## 4 Discussion
### Local Morphodynamics
The wave data used provide offshore wave heights and directions, but onshore, breaking wave characteristics are determined by wave propagation phenomena, such as shoaling and refraction, which depend on the coastal morphology and setting. Nonetheless, analyses of the erosion/accretion indicators (sediment budget and shoreline dynamics) in relation to beach orientation (Figures 12-14), showed: (i) the general temporal trends, with periods of more accretion or erosion, that were also seen in the overall and the spatial analyses; (ii) differences between beach types, with rocky beaches being relatively stable, independent of orientation, and sandy beaches showing marked variability; and, (iii) an apparent tendency towards more erosion/shoreline retreat for segments facing more western directions (despite the scatter). The latter was particularly the case for the periods of the last 5 years and the last year, suggesting that shorelines with less obliquely incident waves presented a higher erosion risk. This could be explained by the dominance of cross-shore transport towards the subaerial beach during wave storms, with the sand deposits occurring at or behind the rocky outcrops. The most westward-facing segments tended to have the highest volume losses and shoreline retreats, and erosion/shoreline retreat increased with more western-dominant wave directions, as was the case in the last year. For the 5-year periods, the wave roses seemed rather similar (as could be expected for such a long period), but the WNW component was higher for the second 5-year period than for the first. In general, more exposed segments (e.g., segments 17, 19, 31; Figure 5) showed frequent sediment loss and shoreline retreat, yet many apparently protected (embayed) segments retreated too (e.g., 18, 20, 23). This may have been due to wave diffraction. The main wave crest orientation was from the NW, inducing a drift current from north to south, which was in some areas inverted due to the presence of obstacles that promoted wave diffraction, causing downdrift erosion. Shoreline retreat was also found downdrift the Canide groyne, which is a typical effect of these structures that are intended to trap sediments in updrift areas. However, even the segments above the groyne did retreat, though less than those downdrift. Segments north (39, 40), as well as south, of the Aguda breakwater (43, 44) moved from accretional during the first years of the survey series to erosional during the last 5 years. This may have been due to changes in wave action, as explained above, but also due to sand beach management operations that removed sediments from north of the breakwater to nourish the beaches of the city Espinho in the south. There were two flood events in the Douro river during the study period, one in February 2010 and another in March 2018, with peak river flows of more than 6000 m\({}^{3}\)/s and 4000 m\({}^{3}\)/s, respectively. These may have affected longshore sediment transport to the sectors south of the river outlet, as the periods comprising these flood events showed consistent erosion in the intermediate segments (16-25) of the study area. However, a detailed hydrodynamic analysis would be needed to confirm this hypothesis. An interesting issue was that volumetric changes did not always correspond to shoreline dynamics, suggesting sediment transport from the beach to the dunes. There are different modes of beach dynamics--shoreline advance/retreat (translation modes) and beach steepening/flattening (rotation modes)--that can occur simultaneously and are often linked to sediment redistribution within the beach-dune system [50]. For instance, segments 39 and 40 (located to the north of the Aguda breakwater) presented shoreline regression and stability (\(-6.1\) m and \(-0.2\) m, respectively) but volume stability and accretion (\(2.1\) m\({}^{3}\)/m and \(265.4\) m\({}^{3}\)/m, respectively) for the decade.