Increasing coincident surge and rain flooding

An important study was just published in the journal Nature Climate Change, on the topic of coincident rainfall and storm surge, termed “compound flooding”.  We knew that storm surges and heavy rainfall events were both getting worse in some places, such as New York City.  However, most storms with rain don’t have storm surge, and vice-versa, and an example of storm surge without much rainfall is Hurricane Sandy.  On the other extreme is Hurricane Irene, which came with both heavy rains and a moderately bad storm surge that spilled over into some neighborhoods.

The main finding of this paper is that they have discovered several cities in the U.S. where cases of storm surge and severe rainfall are increasingly coming at the same time.  They looked nationwide and found several example cities where these problems are increasingly linked, and New York City was highlighted as one of them.

This is important work, because when you live near the ocean and hear about flooding you often are hearing about compound flooding, and more specifically rainfall flooding that will not drain through sewers because the sewer outlets are submerged by high tides.  Many neighborhoods around New York City, such as Hoboken or Red Hook for example, are at an elevation that is not more than a few feet above high tide, and their sewers are several feet beneath the street, so need to be pumped out at their ocean end.  If there is a high tide or a heavy rain event, then the water can’t be removed quickly enough, and floods the streets.

One thing that’s not certain from the paper is whether the increase in joint occurrence of rain and surge is being caused by manmade climate change or whether it is a passing trend.  We have evidence that global warming has led to heavier rainfall in some places, and there is high confidence that warming will lead to more heavy rain events globally (IPCC AR5, 2013).  We also know that sea level rise is creating more coastal floods, and this will intensify as sea level rise accelerates (IPCC AR5, 2013).  Whether this pair of troublemakers will become closer allies in the future is harder to predict.  But this paper is one of the first ever to lay out useful statistical methods and look at the problem over such a broad area, so it’s a step in the right direction for better understanding the science, and monitoring how things change.

As far as solutions go, levees or storm surge barriers can protect against storm surges, but they will generally not stop rainfall flooding when a neighborhood is below high tide, as will be the case for an increasing number of neighborhoods as sea levels rise.  For this problem, cities like Hoboken and Jersey City have been investing heavily in large pumps, but these often cause complaints due to excessive noise.  And they are increasingly seeking to build green infrastructure that can absorb rain water, such as green roofs or bioswales; anything porous that can at least temporarily hold onto rain water.

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