Dam & Levee Safety

Engineering:

Engineers help in the process of building dams. They also have the responsibility of making sure the dams they build are safe. To do that they must plan carefully how and where they are going to build. They must take into consideration environmental issues, features to add to the dam, and the geology of the area. The French engineer Benoit Fourneyron perfected the first water turbine in 1832 which dramatically increased the efficiency of watermills.

Maintenance:

Once a dam is built the job for the engineer does not go away. The engineers must monitor the dam for safety. Routine maintenance is performed on the inside and outside of dams. The environment around the dam must be kept clean; grasses even need to be mowed. Inside valves, doors, and many more instruments vital to the dam must be checked. It is always important to contact someone if there is a potential problem with a dam.

Active Dams Safety Management Programs:

While some dams have failed in the past, engineers have learned from the past experiences and dam failures are not significant now in terms of being a danger to people or causing significant property damage. Engineers use the highest standards during the design and construction of dams. These standards ensure that as dams age they will continue to serve us for many generations. Maintenance is continuous, with owners and government agencies, and private organizations providing periodic inspections. And since 9/11, security has been stepped up even further. The United States Society on Dams was created to bring together professional dam designers, engineers, contractors and operators to exchange information on the latest technological advances to assure top performance and safety for reservoirs and dams.

Dams can safely operate for a long time if properly maintained and improved when necessary. Typical maintenance activities include:

  • Lubrication and testing of gates and equipment
  • Removing vegetation
  • Repairing damaged concrete

Periodic safety reviews are necessary to verify that dams meet current stability criteria under normal operating, flood, and earthquake loading conditions. It is important to note that just because a dam does not meet current stability criteria does not mean that it is unsafe or likely to fail. Stability criteria include a factor of safety depending on the dam type and loading condition. Typical factors of safety for earthfill and concrete gravity dams are:

  • Earthfill dam: normal loading – 1.5; earthquake > 1.0; no overtopping for the design flood
  • Concrete gravity dam: normal loading – 3.0; flood loading – 2.0; earthquake loading – 1.3

The appropriate design flood or earthquake for a dam is dependent on the downstream risk posed by the dam. Dams are classified as High, Significant, or Low Hazard Potential based on the consequences of potential failure of the dam:

  • High Hazard – serious economic damage and danger to human life
  • Significant Hazard – some economic damage and possible danger to human life
  • Low Hazard – limited economic damage and no danger to human life

The hazard classification is a measure of the consequences of potential failure not the probability of failure.

A dam is provided with a spillway to pass flood flows. The spillway must be able to safely pass the inflow design flood (IDF) without endangering the dam. For earthfill dams the IDF must not overtop the crest of the dam, while for concrete dams, some overtopping may be acceptable. The IDF for High Hazard dams is the probable maximum flood (PMF). For Significant and Low Hazard dams, the IDF is typically less than the PMF.

Earthquakes pose a significant threat to dams in many areas of the world. Concrete dams must be able to withstand earthquake ground motions without significant cracking or sliding. Earthfill dams can settle during earthquake shaking or certain materials can lose strength leading to liquefaction.

Dam safety inspections and evaluations require input from a wide variety of engineering and scientific disciplines, including – civil, structural, hydrologic and hydraulics, mechanical, electrical, geotechnical, seismology, and geology. More information on dam safety is available from the USACE or USBR “Training Aids for Dam Safety”