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Thirty U.S. states have a coastline of some kind, and most coastal areas are experiencing an increase in flooding. In this SciLine media briefing, scientists discussed which coastal regions are most vulnerable to different types of floods; the effects of flooding on local wastewater treatment systems and other municipal infrastructure; risks to aquifers and crops from saltwater intrusion; and projections of regional flood patterns for the near future.

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RICK WEISS: Hello, everyone and welcome to SciLine’s media briefing on coastal flooding. A climate-related risk that’s actually relevant to more than half of U.S. states when you consider our extensive coastlines that are oceanic and Great Lakes. I’m SciLine’s director, Rick Weiss. And for those of you who are not familiar with us, SciLine is a philanthropically funded, editorially independent, free service for journalists and scientists based at the non-profit American Association for the Advancement of Science. Our mission is simply to make it as easy as possible for reporters like you to get more scientifically validated evidence into your news stories. And that means not just stories that are about science, but any story that can be strengthened with some science. Among other things, we offer a free matching service that helps connect you directly to scientists who are both deeply knowledgeable in their field and are vetted by us for their communication skills. They’re available to you on deadline. Just go to sciline.org and click on “I need an expert.” And while you’re there, check out our other helpful reporting resources. A couple of quick logistical details before we get started, we have three panelists today, as usual, who are going to make short 5 to 7 minute presentations. Each before we open things up for Q&A. If you’d like to enter a question please, do so during the presentation or after by hovering over the bottom of your Zoom window, that Q&A tab, and enter your name and news outlet and your question. And if you want to pose that question to a particular panelist, please be sure to note that. A full video of this briefing should be available on our website by the end of the day today—maybe tomorrow morning. Time-stamped transcript within a couple of days. But, if you’d like raw copy of this recording more immediately, just submit a request with your name and email in the Q&A box and we’ll get that to you for sure before the end of the day today. You can also use the Q&A box to alert staff to any technical difficulties.

All right, now I’m not going to give full introductions to our speakers, their bios are on the SciLine website. I’m just going to tell you that we will hear first from Dr. William Sweet. He’s an oceanographer at the National Oceanic and Atmospheric Administration—or NOAA—where he spearheads efforts to predict and track changes in sea level and coastal flood risk to support evidence-based decision making. And that’s of course exactly the kind of decision making that we want our public servants to facilitate. So, as a former fed myself, let me just say, Dr. Sweet, thank you for your service in that regard. Dr. Sweet will give an overview of coastal flooding, including a description of what the future may hold given current climate projections. Second, we’re going to hear from Dr. Michelle Hummel, an assistant professor in the department of civil engineering at the University of Texas Arlington. Who will focus on the impacts of coastal flooding on wastewater treatment systems and other local infrastructure. Which is something not all of us think about much but is actually a significant risk here. And third, we’ll hear from Dr. Holly Michael, director of the Delaware Environmental Institute at the University of Delaware, who’s going to focus on the problem of saltwater intrusion into coastal groundwater and the impacts of that on freshwater aquifers and on agriculture. OK, so let’s get started and over to you, Dr. Sweet.


WILLIAM SWEET: Great. Thank you very much. Glad to be here. Share my screen. All right, well, as mentioned, I’ll talk about sea levels and high tide flooding past and future.


RICK WEISS: I don’t see your screen yet, though.


WILLIAM SWEET: Try again. I apologize.


WILLIAM SWEET: How about now?


RICK WEISS: Not showing. We can also show that deck ourselves.


WILLIAM SWEET: Let’s just do that then.


RICK WEISS: OK, there it is. I see it from someone. So you’re good to go.

Overview of coastal flooding


WILLIAM SWEET: All right, super. Hi. My name’s William Sweet, I work for NOAA’s National Ocean Service. I’ll speak today about sea levels and high tide flooding past and future. Sea levels are rising along the United States coastline. On average, they’ve risen about one foot in the last 100 years, and that rate is accelerating over the last 50 years. And that’s concerning. The past is not the indicator of the future. Where is the rise headed? That is what we are here to help answer. I am part of a federal task force that’s focused on providing sea level rise information for the United States federal agencies, as well as states, municipalities, practitioners of all sorts. We just released our last report last year focused on three main deliverables. How much might sea levels rise in the next 30 years? Can we give clearer, concise, confident answers as to how much we should expect? But how much might sea levels rise at the end of the century? Can we establish goal posts, considering the known unknowns? Could be as high as this, could be as low as that. It’s conditional upon future heating and emissions, things that we do not have a crystal ball to determine what we collectively as humans will do. Thirdly, and most importantly, is what does that mean to us as humans that live along the coast and ecosystems? What’s the risk now? And by 2050 with that expected level of sea level rise, what does that mean in terms of three flood types? Minors, typically disruptive. Moderates, typically damaging. And major, which is often destructive. How are those risks going to change?

