The tide comes in, and the tide goes out. And the land in between cycles nutrients, stores carbon, and supports a wide range of species from algae to crabs to migratory birds.
Recently, in Australia, a team used 30 years of satellite data to create 3-D maps of more than 15,000 square kilometers of the country’s intertidal zones.
“With knowledge of the three-dimensional structure of the intertidal zone, we can better understand the complex and biodiverse ecosystems found along our coastlines,” said project lead Robbi Bishop-Taylor, an Earth observation scientist at Geoscience Australia in Canberra.
Moreover, “intertidal zones are faced with increasing threats, including coastal erosion, coastal development, and a rise in global sea levels,” he told Eos. “Accurate elevation data that describes the height and shape of the coastline can help predict when and where these threats will have the greatest impact.”
The Land Between Tides
The intertidal zone is often the odd one out when it comes to elevation mapping, Bishop-Taylor explained. Water stymies most land-mapping techniques, and shallow coasts prevent seafloor mappers from getting close enough to shore. What’s more, many of the world’s coasts, including some of Australia’s, are too remote or too hazardous to survey from the ground.
That’s where satellites come in. Bishop-Taylor and his team looked at 30 years of optical and spectroscopic images of Australia’s coasts from the Landsat program to create the National Intertidal Digital Elevation Model (NIDEM).
“Given the dynamic nature of intertidal environments, NIDEM is a much more cost-effective way of mapping intertidal zone elevation at the regional or continental scale than the current methods used,” Bishop-Taylor said.
For every section of Australia’s coast, the team gathered all relevant archived Landsat data, calculated the exact location of the shoreline, and sorted the shorelines by elevation relative to mean sea level. They calculated tidal elevation contours by averaging the measurements at regular intervals and then smoothed between the contours to make a continuous gradient from low tide to high tide.
The team found that NIDEM’s elevations are accurate to less than half a meter along sandy beaches and flatter shorelines, “which approaches the accuracy of more expensive, higher-resolution approaches such as airborne lidar,” Bishop-Taylor said. The model’s accuracy drops to around 3 meters in elevation for rocky and reef shores, where tide level change is more unpredictable. The team published their results in March in Estuarine, Coastal and Shelf Science.
This new way to map the intertidal zone “operationalizes the way we can monitor the elevation of intertidal ecosystems in Australia using satellite remote sensing,” Nick Murray, a conservation ecologist at the University of New South Wales, told Eos.
“To my knowledge, this is the first time [3-D intertidal mapping] has been achieved at such a large scale,” said Murray, who was not involved with this research.
Conserving Coasts Around the World
“Our results reveal the full three-dimensional complexity of intertidal environments along the coastline of Australia,” Bishop-Taylor said. Many of Australia’s intertidal areas “remain hidden from view in current maps and coastal data sets and [some] are mapped in 3-D in NIDEM for the first time.”
Between 1984 and 2016, about 16% of the world’s intertidal zone was lost to land reclamation, coastal erosion, and changes in river sediment balance. These threats, plus the looming specter of sea level rise, will continue to chip away at the intertidal zone and remove vital and biodiverse habitats, the team wrote.
In Port Paterson, South Australia, the intertidal zone spans a mere 4.08 meters of elevation, but it includes many patches of land that are exposed only at low tide. Credit: Robbi Bishop-Taylor
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The team made its data and mapping code open source and hope that others will use their work to build 3-D maps of intertidal zones elsewhere in the world. According to Murray, the team’s mapping techniques “will transform our ability to monitor sea level rise [and] the impacts of coastal development, and manage intertidal environments.”
The next stages of NIDEM seek to increase the maps’ resolution in both space and time using Copernicus Sentinel data from the European Space Agency. The team also plans to adapt their models to make tidal elevation maps that cover shorter, sequential chunks of time “to track absolute volumes of coastal change across time,” Bishop-Taylor said.
—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer
02 August 2019: This article has been updated to clarify that some of these locations have been mapped before.