CoastLine: Sea Level Rise in the Cape Fear

Dec 16, 2016

North Carolina has a controversial history when it comes to its willingness to accept and plan for sea level rise.  In 2012, the state legislature enacted a multi-year moratorium on considering data from a science panel for future planning and policymaking.  That moratorium has since lifted and a new study out last year, looking at the next 30 years, is now accepted as a reasonable basis for policymaking.

But in local municipalities – particularly small coastal communities impacted by sea level rise, planners and researchers are quietly going about their work.  On this edition of CoastLine, we learn more about what they’re considering where sea level rise is concerned and how they’re preparing.  We also learn about whether we’re seeing evidence of it and how that shows up locally. 



Philip Prete, Senior Planner in the Comprehensive Planning and Design Group, City of Wilmington

Lawrence B. Cahoon, Professor of Biology and Marine Biology, University of North Carolina – Wilmington

The Community Resilience Pilot Project funded by the Environmental Protection Agency can be found here:


Segment One

Rachel Lewis Hilburn: Philip Prete, what impacts are there from the city of Wilmington’s perspective?

Philip Prete: Well, immediately, we don't see drastic impacts like some areas are seeing. We do have increased flooding, and that’s driven partly by increased sea level and also partly by the tidal movement. And in some areas of the city, particularly along the eastern part of the city where the tidal creeks are, a lot of it is driven also by heavy rainfall and the fact that these soils in that part of the city don’t absorb the rain. They are not very good sponges for the rain. So you have a lot of low-lying areas that end up getting inundated. So we see that every time there's a heavy rain, we see some flooding. We’ve seen more frequent flooding of Water Street downtown. But again, it’s something that’s not to the same extent that some other municipalities are seeing because we have such a rapid rise off of the river in the downtown area. Now if you go down south of the downtown area, there’s lot less of a rise and you get more flooding in those areas. And if you look across the river to where the battleship is on Eagle's Island. they routinely get flooding over there. The parking lot is inaccessible at times. The street to the battleship is often inaccessible. So then that’s because it’s a lot more of a gradual rise from the river.

RLH: And Lawrence Cahoon, you recently completed a study that looked at in-flow and infiltration when it comes to local sewer systems. Can you tell us first why you were even looking at sewer systems? And what does this have to do with sea level rise?

Lawrence Cahoon: Right. I got interested in sewer systems because there's been a history locally of sewer system overflows. And one of the things that drives that is excess water in the system. The folks who run central sewage systems call it “I&I”—inflow and infiltration. Inflow is water that's entering the system from above—for example, rainwater leaking through manholes. Infiltration is water infiltrating into the collection system from the ground. Basically, elevated groundwater levels would drive that. All these collection systems are made of pipes and so forth and they leak. A little bit of leakage is normal, it’s factored in, but excess leakage is a problem because it can cause the sewer plant itself to function poorly. It can cause sanitary system overflows where the amount of water in the system actually pushes the manhole covers up because of the pressure and blows raw sewage out. You can have other kinds of failures associated with that—sometimes pipes rupture and that sort of thing. So, all of that’s bad. It is a problem that’s not readily visible. Sewer systems are one kind of infrastructure most people never think about. When we talk about infrastructure, generally we're thinking roads, bridges, highways, utilities like the electric system and so forth, and we rarely think of sewage systems. We flush the toilet, and we assume that stuff just disappears—it goes someplace and somebody else takes care of it. And it’s not quite so simple. Sewer systems are expensive to repair and maintain. They cost about a million dollars a mile to install.

RLH: A million dollars a mile?

Lawrence Cahoon: That's about the average Yeah.

RLH: And when you were conducting this study, what were the geographic boundaries?