So, looking at this again, at a sort of a U.S. level, in the report, the data sets we get down to very local levels but just for a broad stroke here, sea levels, that black line, is the last 50 years of the same plot I just showed. The green is the trajectory. Where are we headed? Well, we’re headed for about a foot of sea level rise in the next 30 years, which would match that over the last 100. The colorfully lines are these very scenarios of future sea levels. Of under high emissions and high warming or low emissions and low warming. Perhaps some instabilities in the ice sheets, repaid disintegration of the Doomsday glaciers, let’s say. But to note, in the next 30 years, there’s not that much distinction. We have a thermal momentum, a pathway that we are on. After this midcentury, emissions really do matter and there’s a lot of uncertainty. But for the next 30 years, it’s much more—we can speak with clarity and confidence of where we are heading in terms of sea level rise. And when we frame this in terms of the types of flooding as mentioned—minor, moderate, major—using the thresholds established by NOAA’s National Weather Service and local emergency managers, used for impact forecasting on a day-to-day basis. National Weather Service has issued a coastal flood warning, or coastal flood watch. Really tuned and calibrated impacts on the ground. We are really trying to graduate risk within FEMA’s floodplain. So, a minor flood is one that’s about 2 feet above average high tide. A moderate is about 3 feet above average high tide. And a major flood is about 4 feet above average high tide. It could be higher, but that’s sort of the threshold of these classifications. And when we look at this, what has sea level rise done? SLR. On the X axis on the bottom is that height, above mean height high water. So that minor flood is about 2 feet, the major flood is about 4 feet. The area under the curve is 365 days on average. And what we see is through time, sea level rise continues to creep up and run amok amongst our infrastructure that is on the ground. And the vulnerabilities, which I just showed you in these flood definitions, major flood’s becoming more likely but it’s still something that happens maybe ever 5 years, every 10 years. Not something that’s happening every year. But the minor floods are happening every year, and we’re starting to see many times dozens of these within communities and it’s getting underneath that fat of the curve mean and it’s accelerating. A little bit of sea level rise goes a long way in terms of the types of impacts that are really starting to grow in leaps and bounds.

And here’s an example. We give warnings now. In the next few days, what should you expect? If you’re blinking, you’re likely to flood. What does that flood look like? So, the green here would be a tide gauge measurement. When you get to about 2 feet above average high tide, we start to get impacts. A community that I live in, firepits are jeopardized. More importantly, piers are going underwater. It’s costly. But downtown is flooding, too. People aren’t shopping. Water is up to the streets. It’s a problem. It’s not just weighing on people’s minds, but it’s weighing on commerce and commutes within communities. And these numbers are growing, as I mentioned, in leaps and bounds. This is the number of days per year with a flood occurring within Annapolis. And so it’s not a smooth, gradual change. It’s growing, it’s accelerating. And so when we look at this, at the three flavors of flooding now—minor, moderate, major—on the left what we show here is on average within the United States. About 3 events per year. Maybe 2 days per event in 2020. Moderate, about .3 events per year. Maybe you have about a 1 in 3 chance in a given year and so forth.

The major, about a 4% chance, .04. So, about once every 25 years. In 2050, in the inner highlights with sea level rise that is expected to occur, what we find is the moderate floods, those that are damaging, take on a frequency even greater than that of the minor. So, in essence, we’re going to have a flood regime shift. Moderate flooding will take over the frequency of minor flooding. So, disruptive flooding is going to become damaging. Damaging flooding is going to become more destructive. It’s still not something that’s happening every year, but it’s headed that way. So, quickly, some maps showing these layers. Red is flooded at minor. Moderate is flooded during the moderate floods. And major is flooded about 4 feet. Red’s always flooded, that’s we give it the red definition. But you can see an area like Norfolk, the largest Naval base in the world. There’s a lot of areas that are at risk now. 2020 numbers, minor flooding, about 5 events per year. By 2050, moderate flooding will be happening 5 to 10 times per year. Minor flooding will become sort of a normal day occurrence. And so, when we look at that moderate flooding in Norfolk, as an example. 8 or 9 inches of water on the road, leading into the largest Naval base in the world, it’s difficult for service members to get onto base at this level. 1990, this would be about a 20% chance in any given year of this kind of flood to occur. Now, it’s about once per year. 2050, 5 to 10 events per year. It’s a problem. So, with that, I will end and I’ll pass it over to our next speaker.


RICK WEISS: Thank you, Dr. Sweet. Really good and little bit scary introduction there. Over to you, Dr. Hummel.


MICHELLE HUMMEL: Thank you. OK, can you see everything?


RICK WEISS: Looks good.

Imacts of coastal flooding on wastewater treatment & other infrastructure


MICHELLE HUMMEL: Great. Hello, everybody. So, today I want to spend a few minutes discussing the challenges that coastal flooding poses, particularly for our critical infrastructure systems. And in particular, I want to focus on the approximately 16,000 centralized wastewater treatment systems in the U.S.. Approximately 74% of the U.S. population depends on these systems to treat and clean their used water before it is discharged back to our rivers and coasts. And this helps to ensure that we have safe water quality that can support recreational use and ecosystem health. In coastal regions, many of these wastewater treatment plants are located at low elevations near the coastline, because this allows wastewater to travel by gravity to those treatment plants, and thus minimizes the need for pumping. While this approach has been economical for the operation of these wastewater systems in the past, it also makes them susceptible to coastal flooding. When plants become flooded, this can lead to failure of mechanical and electrical systems, which could potentially shut down the plant operations. And even if those components remain dry, floodwater that enters other parts of the treatment works can overwhelm the plant’s capacity and potentially require untreated sewage to be discharged directly to nearby waterways, which can lead to impaired water quality.

I also want to note here that the other 26% of the U.S. population who aren’t served by these centralized treatment plants largely rely on onsite septic systems to treat their sewage, and these septic systems, which are buried underground, are also very susceptible to flood induced failures. And may require costly repairs by homeowners after a flood event. So, in the past, hurricane driven storm surge and rainfall have led to severe flood impacts for wastewater systems, like the facility you can see here in Houston, Texas, which was flooded during Hurricane Harvey. While many disruptions to wastewater facilities are relatively short-lived, some flood impacts can extend for weeks or potentially months, thus causing substantial logistical challenges for system managers. So, the graph that you can see here shows the time after Hurricane Harvey made landfill in Texas at the end of August 2017 on the horizontal axis. And the number of non-operational wastewater treatment plants on the vertical axis. And so we see that the total of 40 wastewater treatment plants in Texas were offline for at least some period of time in early September. Most of these plants recovered and were operational again within a week or so. But some plants were still not functional up to four months later. So, this really highlights the lasting influence that flooding can have for these systems.

