Ensure Dam Safety by Knowing Your Failure Modes

Three important areas to address in responding to the FERC letter on spillways

A well-functioning flood-control spillway may not create revenue for a hydropower producer, but its safety function makes it a critical asset. Now that spillways will be under tighter review by the Federal Energy Regulatory Commission (FERC), it is critical that dam owners understand what to look for, and how further testing can ensure the integrity of their infrastructure and the safety of surrounding communities.

In the aftermath of the spillway incident at California’s Oroville Dam, FERC has identified dams under their jurisdiction that meet similar hazard criteria and requested that they undergo a focused spillway chute assessment by the end of this year. FERC’s letter to dam owners also requests a spillway-focused potential failure mode analysis (PFMA) as part of their annual dam safety inspection, or to be conducted separately.

The assessment should include foundation rock or soil composition and quality, geotechnical investigation reports, underdrain system details, inspection reports, performance monitoring and construction photographs. Visual inspections will need to cover typical potential failure modes associated with concrete spillways.

Here are three key areas of concern that hydropower producers should be aware of, and how to address them:

Uplift, Cavitation, and Slab-Jacking

A discontinuity, crack, or defect with an adverse offset can create hydrodynamic uplift on a spillway’s concrete slab. If the uplift is sufficient, it can pull the slab out of position, which creates a stronger uplift force and results in more dislocation. When a high-energy stream of water is forced up and over an adverse offset, reduced pressure creates suction on the spillway surface. A large enough step can cause cavitation, a phenomenon in which local pressure becomes low enough to form vapor pockets or bubbles. Immediately downstream, when the pressure recovers, the bubbles collapse, creating localized pressure fluctuations that can erode the concrete. An adverse offset step on a spillway that lacks the proper water stop and drainage can also create uplift pressure under the concrete slab.

One way to identify potential problems is “sounding” the concrete slab with a hammer or rod to listen for evidence of voids beneath the spillway. A visual inspection can also identify cracks in the slab, areas of settling or adverse conditions at the slab joints. Depending on the spillway design, necessary repairs may include joint filler/water stops, anchors, weep holes, drains and other methods.

Head Cutting Conditions

 Some spillways can form a head cut, or scour hole, at the downstream end, or toe, of the concrete. This scour hole can undermine the spillway, causing the spillway to fall into the hole, which then migrates upstream. FERC is particularly concerned about toe erosion and subsequent head cutting because it can migrate up the spillway chute. To identify the onset of a potential head cut, it’s important to inspect for scour holes at the toe of the spillway and mitigate them before more damage is done.

Gate operation procedures

FERC has also identified gate operation procedures as an important area for review. The intent is to evaluate potential effects of misoperation, human error or mechanical gate failure that could increase flows over the spillway. While many dam owners have analyzed such scenarios, a thorough modeling of the spillway operating with larger-than-intended design flows may be needed for assurance that this situation would not cause the dam to fail in other ways.

Conclusion

Nearly all hydropower providers should be able to meet FERC’s December timeline for conducting a visual analysis of their spillways. However, depending on their findings and the condition of the infrastructure, additional investigations may be necessary to analyze the level of impact, make appropriate repair decisions and ensure safe operation. In this case, a wide array of analytical tools is available, ranging from empirical scour tools, to 3D numeric models and scaled physical hydraulic models. The key to success is ensuring that the team responding to FERC has experience with spillway analysis and an understanding of the relative strengths and weaknesses of the various investigative tools.

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