Overlooked channels influence water flow and flooding along Gulf Coast

The coastal plains are relatively flat, which has kept most research on flood risk and water flow focused on large rivers in the region. But the new research led by scientists with UT Austin and the Water Institute of the Gulf revealed that although the surface elevation is steady, the landscape is covered in narrow but deep channels that play an important role in moving water.

"Typically, flood risk has been characterized in Texas and Louisiana based on how close you are to a river," said the study's lead author, John Swartz, who started the research as a doctoral student at the UT Jackson School of Geosciences and is now a research scientist at the Water Institute of the Gulf. "But we see through things like Hurricane Harvey that what is happening to the broader landscape when there is a lot of water present is really important."

The research is published in the journal Nature Geoscience.

Scientists said that understanding the specifics of how the channels move water -- especially during floods -- in different areas will require more research, and ultimately the findings will need to be incorporated in national and regional flood models.

Channels on their own are not an uncommon sight along the coastal plain. However, researchers were able to show the full extent of the branching networks they form by creating a high-resolution elevation map spanning the Gulf Coast from Texas to Mississippi.

The data for the map came from state and federal agencies -- including the Texas General Land Office Natural Resources Information Service, the U.S. Geological Survey and the Federal Emergency Management Agency -- which collected the information over the years for local and regional use.

As it became public, Swartz and his collaborators used the chance to piece it together and get the most comprehensive and detailed look yet at coastal plain topography.

The map captured every 3 meters of the coastal plain and measured elevation changes within a few centimeters. At this scale, a complex array of channel networks emerged.

The channel networks covered more than 12,000 square miles, or about one-third of the study region, and were separated into more than 40 distinct drainage basins. The depth of these basins can be as large as 22-32 feet, which is comparable to the depths of the larger rivers in the area.

"The surface is far from flat," said co-author David Mohrig, a professor at the Jackson School. "And how the topography is distributed has significant consequences for how extreme precipitation is routed across the landscape."

The study has already led to important geological insights about how these channel networks form and evolve.

For example, in most geological settings drainage basins are formed by erosion hollowing out a landscape over millions of years. But in the case of the coastal plain networks, the basins are built up at their edges by accumulated sediment, which was deposited on the landscape by the region's rivers when they followed different paths hundreds to thousands of years ago. As the rivers' paths change in the future, the sediments they leave behind will likely define the edges to new drainage basins -- and a reshaping of the drainage channels and coastal plain landscape.

According to Douglas Edmonds, an associate professor at Indiana University Bloomington who was not part of the study, these findings indicate that the coastal plain is a much more dynamic landscape than many geoscientists thought.

"It's really thought provoking," he said. "I think it challenges a lot of our models for how we think water sculpts the land surface and how it organizes into drainage basins."

Mohrig said that this study exemplifies a growing research stream at the Jackson School called "environmental sedimentology," which focuses on how the dispersal and buildup of sediments over time affects human communities and larger ecosystems.

The study's other co-authors include scientists from UT Austin and the California Institute of Technology.