As we look to the future, inundation due to sea level rise is a growing concern for many of these low-lying coastal wastewater treatment plants. My team conducted an analysis of the number of wastewater treatment plants across U.S. coastal states that would experience inundation during high tide. And the results are summarized here with the states listed along the first column and the number of plants experiencing flooding with between 1 and 6 feet of sea level rise shown in the subsequent claims. We found that with just 1 foot of sea level rise, approximately 60 wastewater treatment plants serving more than 4 million people across 18 of the U.S. states could experience flood hazards. And when we look at a 6 foot sea level rise scenario, this increases to 394 plants serving almost 32 million people who would no longer be able to depend on reliable wastewater services unless substantial mitigation measures are put into place. Coastal inundation from storm surge and sea level rise also threatens other types of critical infrastructure, as you can see here. Like our roadways, our airports, and power generation facilities. And as with wastewater treatment plants, flood related disruptions to these facilities can lead to widespread and potentially cascading failures that can impact entire communities or have regional or even national economic consequences. These cascading failures can occur within a single infrastructure network, like the road network, where if one road becomes flooded, this can lead to delays in congestion throughout the rest of the transportation network. But these types of disruptions can also occur between two or more critical infrastructure networks. For example, a failure at an electrical substation could cause a loss of power at a neighboring wastewater treatment plant, potentially causing disruptions to the treatment processes if appropriate backup power supplies are not in place.

So, what can we do to address these hazards and to mitigate the impacts of future flood events on critical infrastructure systems? We can consider this at two different scales. So, at the scale of the individual facility, we typically have three options to choose from. The first option and the one that is most commonly used is to protect the facility. Typically using engineered structures like seawalls or levees, or nature based systems like dunes. A second option is to accommodate floodwaters by modifying our infrastructure to lessen the impacts of flood events. So for example, we could elevate structures or roadways so that floodwaters can pass underneath them without causing damage. However, this is likely not a viable option for wastewater infrastructure due to the extensive networks of tanks, pipes, and electrical components. Many of which are buried underground. Finally, relocating vulnerable facilities to higher ground would likely provide the greatest reduction in flood exposure in the long-term, as this could help to completely remove the facilities from the coastal flood hazard zone. But finding and permitting new locations for some of these facilities may be cost prohibitive. So, it’s important that we consider flood risk holistically across the expected lifetime of our infrastructure systems to determine which of these solutions is best for a given facility. And then secondly, we can also consider this at the network scale.

So, considering these infrastructure networks as a whole, one important adaptation option that we have is to try to build redundancy into these systems. A redundant system offers multiple pathways to move materials, whether we’re talking about wastewater or energy or traffic. So if one facility or path is disrupted due to flooding, other facilities or paths are still available to continue providing necessary services. In the context of wastewater systems providing interconnections between treatment plants would allow wastewater to be diverted to another plant if one goes offline due to flooding or other disruptions. A recent example of this is the Cedar Bayou Wastewater Treatment Plant near Houston, which was inundated by more than 6 feet of water during Hurricane Harvey. But this facility was able to pump its wastewater to a nearby sewer that connected to another treatment facility that was still operational in the city of Baytown. And that allowed for continual treatment of the district sewage, despite the catastrophic flooding that occurred. So, solutions such as this can provide substantial benefits by maintaining the functionality of our critical infrastructure systems during flood events. And I have a couple resources here in case you would like to read more. And I will turn it over to our next speaker.


RICK WEISS: Thanks, Dr. Hummel. So many great story ideas there. I want to know right away what my local wastewater treatment plant elevation is relative to flood risk. The prospect of tens of millions of people around the country being vulnerable to this kind of breakdown for a few feet of water is daunting. Dr. Michael, over to you.


HOLLY MICHAEL: Great, thank you. Shared my screen. OK. Can you see the full screen?


RICK WEISS: Perfect.

The problem of saltwater intrusion into coastal groundwater


HOLLY MICHAEL: OK. Thank you for having me. I look forward to talking with everyone today. I’m Holly Michael. I am at the University of Delaware, where I’m a professor of hydrogeology and director of our environmental institute. See if it will advance. Maybe not. There we go. So, when I think about coastal water resources—and in particular groundwater resources—I think of them as being squeezed between land and sea. Between sources of contamination. So, on the land, we have sources of contamination from agriculture. So, fertilizers and pesticides, we have municipal pollution, we have industrial pollution, we have septic systems, et cetera, that can all infiltrate our water resources. And then on the sea side, we have this infinite source of contamination, which is seawater. And so ,this diagram depicts a coastal community. So, hopefully you can see my cursor. This is the ocean over here, and the land side, and then we cut through the subsurface and show the groundwater system. So, groundwater typically exists in the pores of soil and aquifer sands. So, it’s everywhere beneath our feet. Sometimes people think of it as underground rivers, but really, for the most part, groundwater is just in the pores and it can move, slowly. And so, on land, we have fresh groundwater and that’s a major resource especially for coastal communities. And then under the sea, we have saline groundwater. So, the ocean bottom isn’t hardened. The water continues under the ground. And so that’s a natural thing. And seawater is more dense than freshwater. So, freshwater typically sort of floats on top of it, and naturally seawater, saline groundwater comes inland.

