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What Forecasters Really Mean by a "100-Year Flood"

Unknown disasters, environmental disasters, flooding, natural disasters, severe weather

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 Weather forecasters sometimes warn of storms that unleash such unusual rain they are described as 100-year or even 500-year floods.


 Here’s what to know about how scientists determine how extreme a flood is and how common these extreme events are becoming.

<h2>
 What does a 100-year flood mean?
</h2>

 Scientists use math to help people understand how unusual a severe flood is and how to compare the intensity of one flood to another.


 According to the
 U.S. Geological Survey
 , one statistic scientists use is the percentage chance that a flood of a specific magnitude will happen. A 500-year flood means such an event has a 1 in 500 chance, or 0.2%, of occurring in a year.


 Another concept scientists use is how frequently an event of a certain intensity is expected. For example, a meteorologist can look at the average recurrence interval of an anticipated flood and see that a similar event is only expected once every 25 years.


 Agencies have preferred expressing the percent chance of a flood occurring rather than the recurrence interval because that statistic better represents the fact that rare floods can happen within a few years of each other. It's sort of like rolling a pair of dice and getting double six's twice in a row. It's rare, but statistically possible.


 Another term people hear during an impending flood is that it could be a once-in-a-generation or once-in-a-lifetime event, a casual way of saying a flood could be unlike anything many people have experienced.

<h2>
 How flooding is changing with the climate
</h2>

 Houston, Texas, was struck by three 500-year flood events from 2015 through 2017, according to local officials at the time. The events included Hurricane Harvey, the heaviest recorded rainfall ever in the U.S. Homes and businesses were destroyed and cars were swept away by the floods.


 Although math can calculate how often to expect floods of specific magnitudes, nature has its own plans, including irregularity. Many interconnected systems in the environment, such as local weather patterns and larger events like El Nino, can contribute to the changing likelihood of floods.


 Since the early 1900s, precipitation events have become heavier and more frequent across most of the U.S. and flooding is becoming a bigger issue, according to the
 National Oceanic and Atmospheric Administration
 . Gases emitted by humans, like carbon dioxide and methane, are warming the atmosphere, allowing it to
 hold more water vapor
 . For every 1 degree of Fahrenheit that the temperature warms, the atmosphere can hold nearly 4% more water, which is a 7% increase for every 1 degree Celsius, said Victor Gensini, professor of atmospheric sciences at Northern Illinois University. That vapor eventually falls back to the ground as rain or snow. “We’ve absolutely seen a shift in the probability distribution of heavy rainfall over the last three decades,” Gensini said.


 Other regions have experienced drought due to changing precipitation patterns. According to
 NASA
 , major droughts and periods of excessive precipitation have been occurring more frequently. Globally, the intensity of extreme wet and dry events is closely linked to global warming.


 ___


 This story has been corrected to show that the three 500-year flood events in Houston took place over three years, not 24 months. It has also been corrected to show that the origin of the calculations about 500-year flood events in Houston were from local officials, not researchers at the University of Chicago.


 ___


 Seth Borenstein contributed to this report from Washington, D.C.


 ___


 The Associated Press’ climate and environmental coverage receives financial support from multiple private foundations. AP is solely responsible for all content. Find AP’s
 standards
 for collaborating with charitable organizations, a list of contributors and supported coverage areas is provided
 AP.org
 .

What Forecasters Really Mean by a "100-Year Flood"

Unknown disasters, environmental disasters, flooding, natural disasters, severe weather

Weather forecasters sometimes warn of storms that unleash such unusual rain they are described as 100-year or even 500-year floods. Here’s what to know about how scientists determine how extreme a …

Atmospheric River Threatens Devastating 1,000-Year Flood

Unknown disasters, flooding, meteorology, news, rainfall

<img data-document-id="cms/api/amp/image/AA1C8Qeb" data-reference="image" src="https://img-s-msn-com.akamaized.net/tenant/amp/entityid/AA1C8Qeb.jpg"/>

 An
 atmospheric river taking aim at the Midwest
 and parts of the South could exceed the 500- to 1,000-year average for rainfall in the area, meteorologists warned.

<h2>
 Why It Matters
</h2>

 Atmospheric rivers are a "long, narrow region in the atmosphere—like rivers in the sky—that transport most of the water vapor outside of the tropics," according to the National Oceanic and Atmospheric Administration.


 The storms brought by atmospheric rivers
 are known for their heavy snow, heavy rain and strong winds
 . They more
 commonly affect the West Coast
 , particularly during the winter months.

<h2>
 What To Know
</h2>

 But a similar storm system will bring four months' worth of rain in a period of five days to a 1,000-mile swath in the central U.S.


