Flooding
Climate Change Earth Science Feature Feature 2 Hydrosphere Repost

Climate change: Increasing the frequency and intensity of flooding

Flooding has always been with us, but the future holds more frequent flooding and higher intensity events with a greater capacity for catastrophic flooding. Climate change may not be the root cause of flooding, but it will undoubtedly change how we experience it.

The sight of a person surveying the ruins of their muddy, water-logged home after a flood is heartbreaking. But flooding is a common problem shared each year by people living in mountain retreats, rural farms, urban neighborhoods, and along coastal beaches. However, the connection between climate change and flooding is indirect. Flooding is a natural phenomenon, which started when water first appeared on the planet’s surface some four billion years ago, long before Anthropocene climate change. So, what is the connection between climate change and flooding? The critical connection is climate change amplifies flooding by increasing the frequency and intensity of destructive flood events. But there are many types of flooding, and climate change affects each of them in slightly different ways.

Death by storm surge

Ben Duckworth spent the night of August 17, 1969, clinging to an old oak tree in Pass Christian, Mississippi. He held tight to the tree trunk as sustained winds of over 175 miles-per-hour whipped inland off the Gulf of Mexico. Simultaneously, a 24-foot wall of water slammed into the coast, washing away everything in its path. Hurricane Camille came ashore about midnight and started ripping apart the Richelieu Manor Apartments, where Ben huddled with several other people. His escape to the roof of the three-story building was cut short when wind and water swept him from the roof, carried him inland, and slammed him into the tree that saved his life. In the morning, there was nothing left of the Richelieu Apartments except for the foundation slab.

Thirty-six years after Ben’s night from hell, hurricane Katrina charged northward across the Gulf of Mexico, bringing a 25-foot storm surge. At landfall on August 29, 2005, a mountain of water collided with the Louisiana coast, overwhelming New Orleans’ flood defenses and submerging the city. Katrina became the costliest tropical cyclone on record, leaving over 1800 people dead and a final damage bill of $168 billion (inflation-adjusted).

Storm surge is only one of several types of coastal flooding triggered by hurricanes, but its onslaught is rapid, and the damage it wreaks is sometimes unpredictable. Storm surges are killer events driven by low atmospheric pressure and wind, not rain.

How storm surge works

The two factors controlling storm surge are atmospheric pressure and wind strength. Atmospheric pressure in the center of a hurricane is lower than on the outer edges. This pressure differential causes water to mound up beneath the eye of the storm. The lower the pressure, the higher the water rises. But the pressure surge is not as devastating as the wind-driven surge.

Hurricane winds in the northern hemisphere blow around the eye of a storm in a counterclockwise direction. When Camille and later Katrina made landfall, the winds on the east side of the eye blew landward. Caught between wind shear at the ocean’s surface and frictional resistance on the shallowing ocean bottom, the seas mounded up, creating a massive wind-driven surge.

Pressure and wind worked together, producing the total storm surge. When the Richelieu Apartments collapsed during Camille, the storm surge had already inundated the two lower floors. The surge is more akin to a tidal wave than a normal wave. The pressure it applies against any structure is a relentless push inland. A push that lasts until the storm passes. Very little can stand in the way of a massive storm surge.

Storm surges and climate change

Wind, low pressure, and water, not climate change, create storm surges. But climate change makes storm surges more dangerous, mainly from warmer oceans and stronger hurricanes. Recently published research (Kossin et al.) analyzed 40 years of hurricane data showing the frequency of Category-3 storms has increased since the 1980s. This observation doesn’t mean every hurricane is stronger, or more Category-3 hurricanes occur every year. But it does tell us more Category-3 storms are likely in any given year, and storm intensity is increasing on a decade-by-decade basis.

Due to climate change, increasing storm intensity means the risk of severe storm surge flooding is on the rise — bad news for coastal communities on the Atlantic and Gulf coasts.

Pluvial flooding from a guest that wouldn’t leave

Hurricane Harvey made a long arc out of the Atlantic Ocean, skimming off the coast of South and Central America and crossing the Yucatan Peninsula before heading to Houston. Landfall was on August 26, 2017, near Rockport, Texas. The storm weakened and slowed after coming inland, but the rains kept falling. The stalled storm hovered near the Texas-Louisiana border for about four days, delivering over 40 inches of rain to the area and creating epic flooding. Harvey was the guest that wouldn’t leave, and a great example of pluvial flooding.

Houston went under the floodwaters, and the final cost from flood damage reached $130 billion, making Harvey second to Hurricane Katrina as the most expensive USA disaster. Over 100 deaths were attributed to the storm.

Flat is not where it’s at

Floods come in all types, but pluvial floods represent an unfortunate intersection between rain and topography. Mountainous or hilly terrains result from erosion as rainwater flows down the slopes and collects in the valleys. The flowing water carries off soil and debris, continuously eroding the land, creating undulating topography. Erosion builds the pathways to remove excess water from the land and funnel it back to the oceans. But what happens when there are no hills?