But when we talk about seawater intrusion, that’s when this naturally occurring saline groundwater moves inland and salinizes fresh resources. So, what causes this? Well, there are a number of different mechanisms. And now I’m showing just a cross section. A simple diagram. Again, in blue is the fresh groundwater. And the green is the seawater. And underneath that is saline groundwater. And here, I’ve just drawn a simple interface between the freshwater and the saltwater. So, when sea level rises, you can think of it as sort of upsetting a seesaw. So, when sea level rises, then on the sea side, pressures rise and that moves this interface inland. And salinizes fresh groundwater. Also, as the coastline migrates, you can get vertical infiltration of that dense saltwater downward. If on the land side things change, so for example, we have less precipitation or more evapotranspiration, that can cause water tables to drop. And again, that causes this saltwater to move inland. Pumping—we use groundwater and when we use it, we pump it out. And that reduces pressures on land, allowing the saltwater again to move inland. And storm surges. So, with climate change, storm surges are predicted to become more frequent and more intense. And so we expect more saline flooding. And again, vertical infiltration of saltwater. And then lastly, surface water also experiences saltwater intrusion. So, as sea level rises, or as streamflow depletes, the saltwater front can move inland. And you only need a tiny bit of seawater, only 2% or less, to ruin a freshwater resource both for drinking and for irrigation purposes.

Seawater intrusion occurs on all the coasts of the U.S. I’ve just highlighted a few locations here. And I’ll talk about a few later as well. And it occurs globally. And again, I’ve just picked out a few studies here. More than 600 million people live within 10 meters of sea level vertically. And 2.4 billion people live within 100 kilometers of a coastline. So many, many people globally are affected potentially. And this is a big problem for water resources, but it’s also—saltwater intrusion and sea level rise are also causing ecological change. So, I want to talk a little bit about that. And that’s due both because soils and water are salinizing, but also because a rise in sea level causes a rise in the water levels on land. And that can drown the root zone in coastal systems. So, there are effects on first ecosystems. And so, we see the emergence of what we call ghost forests, these are dead trees where marshes are moving inland or migrating. And this is making the news particularly in the mid-Atlantic of the U.S. The same is true for effects on agriculture. So, there are portions of agricultural fields that are just becoming burnt out in saline or flooded.

So, what do we do about these threats to our water resources and to our ecosystems? As a researcher, I think we need to do more research to understand the scope of the problem. To understand what’s happening, the mechanisms, the economics, the social feedbacks. And put all these things together and try and better predict what’s going to happen in the future so we can prevent it. Communication and education is really important. And that’s what all of you are doing, so thank you for that. Monitoring. We need to know what the water levels are, what the salinities are, what kind of contamination is there? So, for example, if we have a pumping well, we can install wells in front of it to monitor and have early detection of saltwater intrusion. We could also do really cool things like fly a helicopter with electromagnetics and actual see down into the subsurface. The USGS has done this in several places, one of which recently was along the coastlines of the Delaware Bay. And then solutions. One is management. We have to be really careful about how we use our water. Because once groundwater becomes saline, it’s really hard to get that salt out. Engineering solutions, some of which Dr. Hummel mentioned. So, there are hydraulic barriers, and hardened infrastructure. I’ll give a couple examples of these. I think a really important one is green infrastructure, which can reduce the feedbacks with climate change. It can actually take some carbon out of the atmosphere. And most importantly, I think, making sure our policies are science based.

So, just a couple quick examples. This is an example of an engineering solution, which is desalination of water. This is happening in Cape May, New Jersey. And in this case, a desalination plant was installed in 1998. One thing to keep in mind is that desal requires a lot of energy and maintenance and I found this article from last year that indicates that Cape May is looking for funding for a new plant. Because of course, infrastructure ages. Another solution for groundwater seawater intrusion is something called artificial recharge. This has been in place in Los Angeles for a long time. Where if this is a pumping well and you have this saltwater moving inland due to pumping, you can actually inject water closer to the coastline. And that forces that saltwater out. Of course, you need saltwater for this, and sometimes that’s treated wastewater. It also requires energy. Green infrastructure. Those are things like living shorelines. So oyster aquaculture that protect shorelines. Or forested and tidal wetlands that mitigate flooding. Et cetera. So, those are options, but of course, there are also options of hardening the shoreline. But there are studies that show that those hardened shorelines can have ecological impacts. So, impacts on fish and crustaceans, but they also are not sequestering carbon the way that, say, wetlands would. And also, this is a study from my group where we showed that allowing marshes to move inland, say, into agricultural land will actually protect the agland behind it from flooding. And so I’ll just end with saying saltwater intrusion is a complex and growing issue in the U.S. and worldwide. And mitigation and prevention will require collaboration among policy makers, scientists, and communities. Thank you, I look forward to your questions, and here are some resources as well.


What is being done well in press coverage of coastal flooding, and where is there room for improvement?


RICK WEISS: Fantastic. Super interesting problem. I’ll remind reporters that these slides will be available on the website for you to look at more closely after the briefing. Also to remind you that you can send your questions into that Q&A icon at the bottom of your screen. And to get started. I always ask the same question in these media briefings just to get started because I want to help reporters do their jobs as well as possible. And what I like to ask is what is one thing that each of our panelists here today, from their own reading of the news on their topics of expertise, think that reporters are either doing well as they’ve been covering this beat, or things they could do better at? Some room for improvement. And why don’t we just quickly go around the horn and do that starting with you, Dr. Sweet.


WILLIAM SWEET: Well, I think one of the things the media could continue to work towards is eliminating the effects of sea level rise in terms of sort of more of the minor events. A lot of the attention usually gets put on big events. Hurricane Sandy and its influence of climate change. But I would argue probably not the best time to talk about climate change with such a big events. Big events do occur and past environments, climates, today’s climates and future climates. But it’s the I would say less salient effects of sea level rise, high tide flooding, we’re calling at NOAA chipping away at infrastructure, stormwater systems, wastewater systems, groundwater, things that we just heard about today. Are the impacts that just aren’t the billion dollar disasters, but are really starting to stack up. And I suspect will be the motivator for change at the coast to really motivate folks to spend money, say we have a problem, Houston. We’re flooding like we didn’t used to flood and it’s causing all sorts of problems. And I think that is one focus that I think could continue to be a spotlight for the media.


