[StBernard] Rebuilding New Orleans: Subsidence and Sea-Level Rise are the Greatest Threats

[Westley Annis]

Wendy had an interesting idea.  I know when the Mississippi flooded Missouri
about 10 years or so ago, some of the farmers had an extra 6 feet of sand
and soils deposited on their farmland.  Other than the destruction the
floods brought, they seemed to be kind of happy about the deposits because
the mix of soils was good for the crops.   In the case of St. Bernard and
lower lying areas along the Mississippi, it would be nice to have the extra
elevation but with the previous development of the land and all the homes
and businesses already established-albeit, currently in bad shape--it just
seems the mess might not be worth it.
 
About subsidence, the last I heard official word, New Orleans was sinking at
the rate of 9mm a year----that's an average.  Some locations aren't sinking
at all and some are sinking at a greater rate.  The areas closest to the
Mississippi river tend to be the areas that don't subside. Because of the
river, they have vast deposits of sand and maintain the highest elevations.
As you go further away from the river, the soils are composed of sediments
that tend to compress as time goes by.  The closest thing New Orleans has to
bedrock is Pleistocene clay.  In Baton Rouge, it is at the surface or close
to it, but in the New Orleans area, in some locations, it is 60 to 90 feet
below the surface.  Yes, New Orleans is sinking and the soils will continue
to compress even more.  This worsens when there is drought or a lowering of
the water table because the water in the underlying soil strata adds to the
mass of the layers and helps the soils maintain their elevation.  When
drought depletes the water from the soils, they lose some of their "bulk"
and naturally compress.  At that point, the soil will never regain its
previous elevation, no matter how much water is put back in.  I am not sure
how the dire predictions in the article will pan out, but subsidence is
another painful realization to take in when deciding to rebuild.  It always
worried me when I lived there.  Kind of reminds me of the song "New Orleans
is Sinking"  by the "Tragically Hip".  The lyrics go, "New Orleans is
Sinking...and I don't want to swim."  Man can only do so much when up
against God and Nature.
 
Michelle