BATON ROUGE, La. (Feb. 28, 2022) – Coastal Louisiana and Texas landscapes may seem very flat, but a new study shows how subtle elevation changes and historical channels can create large differences in how floodwaters move through them during high water events.
The new study, “Tributary channel networks formed by depositional processes,” out today in Nature Geoscience takes elevation maps of basins originally collected for flood modeling and flood risk assessment and reveals the network of narrow branching networks that cover the coastal landscape between the Rio Grande and the Mississippi River.
This more comprehensive look at the structure of the coastal landscape illustrates the need for better understanding of how floodplain channels move water, particularly during compound flood events when there is rainfall upriver and storm surge coming from the Gulf of Mexico. Understanding how water flows out of upland areas, and communities, can help how researchers and decision makers model and prepare for these compound floods caused by events like hurricanes.
The work is a collaboration between John Swartz, research scientist at The Water Institute and researchers at The University of Texas at Austin, examining the similar landscapes and challenges found in both Texas and Louisiana. Swartz began the research as a doctoral student at the UT Jackson School of Geosciences.
Within the coastal zone, rivers create ridges as water over tops the banks and leaves sediment behind. As rivers change paths over hundreds to thousands of years, they leave behind accumulated sediment which then form the edges of coastal drainage basins. When examining the new LiDAR datasets being published by government agencies, the researchers started observing the complex structure and organization of the landscape that has been previously overlooked.
“What makes a high point or a low point isn’t a huge difference, but it does make a huge difference in where flooding can occur and how water flows over the landscape,” Swartz said. Currently these channels and their accompanying ridges are seen as boundaries between two water basins, but this new research points out that they aren’t just boundaries, but are instead part of the way water moves through the coastal system. “Using high resolution LiDAR, these channels can cut 20-30 feet down. They really are impressive features and they make a big difference in water flow.”
National water models look at big river systems, but this study looked at the smaller streams that historically distributed water from the larger systems.
“What we found is a structure to the landscape that made sense, but hadn’t been recognized before,” said David Mohrig, professor in the Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin. “It’s not flat. John showed that we can use tools we currently use for larger scale river systems and apply them to these smaller scale systems.”
In short, it’s not just the large river creating changes in the smaller channels, but that these smaller channels have an impact on how these larger waterways flow during high water events. During times when there is flooding upstream as well as high water from the Gulf of Mexico, these basins connect and help direct where that water will flow.
“It’s a new way of looking at this landscape,” Swartz said. “It changes the way you think about how these systems are constructed and how they’ll perform in flood situations”