Lawrence Cahoon: We did two studies. One’s been published already, and the other one’s in press. The first one looked at four coastal wastewater treatment systems—the two big ones here in Wilmington (Southside, Northside), Carolina Beach, and Beaufort. We looked for both rainfall effects on sewer system performance and sea level effects. Now again, sea level is rising very gradually overall, globally. But what we were looking at was the effect of neap tides versus spring tides. Spring tides, of course, are much higher because of the position of the moon and the sun. And so you see this alternation in tide levels. What we found was, sure enough, tide levels made a difference in the amount of water going through each one of those four systems. And if you have an extra meter of water, for example, at high tide, the Northside plant is getting half-a-million gallons a day of extra water coming through. What we think is happening there is that as the tide comes up, the ground water levels elevate and the sewer system collects a lot more groundwater. In the case of Carolina Beach, what we found was equally dramatic. And I understand that at high tide, they can measure the salinity of the water coming into the plant. It’s not huge, but it’s there. I mean there’s a salt water signal at high tide

RLH: And tell us why that is remarkable. Why is that notable?

Lawrence Cahoon: Well, it means that sea water is getting into your sewage collection system, which is bad for a lot of reasons. It says your system is pretty leaky, for one thing, but sea water is not good for a sewage system. It will cause damage, and you know, we may have time to go through all of the different kinds of damage, but let me skip over to the second study, which was a much broader one funded by the UNC Sea Grant. We looked at all the coastal counties in North Carolina, and we looked at data for 92 wastewater treatment systems in those 27, 28 counties. And what we found was about 90 percent of them had an I&I problem, which means they’re getting more inflow and infiltration than they should be, according to the normal rule of thumb for designing those systems. And when you throw into the mathematical models a rain event like a two-inch rain event in one day, it got much worse real fast. So big rain events make these systems fail at a much higher rate.

RLH: And so Philip Prete, when you consider this as a senior planner with the city of Wilmington, what does that mean in terms of how Wilmington needs to plan?

Philip Prete: Well it’s interesting, Rachel, because Larry and I have shared information on some of the work that we’ve done over the years, and the city actually was funded to do a pilot project a number of years ago. It was during the time when this moratorium was supposedly in place and you weren't supposed to be doing planning. We had federal funding to do it, so it was really the federal government doing this work with us. But we looked at the water and sewer infrastructure in the county and looked at several different scenarios of sea level rise and overlaid on top of those scenarios a couple of different scenarios of storm surge, just to see over the next couple decades, where we are going to have increased potential for the types of things that Larry is talking about with increased infiltration. When you start getting sea level rise, you start putting more of the system at risk. What we found is that there are quite a few manholes that will be subject to inundation in the future. And there are a number of pump stations in this city and county that will be at risk in the future. Now, the thing with pump stations is they’re always located in the lowest lying areas because everything flows downhill and then they pump it back to the wastewater treatment plants. So you basically have put these systems in harm’s way. If we continue doing that without thinking about the future, these are not 10- or 20-year life cycle systems. We were expecting these systems to last for decades and decades, so we've really got to think into the future when we do this. And those are the results that came out of this study. We actually had the [Cape Fear Public] Utility Authority and the county as local partners on this project, and so all the information and models that were generated out of the study are able to be used by the [Cape Fear Public] Utility Authority. They can take those and incorporate them into their future maintenance and repair schedules and in their future planning.

RLH: And so how old are some of the components in the city's sewer infrastructure system that will need to be replaced or that will be replaced first?

Philip Prete: Well, I would say that the pattern for replacement of sewer systems is that it’s done when there’s failure. And some of the system, particularly in the downtown area, is over a hundred years old. And in fact, when they were doing some of the street work downtown, they found clay pipe.

Segment Two

RLH: I've received multiple pictures this morning from one of our listeners, Robert Parr. He has traveled around the community and taken pictures of flooding. There are some additional factors going on right now as we speak. And you're nodding, Lawrence Cahoon. Can you tell us what some of the factors are that are contributing to the flooding that we're seeing today?

Lawrence Cahoon: Sea level is an extraordinarily dynamic factor. There are a lot of things that affect it—obviously, the oscillation of the tides. Right now, we’re having a super moon, a full moon. And so we’re getting a spring tide as a result of that. And so the spring tides are normally much higher than the neap tides would be. There are other factors that affect apparent sea level. Wind effects along this coast can be dramatic. We get wind effects on sea level that can be a couple of feet even without a hurricane or anything like, which can make a big difference. And again, it's a complicated scientific explanation behind that. But on top of that is what we call eustatic sea level rise. That is the global change in the volume of the oceans. And we know that that is changing at about 3.3 millimeters per year going up.

