Colorado Disaster: What Is a 100-Year Flood?
This image from the Suomi NPP satellite's VIIRS sensor from the evening of September 11, 2013, shows the storm system that has devastated towns in the foothills of the Rockies in central Colorado.
Oct. 28, 2012 --
The Earth has the most complicated atmosphere in the solar system, so complicated that meteorologists look to other planets to understand how our weather works. Combined with its constantly active plate tectonics, our planet is truly an adventurous place to live. As the East Coast of the United States braces for a storm of historic proportions in Hurricane Sandy, we take a look back at awesome Earth events past, from earthquakes, to blizzards, to floods.
1. 1900 Galveston Hurricane This was the deadliest natural disaster ever to strike the United States. Approximately 8,000 people lost their lives on Sept. 8, 1900 in what was the biggest city in Texas at that time. Ship reports were the prevalent tool for observing hurricanes at sea, and because the wireless telegraph was in its infancy, there was precious little warning. Plus, Galveston residents had seen such storms before. Or so they thought. Many believed that they had already weathered the worst that Mother Nature could throw at them, but this hurricane brought winds above 100 mph. The storm surge knocked buildings off their foundations, leveling virtually every one in town. When it was over, 3,600 homes had been destroyed. The few buildings that withstood the enormous storm (mostly the more solidly built mansions of the wealthier residents) are maintained as tourist attractions today.
2. 2005 Hurricane Katrina Katrina was the sixth-most powerful Atlantic hurricane ever recorded, and the costliest in terms of damage. Peaking on Aug. 28, 2005, with winds hitting 175 mph, it originated in the Bahamas and was christened "Katrina" on Aug. 24. The early warnings quickly declared the potential devastation, as Katrina would weaken for short periods and then come back stronger than before. When the hurricane's sheer size became apparent, mandatory and voluntary evacuation orders were put into place and state and federal resources were targeted to minimize the impact of the coming storm. However, even today the government's response in preparation and recovery is a controversial issue. In the end, Katrina caused 53 different levee breaches in greater New Orleans, devastated the coasts of Mississippi and Alabama and did more than $80 billion in damages. Nearly 2,000 people were killed.
3. Blizzard of 1888 In the northeastern United States, March is supposed to be well past the peak time for dramatic snowfall. However, in March 1888, one of the most severe blizzards in U.S. history hit the New Jersey, New York, Connecticut and Massachusetts. Sustained 45-mph winds drove the 40 to 50 inches of snow, which fell into snowdrifts 50 feet high. Railroads were shut down, fire stations were immobilized and people were confined to their homes for up to a week. The transportation freeze from this event was partially responsible for the creation of the first underground subway system in the United States, which opened nine years later in Boston. About 400 people died from the blizzard and the cold of the week that followed -- 200 of them in New York City alone.
4. 2004 Indian Ocean Tsunami On the day after Christmas in 2004, the Sumatra-Andaman earthquake struck beneath the Indian Ocean. It was the second-strongest earthquake ever recorded on a seismograph, at a magnitude of 9.1 to 9.3, and it unleashed a killing wall of water that would claim more than 230,000 lives in 11 countries. In deep water, tsunamis travel at great speeds (300 to 600 mph) while creating small, barely noticeable waves. However, as they reach land, they slow down dramatically and those waves become enormous. On this day the waves reached as high as 100 feet, about the height of a 10-story building. Among the dead were about 9,000 tourists (mostly European) enjoying the peak holiday travel season. An estimated one-third of the dead were children.
5. 1906 San Francisco Earthquake The earthquake that hit San Francisco and parts of Northern California at just after 5 a.m. on April 18, 1906, is estimated to have been a magnitude 7.7 to 8.2. It ruptured along the San Andreas Fault, booth north and south, for about 300 miles, with the quake center located two miles offshore from the city. As a result of the quake itself, and the ensuing fires that blazed for days, the city of San Francisco was virtually leveled. At first, city and state officials announced the death toll at 376. This would have been a miraculous total had it been true. Fearing that the true figures would adversely affect real estate prices and investment in rebuilding the city, the officials had simply made up that figure. Also, hundreds of casualties in Chinatown had been ignored and unrecorded. Today the number of dead is estimated at 3,000, with 300,000 (about 70 percent of San Francisco's population) left homeless.
