Scientists have discovered significant variations in local tides, with differences of nearly a meter found across single bays. This finding suggests that coastal communities may face disproportionately higher risks of flooding, saltwater intrusion, and pollution spills compared to their neighbors.
Dr. Thomas Monahan of the University of Oxford highlighted that these localized tidal shifts can dramatically alter flood levels during storms. "For the same storm, two neighboring locations may have very different levels of flooding," Monahan stated, emphasizing the need to account for these hyper-local variations in future coastal infrastructure design.
This research emerges as global sea levels continue to rise, reaching a record high for the 14th consecutive year according to the American Meteorological Society (AMS). Global sea levels are now approximately 111 millimeters above the 1993 average, the starting point for satellite measurements.
Understanding tidal patterns is crucial for predicting coastal flooding, particularly 'compound flooding,' which occurs when high tides coincide with storm surges. Dr. Michael Hart-Davis of the Deutsches Geodätisches Forschungsinstitut explained that a high tide during a storm increases the likelihood of flooding, while a low tide might mitigate storm impacts.
Previous methods for measuring tides, such as tide gauges and conventional radar satellites, have limitations. Tide gauges provide precise data for only one spot, and radar satellites typically have a resolution of tens of kilometers.
The new technique utilizes satellite images of the coastline itself, using the beach as a scale. By tracking the waterline's movement up and down the beach slope as tides change, and factoring in the beach's gradient, researchers can calculate sea level changes with high accuracy, down to a 100-meter scale.
Analysis of 40 years of satellite records revealed substantial tidal variations. In New Zealand's South Taranaki Bight, tides varied by nearly one meter across a 90-kilometer bay. Near Christchurch, New Zealand, tides to the east of the city were found to be 40 centimeters higher than those to the south.
These variations are attributed to factors such as changes in ocean depth and coastline shape. As sea levels continue to rise due to climate change, driven by thermal expansion of ocean water and melting ice sheets, these localized tidal differences will significantly impact flood risk, altering the duration and magnitude of inundation.
The AMS reports that warming oceans contribute about 1.6 millimeters per year to sea-level rise, while melting glaciers and ice sheets add approximately two millimeters annually. If all glaciers outside Greenland and Antarctica were to melt, global sea levels could rise by over 32 centimeters.
Researchers plan to apply this new satellite technique to study historical tidal changes in relation to rising sea levels. This could lead to more precise predictions about when and where coastal flooding events are likely to occur.