RLH: That sounds like a tiny amount.

Lawrence Cahoon: It does, but it’s additive. There are also local effects. For example, in North Carolina, if you go up to Currituck the apparent sea level rise there is about 4.5 millimeters per year, partly because of land subsidence. If you come down to Southport, it’s about half of that. And again, that's because of a little bit of land rising at the coastline there. There are other places in North Carolina, especially inland and marshy areas, where the subsidence of the soils because of seawater intrusion is driving a much higher rate of apparent water level increase.

RLH: Philip Prete, just before we went to the last break, you were talking about looking at some of the infrastructure, specifically sewer systems, some of which would be replaced. There were components of it that were old. You said there is word that some was made of clay. How old would that be?

Philip Prete: I can’t say exactly, but some of the the lines were 100 years old or older.

RLH: And you said that replacement usually comes when there’s failure.

Philip Prete: Typically, it’s either out of necessity or out of convenience. If there’s a project where they’re redoing the road, then you know, they’re stripping pavement and that’s a good opportunity if they have scheduled for water and sewer replacement in that area, it’s a good time to open up the pavement that one time, and that way you don’t have to do it twice. So whenever there’s a major road project like when they are doing the streetscape improvements down on Water Street. It’s taken a long time because the first part of that is to get down there and replace some of the storm water and the water and sewer infrastructure below Water Street while they are in the process.

Bob (email): What future tide levels are part of the new design criteria, and when in the future will these new modifications need to be modified again?

Philip Prete: That’s a good question. And it’s I think it really depends on the area on where we’re where we are. If you’re looking at Water Street, for example, since we’re talking about that, which is already occasionally flooded, then if you’ve got subsurface infrastructure, then it’s difficult to you know— We've got an existing road that basically dictates that elevation. So it is difficult to change that elevation. Now, where we should be thinking about changing elevations or accommodating sea level rise is in new development or new areas where we extend a new water and sewer or new roads. We should be factoring that in. In fact, the Department of Transportation does factor sea level rise now into some of their road planning and design and their bridges because you might have issues with the clearance under a bridge in the future. Again, this is infrastructure that has a long lifespan. It might be 50 years or longer. So you want to factor in at least that much of a time frame in the planning.

RLH: So I know that you’re with the city of Wilmington, but when we look at the recent construction on the Leland causeway and the bridge that goes over Alligator Creek, that was elevated compared to the old bridge, but how much sea level rise are they taking into account there at this point?

Philip Prete: I don't know the answer to that in detail. I know they are looking at models, and I think they’re probably looking at, you know, not the highest scenarios, but they’re looking at middle-of-the-road scenarios, and it’s one of those situations where— And this is one of the big things that drives a reluctance to make policy changes based on the projections of sea level rise into the future. If you look into the near future, we’re pretty certain how much it’s going to rise. There is a range, but we’re pretty certain we can hit [the projection] pretty close. But as you start to go out further into the future, that range gets wider and wider and wider.

RLH: Well, will what are we even talking about when we talk about near term versus a long term view? I mean that the science panel that was ultimately fully rejected by the state legislature, I think they published their report in, it might have been 2010, but it was 2012 that the state legislature said, “We're not taking that into account when it comes to making any kind of policy decisions.” And then in 2015, that moratorium lifted, and that’s when a new report came out that looks at, what, 30 years? Is that short term or is that long term?

Philip Prete: Well, it depends on what you’re thinking about. It’s short term for some investment. I think the reason they picked 30 years is there was a lot of realtor direction behind the legislation and behind the the science panel, and 30 years is the basic lifespan of a typical house mortgage. So you know, from the standpoint of being able to plan for private homeowners, 30 years is a pretty reasonable number. But if you’re talking about infrastructure that is going to be in place much longer than that, then you want to take a longer view. When the first science panel came out with the first report in 2010, they looked clear to the end of this century, so you know, it was quite a bit different horizon. When we did the infrastructure study for the pilot project we did in the city, we used that same kind of scenario. We looked out to the end of the century, and we bracketed the scenarios at the same range that the science panel originally came out with. So you know, that was something that we now have that data, we have those models. The modeling was assisted by the North Carolina Division of Emergency Management, which had a significant amount of FEMA money to revise their floodplain models as well as to do some scenario analysis of sea level rise.

