Review article: Towards consistent estimation of coastal water levels combining satellite altimetry and hydrodynamic modelling

Abstract. Coastal cities are increasingly exposed to flooding as sea level rises and as compound extremes involving tides, storm surge, waves, and river discharge act together. Reliable estimation of extreme coastal water levels is therefore fundamental to flood risk assessment, forecasting, and coastal management, yet it remains constrained by sparse in-situ networks and by the partial, scale-dependent representations that satellite altimetry and hydrodynamic models each provide. Over the past decade, advances in coastal altimetry and in regional-to-global hydrodynamic modelling have expanded the information available for coastal water-level estimation, but their combined use remains dominated by region-specific applications, and globally consistent methodologies are still emerging. Here we review approaches for combining satellite altimetry and hydrodynamic models across four interconnected domains: the detection of coastal sea-level and sea-state extremes from along-track altimetry; strategies for integrating altimetry with hydrodynamic models; vertical referencing and datum consistency; and validation and uncertainty characterisation, particularly in data-sparse settings. We argue that the central challenge is not how to merge two sources of information, but how to ensure that they represent commensurable quantities. No single integration strategy is universally optimal, and differences in process representation, sampling, and vertical reference must be treated explicitly rather than absorbed into residual error. While satellite altimetry does not resolve wave-driven processes at the shoreline and hydrodynamic models often approximate baroclinic and compound contributions, consistent vertical referencing and coherent propagation of uncertainty can render altimetry- and model-derived estimates comparable. Progress therefore depends less on combining datasets more widely than on defining when such combinations are physically meaningful, vertically consistent, uncertainty-aware, and transferable across coastal regimes, laying the foundation towards globally consistent estimation of coastal water levels.