When high or low tide occurs, the water does not necessarily rise and fall uniformly along the entire coastline. In fact, the height of the tide in a bay can vary by up to one meter from one location to another just a few kilometers away.
This is shown in a new study published by researchers from DTU Space in collaboration with colleagues from the University of Oxford and the Technical University of Munich (TUM), as well as the German space technology company OHB. Together, they have analyzed satellite images taken over a period of more than 40 years by the U.S. Landsat program.
Typically, tides are measured using water level gauges located at specific sites or through satellite measurements of the sea surface. Especially in sparsely populated areas, this leaves large areas without direct measurements of tidal height along the coast.
The new method, instead, is based on satellite images of the coastline itself:
"Typically, water level gauges are spaced 50 km apart or more, and measurements are then calculated from their values. With this method, we can instead use satellite images of the coastline to obtain a much more detailed picture of how tidal levels vary geographically along a given coastline," says Professor Ole Baltazar Andersen of DTU Space, a co-author of the study, which has just been published in the Nature journal Communications Earth & Environment .
By tracking the boundary between land and water across thousands of satellite images and combining this data with information about the beach's slope, researchers can calculate changes in water level and thus tidal variations.
This allows for far greater precision than with previous methods, says Michael Hart-Davis, a researcher at DGFI-TUM and the lead author of the article:
"Variations in the coastline have often been regarded as noise that needed to be removed from satellite data. Here, we show that this noise can instead be used as a signal to map tidal patterns on a very local scale," he says.
Improving local models
The extensive satellite data allow researchers to verify and improve existing tidal models, thereby contributing in the long term to more accurate local forecasts of high and low tides.
"The interesting thing is that we can use this new knowledge to improve the models. In the long run, this could give us better local tidal forecasts, even along coastal stretches where we currently have very few measurements," says Ole Baltazar Andersen.
This could be particularly significant in areas where traditional water lavel gauges are few and far between.
"In Denmark, we have many water level gauges and relatively small tidal variations, so here the method isn't quite as significant. But in Greenland, for example, it's completely different. There are only a handful of physical water level gauges, while the daily variation in the tides can be as much as five to six meters. Here, more detailed data along the coast can be of great importance," says Ole Baltazar Andersen.
Better local tidal models may also be relevant in low-lying and exposed coastal areas, such as in countries like Bangladesh. Here, tides interact with factors including wind, currents, and river water, and have a major impact on the risk of devastating floods.
Tides and rising sea levels
The method can also serve as a new tool for researchers studying sea-level rise and changes in the world's coastlines.
Tides are one of the factors that constantly shift the boundary between land and water. If researchers can better determine the local magnitude and variation of tides, it will also be easier to distinguish tidal movements from longer-term changes in the coastline.
This is particularly relevant when researchers are studying how rising sea levels resulting from climate change affect coastal areas.
At the same time, the nature of the tides in terms of both magnitude and timing can change as sea level and local coastal conditions shift. According to the researchers, the more than 40 years of satellite data therefore provide an opportunity to study, in the future, how local tidal conditions change in tandem with rising sea levels.
