Flooding is a regular occurrence and important for both ecosystems and human society. River floods recharge groundwater, deposit nutrients in surrounding soils, and restore ecological health by removing pollutants. Anywhere it rains or waves break it can flood.

But flooding can also be devastating to society. From ancient settlements to modern megacities people have built near water either on riverbanks or seashores. Recognizing the dangers of flooding, societies have erected structures to hold back waters.

We will never be able to remove the risk of flooding, but we can organize and prepare to reduce our exposure as much as possible. The first step is awareness and an understanding of the three types of flooding: riverine, pluvial, and coastal.

Climate change influences all three through more rain falling from the sky and the rising of the sea.

Riverine flooding happens when the water flowing through a river overtops the riverbanks and moves into the surrounding land. This land the water now occupies is called the floodplain. 

Some of the most spectacular river features are produced from floods. Braided rivers are made up of many shallow channels that intersect and appear woven together. Large sediment loads and relatively steeper topographies are usually associated with braided rivers.

People have tried to control water for centuries. Many of these efforts resulted in mounds of dirt, known as levees, to hold back floodwaters. As construction technology advanced, so did the complexity and scale of flood barriers.

The Dutch are renowned for their ability to hold back water. As extraordinary as these flood defenses are, there is always the risk that they will fail and cause catastrophic damage to the populations they protect. While not an example of riverine flooding, the levee breach in New Orleans from Hurricane Katrina in 2005 resulted in more than 1,500 deaths in Louisiana.

Rather than trying to impose our will on water, some communities aim to restore the natural floodplain and live with the water to reduce overall flood risk. Projects such as the Dos Rios Ranch Preserve, where the San Joaquin and Tuolumne rivers meet, in California are restoring the natural floodplain allowing for a greater buffer between floodwaters and people. But not everywhere is conducive to restoration projects.

Highly urban areas will still need large infrastructure if retreating from the floodplain is not possible. In the image above, much of the city of Lawrence, MA is on the banks of the Merrimack River and highly vulnerable to flooding (Woodwell Climate Research Center). 

From the Latin word for rain, pluvial flooding is a local form of inundation that occurs when the soils cannot infiltrate or stormwater systems cannot convey rainwater fast enough to prevent flooding. This type of flooding occurs outside of river floodplains. City streets become canals because there is nowhere for the water to go.

Out of the three main types of flooding, flood modelers tend to ignore pluvial flooding the most. Simulating such localized flooding requires highly detailed data of a city’s stormwater infrastructure: the pipes, gutters, and pumps along every street.

Many cities and towns have incomplete data records and some have no digital records at all. The scarcity of data makes it even harder to estimate how climate change will impact pluvial flood risk.

Coastal flooding can manifest mainly through several processes. We will focus on the two that are climate-related. One is through storms bringing seawater onto usually dry land, a phenomenon called storm surge. Storms do this through several mechanisms. Winds push water towards the shore causing water levels along the coast to rise temporarily.

Another mechanism is the low atmospheric pressure that accompanies storms. Lower air pressures allow ocean levels to rise since there is less force from the atmosphere pushing down on the ocean. 

The second process is called tidal flooding. Abnormally high tides are not necessarily a danger to human life but they can cause significant damage to infrastructure and buildings.

A particularly devastating combination is when storm surge occurs at the same time as a high tide. A good example of this is when Hurricane Sandy made landfall in New York City in 2012. If the storm had hit during low tide, then the water level would have been several feet lower.

about if where you live is at risk 

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https://probablefutures.org/ 

https://riskfactor.com/

https://www.climatecentral.org/

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