RICK WEISS: That’s a really interesting point. And often a weakness in journalism to wait until something huge happens when actually the news is what’s happening worse and worse every day. Great point. Dr. Hummel.


MICHELLE HUMMEL: Sure, I think one of the things that reporters do well is really putting a local spin on these sort of global and national scale problems. So oftentimes, as researchers, we report on big numbers and they can be kind of scary or hard for people to comprehend. Especially how that might impact them locally. But I’ve seen a lot of great journalism that’s really talking to individual residents who are being impacted or operators of infrastructure systems to think about what are the things that they’re seeing on the ground now, and what sort of approaches and solutions are they considering? And I think that really helps to take this large-scale problem and make it more relevant for folks and to really get them thinking about how they could be impacted and how they could ultimately adapt in the future.


RICK WEISS: Great, thank you. And Dr. Michael?


HOLLY MICHAEL: Yeah. I definitely agree with Dr. Sweet and Dr. Hummel. In fact, Dr. Sweet, what you were saying about these smaller events, we’re finding in our research that those are actually the ones that are driving most of the change. Especially the ecological change that we see. But I was going to say that one thing I think the media are doing well is covering flooding in extreme events and trying to tie them to climate change and the bigger picture. But I think that in my view, there is a lot of focus on surface flooding. Because that’s what we all see. And that’s what’s affecting our homes. But I think that we should be concerned also with subsurface flooding, groundwater salinization. I think that’s something that the public might not understand as well as effects that are happening above land surface.

Is relocating wastewater infrastructure to be further away from water a sustainable mitigation strategy?


RICK WEISS: Great. Start looking underground. OK. That was very helpful to get started. We do have some questions coming in so let me give a start here. The first question is from Douglas Glass at the Associated Press. This is directed to Dr. Hummel. But we’ll encourage others to chime in if they like. “Is there any evolution or revolution inciting of wastewater infrastructure to move those facilities away from water, or is it just impractical and mitigation has to happen in other ways?”


MICHELLE HUMMEL: I think a lot of it is also event driven. So for example, in Florida, I’m aware of at least one wastewater treatment plant that was flooded during a hurricane back in the early 2000s. And as a result of that, in the planning effort to decide whether to rebuild or whether to relocate, they ultimately decided that given the potential for future damages from storm surge and the threat of sea level rise, that it would be better to relocate that facility inland. And so, they closed the original damaged facility and built a completely new facility. But again, that was in the aftermath of an event. So, I don’t think there’s as much of this sort of proactive planning. Including a climate informed approach for thinking about how to manage these infrastructure systems. I think a lot of it is responsive to individual damaging events.


RICK WEISS: Dr. Sweet or Dr. Michael, is there any sense of whether people are thinking ahead, yet, as they think about new projects or land use?


WILLIAM SWEET: Well, federal agencies are. Engineers, for one, definitely have been considering sea level rise in their risk reduction efforts and project planning now for a decade or two. So, it’s just not a uniform federal policy, obviously, of how states, municipalities are going to go about preparing for the future. Typically impacts are local and so are the solutions, found and funded locally. So it typically is a challenge. Those communities that had the resources to respond will. And those that don’t, hopefully they recognize change is at hand. That’s one thing we’re trying to do, have governments make sure folks have equal access to the data so they don’t need to spend their time and energy trying to figure out what to plan for that they can use their assets and money to actually do something.


RICK WEISS: Understood.


HOLLY MICHAEL: And we find the same in communities that we work with around the Delmarva Peninsula. That it’s very much local and it’s very variable. And there are some communities that are really planning ahead and changing their regulations and others that aren’t.

How is sea level rise affecting inland waterways?


RICK WEISS: Well, the good news in this from the journalism point of view I think is that it means local reporters have a role here to play. It’s a great opportunity to highlight exactly where the solution needs to happen. So, good opportunity there. Question here from David Mitchell at The Advocate in Baton Rouge. “I work in an area of South Louisiana that has plenty of inland waterways that are tied to the coastal and tidal fluctuations. How is sea level rise effecting these inland waterways? How does a 2 foot rise translate into higher waters in a river upstream? Has FEMA taken into account sea level rise and its risk rating 2.0?” Dr. Sweet, it sounds like it’s got your name on it for starters.


WILLIAM SWEET: I’ll do my best. Risk rating, 2.0, as far as I know definitely is more graduated risk. To my knowledge, we’re still not to the forward looking sea level rise component. Flood risk is difficult, whether it’s storm surge in waves, is it a compound flooding of riverine meets the coast? That’s a scientific—that’s not very well observed or modeled or quantified in rating. Typically, FEMA focus on riverine flooding inland, coastal surge at the coast. And the two of them are not very well modeled in probability sets developed. Meaning big storm surge comes up, rainfall comes down, they meet, flooding, we think of Houston and some of those big events. That’s not very well quantified. So, a 2 foot sea level rise definitely will affect discharge. The hydraulics of the system’s going to need to depend. We know big drops or rises in sea, drops in hydrogeology, we had issues in the Mississippi of saltwater intrusion going much farther upstream and freshwater intakes being jeopardized. So there is a balance there. Every system’s going to be slightly different. But one thing’s for certain. 2 foot of sea level rise in Southeast Louisiana, which is more or less close to what is projected for 2050, there’s a lot of land that disappears. Unless you’re by a wall, you’re wet.

Is there evidence that subsurface flooding results from an influx of seawater?


RICK WEISS: That is very succinct. Thank you. Question from Janet Babin from Bloomberg News. “With subsurface flooding and the phenomenon of ghost forests, is there a definitive measure or way to prove that the problem is coming from an influx of seawater? What metrics are used to determine this other than what’s apparent?”