6. 1887 Yellow River Flood (Hwang Ho River Flood), China Flood For centuries, farmers along the Yellow River have built dikes to contain the flooding caused by silt accumulation along the riverbed. And for centuries those dikes have eventually given way to the mighty power of the rising river. In 1887, after days and days of heavy rain, the river burst through the man-made restraints and covered the low-lying surrounding areas with such speed and at such volume that most people were trapped without warning. Eventually 50,000 square miles of what had been homes, farms and villages were covered in water from the river. After the flood, millions were left homeless. The resulting pandemic and lack of basic essentials claimed as many lives as the flood itself. Total deaths are estimated to be an almost unthinkable number: between 900,000 and 2 million.
7. 1925 Great Tri-State Tornado This tornado has the dubious distinction of being the longest sustained tornado ever recorded (traveling 219 miles), as well as the deadliest (approximately 700 dead). It moved across Missouri, Illinois and Indiana and was on the ground for three and a half hours. On March 18, it touched down at around 1 p.m. just outside Ellington, Mo., and didn't dissipate until 4:30 p.m., near Petersburg, Ind. Along the way, the tornado destroyed 15,000 homes. The U.S. Weather Bureau's forecast for that day called for "rains and strong shifting winds," which one witness to the tornado later described as "a huge understatement."
Courtesy Mt. Washington State Park
8. 1934 Mount Washington Hurricane On April 12, 1934, the highest surface wind measured anywhere on Earth was clocked by the staff of the Mount Washington Observatory located in New Hampshire. The figure recorded -- 231 mph -- has become the stuff of legend. The day before seemed to indicate a typical spring storm to the three meteorologists on duty, but early the next morning that was obviously not going to be the case. By 5 a.m., winds had reached 150 mph. The readings slowly climbed until 1:21 p.m., when the 231 number was recorded. Meteorologist Sal Pagliuca wrote in the official log, "Our first thought was, will anyone believe it?" The beauty of this incredible show of nature's force? No reported injuries, no recorded property damage.
9. 1991 Halloween Storm (aka The Perfect Storm) The locals call it "the Halloween Nor'easter of 1991." In his book describing the almost unbelievable confluence of circumstances, Sebastian Junger called it "The Perfect Storm." Three separate weather events built slowly over the course of five days. First, a seemingly harmless low-pressure system formed over the Great Lakes. On its way east, it met up with an icy-cold high-pressure system from Canada. This combination formed a storm in the North Atlantic just off the coast of Nova Scotia. But it was the third, most unexpected event that made this real-life Halloween horror so historically devastating. Late-season Hurricane Grace blew up from the south to collide with the ongoing storm, and it was as if the forces of heat and cold, summer and winter, were at war. Waves 30 to 40 feet high hit the New England coast Halloween afternoon, and when it was over damages had amounted to an estimated $208 million. Twelve people were dead, six of them from the crew of the Andrea Gail, the fishing boat documented in Junger's book.
10. The Johnstown Flood On May 31, 1889, Johnstown, Penn., was a town of about 30,000 people and growing, known for the high quality of its steel production. After that day it would become synonymous with disaster. The area was (and is) prone to flooding due to its position at the confluence of the Stony Creek and Little Conemaugh River, and there is speculation that modifications made to a nearby reservoir to convert it to a luxury hunting and fishing club increased the vulnerability of the dam. The rain came to Johnstown from the west and would total as much as 10 inches in 24 hours. Creeks became rampaging rivers. By daybreak, railroad tracks and telegraph lines had been washed away. At mid-morning, the water was 10 feet high on the streets. Just after 3 p.m. the South Fork Dam burst, and for the next 40 minutes, 20 million tons of water had its way with Johnstown and the surrounding towns and villages.
A massive amount of rain has fallen in the region surrounding Boulder, Colo., causing widespread flooding that's killed at least three people and taken out roads and houses, according to news reports. The event has sent 20-foot "walls of water" rushing down mountainsides, destroying bridges and isolating entire towns, Boulder County Sheriff Joe Pelle said in a statement.
