Scientists at the University of Oxford have revealed that ocean tides can vary by almost a metre across a single bay, dramatically altering local flood risks.
The discovery means that neighbouring coastal communities face drastically different risks from storm surges, saltwater intrusion, and pollution spills during identical weather events.
"A major challenge along the coasts is flooding, and this can be exacerbated by ocean tides," said study co-author Dr Thomas Monahan of the University of Oxford. "Given the sharp variations of tides over short distances, this means that for the same storm, two neighbouring locations may have very different levels of flooding."
Dr Monahan added that accounting for hyper-local variations will be essential for keeping communities safe when designing future coastal infrastructure.
The University of Oxford, located in Oxfordshire, England, is one of the world's oldest and most prestigious research institutions.
New satellite technique tracks shoreline waterlines
Understanding tide levels is critical for modelling coastal flood risk because natural tidal patterns combine with storm surges to trigger compound flooding.
"If you have a high tide at the time of a storm, there is a higher chance of a flooding event occurring, while the opposite may reduce the impacts of a storm," said co-author Dr Michael Hart-Davis of the Deutsches Geodätisches Forschungsinstitut.
Scientific understanding of local tidal variations has remained extremely limited. Conventional tide gauges deliver accurate measurements for only a single fixed location, while standard radar satellites have limited range and a resolution typically measured in tens of kilometres.
To obtain a clearer view, the research team developed a technique that uses satellite photographs, treating the beach itself as a massive ruler. As tides rise and fall, waterlines move up and down sloping beaches, allowing researchers to convert waterline positions into sea level data using shoreline slope measurements.
By analyzing hundreds of observations across 40 years of satellite records, the scientists constructed a detailed tidal picture accurate down to a scale of just 100 metres.
The Deutsches Geodätisches Forschungsinstitut is a major geodetic research institute headquartered in Munich, Germany.
Pacific coastline study reveals massive variations
Applying the method to beaches bordering the Pacific Ocean revealed significant tide differences within individual bays.
In New Zealand's South Taranaki Bight, a broad coastal bay on the west coast of the North Island, tides varied by just under one metre across the 56-mile (90km) bay.
Around Christchurch, the largest city in New Zealand's South Island, tides to the east of the city were 15.7 inches (40cm) higher than those to the south.
Researchers noted that these differences are generally caused by a combination of changing ocean depths and the specific shape of the coastline.
Rising global sea levels intensify flood threats
The study follows a report from the American Meteorological Society (AMS) warning that Earth's sea levels are rising to record heights due to climate change.
Global sea levels have reached a record high for the 14th consecutive year, standing approximately 111 millimetres above the 1993 average when satellite measurements began.
The AMS, a prominent scientific organisation founded in the United States, estimates that warming oceans have contributed around 1.6 millimetres per year to sea level rise since 2005 through thermal expansion. Melting glaciers and ice sheets added an average of 2 millimetres per year.
A recent study estimates that glaciers outside the ice sheets of Greenland and Antarctica contain around 150,000 cubic kilometres of ice. If every one of these glaciers melted completely, global sea levels would rise by more than 12 inches (32.3cm).
"When we talk about sea level rise impacts, the static sea level alone rarely causes immediate coastal flooding. Instead, it is the variations on top of this baseline that produce flooding," Dr Hart-Davis told the Daily Mail.
Dr Hart-Davis explained that tidal variability varies significantly over short distances, meaning that adding sea level rise creates non-uniform impacts in flood magnitude and duration.
Catastrophic risks from melting ice sheets
Researchers hope to use 40 years of satellite observations to examine historical tidal changes and improve predictions for where and when flooding will strike.
Wider scientific research warns that the collapse of the Thwaites Glacier in West Antarctica could raise global sea levels by as much as 10 feet (3 metres), with collapse potentially beginning within decades.
Sea level rise threatens major international cities including Shanghai, London, New York, and Sydney, alongside low-lying swathes of Florida, Bangladesh, and the nation of the Maldives. Southern US cities such as New Orleans, Houston, and Miami face severe inundation risks.
A 2014 study by the Union of Concerned Scientists, an American non-profit science advocacy group, examined 52 sea level indicators in communities across the United States. It found that tidal flooding will dramatically increase in many East Coast and Gulf Coast locations based on conservative projections.
By 2030, more than half of the 52 communities studied are projected to experience an average of at least 24 tidal floods annually in exposed areas under moderate projections, with 20 communities seeing flood events triple or more.
The mid-Atlantic coast faces some of the greatest increases. Annapolis, Maryland, and Washington, DC, can expect more than 150 tidal floods per year, while several locations in New Jersey could see 80 or more.
Severe submergence risks for United Kingdom coasts
In the United Kingdom, a sea level rise of 6.7 feet (2 metres) or more would put areas such as Hull, Peterborough, Portsmouth, parts of east London, and the Thames Estuary at risk of becoming submerged.
A paper published in November 2016 in the Proceedings of the National Academy of Sciences found that a two-metre rise by 2040 would submerge large parts of Kent almost completely.
South coast locations like Portsmouth, together with Cambridge and Peterborough, would be heavily affected. Cities and towns surrounding the Humber estuary, including Hull, Scunthorpe, and Grimsby, face intense flooding.
