The power of La Niña: its effect on rivers, floodplains and humans

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1. Please give a brief summary of your work.

Floods can have a significant impact on human settlements, mostly because 90% of the human population lives 10km away from rivers and other bodies of fresh water. This motivated me to understand:


Why the natural levees of a river are breached only during certain flood events?

How levee breaching can lead to the formation of new river channels?

Why the formation of new river channels only happens at specific locations?


Based on satellite imagery, local and global climatic data, and geological information from a South American river I found that the frequency of levee breaching significantly increases during La Niña climatic events and that the formation of new channels can be triggered and maintained by extreme La Niña events. I also found that the geologic setting can determine where new channels are formed. This research showcases how climate and the geologic setting can control the dynamics of river systems and highlights that if we want to effectively manage the risks associated with living on floodplains we need to understand how these systems are interconnected.

2. Describe your approach and broader findings.

The first step of this research was to examine worldwide satellite imagery to find a river where the natural levees were frequently breached during floods and to use that site as a natural laboratory. Once I found a suitable river system, The Magdalena River in South America, the biggest challenge was to understand why in some years the floods were more intense and resulted in the breaching of the levees. It turns out that the variability of river flow from one year to another in the Magdalena River is largely governed by El Niño Southern Oscillation (ENSO). ENSO has two contrasting phases El Niño and La Niña (girl in Spanish), droughts tend to occur during El Niño phases while wet seasons are considerably stronger during la Niña phases. Interestingly, similar patterns have also been observed in Australia. This allowed me to use similar statistical methods, that had previously been applied in Australia to understand the effect of El Niño-La Niña in precipitation and river flow, to my South American case study.

 

To test if there was a link between the timing of levee breaching and La Niña phases, I compiled 41-years of satellite imagery from the Magdalena River and recorded the specific locations and dates when natural levees of the river were breached. By comparing that information with local records of rainfall and river discharge and global records of ENSO events I found that the frequency of levee breaching significantly increases during La Niña phases, as a direct result of increases in precipitation and river flow. I also discovered that the formation of new river channels was triggered and maintained by frequent and large flood events related to the 2007-2008 and 2010-2011 extreme La Niña events.

 

The remaining piece of the puzzle was to understand why these new river channels were formed in that particular location. To elucidate this, my team and I went to the field to collect topographic data, river depth and sediment samples and combined this information with geological maps and published geophysical data. We found that the new river channels were formed in that particular location because of changes in elevation caused by a fault. This was an exciting finding because it neatly showcases how the geological setting can control river systems.

3. What is the wider contribution, or impact, to your scientific field(s)?

These results illustrate how global-scale climatic processes can trigger levee breaching and the formation of new river channels, providing valuable information for river management authorities and allowing improved mitigation strategies to assess the risks associated with major flood events. This research showcases how climate and the geologic setting can control the dynamics of river systems and highlights that if we want to effectively manage the risks associated with living on floodplains we need to understand how these systems are interconnected.

 

Because ENSO is a global climate phenomenon I was able to make use of advances in statistical methods developed in Australia and apply them to a South American site. Additionally, this cross-disciplinary approach brought together a team of sedimentologists, geomorphologists, climate scientists and hydrologists. Because this research investigates global processes and floods affect the vast majority of human population, our approach can be broadly applied in other locations by using publicly-available datasets (satellite imagery, ENSO records and local climatic data) and open source software.

4. Are there any potential ideas you would like to explore to take this research further?

I would like to understand why in other regions around the world levee breaching has occurred during moderate flood events, soon after a much larger flood had passed. A current hypothesis is that other critical parameters such as water-surface slope and water pressure on the floodplain levee walls can also control whether levee breaching and channel formation occur, or not. However, because those parameters can rarely (if ever) be measured in the field, the role that those parameters play on levee breaching and on channel formation has seldomly been explored or only speculated upon. To overcome this, I want to use numerical modeling to explore the potential role of those parameters, so that we can better understand the hydrodynamics of levee breaching.

 

Rivers and their floodplains can be preserved in the geologic record, thus I am also investigating the long-term geological controls that allow the maintenance and preservation of river systems. The greatest challenge of that research goal is that these long-term controls occur over time scales longer than human-life spans, so their effects are usually assessed via indirect evidence. I am currently using numerical modeling to try to overcome that shortcoming. This approach has the advantage of unambiguously defining all of the input variables so that the geological controls that allow the preservation of fluvial systems can be directly evaluated. A long-term goal of my scientific career is to improve the quantitative tools and methods that geologist use to better understand and interpret the geological record, specifically by combining numerical modeling and insights from Earth-processes that we observe during human-life spans. This aspect of my research has applications for the exploration resources (e.g. water and hydrocarbons) that are accumulated in rocks that were rivers millions of years ago.

5. Please share a link for researchers to access a relevant publication, data-set, or thesis.

Morón, S., Amos, K., Edmonds, D. A., Payenberg, T., Sun, X., Thyer, M. 2017. Avulsion triggering by El Niño–Southern Oscillation and tectonic forcing: The case of the tropical Magdalena River, Colombia. Geological Society of America Bulletin doi: 10.1130/B31580.1

 

Acknowledgments

This research was funded by Chevron Australia Pty Ltd. SM carried out this research while being a recipient of an International Postgraduate Research Scholarship awarded by the Australian Government. Edmonds acknowledges support from National Science Foundation grant 1249330 and from the donors of the American Chemical Society Petroleum Research Fund. We are grateful to the people who provided logistical and technical support and help with the data collection and analysis: R. Serrano, G. Ribbon, J. Grech, C. Rosero. F., M. Ariza, R. Nanson. N. Hoyos, J. C. Restrepo, M. Clark. We specially thank students C. Ortiz, J. Avellaneda for their valuable help in the field. Satellite imagery is from Imagery is from EarthExplorer (http://earthexplorer.usgs.gov/).

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Sara Morón
12 months ago

A massive THANK YOU to everyone who has voted for me!

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