by James A. Bacon

Atchafala delta, 1984
Atchafalaya delta, 1984

Louisiana’s coastline is shrinking. Humanity’s impact on the state’s massive but fragile wetlands — levees accelerating Mississippi River water flows, the criss-crossing of marshes with canals — has aggravated the natural phenomena of subsidence and sea-level rise to inundate some 1,900 square miles of coast land over eight decades. It’s an object lesson for Virginia, much of whose low-lying Tidewater region also could end up waterlogged as sea levels rise. We’ve seen the maps — I’ve published some on this blog. A hundred years from now, there could be little left of Norfolk and Virginia Beach in a storm surge but a bunch of islands.

Atchafalaya delta, 2014
Atchafalaya delta, 2014

But, wait, the process of shrinking land mass is not inevitable. Portions of the Louisiana coast are expanding. That’s exactly what you’d expect to find in the Mississippi River delta as the nation’s mightiest river deposits massive volumes of silt and sediment into the Gulf of Mexico. An article in Atlantic CityLab shows satellite photos of the Atchafalaya River, which empties west of the Mississippi, in 1984 and 2014. This delta complex is growing at the rate of one square mile per year.

Writes John Metcalfe: “Scientists are quite interested in studying these processes, as they believe they might help counter today’s leading cause of coastal deterioration: rising sea levels.”

There is a widely held assumption that Virginia could lose hundreds of square miles of wetlands as local subsidence and rising global sea levels conspire to flood the Tidewater marshlands. But is inundation inevitable? The James, Potomac, Rappahannock, Susquehanna  and other tributaries dump large volumes of sediment into the Chesapeake Bay — so much so that the silt clouds the waters, blocks sunlight and disrupts the bay ecology. But eventually the sediment settles to the bottom, contributing to the build-up of mud and muck.

It would be interesting to know: Which process is occurring more rapidly in the Chesapeake Bay — sea level rise or sedimentation? A related question: How is the sediment distributed? Accumulation of silt in the middle of the Bay just makes a shallower bay. But accumulation in the marshlands might support the creation of new land mass that we see in the Atchafalaya delta.

craney_islandDredging the sediment build-up in Virginia’s shipping channels costs tens of millions of dollars a  year. Much of the dredge material has been directed to Craney Island, a man-made land mass that has transformed the coastline of Hampton Roads. We have a lot of raw material to work with.

Last summer, Governor Terry McAuliffe appointed a Climate Change and Resiliency Update Commission to prepare Virginia’s coastal communities for the impact of climate change. It strikes me that the sedimentation issue is ill understood and little discussed. How likely are Virginia marshlands likely to survive incremental sea-level rise as the deposition of silt raises the bay bed? To what extent can Virginia productively re-route sediment from channel dredging to build up the most vulnerable sections of the coastline?

There is a strong bias among those who fret about Global Warming toward solutions that entail re-engineering the nation’s energy economy in order to reduce the carbon dioxide emissions implicated in rising temperatures. Any changes we make in Virginia will have an infinitesimal impact on global temperatures, even if, as widely asserted, CO2 emissions are driving them higher. To survive global warming and rising sea levels, we must make our communities more resilient. That’s where our actions can make a difference.

The Governor’s commission is scheduled to submit its recommendations by June 30 this year. Let us hope that it incorporates the insights scientists are gleaning from Louisiana’s Atchafalaya delta.

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14 responses to “Silting, Resilience and Climate Change”

  1. Very interesting, especially since I seem to recall environmental folks in the 1970s being concerned about the Corps of Engineers’ work on the Atchafalaya that was inhibiting delta growth of the main part of the Mississippi delta. The point then was to let the river well enough alone and save money at the same time. We in the WH Council on Environmental Quality urged Reagan, when he arrived, to save federal money and protect the environment but sadly he then cut CEQ’s budget by 2/3 and we all left!

  2. One debate I have with my kids (who believe a worldwide abatement of atmospheric CO2 in response to global warming is grossly overdue) is how we’d ever achieve such a compliance from the rest of the world without fighting what amounts to a war, economic or military, to force compliance; that in effect it’s naive to think we’ll ever get there otherwise merely by our example.

    I remind them that, even if we accept the dogma that our fossil fuel use is responsible for the current rise in temps (I’ll accept that the rise is real, not a blip), we live near a coastline that demonstrates dramatic changes in sea level for what were plainly natural causes, from minus 300 feet to plus 250 feet (relative to today), as indicated clearly to geologists by the ancient eroded river bluffs and sharks teeth and ancient dune lines and river channels of earlier seas. More importantly these huge changes took place over relatively recent times, within the past 1 million years, too recent and too fast for geologic forces of continental drift or subsidence to have contributed much; these were largely climate changes; yet we don’t understand them; which is to say, we don’t understand the natural climate background against which our feeble efforts at climate control would play out. My kids respond, that’s no excuse for not trying to do what we can.

    I’m afraid we have different opinions about what it takes to sustain our national economy in a competitive world. To me it seems obvious, we can’t cripple our economy with carbon controls if nobody else does, even if it’s the right thing to do. Whatever economic leverage we have, we’d lose by the very act of going it alone.

    But then, Jim, you are right, it’s community resilience that we CAN work on. Even my kids support that.

    1. Liberals complain that conservatives have no effective alternate plan to Obamacare. Conservatives can rightly complain that liberals have no effective plan to abate global warming.

      Liberals are victims of the 3 Mile Island Syndrome. At 3 Mile Island it was rumored that so many of the control lights had switched to red during the meltdown that the safety engineers felt overwhelmed and panicked. The were last seen driving at high speed away from the power plant. I have always suspected this was more urban legend than reality. However, the name stuck and it is appropriate here. The liberals have pained such an overwhelmingly panicked picture of global warming that nobody can conceive of a solution that could possibly work.

  3. I am not convinced that the so-called rivers that enter the Chesapeake Bay bring enough sediment with them to change much of anything. In fairness, I do not have the facts here. However, I have observations from many, many years of boating and fishing at the intersection of those “rivers” and the Chesapeake Bay. I put rivers in quotes because the Potomac, for example, is essentially a brackish bay as far north as Mt Vernon. The relative trickle of water that funnels through Great Falls seems barely enough to offset evaporation as the Potomac stretches well over a mile wide further south.

    I could be wrong but I don’t believe the silt clouds are from natural erosion. I believe they are from excessive biological growth occasioned by nutrient rich runoff from rural fields and failing urban and suburban sewage and septic systems.

    1. I’m pretty sure the sediment clouds come from erosion . That’s one of the main emphases of storm water controls are so focused on sediments.

      Whether the excess sediment is sufficient to help the marshes along the Chesapeake Bay keep up with rising sea levels is a scientific question that I don’t pretend to answer. But it’s pretty clear that the sediment in the Atchafalaya delta is keeping the wetlands ahead of sea level rise.

      1. re: ” keeping the wetlands ahead of sea level rise.”


        you realize that the wetlands do not create a “dam” that keeps out the sea – right?

        when the sea rises – all water-bodies connect to the sea also rise.

        the only way you block the sea advancing and rising is if you have a dam.

        otherwise the sea just comes right up the rivers.. no matter how much sediment has been deposited.

        but hey – why do you believe the scientists on delta creation and sedimentation and not climate science?

    2. I think the Chesapeake Bay Foundation data supports what you say. CBF says, “Agriculture is the largest source of nitrogen, phosphorus, and sediment pollution damaging local rivers, streams, and the Chesapeake Bay” and “reducing pollution from agriculture remains the region’s biggest challenge.” CBF 2014 State of Bay Report. But I also believe the Potomac and other Bay tributaries, at least on the west side, while they may be “trickles” most of the time, can make a large silt contribution during floods when their volume and their sediment load is extremely high. In that sense let me differentiate between silt, or sediment, and the clouds of suspended particles from organic pollution that indeed comes from nutrient rich runoff. I have a place on an arm of Mobjack Bay, where there’s essentially no upstream input (nearly every tributary “river” is no more than flooded channel or marsh) and there’s little silt deposited annually, but the water still gets murky every summer and the grasses and marine life suffer.

      1. ” So why are the deltas in Atchafalaya Bay growing while the rest of Louisiana’s coastline is retreating? The key reason is that the Atchafalaya delivers sediment to the coast at a pace that allows it to settle into shallow water and to maintain marshes. In contrast, an extensive series of levees keep the Lower Mississippi’s water flowing in a narrow channel that whisks water and sediment past natural floodplains. Instead of building new land along the mouth of the Mississippi, the controlled river sends jets of sediment-rich water directly into the relatively deep waters of the Gulf of Mexico and toward the edge of the continental shelf.
        With sea level rising rapidly due to natural geological processes, climate change, and human activities, predictions for the future of the Mississippi Delta are grim. Even the land gains in Atchafalaya Bay will do little to offset the losses elsewhere, according to geologists. Most scientists expect the Mississippi Delta Plain to lose roughly 5,000 square kilometers of land over the next 50 years.”

  4. I don’t think you can or should try to compare the Chesapeake Bay with the Mississippi and Atchafalaya River – deltas.

    The Mississippi drains a huge area of the country – and has no dams on it to trap the sediment.

    The deal with the Atchafalaya is worth reading about if one really wants to understand what is going on there. The Atchafalaya is the Red River that drains through Texas all the way to New Mexico and then runs parallel to the Mississippi and would have “captured” the Mississippi, i.e. cut through and re-direct the entire Mississippi into the Atchafalaya and de-water New Orleans and other ports. The Mississippi in spring flood wants to cut a new channel into the Atchafalaya and once down – the river re-directs..

    The Army Corp build a dam between them that doesn’t let the Mississippi cut through but will meter it and in high water let a lot go down that way.

    Just wiki it and you might be fascinated with the natural process that was stopped by the COrp – although it may – over time – be a losing battle.

    I AM saddened that so many of us no longer believe what the science says and prefers to play armchair scientist with data and information provided by folks who themselves are not scientists since about 98% of the world scientists DO more or less agree with the science.. and even that strong agreement is denounced as a global conspiracy.