HOLLY MICHAEL: I can take that one. Thank you for that question. That’s a good one. We actually have a project looking at exactly this question. And so, I would say that part of this is measuring it. So, we have instruments along the marsh upland transition zone, where we’re monitoring water levels and salinity and so we know that when we see salinization of the forest soils, whether it’s coming from the marshes, from the coastal flooding, or the ocean, or another source, in the areas that we’re studying, the two main reasons to see ghost forests are the saltwater coming from the ocean or the marsh side and this water rise. So, the water level, even in the freshwater, is rising in response to the sea level rise. The whole base hydrologic level rises and that causes flooding even from freshwater. Which will also kill trees. So, there are questions about what is causing the change. And in order to really know what’s happening in localized areas, we need to measure it.

Does beach nourishment really work to mitigate the harms of coastal flooding?


RICK WEISS: OK. Question here. “Does beach nourishment where sand is added to eroding coastal beaches really work to mitigate the harms of coastal flooding?”


WILLIAM SWEET: I’ll take that. I think a lot of people have in mind sea level rise being water rising on a sandy beach. And that’s true. That’s really not what’s affecting communities for the most part. Sure, we see houses collapsing on the beach of Rodanthe, grew up in North Carolina, it’s sad. It’s shifting. That’s a geomorphological ocean wave or ocean process, sea level rise is definitely playing its part. Chipping as well as, creating new equilibriums that are moving, transgressing, moving inland. But the bulk of our flooding that’s really causing problems right now are more these urbanized environments. They’re not necessarily the sandy beach that you rent your beach house on. It’s Miami, it’s Charleston, Savannah, it’s Norfolk, it’s Atlantic City, it’s Annapolis. It’s urban areas that for 50, 100 years, reclaimed land, building right along the coast, for maritime commerce. It made sense then. Now it’s starting to flood more often. Garden variety reasons for flooding is sea level creeps up and high tide closes in on our infrastructure. Beach nourishment, sure. It might help at the beach. That’s definitely probably one of the main solutions at hand. In fact, the Army Corps going to build a multi-billion dollar solution to help prevent storm surge. But you can’t really build walls everywhere in these communities. Backfill and elevation, bulkheads, sea walls, those are a start. When it makes sense to protect. But for the most part, as we heard, whether it’s groundwater creep that’s rising up, stormwater systems that are backfilling and flooding into our urbanized environment, beach renourishment isn’t really the discussion in these major urban maritime communities. That’s just not the environment that they’re built on.


RICK WEISS: Interesting.


HOLLY MICHAEL: I might add a little bit to that if I could. That in the rural areas that we’re studying, we see saltwater intrusion as well. And it comes primarily through the tidal channels, the tidal creeks. So, it’s not a big over wash over a dune. Well, it is. Like you said, in some areas. But in the rural areas, too, it’s just kind of creeping in. And there’s no clear way to make a barrier to stop it.

How can reporters understand the vulnerability of their local wastewater treatment facility to flooding?


RICK WEISS: Question here for Dr. Hummel. “Any more advice for local reporters to figure out just how vulnerable to flooding their local wastewater treatment structure is? Is there a government organization that’s typically in charge?”


MICHELLE HUMMEL: I think in terms of actually getting this information from a wastewater service provider, again, they’re very locally managed. So, you could discuss with the local wastewater service provider to see if they’re thinking about climate change or sea level rise in their planning efforts. There are a lot of tools that NOAA produces that could help look at what are the potential flood levels and flood risks for different locations along the coast. But I think it’s very localized in terms of the types of information that these infrastructure operators maintain, as well as what sort of information they’re using for their planning. And this is another challenge, I think, more broadly in understanding risk to our infrastructure systems is that a lot of this is somewhat confidential information and that we don’t want to give away the locations of critical infrastructure. And so that does make it a challenge for the public to understand that risk and even for researchers to try to quantify what the risk might be.

How far inland do the effects of coastal flooding reach?


RICK WEISS: That’s actually a fascinating conflict there. Reporters can find out whether FOIA requests, for example, are not as productive as you might hope if security is in question there. Let’s see. We have another question here. “How far inland do the effects of coastal flooding actually reach? Is it truly just cities right on the coastline that need to be worried? What about suburbs or rural areas that are near to but not right on the coast?”


WILLIAM SWEET: Elevation matters. Plain and simple. It’s all dependent upon the topography of the location. Louisiana, 2 to 3 feet goes a long ways inland. New England, 2 or 3 feet does not go that far inland. So, the groundwater is another issue that we heard about that is separate. But again, every landform’s going to have a slightly different topography, a different interaction to see just how far inland might these effects go. The impacts of which, though, as we’re tied to it financially, these are other, maybe not direct physical costs. But who pays for solutions at the coast sometimes is more of a statewide discussion. Who pays for beach renourishment tax base, generation of income, tourists. So, the effects of sea level rise definitely percolate much farther inland than the actual, let’s say, the economic effects. Than the actual physical impacts, per se. But maybe there’s another answer here at the panel.


MICHELLE HUMMEL: Well, I would just also highlight that infrastructure becomes very important when thinking about this. So I’ve done a lot of work in the San Francisco Bay area. And there are communities that are located right along the Bay Shore that have high flood risk. There are communities up in the hills that have no flood risk from coastal flooding. And yet everybody is using the same transportation network to get around, the power network is highly tied together there. So, when we’re thinking about solutions for this problem, it’s not just the people whose property is being flooded who are ultimately going to be impacted and who will ultimately have to pay. If a wastewater plant gets flooded, that’s going to affect people who live on dry land or on high ground as well as people who live near the shoreline as well.

Are water treatment and wastewater treatment plants equally vulnerable to coastal flooding?


