A Changing Flood Recipe for Las Vegas

A Changing Flood Recipe for Las Vegas

A Changing Flood Recipe for Las Vegas

January 18, 2022
LAS VEGAS, Nevada

Urbanization
Climate Change
Flooding

Above: Las Vegas after thunder storm with flood water in November 2019. Photo Credit: 4kodiak, iStock. 

A new study shows that urbanization and climate change are changing the strength and seasonality of flooding in the Las Vegas region

Las Vegas, with its rapid urbanization and desert landscape, is highly vulnerable to flooding. For this reason, flood managers have built an extensive system of drainage ditches and detention basins to protect the public. Now, a new study shows how intentional engineering and urban development are interacting with climate change to alter the timing and intensity of flood risk.

In a study published Jan. 6 in The Journal of Hydrometeorology, researchers from DRI, the Clark County Regional Flood Control District, the University of Wisconsin- Madison, and Guangdong University of Technology examine Las Vegas’ changing flood regime. Their results show that flood intensity has increased since the mid-20th century, with an abrupt shift occurring in the mid-1990s. Climate change has also shifted flood seasonality, with the storms and their resultant floods now occurring more frequently in winter, in contrast with the historically stronger summer monsoon season.

“When I looked at the data for annual flood peaks, I could see that something is changing,” said Guo Yu, Ph.D., lead author on the new study and hydrologist at DRI. “I wanted to understand the reason for this change as well as the physical mechanisms driving it, because that will help water managers and the public understand whether such a change will continue in the future, given climate and land use changes here.”

Las Vegas is one of the fastest growing metropolitan regions in the country. In 1950, fewer than 35 thousand people resided in the region; by 2020, that number grew to 2.6 million. Like many cities in the arid Southwest, development centers on the valley floor and spreads up into the natural topography of the surrounding mountains. As concrete and pavement replace more porous desert soils, the risk of flooding in human communities rises – catastrophic floods have caused fatalities as recently as 2022. To mitigate this risk, the Clark County Regional Flood Control District constructed a complex series of storm drains and culverts to capture and direct the flow of water away from populated areas and toward Lake Mead.

Over the same period, climate change has led to shifts in seasonal rainfall patterns. The Southwest has two distinct flood seasons: winter floods produced by atmospheric rivers and summer floods linked to the North American monsoon. Since 1950, daily rainfall amounts have increased in winter and decreased in the summer months.

“Historically, people in Las Vegas haven’t paid as much attention to winter floods as to summer floods,” Yu said. “But our research shows that there will be more frequent winter floods happening because of climate change. This is because the warmer sea surface temperatures on the Pacific coast will cause more atmospheric rivers, like what we’re seeing this January in California. And when these are positioned to bypass the Sierra Nevada mountains, they will very likely hit Las Vegas and cause severe winter rainfall and floods.”

The new research demonstrates an overall picture of shifting intensity and seasonality of floods in Las Vegas. The study authors are continuing to refine their understanding of flood risk in the region with an upcoming study, currently under review, that examines changing rainfall patterns in more detail.

“A lot of research focuses on a single driver – either land use or climate – but in Las Vegas, our study shows that both are changing and interacting with each other,” said Yu.

More information:

The full study, The Nonstationary Flood Hydrology of an Urbanizing Arid Watershed, is available from The Journal of Hydrometeorology: https://doi.org/10.1175/JHM-D-22-0117.1

Study authors include: DRI researchers Guo Yu, Julianne Miller, Benjamin J. Hatchett, and Markus Berli; as well as Daniel B. Wright (University of Wisconsin, Madison); Craig McDougall (Clark County Regional Flood Control District); and Zhihua Zhu (Guangdong University of Technology, Guangzhou, China).

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About DRI

The Desert Research Institute (DRI) is a recognized world leader in basic and applied environmental research. Committed to scientific excellence and integrity, DRI faculty, students who work alongside them, and staff have developed scientific knowledge and innovative technologies in research projects around the globe. Since 1959, DRI’s research has advanced scientific knowledge on topics ranging from humans’ impact on the environment to the environment’s impact on humans. DRI’s impactful science and inspiring solutions support Nevada’s diverse economy, provide science-based educational opportunities, and inform policymakers, business leaders, and community members. With campuses in Las Vegas and Reno, DRI serves as the non-profit research arm of the Nevada System of Higher Education. For more information, please visit www.dri.edu

Study Explores Uncertainties in Flood Risk Estimates

Study Explores Uncertainties in Flood Risk Estimates

Study Explores Uncertainties in Flood Risk Estimates

June 14, 2022
RENO, Nev. 

Hydrology
Climate
Flood Risk

Above: The Truckee River in Reno, Nev. during high flow conditions after a storm in late January, 2016. 

