Nuisance Flooding

Diving Deeper: Episode 63

car drives on flooded street in Charleston, South Carolina

What does nuisance flooding look like?

A car drives through a flooded Charleston, South Carolina neighborhood. Nuisance flooding causes public inconveniences such as frequent road closures, overwhelmed storm water systems, and compromised infrastructure.


HOST: Welcome to Diving Deeper, I’m your host Kate Nielsen. Today we’ll talk about nuisance flooding and joining us is Billy Sweet, an oceanographer with NOAA’s Center for Operational Oceanographic Products and Services.

Hi Billy! Welcome to our show.

BILLY SWEET: Hi Kate, glad to be here. Thanks for having me on.

HOST: Billy, to start off our discussion today, can you explain what nuisance flooding is for our listeners?

BILLY SWEET: Nuisance flooding is minor tidal flooding that occurs at high tide often times associated with minor impacts such as old sea walls being overtopped, water in low-lying areas of roads, storm water systems that actually have water coming in through the outtake pipes, so a degrading functionality. Not necessarily widespread or damaging, but just that minor and relatively easy to navigate around, but becoming more of a problem.

HOST: And do we see nuisance flooding in all U.S. coastal waters?

BILLY SWEET: It is a national issue. By definition, I use a threshold established locally by the Weather Forecasting Offices of the National Weather Service. They have, through years of impact monitoring, have established levels at which minor impacts will occur and they do this with emergency managers. They have these levels established around the country, more so in the East Coast. On the West Coast, waves are an issue and tide gauges aren’t really set up to sample waves, but flooding occurs on all of our coasts, but the nuisance flooding is becoming more of an issue let’s say in the Mid-Atlantic right now is probably sort of the hot spot of where we have the highest frequency of these types of events.

HOST: Billy, where do we see the greatest impacts from nuisance flooding?

BILLY SWEET: Generally, wherever you have a vulnerable population, and when I say vulnerable maybe they just don’t have the large sea walls in place, and a population. It’s really a nuisance to people. So it’s sort of a combination of where people live and where impacts are occurring. The greatest rates of change generally are in areas where sea level rise has been highest, but the Mid-Atlantic region is particularly susceptible to these types of events just by the fact that they have a lot of Nor’easter exposure and wide continental shelf that readily allows waters to build up when winds start blowing. And, as you know, it’s a very heavily populated area. So right now, that sort of is the area that is really experiencing the greatest frequency of nuisance flooding.

HOST: Is there anywhere in the U.S. that nuisance flooding is not really an issue or a concern at all?

BILLY SWEET: Well, if you look at the levels, at these nuisance levels that the Weather Service has established, you do see some pockets around the country that they’re relatively high, the New England coast for instance. Minor impacts don’t really occur until waters get over two feet above high tide and that may just be due to the simple fact that they are probably accustomed to fairly regular, strong Nor’easters and their infrastructure has been built in such a way and mitigative strategies are already in place.

There’s a few others in the Gulf that may have some strategies, some sea walls already in place due to large hurricanes that were added to give added protection like Galveston that has a very large sea wall, St. Petersburg has a high level, but then again these are usually in these areas in the Gulf that are probably protecting a small area, you go a couple of miles away and then very possibly they’re susceptible to flooding that some of these other areas may not experience.

HOST: And can we predict when nuisance flooding will occur for a coastal community?

BILLY SWEET: Well, we sure would like to. We do give a couple days heads up and that’s sort of the role of the National Weather Service, they will issue coastal flood advisories, when coastal waters are expected to exceed these local nuisance levels. If it’s a more substantial event, it may be a coastal warning when moderate or major flooding will happen. And they generally do this a few days out in advance. So that’s sort of our first line of heads up for emergency preparedness.

In terms of seasonal or annual type of outlooks that we’re experimenting with now, we tend to use history to guide us and there’s two things sort of driving changes that we notice—sea level rise itself as well as climatic patterns associated with the El Niño is the primary driver on the East Coast and West Coast, that with a strengthening El Niño, the weather patterns tend to be more conducive for nuisance level flooding events to occur. Considering history, we can see that there is some level of predictability, but again, best bets are a few days out when the Weather Service really is able to track these types of events forming.

HOST: When you noted that folks may be able to get notice a couple of days ahead of time how do they get this notice?

BILLY SWEET: They get their notice, if an event looks like it’s going to occur, the Weather Service gives a few days, through their models, they track the development of extra-tropical storms, of course tropical storms are tracked by the National Hurricane Center, but types of events that might materialize and cause nuisance level flooding or even greater, more substantial flooding is the role of the local weather forecasting office and that’s what they do. They track whether sea level is expected to be high from a high tide, a spring tide, a perigean tide for instance, but it might also be coupled with the fact that there’s been large ocean waves or local rain is predicted. So it’s a tool in the forecaster’s tool bag to essentially monitor, what are the tide levels, what are the other factors, and when to issue these heads up. That’s their role and it’s a critical piece for people being prepared.

HOST: And how do we measure the impact after a nuisance flooding event? What can we learn from this before the next one?

BILLY SWEET: And that’s really sort of the key piece in understanding where impacts occur. Historically, big events happen and people are very aware of them, you know the most extreme of them all, the types of Sandys and Katrinas that happen, but lesser events are also important and they may not be a named hurricane, but it could just be a strong Nor’easter or maybe it’s that sunny day flooding. It generally requires feedback from the public. The emergency managers oftentimes are the first level of observation—what roads are impacted, what roads consistently seem to be impacted, how do you delineate hot spots—is a current challenge that we’re discussing here at NOAA and elsewhere is when events happen, can we document them so we have a better spatial understanding as to how impacts actually occur and where they occur and where are the areas that are reoccurring due to nuisance level floods that are more common than the less common like the hurricane.

