Scientists have developed a satellite-based method that measures tides along coastlines at intervals of about 100 metres, revealing that water levels can differ sharply between nearby beaches. The University of Oxford and Technical University of Munich team reported on August 25 that tidal height varied by almost one metre across New Zealand's roughly 90-kilometre South Taranaki Bight, a level of local detail largely missed by existing observation systems.
The peer-reviewed study, published in Communications Earth & Environment, addresses a persistent coastal data gap. Tide gauges provide accurate records but only at isolated sites, while conventional radar-altimetry satellites generally resolve areas tens of kilometres wide and struggle close to land. Yet the coastal zone is where tides most directly affect flooding, navigation, fishing, recreation, pollution spills and public safety.
Instead of measuring sea height directly, the researchers treated each sloping beach as a large ruler. They located the waterline in images from the NASA and US Geological Survey Landsat programme, combined its movement with the known beach slope, and converted the changing shoreline position into sea-level estimates. More than 40 years of Pacific shoreline observations allowed the team to identify repeating tidal rhythms and reconstruct or predict local tides.
Tests across Pacific-facing coasts found substantial differences over short distances. Besides the near one-metre variation in South Taranaki Bight, tides in Pegasus Bay east of Christchurch were about 40 centimetres higher than at beaches south of the city near the Rakaia River. The results show why a gauge or model located some distance away may not accurately describe conditions at a particular beach during the same storm.
The researchers said finer measurements could strengthen models of compound flooding, in which tides combine with storm surge, rainfall or river flow. They could also improve analysis of sea-level variability and saltwater intrusion. Oxford co-author Thomas Monahan explained that short-range tidal differences may determine whether one neighbouring area floods while another remains safe, making beach-specific forecasts a practical resilience tool.
The team said the method could fill gaps where tide gauges are sparse and eventually track changes in tidal timing and magnitude as sea levels rise. Additional imagery from systems such as Europe's Sentinel-2 constellation may broaden coverage and sharpen estimates, including in under-observed African coastal regions. The next task is to mature the technique so forecasts describe conditions at an individual beach rather than only somewhere nearby.
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