Tuesday, December 21, 2010

Slip Slidin' Away

A pretty decent article on sea level rise and the Chesapeake Bay today - Sinking land, not rising seas, a bigger worry

The highlights:

First, the good news: Sea levels around the Chesapeake Bay are not rising as quickly as other places in the world - actually, they are moving about half as fast as the global average.
Now, the bad news: Coastal lands around the Bay are sinking more rapidly than elsewhere around the planet, especially in Hampton Roads.

It is this sinking phenomenon, called subsidence, that makes Hampton Roads one of the spots in the United States most vulnerable to rising sea levels and to events such as flooding, tidal surges and storms. Only New Orleans is more susceptible.

Such are the findings of a study released Monday by the Virginia Institute of Marine Science, a branch of the College of William and Mary.
More after the jump

 Sea level rise is one of those things we hear a lot about in the news, but without much historical context.  Sea level has been rising world wide for since the peak of the last ice age, as the water trapped in the continental glaciers that covered much of North America, Europe and Asia began to recede.
Source: Wikipedia
In the historical period, however, the rate of rise has been fairly low and constant with no appreciable evidence of increased rise due to global warming.

Source: Wikipedia

The article points that out, and fact that about half of the measured sea level rise in the Chesapeake region is due to subsidence of the land, much of it a response to the melting of the glaciers and the rise of the ground previously weighed down by the ice, causing subsidence of nearby areas, much like a bowl of silly putty might (do they still make Silly Putty?).

Now the current rate of sea level rise (about 4 mm a year  here) doesn't seem like much, but over a good lifetime (lets say optimistically 100 years to make the math easy) that's 40 cm or 1.3 ft.  That's enough to make a road or house sited close to the rising tide significantly more vulnerable to storm tides as it ages.

Some lands can respond to sea level rise.  Salt marshes, which rim much of the Bay shore, particularly on the Eastern Shore can add mass, from sediment and accumulate plant material (peat) to match the rise of sea level.  Beaches can gain or lose sand, depending on prevailing currents and waves.  The land near where rivers enter the bay fill with sediment.  Hard shores, rock and sandy cliff (like mine) are slowly engulfed, with the cliff eroding at the bottom, and the top eventually sliding down.  In much of the bay, the banks are eroding back at a rate of about 1 foot per year average.  On one site on our weekend walks, there was reputed to be a baseball diamond on top of the cliff where the beach is now. An old dugout being used as a shed overlooks the cliff

The Bay itself is geologically a transitory feature.  At the height of the last glaciation, there was no bay, just the continuation of the Susquehanna River out onto the continental shelf. Clovis men hunted big game, and left spear point on a hill overlooking the river, on what is now Tilghman Island.  And then the bay came in again.  For the fourth (at least) time.  If we don't get another glacial reset, the Bay is scheduled to fill with sediment in roughly 40,000 years.  Long by human standards, but a short paragraph in geological record.

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