HOST: How does sea level rise affect nuisance flooding in an area?

BILLY SWEET: Well, sea level rise is sort of the main driver, the change in the frequency of these types of events. A half century or so ago, mean sea level was lower, generally in most places in the United States, and it took a much more substantial storm to reach elevations that were impacting. Elevations of our infrastructure really haven’t changed much, sure we might have installed sea walls and we might have made new houses be built on stilts, but the roads and the things that are laying on the ground or sub-surface haven’t really changed in elevation.

So what sea level has done is that it has raised that level of the ocean, it’s reduced the freeboard between let’s say the mean sea level and our infrastructure such that more common tides and storms are now causing impacts when it would have taken a much stronger storm in the past. So, over the last few decades, these types of events have become more frequent and more and more frequent and more and more and more frequent so, it’s oftentimes not a gradual change that we’re witnessing right now.

HOST: Besides sea level rise, are there other factors that may accelerate nuisance flooding?

BILLY SWEET: Well, strictly talking about changes in mean sea level, there are a few components to what causes sea level to change. There’s the climatic portion such as the long-term changes in the ocean, the volume itself due to the thermal expansion, or additions of ice melt that actually increases the mass in the ocean, but there’s also local vertical land motion that may be occurring for non-climatic reasons. It could be natural compaction of sediments or settling of sediments, which can be further exacerbated with withdrawal of fresh water and gas and oil that actually cause the land to sink, causes a relative change in mean sea level and that’s the primary factor. When we look at storms themselves as the storm frequency or storminess changes, there are some cycles that we can note but there really are not long-term changes in the frequency of these types of storms. We tend to get a certain number of Nor’easters each year and so it’s not so much the storminess component, it’s the change in mean sea level, that’s really driving this increase in frequency of nuisance flooding.

HOST: Can you share with us how something like El Niño may make nuisance flooding better or worse in an area?

BILLY SWEET: Well, unfortunately El Niño historically has not been a good thing in terms of the frequency of nuisance flooding. In many places it compounds the effect of sea level rise to begin with. We know sea level rise is causing a long-term increase, in many places this is an accelerating trend to begin with and El Niño compounds that. In any given year, it can be above or below your long-term trend and that’s weather. What El Niño typically does and especially with the stronger/more pronounced El Niños, is it tends to have a direct oceanic signal along the West Coast. You have normal waters, you have higher sea levels for a few reasons which then just raises the elevation of mean sea level such as a few decades worth of sea level rise can happen within a year.

On the East Coast, you tend to have your storms track a little bit more West to East across the U.S. and what this does is actually increases the frequency of storms along much of the mid-Atlantic and in response a higher frequency of storm surges which equate to more nuisance flooding. What we’re actually able to provide is an outlook based upon history with the predicted strength of El Niño provided by NOAA and other modeling efforts around the world when you take sort of an ensemble mean, but we take that into consideration and based upon historical characteristics, areas like Sandy Hook; Atlantic City; Lewes, Delaware; Baltimore; Washington, D.C.; Norfolk; Wilmington, North Carolina; La Jolla, California; San Francisco; Montauk, New York—these numerous sites are expected to increase upwards of 33 all the way up to 125 percent more days with nuisance flooding this coming year than would be expected by the historical trends themselves. Again, it’s sort of a compounding effect which in this case is not necessarily good news and in fact there’s four areas—Sandy Hook, New Jersey; Lewes, Delaware; Washington, D.C.; and Norfolk, Virginia that may very well experience the highest number of days with nuisance flooding that they’ve ever had on record. So, we’ll need to wait and see and hopefully that’s not the case, but if history repeats itself, it means that the outlook isn’t necessarily great.

HOST: Thank you for everything that you’ve shared with us so far to help us understand nuisance flooding just a little bit more. I’d like to shift gears for just a moment and ask how you got involved with this kind of work? For our aspiring oceanographers in the audience, what brought you to NOAA?

BILLY SWEET: Well, I had some experiences putting buoys offshore and working in the field of oceanography and realized that it was something I really enjoyed. I was able to be on the ocean, I was able to use some of the mathematical skills that I developed to apply to real-life issues and I had the opportunity to work for NOAA and I quickly joined forces working for the group that runs all the tide gauges around the country. There was just a wealth of information and still a lot of secrets to be unlocked in the data itself. So, what we’re finding here stems from years and years of very conscientious folks paying attention to data and data quality for a mission that has been so important for this country—safe navigation—but now we’re finding that it’s also telling us about sea level rise and changes in flood frequency and of course these are issues that we’re going to be dealing with for some time and so it’s an opportunity now to help provide for future generations.

HOST: Billy, do you have any final, closing words to leave our listeners with today?

BILLY SWEET: Well, I think, as I sit and look at tide gauge data day after day, I think really what it’s telling us is that we live in a world that’s dynamic—planning for tomorrow doesn’t necessarily mean that you need to assume that today’s world will be the same. We’re living in an area during a time of change and I think it’s important to listen to what history’s told us thus far and plan for the future. So, we’ll continue to monitor changes in flooding as it impacts local communities in the U.S. as a whole and we’ll keep delivering information so it helps people plan for tomorrow.

HOST: Thank you Billy for joining us today on Diving Deeper. To learn more about nuisance flooding, you can visit That’s all for today’s episode, thanks for tuning in.

Diving Deeper podcast icon

Connect with ocean experts in our podcast series that explores questions about the ocean environment. Get ready to Dive Deeper!

Subscribe to Feed | Subscribe in iTunes

Search Our Podcasts
Get Social

More Information

Contact Us