Bailing In: Subsidence and Sea Level Rise

Since 1880 sea levels have risen about 8 inches. For most of us, that has not posed a problem.

bailing iin

Where it has posed a problem it is combined with both natural and human-caused subsidence. In parts of the globe threatened by rising tides, subsidence has played a bigger role than the rising seas.

Subsidence is a common cause of amplified relative sea-level rise, flooding, and erosion in coastal environments. In particular, subsidence due to sediment consolidation can play a significant role in relative sea-level rise in large deltas. We use a combination of InSAR (interferometric synthetic aperture radar), leveling, and global positioning system data to map absolute vertical land motion in the Fraser River delta, western Canada. We show that primary consolidation of shallow Holocene sediments is the main cause for the slow subsidence (−1 to −2 mm/a) affecting the delta lowlands. In addition, parts of the delta undergo increased anthropogenic subsidence. Rapid subsidence rates (−3 to −8 mm/a) are associated with recent artificial loads and exhibit a first-order exponential decrease with a time constant of ~20 years, consistent with the theory of consolidation. Assuming two sea-level rise scenarios of 30 or 100 cm by the end of the twenty-first century, natural subsidence will augment relative sea-level rise in the Fraser Holocene lowlands by ~50% or ~15%. Anthropogenic subsidence will augment relative sea-level rise by ~130% or ~40%, potentially raising it to as much as 1–2 m. In deltaic, lacustrine, and alluvial environments, anthropogenic sediment consolidation can result in significant amplification and strong spatial variations of relative sea-level rise that need to be considered in local planning.”

Subsurface fluid-pressure declines caused by pumping of groundwater or hydrocarbons can lead to aquifer-system compaction and consequent land subsidence. This subsidence can be rapid, as much as 30 cm per year in some instances, and large, totaling more than 13 m in extreme examples. Thus anthropogenic subsidence may be the dominant contributor to relative sea-level rise in coastal environments where subsurface fluids are heavily exploited.”

“Erban et al (2014) use satellite-radar interferometry (InSAR) to document recent subsidence rates of 1–4 cm yr−1 over large parts of the Mekong Delta, and show that this widespread subsidence is likely caused by groundwater pumping and associated water-level declines. They project ∼0.9 m of human-induced subsidence by 2050, versus ∼0.1 m of expected sea-level rise.”

“In some areas of the world, the obvious and expensive damage caused by anthropogenic coastal subsidence has prompted concerted efforts to arrest and reverse groundwater-level declines, often by importing additional surface water. For instance in the greater Houston (USA) area, nearly 3 m of coastal subsidence due to aquifer-system compaction caused billions of dollars in damage and in 1975 prompted establishment of a Subsidence District with regulatory authority (Galloway et al 2003).”

5 responses to “Bailing In: Subsidence and Sea Level Rise

  1. An interesting post, thanks.

    For info – the north of Britain rises while the south sinks:

    Southern England is slowly sinking while Scotland is rising up, according to a new map created by Durham scientists that shows how land and sea levels around the British coastline are rising or subsiding.

    As the ice sheets melted at the end of the last Ice Age 20,000 years ago, the release of their enormous weight resulted in the landmass tilting up in the north and down in the south. The Coastland Map, which has produced the most accurate predictions to date of land uplift and subsidence in the UK, shows that the tilt is continuing.

    It projects that land levels could rise by up to 10cm in some parts of Scotland over the next century, a rate that could offset the effects of sea level rise caused by climate change. In contrast, parts of England, Wales and Southern Ireland could sink by up to 5cm over the next century, adding between 10 and 33 per cent to projected sea level rises.

    • There is a similar boundary in eastern North America. Canada and northern New England are rising while New York City and the Central Atlantic States are sinking. The land around Hudson’s Bay is rising faster than sea level is, so sea level locally appears to be falling. On the flip side, the Chesapeake region is sinking, so even if sea level were to stop rising tomorrow that area would have an ongoing problem.
      There are many coastal areas where geological processes are causing the land to rise or fall at a rate greater than global sea level rise.

  2. The famous Dutch dikes are a result of subsidence (at least the older ones). They did not decide to build below sea level, they drained low lying marshy land, then had to build the dikes when the land subsided due to removal of water from the soil. The same is true for much of New Orleans. For the same reason, Chicago had to raise the entire city in the late 19th century.

  3. Living in Houston and being raised on the Gulf Coast of the US, where subsidence is real and sometimes dramatic, it is just another thing to deal with. If you plan poorly you get some flooding. When tings flood, you fix them, make them flood resistant, raise them up or tear them down and turn the land into a park. Not really that big of a deal. It costs lots of money, but the money gives us lots of green spaces for an urban environment. If Houston City government was not squandering money on sideshow issues and focusing more on infrastructure quality it would be an even smaller problem.
    Coastlines shift for many reasons. The ancient city of Troy, once a port, is now miles inland due to river flow changes. Lands subside, erode, rise up, etc.
    Sri Lanka and India were once connected by land.
    If we were not wasting so much money and effort on the climate obsession we could continue to improve the infrastructure needed to manage subsidence and the other things that impact where lands we care about meet the oceans.
    When we swing back to a rational, optimistic and progressive age people will be puzzled by the reactionary fear mongers using climate and weather as excuses to hold people back.

  4. The only real trouble i see is Miami. But that can be solved by raising street level and houses by 1 meter in new developments. Since sea level rise is so slow, as old houses/buildings are demolished, the replacements can be raised one meter. That gives about 100 years breather.

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