The levee at Alton is in trouble, not because its construction was faulty, but because the construction of the Melvin Price Dam two miles south of the old dam created a permanent flood behind dam and raised the water table in the wetlands behind the levee. Water is seeping under the levee and forming sand boils in the wetlands on the other side.
The Corps of Engineers built and paid for the levee sixty years ago, from material dredged from the river, and turned its maintenance over to the Wood River Levee and Drainage District. This is standard procedure: the Corps builds the levee; the beneficiary maintains it. Were the levee built today, the beneficiary would pay 35% of the cost and the Corps 65% of the cost. There is a move afoot to flip that ratio.
First, you have to understand the geology of the land that supports the levee: a thin clay blanket overlays a thick sand aquifer, which rests on top of bedrock. It’s the thinness of the clay blanket at Alton that is the problem. Water is seeping under clay blanket, moving through the sand, and carrying it to the land side of the levee. Seepage, water seeping under the levee is a given in all levees, particularly when the river is flooded.
Then you have to understand hydrostatic pressure. My husband, the architect, who deals with hydrostatic pressure every time he builds a retaining wall and said hydrostatic pressure does anything it wants, helped me understand it. In the retaining wall the dirt behind it wants to push it over from the top or shove it out from the bottom. That’s one form of hydrostatic pressure.
When you dive deep into a swimming pool and your ears hurt: That’s hydrostatic pressure. Which brings us to levees.
Build a levee and the water pushes against it when the river floods: hydrostatic pressure. The pressure is stronger at the base of the levee than at the surface of the water. The construction of the Melvin Price dam created a permanent flood next to the levee and raised the water table landward.
When the pressure meets permeable soil under the levee, say sandy layer at Alton, it pushes water through the sand to the land side of the levee. That’s seepage. That’s what is happening at Alton . And it’s carrying the sand with it and bubbling up in sand boils in the wetlands landward of the levee.
There are several ways to counter hydrostatic pressure.
Push back: The seepage berm, I have always thought of it as a buttress, but it is designed to balance the press of the water outside the levee with the weight of the berm on the inside. If the water seeps under the levee at a great rate, it will create cavities under the levee, into which the levee can collapse, allowing the river to pour through resulting crevasse.
A second way is to built relief wells, an internal drainage system set at the toe of the levee that reaches down into the sandy layer under the clay blanket, removes the seepage, and spills it into collector ditches, gutters, that carry pump stations, where it is pumped back into the Mississippi.
A third way is to build cutoff walls: excavate a trench down to the bedrock and fill it with bentonite, stopping seepage altogether.
The levee at Alton has a berm on which Illinois 143 travels and relief wells set in the toe of the berm. The Mississippi Levee Board has fairly clear illustrations of seepage berms and relief wells.
When the engineers constructed the dam in the 1980s and early 1990s, they modified the relief wells to counter the increase hydrostatic pressure that came with the permanent flood behind the dam.
I am grateful to Chris Wilson of the St. Louis District of the Corps of Engineers for helping me understand what is going on with the Alton levee. Mr. Wilson is also in charge of the upgrade of all the levees from Alton to Columbia, Illinois, all of which have water seeping under them and all of which have been decertified in the wake of the levee failures at New Orleans, during Hurricane Katrina.
Copyright © Quinta Scott, 2009, All Rights Reserved