Scientists reveal predictive tools to combat catastrophic river floods

Creating a new river channel (avulsion) is a natural geomorphological process. Catastrophic changes in river courses can threaten millions of people around the world. New research allows for better prediction of these phenomena and taking appropriate actions reports the journal "Nature".

Thanks to pioneering research, scientists from Indiana University have discovered a way to predict when and where rivers might drastically change their course. This research shed light on a process that has significantly impacted human history. It contributes to the creation of destructive floods and continuously poses a threat to millions of people worldwide.

Water flows downward, following the path of least resistance, often creating bends and meanders, especially on plains, which isn’t always synonymous with flowing straight. In mountainous areas, rivers may find new, steeper channels. This process causes erosion on one bank of the river and sediment accumulation on the other. Water flow depends on climate variables, such as rainfall and air temperature. River channel formation is also influenced by local topography, land use, and other variables.

Meanders typically occur in lowland areas, in rivers' lower and middle sections. Over time, they can shift towards the river mouth, increasing their curvature. When the water level rises, they can be cut off from the main river channel, forming oxbow lakes.

A research team led by James "Jake" Gearon, a PhD student in the Department of Earth and Atmospheric Sciences (EAS) at Indiana University Bloomington, for the first time developed conditions that lead to changes in river courses. The team also includes Harrison Martin, Clarke DeLisle, Eric Barefoot, and Professor Douglas Edmonds. The research utilised advanced satellite technology for terrain imaging. The scientists emphasised that measuring topography around a river is challenging due to vegetation. To this end, they used a satellite that utilises lasers for measurements.

Changes in river courses, which could have inspired ancient flood myths, have caused the largest floods in history and still pose a threat to many people. Edmonds emphasised that "As climate change alters global water cycles and human expansion into flood-prone areas increases, understanding and predicting avulsions has never been more critical."

River course changes occur when the water level surpasses the surrounding landscape, often due to sediment accumulation. This can result in uncontrolled water spilling over the banks and creating a new channel. This leads to flooding because the new channel often appears in areas unprepared for such a volume of water.

Scientists previously believed that sudden changes in river courses resulted from two main factors: the riverbed rising above the floodplain or a steeper path on the sides of the river. Edmonds stated that "We can now actually test these two 80-year-old ideas with the topography data we have collected from space." The data showed that both factors interact and depend on the river's location.

Analysis of data from 174 river avulsions worldwide, using satellite images, reveals that course changes are more common near mountain ranges and coastal areas, where sediments accumulate rapidly. In these regions, 74 per cent of avulsions occurred.

Utilising topographic data, the research team developed a model for mapping "river avulsions" – potential paths rivers might follow if they change their courses. This model can help governments and planners identify high-risk areas of sudden flooding, especially in regions with limited resources.

– Traditional flood models focus on rising water levels from heavy rains, but avulsions can occur without warning, even in areas where rainfall isn't a major concern. This makes them particularly dangerous and difficult to predict, much like earthquakes – said Gearon.

The research results can be valuable for the Global South, where sudden changes in river courses are more common and can have more catastrophic effects. The flood associated with the avulsion of the Indus River in Pakistan in 2010 affected over 20 million people.