A national atlas of design-wave and water-level conditions for Turkish coastal and offshore structures

Coastal and offshore structures are sized from a few metocean quantities: wave height and period, direction, storm duration, workable weather and water level. This study assembles them for Türkiye in one open atlas, updating the 2002 national reference with 28 years of ERA5 at 50-m coastal nodes. Peaks-over-threshold generalized Pareto models are fitted to declustered peaks of hourly significant wave height Hₛ at 410 ERA5 cells; 145,426 nodes inherit the 61 coastal cells, adding no wave resolution. Outputs fall in three tiers of unequal authority: return-level (50-year a high extrapolation), climatological and screening-level; Sea of Marmara values are indicative (five cells). In the top decile ERA5 is 4.3–10.5% below calibrated altimetry; 20-year basin medians of Hₛ are 3.5–6.1 m. Nearest-cell representativeness uncertainty, 17.4% of that level, exceeds the 7.7% sampling half-width at 89.9% of nodes. Hours with Hₛ below 1.5 m are 92.8–97.2% annually, 24-h window availability falls to 70.4–87.8% in winter, and screening-level 20-year total water levels reach 0.90–1.69 m above local mean sea level. The atlas gives one traceable baseline for feasibility and nearshore-model boundaries, and shows where site measurement matters most: the Sea of Marmara, the straits and narrow gulfs.