Tides can vary by almost a meter across a single bay, four decades of satellite images reveal

New method extracts beach-scale tide measurements from satellite images, revealing significant variations between beaches missed by existing systems.

Tide levels can vary by almost a metre across a single bay, according to a new study that uses decades of satellite images to measure tides at the scale of individual beaches.

Researchers at Technische Universität München and the University of Oxford have led the development of a new technique that can estimate tides along the coast at intervals of just 100 metres. This provides a level of local detail that is difficult to capture using existing tide gauges and satellite measurements.

The study has been published in Nature Communications: Earth and Environment.

Accurate information about tide levels is important for everything from modelling coastal flood risk and planning navigation to understanding how tides are changing as sea levels rise. But up to now, our ability to measure tides has been severely limited. Observations from tide gauges are restricted to individual locations whereas conventional satellite measurements using radar have limited coverage close to shore, with a resolution typically in the region of tens of kilometres.

The new approach instead uses satellite photographs of shorelines provided by the Landsat programme, run by NASA and the US Geological Survey. This provides the longest continuous space-based record of Earth’s land that exists.

Crucially, the researchers did not measure the height of the sea directly from the photographs. Instead, the technique uses the beach itself like a giant ruler. As the tide rises and falls, the waterline moves up and down the sloping beach. Satellite images record where that waterline sits, and, using the slope of the beach, the researchers can translate this movement into changes in sea level.

By analysing measurements of shorelines bordering the Pacific Ocean collected over more than 40 years, the researchers could identify the characteristic repeating rhythms of the tides. Once these tidal patterns have been identified for a particular location, they can be used to predict the tide there at different points in the past or future, and potentially investigate how those patterns are changing over time.

Applying the method across countries bordering the Pacific Ocean enabled the researchers to estimate tides for every 100-metre segment of coastline studied. In New Zealand's South Taranaki Bight, for instance, the technique revealed differences in tidal height of almost one metre across the approximately 90km-long bay.

For the beaches around Christchurch, New Zealand, the researchers found that tides for Pegasus Bay to the east of the city were around 40 cm higher than those for the beaches south of the city, around Rakaia River.

These findings demonstrate that tides can change substantially over relatively short distances along the coast, meaning a prediction based on observations or models some distance away may not always capture conditions at a particular beach.

Study co-author Dr Thomas Monahan (Department of Engineering Science, University of Oxford) said: “Our research shows that tides can vary substantially over relatively short distances. This isn’t just important for activities such as fishing or surfing, but it has big impacts on coastal flooding. Short scale variations in tides can mean the difference between two neighbouring regions being safe or flooded for the same storm. As these methods mature, they could contribute to much more localised tide forecasts, telling people not simply what the tide is doing ‘near here’, but what it is doing at their beach.”

The researchers say the approach could be applied to a range of both tide and sea-level studies. This would help to fill important gaps in the global ocean observing system, particularly in coastal areas where tide gauges are sparse.

Better measurements of coastal tides could also improve ocean and tide models, with potential benefits for understanding and predicting processes including compound flooding (where several factors combine to increase flood risk), sea-level variability and saltwater intrusion. The 40-year satellite record could ultimately help scientists investigate how the magnitude and timing of tides are changing as sea levels rise.

Lead author Dr Michael Hart-Davis (Deutsches Geodätisches Forschungsinstitut, DGFI-TUM) said: “Tides are a key driver of the ocean and greater climate system and have been studied for centuries. However, the coastal zone, where they have the greatest impact on navigation, pollution spills, safety and flooding, remains a gap in oceanographic knowledge. By showing that satellite images of the shoreline can be used to monitor ocean tides, our new study opens up important applications, from improving scientific models and forecasts to coastal protection and helping communities become more resilient in a changing environment.”

As more imagery satellites provide data, such as the Copernicus Sentinel-2 satellites, the researchers hope that this could enable greater coverage and improved tidal estimations, particularly for resource-poor regions such as Africa.

“This work is not only of interest to the scientific community, but to all who enjoy visiting coastlines and beaches,” added Dr Hart-Davis. “Having spent my life surfing, swimming in the sea, walking dogs on the beach and lounging around, I know too well how reliant we are on accurate tide tables and predictions to have the best beach going experiences.”