Pluvial Flooding
Climate Change Daily Earth Science Hydrosphere Repost

Pluvial flooding from a guest that won’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 the 26th of August, 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 was about $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 are the result of erosion, where rainwater flows down the slopes and into the valleys, collecting in rivers and streams. As the waters flow, they carry off soil and debris bits, eroding the land to create hills and valleys. 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 terranes 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 and ponds on the surface and slowly drains towards any subtle low spots in the topography. But slopes are low to non-existent, and rainwater starts accumulating where it falls. When water drains off the flatlands more slowly that it accumulates you get 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 ocean 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.


ArcheanWeb:

Flooding and climate change: Death by storm surge (Source: ArcheanWeb) – https://archeanweb.com/2020/08/03/flooding-and-climate-change-death-by-storm-surge/  Also:

Cascading natural events: The perfect storm (Source: ArcheanWeb) – https://archeanweb.com/2020/06/02/cascading-natural-events-the-perfect-storm/  Also:


Sources:

Feature Image: Hurricane Harvey Flooding and Damage (Modified) – By Jill Carlson (jillcarlson.org) from Roman Forest, Texas, USA – Hurricane Harvey Flooding and Damage, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=62246940  

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.

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