    So, it’s damned if you do and damned if you don’t!

    we did not seem to have this problem with the Ozone Holes. Only the scientists really understand the dynamics and they agree that something had to be done – and quickly or we could face a catastrophe. Enough people did believe them – we did act – and Acbar – if you LEAD others WILL follow – like they have with other science. We ever get there if we say we can’t go first. I think the science confirms we made the right decision with the Ozone Holes – and most of the skeptics are also climate skeptics.

    Virtually all the earth’s major rivers cut deltas and build and reshape the deltas – ironically some of it due to mankind’s activities with farming and land development but the deposits are not stable land – it’s more like warm jello… than firm land… and you can see this for yourself – just go to a wetland in most estuaries and walk on it. you’ll lose your shoes!

    BEFORE – we ever get flooded and under water – we are going to see more and more exceptionally destructive storm surges.

    and the funny thing is – even the doubters believe it if they own land in the inundation area – they are opposed to maps being drawn – even very conservative maps because just drawing those maps make the properties no longer insurable even with Govt-subsidized flood insurance. Basically – they’re done – and billions upon billions of dollars is already evaporating. Houses that sit on the ocean side of Nags Head are sitting ducks waiting for the next storm.

    again though, in the age of the internet and knowledge economies.. we’ve turned a virulent Luddite streak – and what’s going to happen – is – more and more people will reluctantly agree that the science is correct – so what we’re doing – we’re screwing around wasting valuable time until enough knuckleheads come off their Luddite kicks.

  5. re: ” Any changes we make in Virginia will have an infinitesimal impact on global temperatures, even if, as widely asserted, CO2 emissions are driving them higher. To survive global warming and rising sea levels, we must make our communities more resilient. That’s where our actions can make a difference.”

    if we had taken that approach with the Ozone Holes what would have happened?

    several subway tunnels were flooded in New Jersey and New York from Sandy. All told, about 30 billion dollars in damage was done and the fixes are not going to be easy because changing the tunnel entrance elevations is not going to be tidy affair.

    Imagine what it might cost to make the Chesapeake Bay Bridge Tunnel more “resilient” … or consider NC 12 on the Outer Banks… where virtually every new storm breaks through another place.

    I think we’re living in denial here.

    It appears what we’re doing instead is not disputing that sea level will rise but disputing why and even if true – we can’t do anything about it just by ourselves.

    I remind folks here – the rest of the world believes – it is we that do not.

    but much of the rest of the world basically does not have the infrastructure that we do – so they will just retreat inland.. and probably abandon what they have.

    we, on the other hand – cannot just abandon Hampton …

    Here Jim have fretted to and fro over our fiscal ineptitudes but where in Boomergeddon did Jim talk about all these humongous unfunded liabilities from sea level rise which are going to make our 17 trillion in debt look like peanuts!

  6. one of the big differences between the Mississippi and the Atchafalaya is that the delta of the Mississippi has had lateral dikes put in to restrict and force the flow to the center ship channel to move the sediment further out to the sea.

    Before that – the Army Corp had to continually send dredges to clear out the sediment and the sediment during spring floods would lay down new deposits – by the foot!

    so on the Mississippi River delta , there is no real chance to build up wetlands – it’s basically been re-engineered as a ship channel.

    The Atchafalaya does not have a that kind of ship channel .

    here is a report with some graphic maps of changes to both deltas

    10 Fundamental Questions about the Mississippi River Delta

    but you also cannot compare the Mississippi/Atchafalaya delta complex to the Chesapeake Bay – there is 3 times or more as much flow. The Mississippi is 500,000 cfs and Atchafalaya 200,000cfs while the Susquehanna is about 100,000 cfs and it’s half the total flow when other rivers are included so it’s probably about 200,000.

    There is also no delta – and it’s probably because of how the bay itself formed but also – not every major river has a delta but the ones that do – the delta if left in natural condition is an ever changing thing.

    the river essentially breaks up into channels called distributaries and as sediment clogs one channel it re-directs to other channels.

    Because of that, rivers with natural deltas not-engineered for ships – are – not good for ships and even estuaries without deltas – like the Chesapeake bay have to be dredged regularly to keep enough depth for larger ships.

    How all of this works with

  7. Turns out – USGS has actually published a document on this very subject in 2013:

    Land Subsidence and Relative Sea-Level Rise in the Southern
    Chesapeake Bay Region

    Now I realize that these are real scientists and govt ones at that – so it may be those facts seriously undercut their credibility with some in BR, but give it a read if just for yucks…

  8. Just due to land mass “sinking” at the shore (related to ice age glaciers which flexed the land upwards) we will have to decide how to manage subsidence, and of course sea level has been rising slowly for several hundred years, and could get worse due to climate change. So even in the base case without climate change, we have to adapt to the apparent changes. As far as how to adapt, the options will range from engineering controls to “do nothing” in areas where there is no cost effective fix. The only “good” thing about climate change is that it forces authorities to address the adaption change issue, whereas the normal approach might have otherwsie been wishful thinking (no action).

    1. just for yuks..

      does anyone consider these costs to be “unfunded liabilities”?


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