May 17, 2011

Hydrologic hills of Lake Okeechobee

How low is Lake O?

This hydrograph sort of explains.

All the Lake's interior-levee wetlands have gone dry

Current stage – at just around 10.6 ft above sea level – is 12 feet below the level it once needed to be to naturally “overflow” south into the Everglades. That stage was 22 feet above sea level.

In modern times, of course, water no longer overflows its banks. There’s a 35 foot levee and engineered structures through which water is routed instead. Nor does it go south, or not mostly: The majority of water, when it’s there to be had, is released to the west into the Caloosahatchee and to the east down the St Lucie where it’s lost, regrettably, to tide.

That keeps the water line in check so it doesn’t get too high.

Of course half a year later drought strikes and we wish we could have it all back.

Some views of the Lake's major structural features

But 35 ft high would be impossible to over top, right?

The trick in this case is that all it has to go up is to 17 ft above sea level. At that level (and higher) the force of the water on the lime rock levee can (and has) dissolve its way underneath top of the levee in the form of wormholes which, if left unchecked, can threaten a catastrophic breach, sending water flooding south once again.

How can water actually flow south onto higher 22-ft land?

(After all, everyone knows that water can only flow downhill.)

The trick in this case is that the peats lands downstream of the Lake’s south rim have sunk a good ten feet down from its pre-drainage 22-ft high perch. The reason for that is that the surface of the Everglades is made of peat which, over the course of five thousand years (i.e., age of the Egyptian pyramids), built up into a thick mat of organic matter from being constantly submerged. But take away the water and the peat slowly (oxidation) and quickly (fires) vanishes. On the ground, that process – called subsidence – has manifested itself as a sinking and more and more flood prone land.

The levee is in the process of being strengthened as a result. (see article.)

Caloosahatchee's estuary swings between too little and not enough freshwater flow

How low is the Lake?

If waters drop another foot and a half lower, gravity becomes ineffectual: Or in other words, water will no longer free flow Lake water to the outflow structures where managers need it to be in order to send it into the farmlands and farther south.

As a result, pumps will be installed to help it flow uphill instead.

Who knew? So water doesn’t always flow downhill? (I had no idea!)

Here’s an article that explains more.


Janie said...

Levees and dams have upset nature's course in a lot of ways. Draining swamps and building on flood plains may not have been such a good idea after all.

Robert V. Sobczak said...

Once it gets messed up it takes a King's ransom to get it right ... and even then problems persist.

Robert V. Sobczak said...

I just read a newspaper that puts the line where low-water pumps are used at 10.5 ft above sea level. If that's the case I will have to modify my graph. Amazing how much information is out there ... constantly tweaking.