Nancy (caller): Why do they have sewage pumping station at the heads of some of our creeks? In particular, Hewletts Creek. It’s spilled over from that area many times. Why don't they move them? Why were they put there the first place?

RLH: Nancy, I'm sure a lot of people have the same question. We’ll toss that to Philip Prete.

Philip Prete: The basic reason is basic physics. When you're dealing with sewage, it’s a liquid that’s going to flow downhill, and most of the sewer lines that we have in place are what’s called gravity flow sewer lines because it’s much cheaper than the lines that require pumping. So you get as much of it to go down to as low of a location as you can, and then you pump it up to the wastewater treatment plant. So just by cost and the physics of the whole thing, these pump stations get located in the lowest lying areas, and it’s kind of a double whammy because you do have that potential for sea level to impact the pump stations, but you also have very little buffer if they are to fail because you end up with your sewage right there flowing into the creek. So you know, with the pilot project that we did, we ended up with a number of different recommendations for how to deal with this going into the future. So if the utility authority begins to upgrade one of the pump stations or do maintenance on it, if they elevate it by putting a berm around the pump station, it can do a lot to reduce the risk there, or if they raise their power boxes on the pump stations, that can do a lot to reduce the risk of failure. That doesn’t get them out of those low-lying areas, but it does make them a little less susceptible to failure. And as far as moving them, it would be very expensive to move a pump station. It’s not a cheap piece of infrastructure.

RLH: Lawrence Cahoon, a couple of times during this discussion you’ve alluded to the fact that rates of sea level rise are different in different parts of the state. Why is that? Why is this a dynamic thing, and why is it different in, say, the northern part of North Carolina versus our region?

Lawrence Cahoon: Right. A lot of that is the underlying geology. You know, we think of the coastline as being cast in stone, so to speak. In fact, there’s very slow geological effects that are causing the land itself to rise in certain areas of the country and of the world, and other places where geological effects are causing the land to sink, usually very slowly.

RLH: And the land subsidence, that’s the land sinking.

Lawrence Cahoon: That's right.

RLH: And that's what I've heard about in conjunction with sea level rise, but the land is also rising in places?

Lawrence Cahoon: That’s right. There are places where you're having a little bit of geological uplift. For example, I mentioned the South Port area. That's an area that has seen some uplift. It's very gentle. It's about a millimeter per year, something like that, but it counteracts roughly one third of the global sea level rise effect. There are places like Norway and Sweden where, after the glaciers melted, the landscape actually rose after all that weight of ice was removed. So there are places where sea level appears to be falling. But if we take the global measurement and integrate all of that over the entire planet, the sea level, the volume of water in the oceans, is rising again at about 3.3 millimeters per year, something like that.

Philip Prete: Just to add to that, we talked a little bit about the science panel's report that they came out with in 2010, which pretty much got scrapped, and then there was a new direction by the Coastal Resources Commission to develop a new report, which they've published. One of the positive things that came out of the new report was looking at the types of variations in sea level rise along the North Carolina coast. For the 2010 report, they used projections of global sea level rise. And Larry mentioned that it's based on a global mean, but with the more recent report that was published in 2015, they were required by the Coastal Resources Commission to look at four different regions of the North Carolina coast and take into consideration the lands subsiding in those different regions with the different rates. So they came back with—and again, this is what Larry alluded to or mentioned—that up in the northern part of the coast, the rate of increase will be higher—partly because of soil subsidence at a faster rate—than the southern part of the North Carolina coast.

RLH: So what causes land subsidence? Why is the land sinking?