Flash flooding in hilly terrains results from streams or rivers flooding out of their banks because they received too much rainwater and can’t drain it away fast enough. Pluvial flooding, however, is not about water coming from the rivers, it is about water trying to get to the rivers. Water on the surface of the ground relies on slopes and gravity to flow. But flatlands are not conducive to this process.

The first stage of flooding in the flatlands is soil saturation. Rainwater sinks into the local soil and enters the groundwater system. Once the soils are saturated, rainwater pools on the surface and slowly drains towards any subtle low spots in the topography. But when slopes are low to non-existent, rainwater accumulates where it falls. When water drains off the flatlands more slowly than it accumulates, you have pluvial flooding.

Urban areas are particularly sensitive to pluvial flooding since the abundance of concrete pavements and roofs eliminates the ability of soils to absorb any water. So, water ponds in the lowest neighborhoods and progressively moves higher as more rain falls. A city like New Orleans, below sea level and surrounded by levees, is akin to a huge punch bowl that fills with water faster than pumping stations can remove it.

Climate change and pluvial flooding

Catastrophic pluvial flooding relies on massive rainfall and flat topography, making low-lying coastal cities in hurricane-prone regions a prime target for this type of disaster. Harvey hit land as a category four hurricane, drawing moisture from overly warm Gulf waters. Once it made landfall, the storm weakened and stalled, but its water circulation pump was still running, pulling moisture from the warm seas and releasing it inland.

Oceans are warming on a global scale, and warmer water is the fuel that feeds monster cyclones and hurricanes. Average hurricane intensity has increased over the past four decades, leading the charge towards larger, more powerful storms. For low-lying coastal cities, the threat from pluvial flooding is greater than the threat from storm surges, and a warming climate only magnifies this risk.

Fluvial flooding: The river runneth over

Today, in Nelson County, Virginia, nestled in the Appalachian Mountains, you can wander upon patches where bare rocks appear on an otherwise densely forested mountain slope. These are grim reminders of the night of August 19, 1969, when over two feet of rainwater fell in just eight hours. The remnants of Hurricane Camille had drifted northward from the Gulf Coast until they encountered the edge of the jet stream and moisture saturated air. The unique conditions needed for a weather catastrophe developed over the Appalachian Mountains. Rainfall was so intense the runoff stripped portions of the mountain slopes down to bare rock. The next morning, when the sun rose, over 150 people had perished in the flash flooding.

Camille had previously decimated the Gulf Coast when the Category-5 storm came ashore with a 24-foot storm surge. But the storm dumped less than 10 inches of rain in most areas. Storm surge was the purveyor of destruction in Mississippi, but in Nelson County, it was the rain.

Flash forward in time to 2019 and midwestern America experienced over five months of continuous flooding. The late winter season was cold with record snowfall, leaving the ground frozen. But as spring rolled in, temperatures rapidly rose, and heavy rains fell. The thick snow cover quickly melted and combined with the falling rain, producing copious amounts of runoff into the streams and rivers. This process was exacerbated by the frozen soil, which could not absorb any water.

Heavy rains continued through the summer and kept floodwater levels high for months-on-end, affecting over 14 million Americans and costing over $6 billion in damages.

Cumulative flow

Flooding comes in many varieties like storm surge, tidal flooding, pluvial flooding, and fluvial (river) flooding. Both Nelson County and the Midwest were at the mercy of fluvial flooding. This type of flooding is a cumulative flow phenomenon. Mighty rivers like the Mississippi or Amazon pour billions of gallons of water into the oceans each day, but the flow from these rivers represents a collection of water from thousands of miles of inland rivers and streams.

The fluvial flooding process starts in small gullies that collect rain runoff from storms. These normally-dry gullies feed into tiny streams, which gradually combine with larger creeks and small rivers. Smaller rivers then feed into larger rivers, and at each stage of the process, the volume of flowing water increases.

Over millions of years, rivers, both small and large, adjusted their size to accommodate the cumulative flow of their tributaries. But when precipitation is higher than usual, the rivers can’t handle the extra flow, and water spills over the banks, causing flooding. When weather creates extreme conditions, then flooding becomes catastrophic.

Extreme weather

Extreme weather events are increasingly common, and scientists predict that climate change will lead to a higher frequency of catastrophic fluvial flooding. Many factors drive this trend, but two stand out: hot air and a meandering jet stream. A warmer atmosphere holds more moisture than cold air. This higher moisture content inevitably leads to increases in precipitation, and we can expect higher annual rainfall in many areas from global warming.

Temperature issues also drive a meandering jet stream. The Arctic is warming faster than the rest of the globe. A warmer Arctic decreases the temperature differential between the North Pole and the tropics. But the polar vortex, which controls the jet stream, derives its strength from this temperature differential, and so the vortex weakens as the Arctic rapidly warms.