RICK WEISS: That’s a super important point. Berkeley Hills are not immune just because they’re up a little bit. Great. Let’s see. We have a question here from Bob Mudge at the Venice Gondolier in Florida. “Are water treatment plants equally vulnerable? Our wastewater treatment plant was relocated away from the gulf, but our water treatment plant, which the city is starting to talk about moving, is in a flood zone.” Sorry. So, our wastewater—if I read that correctly. “Wastewater treatment was relocated, but water treatment is in a flood zone.”


MICHELLE HUMMEL: So, I haven’t looked into water treatment plants. I mean, if they are located in these flood zones, then they have similar vulnerabilities in terms of the types of infrastructure. So there could be mechanical and electrical disruptions, as well as just disruptions to the capacity of the plant. I think what makes wastewater treatment plants uniquely vulnerable is, as I said in my presentation, oftentimes they’re sited in these coastal areas so that water can move by gravity to those plants. And then when the water is cleaned and discharged, it doesn’t have to be pumped very far, it can just be discharged into the neighboring waterway. And so that has made the siting of those plants in coastal regions or right along the coast economical. I don’t think there are necessarily the same benefits from a water perspective. A water treatment perspective. But it would be interesting to look into the risk for those plants as well.

Are there any communities in the Delmarva area that are noteworthy in their planning for sea level rise?


RICK WEISS: All right. Here’s a question for Dr. Michael. “You mentioned communities in the Delmarva area doing the planning right. Could you tell us which ones and what are they doing that’s caught your attention?”


HOLLY MICHAEL: Yeah. I guess, I didn’t want to suggest that some communities are doing things right and wrong. But one community that comes to mind that is actively trying to incorporate sea level rise into their planning is the town of Oxford, Maryland. They have a town planning committee, a town planner, and are actively considering changing their building guidelines. And yeah, so that’s one that comes to mind.

How effective is green versus grey infrastructure in mitigating storm surge and coastal flooding?


RICK WEISS: Great. Eastern Shore, is that correct? Oxford. Right. OK. A question here from Marlowe Starling, freelance reporter based in New York. “Do we have any concrete data or case studies on the effectiveness of green infrastructure versus grey infrastructure in mitigating storm surge and coastal flooding? When is grey infrastructure more valuable than green, and vice versa?” So we know there was mention of the carbon benefits of green. But I guess beyond that, what about effectiveness itself?


HOLLY MICHAEL: I don’t know if this question is for me. I don’t know if Dr. Sweet would like to take this. Or Dr. Hummel. But there are a lot of studies on this is my understanding. This is not my area of expertise. But I think that there are certainly studies looking at the effectiveness of all of the various options for mitigation. But my sense is that it’s very much a local question, right? It’s not one size fits all. It’s going to depend probably almost entirely on what the system is like. And so, there are always benefits and drawbacks to the various options. In terms of green infrastructure, a big one, like you said is carbon sequestration where you have living shorelines and things like that. And then also benefits of say, oysters, and other various ecosystem services of different types of green infrastructure. But that’s of course balanced by costs and effectiveness and all the other aspects.


WILLIAM SWEET: Yeah, and I’ll just add one point. I think it wasn’t necessarily posed in the question, but the differences of green and grey for sea level rise related coastal flooding, we know that barriers green infrastructure provide great storm surge barriers. But as sea levels continue to rise, I think it was mentioned earlier, that the tides go where the tides want to go. They go up creeks, they go up estuaries, any connectivity body that’s connected to it’s going to swell during the tides getting higher and higher. So, you’re not really providing barriers. It’s really more of an elevation. So once you really start to figure out areas that are starting to flood, by all means, it’s better to kind of let that transition back to a green, natural area. Otherwise it’s urban area, as we’ve created, it’s pretty dirty. So it’s better to have a green environment than a grey, inundated, dirty environment. So I think there are reasons to have both green for storm surge mitigation purposes, slow the water down, frictional effects any way you can. But then with the sea level rise, the creep, the high tide flooding, there are other ways of approaching it with green infrastructure versus grey, but ultimately it comes down to elevation gain. Oftentimes there’s just not—the habitat doesn’t have anywhere to sort of have that successional transition to go up to. So you eventually start getting squeezed and unfortunately start to lose your ability to support a green ecosystem around these urban areas.


HOLLY MICHAEL: Yeah, I would agree with that. Possibly with the exception of salt marshes. Because salt marshes can, to some extent, accrete or gain elevation with sea level rise. But only to a point, it depends on the marsh itself, but that is one ecosystem that can rise. But of course, whatever’s behind it, still can’t. So.

Why do sea-level rise projections vary so much from one area of coastline to another?


RICK WEISS: Right. OK. Got one more question here that looks like it needs an oceanographer to answer. This is from Penelope Overton at the Portland Press Herald. “Why do sea level rise projections vary so much from one area of the main coastline to another? Doesn’t the ocean rise uniformly? I can understand why coastal flooding projections vary, but why would sea level rise vary?”


WILLIAM SWEET: Well, water is not rising like it is in a bathtub. So thank you for that question. Currently, without future projections, we know that rise is basically two parts melt of land-based ice and one part thermal expansion. But it’s not uniform. We see this from space. We know that the East and Gulf Coast are rising faster than the West Coast. And this is due to warmer ocean [inaudible] So, you add warmth into already an ocean that’s warmer, it’s going to expand, have a greater expansion coefficient. When we make projections, there’s also circulatory heating changes, slowing down of the gulf stream, which is expected, recalls additional sea level rise on the East Coast versus the West Coast. Where ice melts really matters, and if you’re close to a source of—I won’t get into that details. But the last very important piece is what’s land doing? Is it sinking? Or is this—which a lot of the East and Gulf Coast is sinking for both natural and unnatural reasons. Natural being compaction of sediments in the Mississippi River delta. Could be an adjustment from the last Ice Age in the Mid-Atlantic. But extraction of fossil fuels and groundwater for consumption is causing a lot of land subsidence. It’s not really an issue on the West Coast. There’s earthquakes that occur, very punctuated change. But that is one of the main reasons why relative sea level is not uniform. It hasn’t been and we don’t expect it to be.