Lawrence Cahoon: Well, there's a couple of factors there. One of them is the underlying geology. The earth's crust actually is dynamic in that way. And again, when you've had glaciers cover, for example, a large portion of North America, a lot of that is rebounding. And you're seeing regional effects where some areas are rebounding and others are actually tilting downward in compensation, so there's that kind of background effect. In addition to that, in the coastal region in North Carolina, we have a lot of places where the soils are largely organic, made of peat from marsh accumulation over time, and as seawater water intrudes into those soils, the salinity itself can foster the growth of bacteria that decompose that organic matter. So the soils themselves are actually decomposing and therefore settling. And so in some of the inland areas where you've got salt water now getting into places that used to be freshwater marshes, you're seeing a very high rate of land subsidence. Again, the land sinking and therefore sea level rise apparently exceeding the rate you see elsewhere and accelerating the process. So there are places in coastal North Carolina that are in very big trouble.

Philip Prete: Another factor is that when you are pumping groundwater for use, you're also removing some of the subsurface pressures that help to keep the earth elevated. So either groundwater extraction for our drinking water supply, which we pump quite a bit out of the wells here and in this county, but also if you had oil or gas extraction—again, that can change the dynamics and cause additional soil subsidence.

RLH: Well, that's interesting. And where might we see oil and gas extraction?

Philip Prete: You might see it in some of the coastal areas in the Gulf of Mexico, for example. And depending on what happens in the future, you may see it along our coast as well.

RLH: Are you talking about offshore drilling or are you talking about hydraulic fracturing?

Philip Prete: Well, I'm not talking about hydraulic fracturing. That's another issue. But the offshore drilling is still— I mean, it's not like there’s a subsurface wall underground between what's under the ocean and what's under our land surfaces, so it's all connected, meaning it can potentially impact the soil subsidence rates

Lawrence Cahoon: It's certainly the case in Louisiana, where oil extraction gas extraction on what was land has caused local subsidence. They're seeing very rapid declines in the elevation of the landscape in coastal Louisiana. It's almost the poster child for that sort of effect. When you get farther off shore, especially North Carolina, it looks like the primary oil and gas interests would be out on the edge of the continental shelf—fairly far out and fairly deep. So I would expect the land effects to be fairly small from extraction of those resources out there because they are so far away and because the underlying geology probably doesn't leave a resource for them to extract that comes very close to shore.

RLH: Okay, and we're going to talk more about how we're seeing evidence of that here. I know, Lawrence Cahoon, you've talked about an accelerated rate in Dare and Hyde counties. We’ll talk about southeastern North Carolina when we come back from break.

Segment Three

RLH: Before we went to the break, Lawrence Cahoon, you were kind of explaining land subsidence and some of the geological causes of it, and you've said that it's more observable, that it's happening at a faster rate north of us in Dare and Hyde County, specifically. But where, here, are we seeing land subsidence?

Lawrence Cahoon: [We’re seeing land subsidence] where you've got organic soils, wetlands in particular. Those soils, as they receive more input of salt water than they're used to, change in character—for one thing, from our freshwater flora to saltwater flora. But the soil bacteria are able to use the sulfate iron in seawater. I don't want to get too technical, but they can use that to metabolize that organic matter in those organic soils, and they basically decompose those soils. So they begin to settle and subside simply because they're being destroyed. And you see that taking place in the northern part of the county, in the wetland areas up there. Matter of fact, you know, Philip,

RLH: You're talking in the northern part of New Hanover County.

Lawrence Cahoon: New Hanover County, yes. You know, when you talk about how the access roads to the battleship and the parking lot are being flooded, it isn't just that the tides are higher. They are, but it's also that that land is wetland, and it's subsiding. And so they're getting a much more amplified effect of sea level rise than they would otherwise.

RLH: We see lots of coastal flood advisories affecting Battleship Road and downtown Wilmington, particularly a couple of blocks on Water Street. Philip Prete, is that land subsidence also on Water Street or is that sea level rise? What's causing that, and why is it happening more often?

Philip Prete: Obviously land subsidence plays into it, but that's a long range, long term change. I think what we're seeing more of is that there is an increasing level of the river, driven partly by sea level rise, but we also see the tides themselves. And we get daily low and high tide fluctuations, and across the river, the flooding will be a lot more frequent with a lot less of a tide. And on Water Street, it's going to be more likely when we have some of the king tides or some of the higher high tides, but that's part of the of the cause.

