanian nearshore, located on the western side of the basin. The resolution in the geographical space is in this case 0.02\({}^{\circ}\)\(\times\) 0.02\({}^{\circ}\). Finally, the third level defined considering the spherical coordinates (L3) comprises three different SWAN domains (denoted as Sph3, Sph4 and Sph5), all of them having the resolution in the geographical space 0.01\({}^{\circ}\)\(\times\) 0.01\({}^{\circ}\). Sph3 is focused on the area at the mouths of the Danube River, while Sph4 and Sph5 is focused on the southern part of the Romanian nearshore--they are denoted as Southern RO1 and Southern RO2. The reason for considering two domains, one larger and another one smaller, focused on the same area in the nearshore of the main Romanian littoral cities is that for the domain Southern RO1 a local multiparameter data assimilation scheme has been implemented [40; 41] considering the in situ measurements from the Gloria drilling unit, which is located close to the western boundary of the computational domain. On the other hand, from operational considerations, it was also found useful to define another SWAN domain, computationally more effective, which is focused especially on the Romanian littoral cities. The geographical spaces corresponding to these five computational domains (Sph1-Sph5) are illustrated in Figure 1.  
The fourth computational level (L4) is related to three Cartesian SWAN domains (Cart1, Cart2 and Cart3). The characteristics of these Cartesian areas are given in Table 2. The first two correspond to the high resolution coastal areas in front of the Sulina and Saint George arms of the Danube, while the third to the coastal environment close to Mangalia city, located in the extreme south of the Romanian nearshore. The physical processes activated in the SWAN simulations, corresponding to the eight computational domains considered are presented in Table 3. In this table: _Wave_ indicates the wave forcing, _Tide_ the tide forcing, _Wind_ the wind forcing, _Curr_ the current field input, _Gen_ generation by wind, _Wcap_ the whitecapping process, _Quad_--the quadruplet nonlinear interactions, _Triad_ the triad nonlinear interactions, _Diff_ diffraction process, _Bfric_ bottom friction, _Set up_ wave-induced setup, and _Br_ depth-induced wave breaking.