 The incoming storm is a one-in-1,000-year event. An AccuWeather spokesperson told
 DIwida.Blog & News
 that floods like this have a 0.1 percent chance of happening in any given year.


 On Monday, AccuWeather published a report about the "conveyor belt of moisture," which is going to bring hours of heavy rain from Wednesday to Saturday night from Arkansas through the Mississippi Valley and into the Ohio Valley.


 "That moisture plume, known as an atmospheric river, will be tropical in nature and originate from the Caribbean," AccuWeather Senior Storm Warning Meteorologist William Clark said in the report. "Tropical moisture raises the risk of excessive rainfall."


 Occasionally, the precipitation intensity might surpass multiple inches within an hour, leading to swift flash floods. In certain regions, over twelve inches of rain could accumulate throughout the span of five days.


 Scott Homan, a senior meteorologist at AccuWeather, informed
 DIwida.Blog & News
 The weather patterns will make the storm stationary over the area, resulting in multiple episodes of intense rain showers. He cautioned that “substantial flooding” will affect southeastern Missouri, southern Indiana, Illinois, Kentucky, Tennessee, Arkansas, and northwestern Mississippi.


 Floodwaters can rapidly turn hazardous and potentially deadly as the influx of moisture fills up local rivers, creeks, and streams. According to AccuWeather, major rivers anticipated to experience notable increases in water levels encompass the Ohio, Wabash, White, St. Francis, Kentucky, and Tennessee Rivers.


 National Weather Service (NWS) offices across the region have already issued numerous flood watches ahead of the storm. Flood watches are in place for Ohio, West Virginia, Indiana, Kentucky, Illinois, Tennessee, Missouri, Arkansas, Mississippi and parts of Oklahoma and Texas.

<h2>
 What People Are Saying
</h2>

 
 Clark said in the AccuWeather report:
 
 "Should the amount of rain occur that we anticipate over the middle of the nation, it would exceed the 500 to 1,000-year average. Truly, the potential is there for a historic flash flooding event."


 
 The NWS office in Louisville, Kentucky:
 
 "This is expected to be a high end event with life-threatening flooding. Major flash flooding and river flooding will be possible. Rush to completion any flooding preparation by Wednesday."

<h2>
 What Happens Next
</h2>

 The storm is expected to bring excessive rainfall across the region by Wednesday night and the deluge could continue through Saturday.


 To best prepare, Homan said, people in the forecast area should ensure their cellphones are charged and that they have access to a radio in case power goes out. He urged them to heed any warnings or evacuation orders and take shelter as the storm progresses.


 
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Atmospheric River Threatens Devastating 1,000-Year Flood

Unknown disasters, flooding, meteorology, news, rainfall

An atmospheric river taking aim at the Midwest and parts of the South could exceed the 500- to 1,000-year average for rainfall in the area, meteorologists warned. Why It Matters Atmospheric rive…

Empowering Rivers, Enriching Lives: Rethinking Flood Management for People and Nature

Unknown environmentalism, flooding, sustainability, urban and regional planning, water management

When considering flood management strategies, ideas typically revolve around constructing taller embankments, reinforced levees, and cement walls. However, could the optimal approach – beneficial for both communities and ecosystems – be to allow rivers more room rather than restricting their flow?


 This alternate solution is gaining traction as a method for reducing flood risks. However, giving rivers more space to flow also provides additional benefits.
 ecological benefits
 well past just reducing flood risks. It aids in biodiversity, enhances water quality, and captures carbon.


 With climate change exacerbating the occurrence and severity of
 extreme floods
 , reassessing our strategy for handling floodplain rivers has never been more pressing.

<h2>
 Climate change, flooding, and restricted rivers
</h2>

 Climate change is amplifying flood risks worldwide, and Aotearoa New Zealand is
 no exception
 Large-scale flooding is anticipated to become
 far more regular and intense
 , posing risks to communities, infrastructure, and ecosystems.


 A lot of these risks are
 exacerbated by previous leadership choices
 those with rivers restricted into tight channels artificially, isolating them from their natural floodplains.


 Floodplain river systems have
 historically been dynamic
 , transitioning through different terrains over time. However, extensive retaining walls, alterations to river channels, and land development have limited this natural variation.


 Choking rivers in this manner redistributes and amplifies flooding hazards further downriver by pushing water through narrowed passages at higher velocities. This practice also deteriorates the ecosystems dependent on the natural rise and fall of river dynamics.

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<h2>
 Allowing rivers room to spread
</h2>

 The concept of permitting rivers to resume control over areas on their natural floodplains is not novel.


 In the Netherlands, the
 Room for the River
 The program emerged as a reaction to the floods in 1995, which resulted in extensive evacuations of both residents and livestock. In England, forecasts indicating that the financial hazards linked to flooding could rise twentyfold during this century have sparked initiatives.
 Making Space for Water
 strategy.