RLH: And tell us briefly what a king tide is, the difference between that and high tide.

Philip Prete: The king tide is when you have the super moons, which drive a larger fluctuation in the tides than your typical full moon.

RLH: Which is what we're seeing right now.

Philip Prete: Correct.

Lawrence Cahoon: Let me throw in another factor that's relevant for us here in Wilmington. Of course, we have a state port here, and the river has been dredged to deeper and deeper depths, which allows more ocean water to get farther upstream. That's a big factor. It probably also amplifies the tidal range here at Wilmington just because it's easier for high tide to get here. You know that water's got to slosh all the way up the river and all the way back out again. And so certainly that is a factor in all of the effects that we've been talking about. But on top of that, you have sea level itself going up and the land subsiding. So we've actually engineered ourselves into a lot more trouble than we would have had otherwise.

Philip Prete: One other factor at play here is the fact that we often have large rain events, and in fact, we're getting larger and larger rain events and more frequent large rain events in our area and other parts of the coast. And so a lot of what you see is sort of a combination of the tidal fluctuations, sea level rise, plus the runoff from these large rain events. And as we've developed, we've added more and more impervious surface within our city, which means the water runs off faster but it goes to low areas—for example, in the eastern part of the city along the Tidal Creek, Bradley Creek, Hewletts Creek, etc. Those are the low-lying areas where we see the most flooding, and the city’s Stormwater Services Department that spends a lot of time and money trying to fix that problem. But that’s not just the tidal changes or sea level rise. It's also just the fact that we've got a lot of development in areas that used to be soils that would absorb that rain, and it's no longer there. It's running off of pavement. But we also have a lot of those low-lying areas where some of the most poorly drained soils are, the wetland areas and the areas with organic soils, so they don't absorb the water as much and that's where we see a lot of the flooding.

RLH: You talked about looking out one hundred years yourself for the sake of local planning. We recently got new information about vulnerabilities of Greenland's ice shelf, things that we didn't know, assumptions that had been made for a long time. This was just published in the journal Nature. The ice shelf could be more vulnerable than we thought, melting faster and contributing to sea level rise. And then just this week, a report was published that was looking at what they called the tri-state area up north—New York, New Jersey, and Pennsylvania. They found sea level rise could permanently flood parts of New Jersey—the Jersey Shore, half of Hoboken and the Teterboro Airport—by the early part of the next century. So I think we're talking about 100 years. Are there areas here, when we look out one hundred years, where you could imagine such a thing? And I realize the science is less stable than looking out 30 years, but what might we see? Can you name things like that?

Lawrence Cahoon: Let me take that opportunity to cast the the big picture here. We know that the ice sheet of Greenland is melting at an accelerating rate. And I gather from what you said about this latest study that there are things at work that we hadn't previously known about. Let's put it this way. When the earth was this warm in geological history, sea level was six to seven meters higher than it is today. Okay? Because of what we call climate inertia, the ice melt effects that the current levels of global temperature will drive is going to get us to that six or seven meters. How soon, we don't know. The panel that got into trouble, the Coastal Resources Commission science panel—

RLH: Back in 2010.

Lawrence Cahoon: Correct. They were using an estimate of about one meter of global sea level rise by 2100, and that was, at that time, considered to be more or less a consensus of the projections. Now, all projections are risky, many things can happen, but if you extrapolate current trends and that sort of thing based on what we now know, it looks like one meter or so is not at all unreasonable.

RLH: And what does that put under water in our area?

Philip Prete: Just from the modeling that we've done and had done here, when you start looking at that one meter of rise and you start looking towards 50, 100 years out, then some of the areas that are lower lying, like along the tidal creeks, you're going to see those start to get inundated, and there's a lot of very nice homes in those areas.

RLH: So we're talking about people who live near Bradley Creek, for example.

