Last month my flight home from Chicago was canceled because of an intense rainstorm. In CT the storm was intense enough to cause flash flooding, which washed out roads and bridges and shut down traffic in many areas. The epicenter of the rainfall was in Oxford, CT (where my brother happens to live), which qualified as a 1,000 year flood (on average a flood of this intensity would occur once every 1,000 years). The flooding killed two people, with an estimated $300 million of personal property damage, and much more costly damage to infrastructure.

Is this now the new normal? Will we start seeing 1,000 year floods on a regular basis? How much of this is due to global climate change? The answers to these questions are complicated and dynamic, but basically yes, yes, and partly. This is just one more thing we are not ready for that will require some investment and change in behavior.

First, some flooding basics. There are three categories of floods. Fluvial floods (the most common in the US) occur near rivers and lakes, and essentially result from existing bodies of water overflowing their banks due to heavy cumulative or sudden rainfall. There are also pluvial floods which are also due to rainfall, but occur independent of any existing body of water. The CT flood were mainly pluvial. Finally, there are coastal flood related to the ocean. These can be due to extremely high tide, storm surges from intense storms like hurricanes, and tsunamis which are essentially giant waves.

How does global warming contribute to flooding? First, there has been about 6-8 inches of sea level rise in the last 100 years. As water warms in expands, which causes some of the sea level rise. Also, melting glacial ice ends up in the ocean. Sea ice melt does not contribute, because the ice is already displacing the same amount of water as it would occupy when melted. Higher sea levels means higher high tide, resulting in more tidal flooding. Increased temperature also means there is more moisture in the air which leads to heavier rainfall – more fluvial and pluvial flooding and storm surges.

In terms of flooding damage there are other factors at play as well. We have been developing more property in floodplains – in the US we developed 2 million acres of property in floodplains in the last two decades, half of which was in Florida.

In addition there have been two development trends that can worsen flooding. We also put down a lot of concrete and asphalt. When it rains or there is a storm surge, the water has to go somewhere. Flooding results when the water in exceeds the water out. Water out includes rivers carrying water to the sea, but also the land absorbing water. The more land that is covered with concrete, the farther the water has to spread before it gets absorbed. The result is increased flooding.

Further, local communities often build damns and levies in order to protect themselves from flooding. This can be coastally or along rivers. However – this can make flooding worse. It actually extends the floodplain deeper inland or farther from major rivers, and intensifies the flooding when it occurs. Again, the water has to go somewhere. This means that even communities dozens of miles inland may still be in a coastal floodplain, even if they are not aware and don’t have proper protections (including flood insurance). The result is a predictable increase in flood damage. According to FEMA:

“From 1980–2000, the NFIP paid almost $9.4 billion in flood insurance claims. From 2000–2020, that number increased over 660% to $62.2 billion.”

What can we do? We can’t change the laws of physics. Water is heavy, and flowing water can have massive momentum, capable of causing extreme damage. People caught in a flood learn the hard way how powerful water can be, which is why so many people are just “swept away” by flood waters. Also, once flooding occurs, flowing water will likely carry a lot of debris, which just adds to further damage. We also can’t change the physics of the water cycle – water will evaporate and then rain back down, and will have to flow to bodies of water or get absorbed into soil.

What we can do is everything possible to slow and hopefully stop anthropogenic climate change. This is just one more reason we need to transition to a green economy. Increasing flood damage (and the cost of mitigation) needs to be factored into the cost of emitting CO2.

But we already have the effects of existing climate change, and a certain amount is already baked in over the next century regardless of what we do (it will just be degrees of bad depending on how quickly we decarbonize our industries). This means we need to think about flooding mitigation. This is economically and socially tricky. There are existing communities in floodplains, and it would be no simple matter to uproot and move them. There are also a lot of economic incentives why states and communities would want to expand into floodplains. Lakeside and coastal properties are often attractive.

It does seem reasonable, however, to set limits on development in high risk floodplains, and to encourage shifting to lower risk areas. I don’t think we should uproot communities, but arranging incentives and regulations so that trends over time shift away from floodplains is feasible. Also, if a community is devastated by a flood, perhaps we shouldn’t just rebuild in a floodplain. If we have to rebuild anyway, why not somewhere safer. I know this is massively complex and painful, but just rebuilding in a high (and increasing) risk floodplain does not seem rational.

Local regulations can also require building standards that are resistant to flooding, such as putting homes on raised foundations, and putting structures on relatively high land while leaving lowing lying lands for water flow. Communities in floodplains, in other words, need to be engineered with flooding in mind. Have lots of open soil to absorb water, have adequate drainage to accommodate heavy rainfall, and raise up property as high as possible.

Finally, civil engineers need to continue to study the dynamics of floodplains to make sure, at least, we aren’t making the problem worse when each community just tries to protect themselves. We need an integrated plan to manage the entire floodplain.

It’s a difficult problem, and there is no simple solution. But I have been reading about this topic for years, and it seems like we are still having the same problems and wrangling over the same issues. There are efforts on the Federal level to address flooding, but they all seem either reactive or small scale. We may need an aggressive national-level strategy to properly address this issue. Otherwise – get ready for 1,000 year flooding.