Coastal tides and tidal Bore in the Severn Estuary from fast-sampled SWOT data

Satellite altimetry is instrumental in deciphering the dynamics of oceans and coastal regions. It yields indispensable data critical for monitoring global sea levels, predicting wave heights, and charting the courses of ocean currents and river elevations. These insights are pivotal for advancing climate research, ensuring navigational safety, and managing water resources effectively. Nonetheless, in coastal settings, the efficacy of conventional altimetry is constrained by its spatial resolution and the interference of land in the radar signal near the coastlines. These limitations hinder its ability to accurately capture the nuanced characteristics of dynamic and intricate coastal environments.The launch of the Surface Water and Ocean Topography (SWOT) satellite represents a monumental leap in the technology of satellite altimetry. With advanced high-resolution wide swath altimetry and innovative use of the phase difference between dual onboard antennas, SWOT drastically reduces the limitations of traditional radar altimeters. SWOT provides a 2D measurements grid with a detailed 50m grid spacing not degrading towards the coast. This marks a substantial enhancement compared to the 7-km across-track spacing along a 1D trajectory offered by conventional altimetry.This enhancement allows for the precise and detailed monitoring of dynamic coastal phenomena such as tides and tidal bores, even in estuaries.Tidal bores, characterized as sudden and powerful water surges against the river's current, are critical for local ecology, navigation, and flood management. Despite their importance, their dynamic and transient nature has made them challenging to study using conventional methods. The Bristol Channel, with its extreme tidal range and the presence of the Severn Bore, presents an ideal case study to demonstrate SWOT's capabilities.We use SWOT 50-meter pixel cloud data during the 1-day fast sampling repeat period in April 2023 to study the high-resolution tidal signal in the Bristol Channel - Severn Estuary and the Severn tidal bore. The results demonstrate that SWOT can capture both complex tidal signals associated with wetting and drying close to the coast, but also the tidal bore sweeping up the Severn River from the mouth of the river and some 20 km upstream.