Philip Prete: Exactly. And also the along the river, there's certainly lower lying areas of the river once you get down south of the bridge. And when you start looking out into the future, you start to see that our state port is somewhat vulnerable and has some potential areas that would potentially be at risk. Some of the southern areas along River Road could potentially be at risk too. And if you look at other areas where they are focusing on trying to make some changes because of sea level rise, the one that really comes to mind where they're really taking a serious look at is the Norfolk, Hampton Roads area, and that's because they've got the naval base there and they realize that this is something that could put our national defense at risk because it could affect the naval base. So they're pumping a good bit of money into working with the local universities, local communities, and the federal and state government to look out into the future and try to figure out ways that they can take care of that issue.

Tony (caller): With a one-meter sea rise, I'm just wondering what is the possibility of changing this whole thing? If we take some reasonable actions or if we look at reasonable actions we can take to reduce the warming and the sea water rise, are we going to be able to turn it around, or do we have an elephant we can't get back into the compound?

RLH: Great question.

Lawrence Cahoon: It's a matter of opinion, but physically speaking, that much sea level rise is already baked into the system. The current temperatures aren't going to go down without massive intervention—you know, global geoengineering and those kinds of things. I see no prospect for that in the immediate future. I'd be surprised if sea level rise is only one meter by the end of the century. But I think we have a major problem on our hands, and we're going to have to adapt to it.

Amy (email): I live off Masonboro Sound Road. With the expansion of the sewer and water within city limits, new pumping lift stations have been placed in areas that seem to be on the floodplain. Am I wrong to think it's a problem?

Philip Prete: No, you're not wrong to think it's a problem, and we did talk about that a little bit in the last segment, where there are a number of pump stations that would be vulnerable with increasing sea level rise. The trick is to either build berms around these pump stations or do something to shore them up.

RLH: Which sounds like something of a stopgap. I mean, she says new pumping lift stations. Why new ones?

Lawrence Cahoon: Well, every time we have a new development, we're going to have to do something to handle the sewer. Why are the new ones going in those areas? It's still the same reason: As long as we've got development in those low-lying areas, there's going to be sewer infrastructure that serves those low-lying areas. And again, due to basic physical principles, it has to be in the lowest lying areas so that you get the gravity feed.

Ben (caller): I have a two-part question. Earlier in the segment, you guys were describing that the extraction of oil or groundwater can lead to subsidence. Now, I'm not aware of any local oil extraction in coastal North Carolina, so that leads me to my first question: Do you think the population—not just in New Hanover County but all up and down the coast of North Carolina—that relies on well water is large enough to lead to localized subsidence?

RLH: That’s a good question, Ben. Philip Prete, you have an answer for that. And do large industrial operations also contribute to this problem?

Philip Prete: Yes, industrial operations can contribute to it. And that was one of the major issues that a lot of folks were concerned about with the cement plant that was going to go in along the northeast Cape Fear River because they were going to pump tremendous amounts of groundwater, which could help to exacerbate this problem.

RLH: And there are current operations in that area—

Philip Prete: There are current operations in that area, but we also have some large well fields that the utility authority operates to get drinking water for the residents and for businesses. Basically we're dealing with a perfect storm situation because we do have population increases. There could be as many as 50,000 new people coming to our area. That's going to be more demand for the water. Also, with sea level rise you're potentially putting the groundwater supply at risk because you get saltwater intrusion into the wells, and we're already there, already experiencing that if you go a couple of counties to the north of us.

Harper Peterson (email): Lawrence Cahoon, for city and county planners and developers, regarding long range land use and development, are there certain parts of this city, this area that we just need to take off the table?

Lawrence Cahoon: Hi, Harper. I'm not sure we can do that politically, as I may have said a while back, it may have been our private discussion. I think what's going to drive a retreat, which Phil says is a dirty word, is probably going to be storm events, things like big floods where the economics of a given situation dictate that you're going have to pull out rather than a policy-driven kind of response. So I suspect we're going to see this happen sort of by surprise. We can predict with some certainty that there will be more floods, there will be more storms and so on, but it will be a surprise when it happens.

RLH: Lawrence Cahoon and Philip Prete, thank you both so much for joining us today.

Philip Prete: Thank you.  

Lawrence Cahoon: Thank you.