Monday, December 12, 2005

Sacramento Flooding, Part Nine

What is revealing about this report in yesterday’s Bee is that it takes 80 foot deep slurry walls (sunken alongside the levee as protection barriers) to ensure against levee seepage, and many of the most populous areas do not have anywhere near that deep protection.

As tragic the floods in New Orleans were and continue to cause great hardship there, it has garnered well-deserved local attention as we are the least protected from flooding of any major metropolitan area in the United States.

Seepage threatening area levees
Experts drill next to earthen walls to pinpoint problems.
By Deb Kollars -- Bee Staff Writer Published 2:15 am PST Sunday, December 11, 2005

Investigators studying the New Orleans flooding disaster have identified underground water seepage as a key reason certain levees collapsed during Hurricane Katrina.

A mostly invisible problem until it is too late, deep water seepage also is an urgent concern in Sacramento and throughout the Central Valley.

For years, flood control experts have had their hands full designing and maintaining above-ground levees that contain the rivers flowing across the landscape.

But now, they also are trying to get a grip on unseen waters traveling silently beneath those levees.

The Sacramento Area Flood Control Agency recently concluded a massive drilling project along levees in Natomas, involving 110 deep drilling sites, to determine the depth and extent of seepage there.

In Yolo County, flood control officials in West Sacramento are evaluating similar geotechnical data already on the books, and are planning more investigative drilling next spring.

At the same time, the state has identified underground seepage as a major concern across the Central Valley, and is working on a plan for evaluating the layers of ground and the water flows beneath levees.

In New Orleans, preliminary reviews have found that such underground flows undermined certain levees and caused them to crumble after Hurricane Katrina hit. The seepage occurred below walls of sheet pilings that had been driven 10 feet into the earthen levees, far shallower than what engineers now say is necessary to protect the city.

Les Harder, California's acting deputy director for public safety for the Department of Water Resources, was part of a National Science Foundation team that investigated the New Orleans flooding disaster. He came home shaking his head - both at the losses in the South and at the potential for losses in California.

"Do we have those cases here? You bet," Harder said of the underground seepage threat. "Have we had past failures because of it? Absolutely."

For the rest of the story: