Monday, October 27, 2008

Sand Boils

Sand boils are defined as underground seepage that rises to the surface, and is oftentimes associated with levee failures (see this Corps of Engineers document). It turns out they are not uncommon down here on SPI, although perhaps the physics is a little different.

The simplest description is when a water main breaks and the water shoots upward to the surface with a pressure called "hydraulic head" (cool name for a pound puppy?). It's a powerful force that can blow asphalt and even concrete roadway apart. When sand and clay rise to the surface in a slurry like that, it is called a sand boil. The first sign of a sand boil is many pencil-sized dribbles of water leaking to the surface. Every year our water district gets about a half-dozen water main breaks, each resulting in a sand boil.

There are several spots on the bayside up the island to the north known as being "quicksand." Yes, quicksand is just another kind of sand boil, where water is rising up from the depths. We almost suspect that the Leaning Tower of Spizza might have been built on a sometimes sand boil.

To explain, there are various strata of sand which a large clay deposit fittingly known as Rio Mud. Under this layer is some unconsolidated sandy/shelly/shale and salt formations, not solid rock. OK, there is a shallow water lens on top of the Rio Mud, water is trying to seep from Laguna Madre towards the sea, and saltwater from the Gulf is invading at the lower levels - all the reason why water wells do not work here on the island. Given the differentials in clay strata and rising or falling water levels, sand boils can routinely occur at any time. So now you know why piers are put down as high as building it tall! We're floating on what is basically ketchup.

And just like ketchup, you never know when that buddy is going to pour out all at once.

If we lived along the Rio Grand or Arroyo Colorado one might be concerned about sand boils and levee damage, so here it is more of an occasional thing that is natural - unless the dang water main breaks again. Interesting phenomena, and if I see a good one I'll snap a picture for ya.


Lucinda said...

As one who works in sand, I don't much care for the title of this post... ;-)

I do really like the term "leaning tower of SPIzza" however!

Sam said...

Now that you say it, it does sound sorta like a creepy infectious disease or something. Too funny. OK, I'll try a new concept mebbe...