What is one key take-home message for reporters covering this topic?


RICK WEISS: Right. Great point. Land is sinking in some places, which does not help. I think the Gulf Coast, definitely one of them, as we pump fossil fuels at. So, we’re just about the top of the hour. And I do want to give all of our panelists the chance to drive home a final point. I’d like to ask at this point if there’s a take-home message. Often the most succinct and clearest summary of a point that we can help get across to reporters. Before I go around the horn and do that. I just want to remind reporters who are on this briefing that when you do log off, you will be presented with a very short survey. It’s just three questions. It helps us a lot. If you would take that half a minute and give us a little bit of feedback so we can keep these briefings as useful to you as possible. But now, let’s just go once around. And I’d like to ask each of our panelists if there’s one take-home message, if there’s one thing you want reporters today to walk away with and use in their reporting, what would that be? And I will start with you, Dr. Sweet.


WILLIAM SWEET: Well, I would say one threshold doesn’t fit all. So the systems that we live with in our communities, whether it’s roadway, wastewater, agriculture, power, property, there are different elevations. And we’re all connected, but each system’s going to have a tolerance that it can tolerate. How wet can it get for how long? How often? So with sea level rise, what we’re really trying to do at NOAA is try to get some actionable thresholds that mean something to people on the ground. So we can contextualize sea level rise with impacts that make sense. But I think from a media standpoint, certain things are going to fail, our functionality’s going to degrade quicker than others. We heard about stormwater systems. We could hear about power. Areas that are affected that need cooling areas along the water as sea levels rise. So I think it’s a problem that really needs to be dissected and understood. The systems that we live by, and they’re all affected as sea levels rise. It’s simply different manners, but understand the tolerances of thresholds and being able to report out on that is key.


RICK WEISS: Great. Thank you. Dr. Hummel.


MICHELLE HUMMEL: I’ll just reiterate what came up with one of those great questions, that if you live in a coastal community or a coastal region, you’re likely to feel the impacts even if your property is not right along the coastline. So because we have this interconnected system of infrastructure, if one road floods, or if one substation floods, that’s likely to have wider spread impacts. And so we need to think strategically about these systems as a whole in order to understand which components can we protect, which ones do we need to relocate, and how can we manage those systems holistically so that they can continue to operate both during short-term disruptions from hurricanes or other types of storms, and then looking longer term over the lifetime of the project with sea level rise.


RICK WEISS: Thank you. And Dr. Michael.


HOLLY MICHAEL: I would just say that we should keep in mind that our coastal water resources are precious and they’re fragile. And once they’re compromised, it’s really hard to reverse that. And not only do we depend on freshwater resources, but our agriculture depends on it. And our ecosystems depend on it. So, that we should keep in mind that the effects of climate change and then revise the way that we manage and think about our freshwater resources along those lines.


RICK WEISS: Fantastic. I think this has been such an interesting briefing, I have to say. With so much of our climate reporting in journalism today focused on extreme weather events and these very notable events, and I at least have heard a picture painted today of something, for the most part, a little slower and a little more subtle, but super dangerous and damaging as well. And a great opportunity space for local journalists to do some digging and figure out what’s going on in their communities. I want to thank our panelists for really such an informative and interesting briefing today. Thank you, journalists, for the work that you’re doing to cover these topics. And I do encourage you to visit our website. SciLine.org. Fellow us on Twitter at RealSciLine. And please do hit that survey as you log off today and help us continue to make these briefings as useful to you as possible. Thanks, everyone. We’ll see you at the next SciLine Media Briefing.

Dr. Michelle Hummel

University of Texas at Arlington

Dr. Michelle Hummel is an assistant professor of water resources in the department of civil engineering at the University of Texas at Arlington. Her research focuses broadly on understanding how coupled human-natural systems respond to climate-driven disturbances in coastal regions. She has developed and applied numerical models to study how shoreline management decisions affect estuarine hydrodynamics and flooding and to quantify impacts on populations and interdependent critical infrastructure systems.

Declared interests:


Dr. Holly Michael

University of Delaware

Dr. Holly Michael is the Unidel Fraser Russell Chair in the Environment and professor in the departments of earth sciences and civil and environmental engineering at the University of Delaware. She is also director of the Delaware Environmental Institute. She is an associate editor of Water Resources Research and served as the Geological Society of America James B. Thompson, Jr. International Distinguished Lecturer in 2019. Her research interests include saltwater intrusion, water resource management, contaminant hydrology, coastal hydrogeology, groundwater-surface water interactions, and geostatistics. Some of her current projects include investigating the coastal critical zone, measuring groundwater flow into estuaries, modeling groundwater salinization due to climate change, evaluating sustainability of arsenic-safe groundwater in Bangladesh, and application of experimental economics to groundwater resources.

Declared interests:

I have federal grant funding to study coastal salinization and flooding due to
sea-level rise and storm surge.

Dr. William Sweet

National Oceanic and Atmospheric Administration

Dr. William Sweet is an oceanographer at the National Oceanic and Atmospheric Administration, where he spearheads efforts to track and predict changes in sea level and coastal flood risk to support sound decision making. He leads NOAA’s annual high tide flood assessment, and he is the lead author for the U.S. Interagency Sea Level Rise Task Force’s 2017 and 2022 reports. He also co-leads the U.S. Department of Defense’s Coastal Assessment Regional Scenario Working Group and is an author for the 4th and on-going 5th National Climate Assessments.

Declared interests:


Dr. Michelle Hummel slides


Dr. Holly Michael slides


Dr. William Sweet slides