Credit: Kelsey Fitzgerald/DRI.

Results show a need to revise existing methods for estimating flood risk

Flood frequency analysis is a technique used to estimate flood risk, providing statistics such as the “100-year flood” or “500-year flood” that are critical to infrastructure design, dam safety analysis, and flood mapping in flood-prone areas. But the method used to calculate these flood frequencies is due for an update, according to a new study by scientists from DRI, University of Wisconsin-Madison, and Colorado State University 

Floods, even in a single watershed, are known to be caused by a variety of sources, including  rainfall, snowmelt, or “rain-on-snow” events in which rain falls on existing snowpack. However, flood frequencies have traditionally been estimated under the assumption these flood “drivers,” or root causes, are unimportant. 

In a new open-access paper in Geophysical Research Letters, a team led by Guo Yu, Ph.D., of DRI examined the most common drivers (rainfall, snowmelt, and rain-on-snow events) of historic floods for 308 watersheds in the Western U.S., and investigated the impact of different flood types on the resulting flood frequencies. 

Their findings showed that most (64 percent) watersheds frequently experienced two or three flood types throughout the study period, and that rainfall-driven floods, including rain-on-snow, tended to be substantially larger than snowmelt floods across watershed sizes.   

Further analysis showed that by neglecting the unique roles of each flood type, conventional methods for generating flood frequency estimates tended to result in under-estimation of flood frequency at more than half of sites, especially at the 100-year flood and beyond. 

“In practice, the role of different mechanisms has often been ignored in deriving the flood frequencies,” said Yu, a Maki postdoctoral research associate at DRI. “This is partly due to the lack of physics-based understanding of historic floods. In this study, we showed that neglecting such information can result in uncertainties in estimated flood frequencies which are critical for infrastructure.” 

The study findings have important implications for estimating flood frequencies into the future, as climate change pushes conditions in snowmelt-dominated watersheds toward increased rainfall. 

“How the 100-year flood will evolve in the future due to climate change is one of the most important unanswered questions in water resources management,” said Wright, an associate professor in Civil and Environmental Engineering at University of Wisconsin-Madison. “To answer it, we need to focus on the fundamental science of how the water cycle, including extreme rainstorms and snow dynamics, are and will continue to change in a warming climate.” 

The study team hopes that this research is useful to engineers, who rely on accurate estimates of flood frequencies when building bridges and other infrastructure. Although many engineers realize that there is a problem with the conventional way of estimating flood frequencies, this study provides new insights into the level of inaccuracy that results.  

“This study shows that taking into account different physical processes can improve flood risk assessment,” said Frances Davenport, Ph.D., postdoctoral research fellow at Colorado State University. “Importantly, this result suggests both a need and opportunity to develop new methods of flood frequency assessment that will more accurately reflect flood risk in a warming climate.” 

More information: 

The full study, Diverse Physical Processes Drive Upper-Tail Flood Quantiles in the US Mountain West, is available from Geophysical Research Letters: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022GL098855  

This project was funded by the DRI’s Maki Postdoctoral fellowship, U.S. National Science Foundation Hydrologic Sciences Program (award number EAR-1749638), and Stanford University. Study authors included Guo Yu (DRI/University of Wisconsin-Madison), Daniel Wright (University of Wisconsin-Madison), and Frances Davenport (Stanford University and Colorado State University).  

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About DRI 

The Desert Research Institute (DRI) is a recognized world leader in basic and applied environmental research. Committed to scientific excellence and integrity, DRI faculty, students who work alongside them, and staff have developed scientific knowledge and innovative technologies in research projects around the globe. Since 1959, DRI’s research has advanced scientific knowledge on topics ranging from humans’ impact on the environment to the environment’s impact on humans. DRI’s impactful science and inspiring solutions support Nevada’s diverse economy, provide science-based educational opportunities, and inform policymakers, business leaders, and community members. With campuses in Las Vegas and Reno, DRI serves as the non-profit research arm of the Nevada System of Higher Education. For more information, please visit www.dri.edu. 

About Colorado State University’s Walter Scott, Jr. College of Engineering 

Colorado State is one of the nation’s top public research universities with about 33,000 students and $447 million in annual research funding. The Walter Scott, Jr. College of Engineering at CSU prepares students to solve global challenges to shape a better world through research, education, innovation, and outreach. In addition to a top-ranked graduate program in atmospheric science, the college conducts cutting-edge, interdisciplinary research that provides students hands-on learning in biological, biomedical, chemical, civil, computer, electrical, environmental, mechanical, and systems engineering. The college attracts about $80 million in annual research dollars, placing it in the top tier of public institutions of similar size, and is a campus leader in patents, startups, and technology transfer. For more information, please visit www.engr.colostate.edu.