The extreme rain and flooding in Colorado was caused when a slow-moving weather system sucked in an unusually large mass of moist air from the Gulf of Mexico, and has been called a "100-year storm." That terminology is a little confusing, though, and requires some explanation.
A 100-year flood or storm is an event that has a 1 percent chance of occurring in any given year, said Robert Kimbrough, a hydrologist at the U.S. Geological Survey (USGS) in Colorado. Provisional data from a gauge on Boulder Creek, which runs through the town of Boulder, suggests that the flash flood that occurred there had about a 1 percent chance of occurring -- thus, a "100-year-flood."
But the USGS doesn't use that term anymore. "The reason we got away from '100-year flood' terminology is because people mistakenly thought it's one that only happens every 100 years," Kimbrough told LiveScience. Instead, the new term is a "one percent annual exceedance probability flood," he said. Though he acknowledged the term doesn't exactly roll off the tongue, it more accurately describes how the term is calculated, he said. (In Images: Extreme Weather Around the World)
Likewise, a 1,000-year flood has a 0.1 percent chance of occurring each year. And a 50-year flood has a 2 percent chance of happening yearly in a given location, Kimbrough added.
While provisional data has been collected from one river gauge, it will take a while to analyze the data collected at scores of gauges around the state, Kimbrough said. Until that happens, it will be difficult to properly quantify the flooding, he said. But Kimbrough did say that it's been the worst in the area since at least the late 1930s.
So-called 100-year floods or rains are very specific to certain locations, said Nolan Doesken, the Colorado state climatologist. Certain areas around Boulder are likely experiencing "100-year floods" or worse, but others aren't, he said. When it comes to rainfall (or snowfall), the duration of the event also must be mentioned. The 10 inches (25 centimeters) of rain that Boulder received in the last 24 hours definitely has less than a 1 percent chance of occurring, Doesken said. (The rain gauge at the National Weather Service's Boulder office had measured 12.3 inches (31 cm) of rain over about two days as of yesterday -- their previous record was 5.5 inches (14 cm) that fell over an entire month.)
Massive Flooding from non stop raining closes down the intersection of Parker and Mississippi in Aurora, Colo.Hector Acevedo/Corbis
Destroyed river gauges
The Satellite Blog of the Cooperative Institute for Meteorological Satellite Studies has said the downpour could be classified as a 1,000-year event, or one that has a 0.1 percent chance of happening. But Doesken noted that this kind of terminology can be problematic, because one needs a lot of data to be able to talk about how likely something is to happen over a long period.
"It's argued that to have a reasonable estimate of a 100-year storm, you'd like to have a couple hundred years of data," he said. "And of course that's a rare thing in the United States." By extension, that means that it would be best to have at least 1,000 years of data before accurately calling something a "1,000-year storm." (Colorado Flood Photos: 100-Year Storm)
Perhaps ironically, the flooding has been so bad that it destroyed three to five river gauges designed to measure floodwaters, Kimbrough said. "One gauge was totally swept away when the river bank collapsed," he said.
Flooding has also prevented researchers at the University Corporation for Atmospheric Research (UCAR) from getting to work — the offices of this large climate research center are closed.
Whence the Rain?
Jeff Weber, a scientist who works at UCAR and lives in Boulder (and whose basement flooded), said all this rain has come as the result of an unusually stationary low pressure system centered over Utah, paired with a high pressure system to the east. This confluence of patterns has sucked in a huge mass of warm, tropical moisture from the Gulf of Mexico and pushed it toward the west. But the Rocky Mountains happened to get in the way — as the air is pushed up, moisture has condensed and fallen out as rain.
"We're dealing with a tropical air mass in a mountainous environment, and that's just a disaster," Weber said. "As the air is pushed up by the mountains, it squeezes out all the rain."
The heavy rain looks likely to subside by this afternoon (Sept. 13), and while some isolated storms are expected this weekend, next week should be dry, he said.
This originally appeared on LiveScience.com. More from LiveScience.com:
Mightiest Floods of the Mississippi River
Natural Disasters: Top 10 U.S. Threats
Colorado Flooding Leaves 3 People Dead
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