While these phrases can make it sound like the flooding is cyclical, only happening once every 100 or 500 years, it is not. Storms are not on a schedule. Rather, these terms refer to statistical probabilities. For example, the “100-year flood” means there is a 1% chance of a flood of that magnitude happening in any given year in a given location. For the 500-year flood, the probability is 0.2%.
Given the right conditions, it is possible to see multiple 100-year or 500-year flood events in a relatively short period of time.Ellicott City, MD, for example, recently saw two 1000-year flood events only two years apart. For more information, watch the video below from the NWS.
For the second time in less than two years, heavy rain unleashed catastrophic flooding in Ellicott City, Maryland this Sunday.
According to the NWS, the area received between 8 and 12 inches of rain in less than four hours. On average, the area gets 4 inches of rain for the entire month of May. This massive amount of precipitation in such a short period overwhelmed streams throughout the region and turned Ellicott City’s historic Main Street into a raging river. The floodwater, which reached as high as the second floor of most buildings, damaged or destroyed numerous businesses and swept away dozens of cars and trees. Local officials say that one man, a sergeant with the National Guard, was killed while trying to rescue people from the fast flowing water.
This type of rainfall is considered a one in one thousand year event. However, that does not mean it can only happen once every thousand years. It is the recurrence interval, a statistical calculation that means an event has a one in one thousand chance (0.1%) of happening in any given year in a given location. Ellicott City experienced an eerily similar event in July 2016.
There were several drivers behind this deadly deluge. First, “training” thunderstorms developed along a stationary front. This is a situation where strong thunderstorms continuously form over the same area – like train cars traveling along a track – dumping excessive amounts of rain.
Although climate change did not cause these storms, it has altered the environment in which they form and is making them more common. As greenhouse gases warm the atmosphere, the air is able to hold more water vapor. More specifically, according to the Clausius–Clapeyron relation, for every increase of 1°F, the saturation level of the atmosphere increases by about 4%. That means there is more evaporation from oceans, rivers, and lakes, and therefore more water vapor available to condense and fall as precipitation.
Another major player in Sunday’s flood was the area’s topography. Founded as a gristmill town in 1772, Ellicott City sits in a valley surrounded by several streams that feed into the Patapsco River. Just ten miles outside of Baltimore, it is a highly urbanized area with extensive amounts concrete and asphalt. These impervious surfaces leave the rainwater with no place to go but racing downhill and through the town.
All of these factors will have to be considered as Ellicott City decides how to rebuild for the second time in two years.
Torrential rain turned Main Street in Ellicott City, MD into a raging river. Credit: S. Baranoski
It is no secret that heavy rain can cause flooding. However, it can be surprising to learn how little water is required to create significant impacts.
As anyone who carries a water bottle knows, water is heavy. In fact, just one cubic foot of fresh water weighs 62.4lbs (28.3kg). Multiplied many times over, raging floodwater can carry away or destroy most things in its path. Moving at just 4-mph, water has enough force to cause structural damage to an average home.
Flowing floodwaters can also pose a danger when hiking or driving. According to NOAA, it only takes six inches of fast moving water to knock a person off their feet. Twelve inches of water can sweep a small car off the road and eighteen to twenty-four inches can float most large vans and SUVs.
Since it is impossible to know how deep water is just by looking at it, it is best to err on the side of caution. As the saying goes, “Turn around, don’t drown!”
Hurricane Nate, the 14th named storm of the 2017 Atlantic Hurricane season, made two landfalls along the US Gulf Coast this weekend. The first was near the mouth of the Mississippi River in Louisiana and the second was near Biloxi, Mississippi.
Coming ashore with 85mph winds, this category-1 hurricane generated a significant storm surge that swamped parts of the region from Louisiana to the Florida Panhandle. One of the hardest hit areas was Pascagoula, MS where a storm surge of 6.3 feet was reported. The storm also downed trees and caused widespread power outages.
Moving inland, Nate was soon downgraded to a tropical depression. However, it still spawned a number of destructive tornados from Alabama to North Carolina. They ranged in strength from EF-0 to EF-2.
Nate was the first hurricane to make landfall in Mississippi since Katrina in 2005.