A knock-on effect of a weaker polar vortex is a slower jet stream that tends to meander. Meandering loops of the jet stream do things like draw hot air into Siberia, or force rapid warming in the Midwest, triggering massive snow melts.

The combination of wetter weather and extreme temperature shifts from a meandering jet stream will bring us increased fluvial flooding.

Coastal inundation: Death by a thousand cuts

I recently listened in on a nearby conversation about seal level rise. I wasn’t snooping. They were just loud. “Miami is toast. They’re going under,” is the phrase I remember. Yes and no, I thought. Eventually, they will go under, but it will be death by a thousand cuts. The end won’t come as a catastrophic inundation below floodwaters. Instead, it will creep up slowly, wave by wave, relentlessly sapping life from the city.

Oceans are on the rise from two factors: heat and melt-water. The heat from global warming melts ice caps, adding water to the oceans, and elevating sea levels. But heat is also a factor in-and-of-itself. Water, like many other materials, expands as it warms.

NOAA, in a 2019 report, tracked a global sea-level rise of 9 centimeters over a 25-year period. Thermal expansion from increasing ocean temperatures accounted for 40% of the rise, and the remaining 60% was new melt-water from ice caps.

The report also discussed the effects of rising oceans on coastal communities. Disaster flooding from major storms and nuisance flooding (high-tide flooding) plague homeowners and businesses in low lying coastal areas. However, nuisance flooding provides the most direct indicator of a changing climate. NOAA reports: “In many locations along the U.S. coastline, high-tide flooding is now 300% to more than 900% more frequent than it was 50 years ago.” So, ten days of high-tide flooding in 1970, translates into 90 days of nuisance flooding in 2020.

A slow spiral downward

High-tide flooding is a growing problem, which is exacerbated when the earth, moon, and sun align, and the extra gravitational forces create exceptionally high tides we call “king tides.” This constant barrage of low-level flooding leads to a slow death for some coastal communities — death by a thousand cuts.

At-risk neighborhoods initially experience high-tide flooding in the form of inundated streets and front lawns. The inconvenience factor is high, and salt-water corrosion from regularly driving through the flooded streets reduces a vehicle’s life. These salty waters also make their way into the ground, corroding water and sewage lines throughout the neighborhood. But this is just the beginning.

Eventually, as sea levels creep higher, nuisance flooding makes its way to the foundations of homes, maybe even lapping under the front door. Rot takes hold in the home’s wooden supports, and flooring, carpets, and drywall need replacing. Some homeowners have flood insurance for covering repair expenses. However, others don’t and must pay to mend the damage. Then there are those without insurance or extra money, and they continue living with the mold and damage.

By this point, real estate values in the neighborhood have fallen — buyers are spending their money on homes in dryer locations. Homeowners either lose equity in their homes or their mortgages go underwater as the value of the home drops to less than what they owe the bank. The neighborhood suffers an irreversible decline, and abandoned homes become the norm.

But sea levels keep rising, and the next low-elevation neighborhood finds its streets inundated more often by high-tide flooding. The city doesn’t fall to a single flooding event, sending it to a watery grave. Instead, thousands of small floods gradually force a long, slow, and painful decline into oblivion.

It’s all about heat

Global warming is the common factor that binds together environmental changes, which lead to both higher flooding frequency and increasingly intense flooding events. The oceans rise because heat melts our ice caps, and thermal expansion causes seawater to physically expand. As oceans rise, tidal flooding increases.

Hotter oceans provide the energy for more hurricanes and cyclones to develop into monster storms, leading to increasing devastation in low-lying coastal areas. A warmer atmosphere absorbs more moisture resulting in higher rainfall, increased runoff, and more fluvial and pluvial flooding.

A rapidly warming Arctic weakens the polar vortex and slows down the jet stream, so it meanders. These meanders push warm moist air further north and cold Arctic air further south, causing extreme temperature changes and driving rapid snow melts. Flooding has always been with us, but the future holds more frequent flooding and higher intensity events with a greater capacity for catastrophic flooding. Climate change may not be the root cause of flooding, but it will undoubtedly change how we experience it.


Sources:

Hurricane Camille (Source: PassChristian.net)

Global increase in major tropical cyclone exceedance probability over the past four decades (By James P. Kossin, Kenneth R. Knapp, Timothy L. Olander, and Christopher S. Velden; PNAS)

Hurricanes more dangerous than in the past (Source: ArcheanWeb)

Rising seas: Let’s look at the numbers (Source: ArcheanWeb)

Climate Change: Global Sea Level (By Rebecca Lindsey; NOAA)

Feature Image: Flooding the streets (Modified) – Credit: Chris Gallagher; Unsplash

William House
William is an earth scientist and writer with an interest in providing the science "backstory" for breaking environmental, earth science, and climate change news.