 However, these initiatives typically
 stay concentrated on flood defense
 , missing chances to optimize ecological advantages. Our
 new research
 demonstrates that thoughtfully crafted strategies can provide both environmental benefits and flood defense.


 This is essential as floodplain river systems rank amongst the most
 valuable ecosystems
 They offer approximately one-fourth of all terrestrial ecosystem services like water retention and contaminant filtering, along with providing educational opportunities, recreation, and cultural benefits.
 benefits
 .

<h2>
 Managing rivers for variability
</h2>

 A key transformation in managing rivers entails recognizing and adjusting to their inherent variability. Floodplain rivers do not remain constant; they evolve over space and time, reacting to seasonal water movements, sediment transport, and ecosystem dynamics.
 processes
 .

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 Our research
 summarizes the ecological activities triggered when floodplain rivers have space to spread out.


 Rivers that have not been artificially restricted usually
 more physically complex
 For example, alongside the primary river channel, there could be narrower tributaries, spots where the flow collects and decelerates, underground springs emerging to merge back into the above-water currents, or even oxbow lakes within the surrounding plains.


 A wide variety of environments sustains a
 abundant diversity of flora and fauna
 . Even gravel bars, which appear along free-flowing river systems, serve as crucial nesting grounds for endangered bird species.


 Biodiversity isn’t uni-dimensional; rather, it functions across various levels, ranging from a tiny marshland pool to an extensive entire watershed or even broader areas. Within this evolving riverside environment, we could observe variations in the genetic makeup of a species differing throughout distinct sections of the river, or perhaps notice identical fish species showing regional disparities.
 differing in their bodily dimensions
 , based on the habitat conditions.


 These instances of inherent biological variation allow species and ecosystems to remain adaptable when faced with unpredictable future circumstances.

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 On a broader scale, the variety and quantity of species inhabiting distinct floodplain river environments differ as well. Such biodiversity among ecological communities leads to this variation.
 variation in the functions
 Ecosystems function within the river, like absorbing nutrients or breaking down organic material. This capability can even assist with
 diversify food webs
 .


 These differences imply that various species or groups within the river may not experience the same disruptions like droughts or floods simultaneously. The reason for this lies in how different plant and animal populations in rivers operate.
 evolved to take advantage
 In various manners, these patterns examine the extended cycles of flooding and water scarcity occurrences.


 For example, the cottonwood trees in the southwestern United States synchronize their seed dispersal with the reliably recurring patterns of spring flooding caused by melting snow in that region. In Aotearoa New Zealand, certain whitebait fish species usually lay their eggs during strong autumn currents. These eggs later hatch into larvae that are carried out to sea via powerful winter flows.


 Certain creatures require various environments along the river.
 different stages of life
 . Various animals journey long distances to utilize the river floodplains temporarily. This group encompasses the banded dotterel (
 Charadrius bicinctus
 ), native to Aotearoa New Zealand. This bird
 journeys up to 1,700 kilometers for nesting
 Each spring, banded dotterels can be found on braided river gravels. These birds are declining in number, and they depend on river habitats that offer ample room to move around.

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<h2>
 An appeal for enhanced eco-friendly river governance
</h2>

 With the acceleration of climate change, it becomes essential to reconsider our approach to managing water systems. While strengthening embankments and increasing channel depths might appear as sensible solutions for heightened flooding threats, such methods frequently
 exacerbate long-term vulnerabilities
 and transfer risk elsewhere.


 We urge professionals to take action.
 expand the range of values
 incorporated into river management policies and programs to account for ecological variation.


 Nature-based solutions
 These methods aim to enhance benefits for both humans and nature. By cooperating with natural processes instead of opposing them, we can develop environments that are stronger, more adaptable, and conducive to supporting both human needs and biodiversity.


 The moment has come to adopt a fresh approach to managing our rivers—a strategy where we view these waterways not as hazards to subdue, but rather as vital resources essential for protection and rejuvenation.


 
 Christina McCabe is funded via an Aho Hīnātore doctoral research scholarship at the University of Canterbury.
 


 
 Jonathan Tonkin is funded by a Rutherford Discovery Fellowship and the Centre of Research Excellence Te Pūnaha Matatini. Additionally, he receives support from the Antarctic Science Platform as well as the Ministry of Business, Innovation and Employment.

Empowering Rivers, Enriching Lives: Rethinking Flood Management for People and Nature

Unknown environmentalism, flooding, sustainability, urban and regional planning, water management

When considering flood management strategies, ideas typically revolve around constructing taller embankments, reinforced levees, and cement walls. However, could the optimal approach – beneficial fo…