Expert Interview 

Mr. Hall is Tetra Tech's senior flood control engineer, overseeing engineering studies, reports, and designs.

He has nearly 40 years of experience in the field, including 30 years with the Los Angeles District of the U.S. Army Corps of Engineers (USACE), working primarily with flood control projects in Southern California, Arizona, and Nevada. While with USACE, Mr. Hall served as Program Manager for the design of major dam projects, including the Seven Oaks Dam, a 550-foot-high earth and rockfill dam on the Santa Ana River in San Bernardino County, California, that is considered one of the largest such dams built in the United States in the past 15 years.

His previous positions with the Corps include Chief of the Hydrologic Engineering Section, the Flood Plain Management Section, and the Design Branch, as well as the Assistant Chief of the Engineering Division.

Q:  Why are flood control projects important today?

A:  Most of our nation's dams, levees, and channels were constructed long ago. They are aging and need to be modified over time; in fact the American Society of Civil Engineers (ASCE) has given them a rating of a "D" on a scale of A to F. There is a significant need for modernization and upgrading of these facilities.

Q:  Where do you see the most need for modernization?

A:  Here in California, people hear about the drought problem more often than the threat of flooding. But we have an extensive levee system that was mostly constructed by farmers 100 years ago, and is therefore very vulnerable. Recently the state of California looked at the levees in the Central Valley and decided that the risk there is considerable. Nationwide, the levees in the delta of California are now considered the most dangerous, posing a risk as great as what happened in New Orleans.

Q:  Why is that?

A:  The real risk is the next major earthquake. Whatever earthquakes do happen in that area tend to shake these levees down, and of course there is water continually up against them. They’ve been there a long time, but there are under-seepage problems and through-seepage problems. The risk is that many of the areas behind the levees are actually lower in elevation than the water, so once a levee fails, the water will just flow into those lower areas.

Q:  What’s driving this new focus on modernization of flood control systems?

A:  When Hurricane Katrina happened and the levees failed around New Orleans, that got everybody’s attention and made them realize that flood control projects can fail. It got people looking at levees throughout the country, and as a result, FEMA is now requiring all levee owners to certify their levees. That has put a large amount of work out there for consulting engineers, because the only people who can do that certification—other than the owners—are consulting engineers.

Also, a National Committee on Levee Safety was set up and it has just released its report to Congress, recommending that a National Levee Safety Program be established. If Congress approves and funds this effort, it could dramatically change the way levees are perceived by the general public, and affect the way the levees are analyzed and treated.

Q:  And that’s where Tetra Tech comes in?

A:  We have a very strong water resources group that can perform the hydrology, the hydraulics, the risk analyses for the levees, as well as the scour protection and revetment that’s required. On the geotechnical side, we have some great expertise in people who are able to analyze the foundation conditions and levee conditions and do seepage analyses that demonstrate whether a levee is adequate or needs to be repaired. Tetra Tech is very strong in the technical skills that are needed to provide levee certifications.

Q:  Give me an example of an interesting flood control project you've worked on.

A:  The Santa Ana Mainstem project is a really interesting one. This $2 billion project consists of a number of elements, including 24 miles of channelization work, Prado Dam, Seven Oaks Dam, a wetlands area, and others. It’s a project that the Corps of Engineers has done the planning, design, and construction on for about 25 years, or for much of the time I worked for the Corps. I designed significant portions of that, and am the engineer of record for Seven Oaks Dam and the downstream channel project.

Prado Dam is an existing dam that’s been there since 1941—it had to be enlarged. It was raised 27 feet and new outlet works were put in, which increased the capacity for releases from Prado from 10,000 cubic-feet-per-second to 30,000 cubic-feet-per-second. The spillway also had to be enlarged. The perimeter of the reservoir itself went up because the dam was raised, and so dikes had to be placed in strategic areas to protect facilities in that area, including Redlands Airport, Chino Women’s Prison, and a number of other facilities.

If you go upstream to San Bernardino County, a new dam was put in—Seven Oaks Dam—and that’s a really interesting project. Tetra Tech performed analyses for this 550-foot-high dam that sits right astraddle the San Andreas Fault. It’s a big earth- and rock-filled dam, and it had to be carefully designed to deal with not only the ground motions from major earthquakes along the San Andreas, but also the potential of up to four feet of actual shear through the embankment and outlet works.

Q:  How do you design for a dam like that—one that is subject to earthquakes?

A:  The dam foundation is in a zone of compression, so in the event of an earthquake, you won’t have a separation four feet wide in the middle of the dam; it slides on itself. The zones within the embankment—and there are about five of them—are designed so that if this shear were to take place right through the embankment, the material in the various zones would tend to seal the dam after the shear had taken place. And if the shear were to occur through the outlet works, you would still be able to empty the reservoir through the remaining tunnel capacity.

There are numerous other elements of the Santa Ana Mainstem project, including a major feature at the mouth of the Santa Ana River where it discharges into the ocean, where you'll find a 92-acre mitigation wetlands that was created by the project. Tetra Tech did the design of the tidegates and layout of the marsh, as well as the design of the relocation of Talbet Channel in the same area.

Q:  What else does this project do for the communities around it?

A:  This is a flood control project, but it also provides recreation, mitigation (in addition to the 92 acres there are other mitigation features all the way along the river), and conservation. Because Prado Dam was raised, there is significant water conservation benefit, as well as from Seven Oaks Dam. Both of those are adding to the water conservation in our area. The completion of the project will also result in a bike path that goes from the ocean along the project all the way to San Bernardino.

Q:  What are you working on for Tetra Tech today?

A:  We're doing a lot of levee certification, which is very much a growing field and Tetra Tech is right at the forefront of it. Another project I'm working on is the Tucson Drainage Area Project, which is currently in construction. Tetra Tech is providing engineering services for the project, which will reduce damages that result from frequent flash floods in the area. Tetra Tech completed the plans and specifications and final design for this project several years ago and we are currently under contract to the Corps during construction.