Forecasting Floods-Induced by Earthquakes
The calm and peaceful Reelfoot Lake in northwestern Tennessee is now known as a sanctuary for migrating birds. However, its origins were marked by a violent and sudden birth. After a series of earthquakes in 1811 and 1812, the flow of the Mississippi River was temporarily diverted, leading to the rapid flooding of the surrounding area and the creation of the lake.
Throughout history, there have been numerous instances of rivers undergoing abrupt course changes as a result of surface-rupturing earthquakes. In an effort to enhance our understanding of the risks associated with these events, a team of researchers has successfully simulated and modeled such occurrences. Their findings, which were published in May in Science Advances, provide valuable insights into the hazards posed by earthquake-induced river course alterations.
Devastation unleashed: Aerial view of the aftermath of an earthquake-induced flood as floodwaters engulf homes and infrastructure leaving a trail of destruction in their wake
A Sudden Lurch
Rivers change course all the time. Avulsions, which are gradual changes in river course driven by erosion and sediment deposition, typically occur over extended periods of time, spanning decades or even centuries. However, the river avulsions examined by Erin McEwan, a tectonic geomorphologist at the University of Canterbury in Christchurch, New Zealand, and her research team, occurred at a significantly accelerated rate.
River Diversion by Fault Scarp: Rapid Course Changes Triggered by Fault Movements-
When a fault abruptly shifts and causes movement in the Earth’s surface, whether horizontally or vertically, it can create a fault scarp that alters the course of a river intersecting the affected area. The fault scarp, formed by the displacement of the Earth’s surface, has the potential to redirect the flow of a river.
“The fault scarp presents an immediate obstacle,” said Timothy Stahl, a geologist at the University of Canterbury and a member of the research team.
A river’s water obviously still has to go somewhere, McEwan said. “It may pool up against the fault scarp and then flow outside of its channel.”
Numerous river avulsions resulting from fault ruptures have been documented in the last few centuries. The likelihood of such events occurring in the future is considerable, considering that there are over 25,000 locations worldwide where active faults intersect with rivers. When human presence is added to this equation, the situation becomes precarious.
McEwan said. “Anywhere [there are] highly active faults, large populations, and rivers overlaying those faults, there’s going to be an issue.”
A Lake in Farmland
With the goal of forecasting the characteristics of fault rupture–induced river avulsions, McEwan and her colleagues conducted a case study of an event that occurred in New Zealand just after midnight on 14 November 2016.
When a magnitude 7.8 earthquake struck the country’s South Island, the Papatea fault suddenly moved roughly 7 meters vertically and 4 meters horizontally where it intersected the Waiau Toa/Clarence River. As the river’s water—flowing at nearly 200 cubic meters per second—encountered that fault scarp, it was forced to change direction by about 45°. It spilled out of its existing channel and coursed for more than a kilometer across farmland.
Those floodwaters went on to form Lake Murray, which inundated more than 80 acres for several months. Even today, more than 6 years later, parts of the Waiau Toa/Clarence River are flowing hundreds of meters from their old channels,
It’s impossible for anyone to know the exact parameters or flow of a river at any moment.
McEwan and her team conducted a modeling study on the Waiau Toa/Clarence River incident. Initially, they aimed to determine if they could replicate the magnitude of the river avulsion by utilizing river discharge data collected shortly before the event and a digital elevation model constructed from lidar imagery of the landscape after the earthquake. The researchers discovered that their 2D simulation achieved a 94% accuracy when compared to actual observations of the river avulsion.
The researchers aimed to test the accuracy of reproducing river avulsions using only pre-event information. They created a digital elevation model of the landscape based on lidar data from 2012 and incorporated a synthetic fault scarp reflecting the actual fault’s movement. By feeding these inputs into their simulation, they achieved an 89% accuracy in replicating the avulsion.
Finally, the researchers simulated a range of possible flooding scenarios by considering five different fault scarp displacements and five different water discharge rates. Researchers simulated possible flood scenarios based on five different fault scarp displacements, and five different water flow rates. The researchers then simulated different flooding scenarios, taking into account five different fault displacements and water discharge rates.
An Avulsion in the Future
Out of the 25 scenarios modeled by the researchers, the Waiau Toa/Clarence River underwent complete abandonment of its original channel and formed new pathways in seven cases. The study revealed that larger fault displacements and higher discharge rates led to increased flooding. Interestingly, the researchers also observed that fault displacement could act as a precursor to future avulsions, which may occur later when the river’s flow rate rises. According to Stahl, the occurrence of an avulsion is not necessarily synchronized with an earthquake and can happen either instantaneously or with a delay.
In certain cases, the researchers observed that fault displacement acted as a catalyst for a future avulsion, which would occur at a later time when the river’s flow rate increased. It became apparent that an avulsion does not always happen immediately after an earthquake.
Stahl said. “It could be instantaneous or delayed.”
The same modeling approach could be applied to evaluate the susceptibilities of other rivers prone to earthquakes.
McEwan said. “That’s what we’d do in places where this hasn’t occurred yet and we’re trying to assess the risk.”
McEwan stated that the findings could provide valuable insights for hazard planning and the establishment of regulations regarding mandatory building setbacks from fault lines.
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