Study seeks to explain stability of ‘loops’ in coastal canal networks
Satellite images of the Mississippi Delta in Louisiana (left) and the Ganges-Brahmaputra Delta in Bangladesh (right). The study concerned the lack of loops in the Mississippi Delta and the numerous loop channels in the Ganges-Brahmaptura Delta.
How are coastal channels formed and what are their stable configurations? These are the questions a team of researchers, including University of Arkansas assistant geoscience professor John Shaw, attempted to answer in a recent paper.
Answering these questions will be necessary to anticipate coastal changes caused by climate change and human activity, including sea level rise, construction of river dams and other forms of engineering.
“We removed channel networks in a way that we hadn’t anticipated,” Shaw explained. “Because so many people live around these networks, destabilizing them can be dangerous and difficult to adapt.”
As an example, Shaw compared the Mississippi River Delta, where all river channels eventually lead to the ocean, to the Ganges-Brahmaputra Delta in Bangladesh, where the network of channels is so full of loops – a kind of island formed by branching canals – which a boat driver could drive endlessly through the maze of canals.
Both regions face a range of economic challenges as some channels leading to ports are clogged with sediment, while others are growing out of control. The document argues that “a clear understanding of the evolution of the canal network is essential for a sustainable coastal future”.
Loops can be hundreds of square kilometers in size and prime locations for aquaculture, so the question of their stability in the larger network is important. But while they are a common feature of channel networks, little is known about how they form and why they persist over time, i.e. why the less efficient channel does not silt up. not simply.
To examine loop stability, the researchers developed a model inspired by vascular biophysics. Comparing capillaries and veins to rivers and streams, Shaw noted that both are adaptive systems. He explains, “If you put more flow through them, they grow. If you put less, they tend to shrink. This is what happens on a large scale with rivers.
Ultimately, using field data and satellite imagery from 21 different channel networks in their modeling, they linked the stability of the loops to systems where there was a strong interaction between rivers and tides. In relatively loop-free networks, such as the Mississippi Delta, flows do not fluctuate significantly.
The trick to maintaining loops in the network, says Shaw, is that the flow through the system constantly rearranges itself due to different flood and tidal inflows. In this case, all the channels of the loop are sometimes crossed by a significant flow, which allows them to remain open.
Shaw and his colleagues now have a better understanding of the conditions likely to generate loops, even if they cannot predict their exact size and location. Nevertheless, now that they understand the basic framework of their creation, they can begin to assess what precise circumstances will lead to their destruction – and how these in turn may affect human settlements.
The paper, titled Interaction of fluvial and tidal forcings promote loops in coastal channel networks, was published in the Journal of the American Geophysical Union, Geophysical research letter. Shaw, the corresponding author, co-authored the article with Adam Konkol, Jon Schwenk and Eleni Katifori. Their work was funded by the Department of Energy, National Science Foundation, Simons Foundation, and Los Alamos National Laboratory.
About the Department of Geosciences: The Department of Geosciences has its origins in 1873 when the first mineralogy course was offered at the University of Arkansas. Our faculty and students examine the processes that form and shape the Earth’s surface, the natural resources we use, how water and ecosystems are interconnected, variations in climate and paleoclimate, the use and development of geospatial methods and the human geography of ethnicity, gender, social class, social inequalities and religion. The department won $2 million in research grants in fiscal year 2020, and our students benefit from more than $3 million in scholarships donated by generous alumni and friends of the department. To learn more about the Department of Geosciences, please visit our website.
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