Daily Earth Science Environment Hydrosphere Repost

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 encountered the edge of the jet stream and moisture saturated air, producing the unique conditions needed for a weather catastrophe. 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 had been cold with record snowfall, leaving the ground frozen. But as spring rolled in, temperatures rapidly rose, and heavy rains fell. 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 of the 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 (ponding) 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 future catastrophic fluvial flooding. Many factors drive this trend, but two stand out: hot air and a meandering jet stream. Global warming creates a warmer atmosphere, and warm air 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.

Temperature issues also drive a meandering jet stream. The Arctic is warming faster than the rest of the globe, therefore decreasing the temperature differential between the Arctic and the tropics. But the polar vortex, which controls the jet stream, derives its strength from this temperature differential, and 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, creating rapid warming and snowmelt.

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

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.