Let’s Build a Seawall

May 17, 2009

I read with great surprise and pleasure that ClimateSanity has been added to the blogroll at  WattsUpWithThat.  Anthony, thank you, it is an honor!

The thing that brought this on was one of my previous posts about the effect of sea level rise on Boston, which Anthony was kind enough to share with his huge readership.

Just to stir the pot a little, here is a simple back of the envelope calculation that may serve as a response to some of the comments at WUWT:

Click to enlarge

Click to enlarge

Suppose you felt the need to build a sea wall three meters high and three meters thick to protect the Boston area from the rising sea.  Lets say you built it from Lynn in the North to Quincy in the south.  These two communities are about 25 kilometers apart in a straight line.  But let’s say when you account for the curviness of the coastline its really 100 km.  So,this sea wall will be 3 meters high, 3 meters thick, and 100 kilometers long.  That means the volume of the sea wall will be 900,000 cubic meters, or about 1,000,000 cubic meters. 

Since the ocean is rising quite slowly, we can allow ourselves 100 years to build this sea wall.

The population of the greater Boston area is about 4 million. Let’s assume that the population remains unchanged for the next hundred years (of course, it will probably grow a lot).

Then the volume of seawall that needs to be built per person per year is:  0.0025 cubic meters per person per year.  Oh my,  how will you every do it?

But if you manage to get it done in 100 years you will be able to look at the accomplishment with pride.  And assuming that the sea level keeps rising at its current rate of about 2.5 mm per year, you will know that the 3 meter tall seawall will keep you safe for another 1000 years.


  1. But the shore birds will be unable to breed. It would be the end of the horseshoe crabs in that area. All beach life would cease.

    Plus there is the EPA paper — the issue for Boston Harbor isn’t the height of the seawall, but how the other structures will be able to withstand the other effects, the weather and the heat.

    And the next New England hurricane of 1938 will wipe part of it out, open a couple of new water channels, and maybe build another island.

    If only the issue were so simple as building a seawall, a seawall would be the way to go.

  2. I think Ed worries in vain.

    I don’t sew what keeps the sea water from running around the ends of the wall, or am I mistaken about the altitudes of the terrain at the anchors?

    I’m a Watt import–nice looking place here. Mind if I look around?

  3. Ed and Larry,

    The engineering required to protect a place like Boston is small potatos for four hundred million manyears of ingenuity and labor. (4 million people for a hundred years)

    No need to sweat the details. This was just a fun, back of the envelope exercise. But it makes the point.


  4. How deep is the ocean at the location of your projected dike?
    If it’s only a few meters, you could find some inspiration here http://images.google.nl/images?q=afsluitdijk to see what’s really needed to protect that area (or even turn it into polder land). That dike was built in 1932 and is 32 km long, through the North Sea.

  5. If we assume the Dutch sea-dikes are good reference designs, we can see that the ‘Afsluitdijk’ (32km of dike, in 2 to 4 meters deep sea) has a width at its base of 90m (270ft)! Its height above sealevel is 7m (21ft). A smaller dike that used to hold out the North Sea is the Wieringermeerdijk, at about 60 at its base and 4.4m above sea level. That dike now holds in a fresh water lake.

    From a rough guess of the crossection of about 560m2, we find that that dike has about 18 million cubic meters of clay and rock in it.

    So even the straight line solution has 20 times more volume than your long way around dike. And this is assuming the Boston Bay is as shallow as the North Sea where it was cut off by the Afsluitdijk (2m, 4 meters in the stream channels).

    I applaud your optimism, but your figures are wildly wrong, and a seawall that’s only 3 meters above the ocean without a slope to absorb wave impact will be gone with the first winter storm, when the waves go over the top of the seawall, and erode its foundation from behind.

    Come over here for a tourist trip. It’s quite worth it.


    • Here’s an url for Dutch dikes crosssections


      • Dear Bart van Deenen,

        thank you for you very interesting comments. I looked at the websites concering Afslutdihk you suggested and several others.

        Your are right about the volume of the dike being about 20 times my hypothetical seawall. Yet it only took about 6 years to built it (1927-1933), 1/20th of my hypothetical seawall timeline. And it was built with 1920s era equipment.

        At twenty times the volume, my 100 year constructed seawall would only require the movement of 0.05 cubic meters of material per person per year. Still an easily accomplished project.

        I was thinking more along the lines of the Galveston seawall, which has a cross section of about 17 feet X 16 feet (call it 5 meters X 5 meters), but with a concave profile. In this case the area of the cross section is back down to about 16 square meters. Such a 100 kilometer seawall has a volume of only 1.6 million cubic meters. At 100 years and 4 million people it only requires the construction of 0.004 cubic meters per person per year.

        Again, this is just an order of magnitude quick calculation. The point is that when there are many free and ambitious people, a few mm of sea rise every year is not such a daunting thing.

        Best regards,

  6. Won’t you need to build a similar one on the West coast, or are you relying on the Rockies to keep the water out..?


    • Dear James P May,

      Thank you for your comment.

      “California’s general coastline is 840 miles long. Along much of the coast, the Coast Ranges rise from the shore in steep cliffs and terraces…”

      Oregon and Washington state are very similar. Nobody is going underwater due to sea level rise at these cliffs and terraces.

      There are some places in California where this is not the case. Look at this map of the “Impacts of Sea Level Rise on the California Coast.” Now zoom in; zoom in some more; keep going. Soon you will see very small areas of the California coast that are at risk due to sea level rise. Now, take a look at the report that accompanies the map any you will notice in the very first sentence:

      “In an analysis prepared for three California state agencies, the Pacific Institute estimates that 480,000 people; a wide range of critical infrastructure; vast areas of wetlands and other natural ecosystems; and nearly $100 billion in property along the California coast are at increased risk from flooding from a 1.4-meter sea-level rise – if no adaptation actions are taken.

      First, note that this is based on a 1.4 meter sea level rise in 100 years. The sea level rise rate over the last 100 year in San Francisco, for example, has been 2 mm per year. Look closely at that sea leve rise link for San Francisco and you will see that the rate of sea level rise rate is simply not increasing. At the current rate, the sea level rise in San Francisco will be 200 mm (0.2 meters, or about 8 inches) in the next hundred years.

      So, 1% of the california population is at risk. The adaptation actions are quite simple, especially when distributed over 100 years and a population of 37 million people (3.7 BILLION man years!!!)

      What do you propose? Should we restrict a economy of California, the United States, and world in order in order to protect 1% of the population of California from an easily mitigated problem that is likely to not happen?

      best regards,

      P.S. Thanks for the smiley face at the end of you comment. That was a nice touch.

  7. […] the magnitude, consequences and proper response to sea level rise here, here, here, here and here. The alarmists would like to dismiss the evidence of man’s ability to cope. I have wondered why […]

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