The U.S. Army Corps of Engineers is a leader in developing engineering criteria for safe dams, and conducts an active inspection program of its own dams. The Corps also carries out inspections at many dams built by other federal, state and local agencies and private interests by request. The primary objective of the Corps’ Dam Safety Program is to maintain public safety by making sure the dams it owns and operate are safe and risks to the public are minimized.
Don Smith of the Wilmington Regional Engineering Center is the Dam Safety and Bridge Inspection Safety Program Manager for the Charleston, Savannah, and Wilmington Districts located at the Savannah District.
“We take our dam and bridge inspections seriously, and I feel the Corps has a good inspection program,” he said. “We look at dams primarily every five years for a comprehensive inspection, and we have annual inspections for embankments and other dam features. At all projects, our Corps people check dams monthly, reading instrumentation and getting good visual observations.”
Smith said that in 2005 the Corps started screening dams for a risk assessment program. Each USACE District identified dams within their jurisdiction that they believed had the highest potential risk based upon their internal evaluation. From this group of structures, experts evaluated the structures and classified them into five basic risk categories:
1. Urgent and Compelling – Active failure in progress or extremely high risk of failure possible
2. Urgent – Failure expected as the result of an event such as a flood or earthquake or very high risk of failure.
3. High Priority – High risk to property loss and/or loss of life if failure occurs
4. Priority – Probability of failure is low but not tolerable.
5. Normal – Adequately safe, meeting USACE guidelines.
“Our projects in the Wilmington District have been screened,” he said. “Of the projects screened to date, Fall Lake Dam, has been identified as a Dam Safety Action Classification (DSAC) Category 3. What this means is that although the dam is structurally sound it does have significant risk for economic loss and loss of life in the downstream areas if a failure occurred. Routine and periodic inspections and evaluations will continue, and we’re currently drafting an Interim Risk Reduction Measure Plan (IRRMP). The resulting actions from the IRRMP would result in lowering the risk classification to a category 4.”
In the South Atlantic Division, Smith added, the Jacksonville District’s Herbert Hoover project has serious problems and is a DSAC I dam.
Dam inspection is thorough, and no shortcuts are ever taken. Ann Hinds, a civil engineer assisting Smith with the Wilmington District’s Dam Safety Program, said when doing physical dam inspections she looks for obvious signs like trickles of water or small cracks.
“We look for any seepage or instability and we look for cracks or irregularities in the slopes. But we have a very good front line of defense. Our project managers and rangers are at the site all of the time. They basically do an inspection every time they’re on the dams. They get training every four years, so they know what to look for. Also the supervisors at projects train new employees once they come on board.”
Hinds said if a leak or crack developed into something greater there would be an immediate fix to thwart an even greater problem.
“A dam could have a small hole and depending on the type of material that dam is made from it could grow larger. Seepage occurs when there is pressure from the water in the reservoir that’s constantly pushing against the dam. But most dams are built in different layers. The core layer, usually clay, is less porous to restrict water flow. Behind that is a filter layer, so if any seepage does get through it goes through sand that will filter any particles and prevent any movement of material.”
Hinds emphasized that if there is out-of the-norm seepage there is immediate notification. However, inspectors know the difference between good and bad seepage.
“At John H. Kerr, for example, we have what’s called a controlled seepage. It’s natural for water to seep through. Water will always find a way to get through, so that’s why we have a filtering system to control the seepage a prevent particle movement. And this controlled seepage is monitored for any changes in flow or clarity.”
Then there are the unseen workings in the interior of a dam that could pose potential problems. But like a physician with a stethoscope on a human body, engineers have devices to check a dam’s inner body.
“We have instrumentation on all of our dams that monitor water levels. Those levels are checked monthly or at other cycles every so often. There are numerous checks and balances.”
Smith said that constant observation is key to ensuring dam safety. In addition, USACE headquarters keeps upgrading safety measures.
“Each year there are more and more guidelines about dam safety. In fact our division office is involved now in more detailed methods of safety. And again, the most important people who are monitoring dams are the people at the projects themselves. They’re there every day, and if they ever see something unusual they know to call Ann or myself.”