Days after making landfall as a category-4 hurricane, Harvey, now a tropical storm, is still battering southeast Texas. Relentless rain has unleashed catastrophic flooding across the region, including Houston – this country’s fourth largest city.
While this devastating event is still unfolding, rainfall totals in some areas have already exceeded two feet. Dayton, TX, northeast of Houston, has reported a staggering 39.72 inches of rain since Thursday. Houston Intercontinental Airport (IAH) has posted 25.66 inches of rain so far and reported its wettest calendar day on record on Sunday with 16.07 inches.
This intense rainfall is causing the many rivers, creeks, and bayous in the metro Houston area to overflow their banks and flood homes, businesses, and major roadways – effectively paralyzing the area. Rainfall rates reached as high as 3 inches per hour on Sunday in some areas, prompting the NWS to issue a flash flood emergency – the highest level of a flood alert.
No evacuation order was issued ahead of the storm in Houston and residents were told to shelter in place. Now, thousands of high water rescues are taking place via boat and helicopter to save people trapped in their homes. The situation has many people comparing it to scenes that played out in the aftermath of Hurricane Katrina in New Orleans. The mayor of Houston, Sylvester Turner, has opened the massive George R. Brown Convention Center to be used as a shelter. As of Monday, eight storm-related deaths have been reported. But sadly, local authorities expect this number to increase as the water continues to rise.
Southeast Texas is no stranger to flooding. However, officials in Houston say this event is the worst they have seen since Tropical Storm Allison in 2001. That storm dumped more than 35 inches of rain on the metro area over the course of five days and caused $5 billion worth of damage.
Harvey, wedged between two areas of high pressure, is expected to linger over the region for several more days and dump an additional 15 to 25 inches of rain. Forecasters at the NWS say the rainfall total for this storm could reach an unprecedented 50 inches in some spots.
Eighteen counties in Texas, according to Governor Greg Abbot, have been declared federal disaster areas because of this storm.
Coming ashore with winds measured up to 130mph, Harvey was classified as a Category-4 hurricane on the Saffir-Simpson Scale. These powerful winds along with storm surge flooding and torrential rain caused catastrophic property damage across the Texas Coastal Bend region. It also downed trees and left hundreds of thousands of people without power. To date, according to local officials, only two storm-related deaths have been reported.
Moving inland, toward Houston, Harvey weakened to a Tropical Storm on Saturday afternoon. However, despite this downgrade in wind speed, its rain bands are still drawing moisture from the Gulf of Mexico and unleashing massive amounts of precipitation. Wedged between two areas of high pressure, the storm is essentially stalled over the region. Therefore, even more flooding rain is expected over the next several days.
Harvey was the first major hurricane – category 3 or higher – to make landfall in the Lone Star State since Hurricane Carla in September 1961.
Satellite imagery from NOAA shows Hurricane Harvey along the Texas Coast. Credit: NOAA
Tearing up the coast as it trekked northward, Matthew made landfall near McClellanville, SC on Saturday as a category-1 hurricane with 75mph winds. It had reached category-5 status in the Caribbean – the first storm to do so since Hurricane Felix in 2007- but weakened as it moved toward the US.
Despite this downgrade, Matthew still packed a powerful punch. Its strong winds, flooding rains, and storm surge caused significant property damage and widespread power outages throughout the region. The death toll from this storm currently stands at 26 people from across five states and is expected to increase in the coming days.
With successive bands of heavy rain, Matthew also caused catastrophic inland flooding. In Fayetteville, NC – 100 miles from the coast – 14.82 inches of rain was reported. As a result, several rivers in the region rose to record or near-record levels and overflowed their banks, inundating communities.
All told, Matthew dumped 13.6 trillion gallons of water on Florida, Georgia, South Carolina, North Carolina, and Virginia before heading out to sea as a post-tropical storm. That is enough water to fill over 20 million Olympic-size swimming pools. The highest rainfall total, 17.49 inches, was reported near Savannah, GA.
The damage caused by Matthew is currently estimated at $6 billion.
Hurricane Matthew batters the southeastern US. Credit: NOAA/NASA
Hurricane Hermine, the eighth named storm and fourth hurricane of the 2016 Atlantic Hurricane Season, made landfall in the Big Bend region of Florida early Friday morning. It slammed the Sunshine state’s west coast from Tampa to Tallahassee with heavy rain and winds measured up to 80 mph.
The category-1 hurricane generated a 9-foot storm surge in Cedar Key and dumped more than 22 inches of rain in parts of Pinellas County, flooding many communities. The storm also downed trees and knocked out power to over 250,000 people. Only one storm related death was reported.
Traveling across Florida, Hermine was downgraded to a tropical storm. It is now in the Atlantic moving north along the eastern seaboard. Impacts such as powerful winds, heavy rain, coastal flooding, and dangerous rip currents are expected to be felt from Georgia to Connecticut this holiday weekend.
Hermine was the first hurricane to make landfall in Florida in eleven years.
Hurricane Hermine makes landfall in Florida on September 2, 2016. Credit: NOAA
The Thames Barrier protects millions of people and billions of dollars worth of property in greater London from flooding. As a New Yorker who experienced Superstorm Sandy and its record storm surge first hand, I made a point to visit this crucial piece of engineering during a recent trip to the UK.
Completed in 1982 at the cost of £535 million (about £1.6 billion today), the barrier was built in response to the catastrophic North Sea Flood of 1953. Often called the worst natural disaster to hit the UK, the floodwaters claimed the lives of 307 people and caused widespread damage estimated at £50 million (£1.2 billion today).
According to the UK Met Office, the deadly flood was caused by the combination of a high spring tide and an intense extra-tropical storm in the North Sea. Together, they generated a storm surge of 18.4 feet above average sea level. Moving upstream during the overnight hours of January 31, 1953, the high water overwhelmed the existing floodwalls and inundated communities along the Thames Estuary with little or no warning.
Situated downstream of central London, the barrier consists of ten individual steel gates that span a section of the river that is 1700 feet wide. It is the second largest movable flood barrier in the world, after the Oosterscheldekering barrier in the Netherlands. The Netherlands were also hard hit by the 1953 storm, with over 1800 lives lost to floodwaters.
When a storm surge or an exceptionally high tide is expected, all of the individual gates of the Thames Barrier are closed creating a solid steel wall, approximately five stories high, across the river. While this protects London from flooding from the sea, the barrier can also be used to help reduce fluvial flooding caused by heavy rainfall. When a high amount of water is forecast to flow downriver, the barrier is closed just after low tide. This creates a volume of space behind the barrier – sort of like a temporary reservoir – for the extra water coming downstream to fill. Without the barrier, the incoming tide would take up this space and cause the river water to rise even higher and spill out of its banks.
To date, according to the UK Environment Agency, the Barrier has been closed 176 times since it became operational 34 years ago. Of these closings, 89 were to protect against tidal flooding and 87 were to help alleviate fluvial flooding. At the time it was built, it was only expected to be used 2 to 3 times per year.
Looking ahead, as the climate warms, heavy precipitation events in the UK are expected to increase and sea levels will continue to rise. This means the barrier will most likely be called into action even more often.
While there was debate about the feasibility and cost of building the barrier, as there is with any large government project, it has repeatedly proven itself to have been a worthwhile investment. It is expected to remain a viable flood defense tool through the 2060’s.
The Thames Barrier protects London from flooding. Credit: Melissa Fleming.
The River Thames is as much a part of London as the buildings and palaces that sit along its banks. During a recent visit to the UK’s capital city, I learned more about this storied river and its long history of flooding.
London is situated along the tideway – the part of the Thames that is subject to tides. As such, it faces a flood threat from exceptionally high tides and powerful storm surges sent up river from the North Sea. Heavy rains that fall west of the city can also send torrents downstream.
These are the bronze lion heads that line both sides of the Thames at Victoria Embankment in central London. Installed in the late 1860’s as part of the Great London Sewage Works project, they are often overlooked today as decorative moorings.
While they do not offer any protection from floods, the lions serve as visual markers for rising water. Basically, if the water level reaches their mouths, the city is at risk of a flood. Or, as the locals like to say, “When the lions drink, London will sink.”
Bronze lion heads line the wall of the River Thames, London.