Dam Safety Forum

The Dam Safety Forum – hosted by the National Hydropower Association’s Hydropower Technical Community (HTC) – is open to all Clean Currents attendees with an interest in dam safety-related hot topics, challenges, and solutions.

Featured Presentations include the following (keep scrolling for details on each!):

– Tainter Gate Trunnion Friction and Seizing Leading to Gate Failure

– Estimation of Hydraulic Forces during Head Gate Emergency Closures under Flow using CFD

– Masonry Overflow Dam – Seepage Mitigation

– Using the Simplified Stochastic Event Flood Modeling Approach to Support the Risk Informed Decision-Making Method to Evaluate Dam Safety

In addition, discussions among attendees will cover these themes:

– Water control and stream/creek/river bypass for dam construction.

– Drilling plans and measures to consider in concrete dams when holes need to be drilled, approval challenges and schedule impacts, including getting clarification on what is needed including appropriate level of effort for various projects (drilling in an artesian aquifer vs installing a fence post).

– Lessons learned from new FERC regulations — key take aways and trends from the first round of comprehensive assessments.

– A look back at the impacts of focused spillway inspections, findings, and resulting rehab/retrofit projects.

– New trends in the industry.

– New technologies.


Details about each Featured Presentation and the faculty are below:

Tainter Gate Trunnion Friction and Seizing Leading to Gate Failure, taught by Paul Schiller, Barr Engineering Company

In 2011, during a routine Tainter gate inspection at the Tomahawk Hydro plant in northern Wisconsin, the operations staff noted what appeared to be buckling and fractures in one of the gate arms near the trunnion assembly. Emergency response was initiated, and a forensic evaluation was undertaken to determine the root cause of the gate arm failure.

The root cause was found to be extremely high frictional resistance at the interface of the phosphor-bronze bushing and the steel trunnion pin, which led to local flexural buckling of the double-angle gate arm near the trunnion assembly at a previously unknown discontinuity in the composite cross-section of the gate arms.

The instructor will share the field data and the scientific method employed by the dam owner/operator and its consultant to determine the root cause of the gate arm failure and to evaluate the potential for similar failure of other gates at the site. The hypothetical root cause was then confirmed through destructive testing on a trunnion assembly of the failed gate. The instructor will share details on the testing program and results.

Several technical phenomena of interest to owners, operators, and dam safety consultants will be presented, including seizing of a pin-to-bushing interface and the design and in-service bushing clearance; gate arm flexural failure during the closing of a gate (due solely to the self-weight of the gate); and local buckling potential at discontinuities in built-up members.

Estimation of Hydraulic Forces during Head Gate Emergency Closures under Flow using CFD, taught by Adrian Strain, HDR, Inc.

The headgate gate closure process appears simple on paper. Lift, move, and lower the gate leaf into place – easy under static flow conditions! In practice and in an emergency closure under flow scenario, it can become significantly more complex. The flow rate and resulting hydrodynamic forces can change instantaneously resulting in a failure to seat the gate leaf. Hydraulic lift and down pull can subject the lifting mechanism to unusual forces and strain.

Multiple computational fluid dynamics (CFD) models were developed to gain an understanding of the hydrodynamic forces and their impact on the closure process. The CFD modeling process was based on previous numerical hydraulic and physical model studies and looked to produce insight on the potential failure during high-head, high-flow scenarios.

Five different intakes were evaluated during the model study. Single gate leaf and multi-gate leaf configurations were analyzed with their respective head and flow rate. The CFD model results were paired with analytical methods to estimate hydrodynamic loading on the gate leaf and lifting mechanisms.

The compiled results were used to inform the site operations staff and allow for understanding of potential failure during the modeled scenario.


Masonry Overflow Dam – Seepage Mitigation, taught by Ian Radcliffe, J.F. Brennan Company, Inc.
Background
Located in International Falls, Minnesota, on the U.S.–Canadian border, a gravity overflow dam lies on the Rainy River. The dam is composed of large masonry blocks, placed in formation over 100 years ago. Over the years, the joint mortar used in construction deteriorated, causing seepage through the dam. In 1997, unsuccessful attempts were made to construct a grout cutoff wall to reduce the leakage emanating from the downstream face. Additionally, a 12-inch-thick unreinforced “skin” was placed to try to control the seepage.

Design/Solution
Brennan went through several design iterations with the engineer/owner before settling on a final solution. This solution involved stay-in-place interlocking vinyl sheet piles along the upstream face, anchored to the dam with tie-back rods and whalers, then backfilled with a flowable fill grout mix. Tie-back rods were required to be drilled a minimum of 2 inches into the masonry blocks to reduce the chance of blocks pulling out during backfill placement. Horizontal and vertical layers of reinforcing steel were also incorporated into the design.

Results/Lessons Learned
Following installation of the vinyl sheeting and grout backfill, the seepage was significantly reduced across the dam. Photographs taken before and after the construction phase undoubtedly showed a difference in the water leakage. Not only was seepage mitigated, but the owner was left with a new vinyl sheeting face on the upstream portion of their dam to resist ice jacking of the masonry joints and provide long-term protection.

Using the Simplified Stochastic Event Flood Modeling Approach to Support the Risk Informed Decision Making Method to Evaluate Dam Safety taught by Omid Mohseni, Barr Engineering Company

One of the objectives and supporting strategies in the Federal Energy Regulatory Commission (FERC) Strategic Plan for fiscal years 2014-2018 is to minimize risk to the public by using Risk-Informed Decision-Making (RIDM) for evaluating dam safety in parallel to traditional dam safety methods.

Resulting risk estimates can be used, along with standards-based analyses, to decide if dam safety investments are justified.

Consumers Energy Company identified a concern at its Alcona Dam in Michigan regarding potential erosion of the unlined, earthen auxiliary spillway, and the potential subsequent failure of the dam during flood events more frequent than the inflow design flood. However, given the possible consequences downstream, the estimated dam fragility, and the proposed and completed risk reduction measures, the risk may be low enough such that modifications to the auxiliary spillway are not warranted. Therefore, in 2017, Consumers began a RIDM study of the Alcona Dam auxiliary spillway for submission to FERC.

RIDM requires a set of hydrologic hazard curves (HHCs) to estimate the overall risk. A Simplified Stochastic Event Flood Modeling (SSEFM) approach was used to develop the HHCs for Alcona Dam. The SSEFM method is a compromise between a purely deterministic approach, which tends to be conservative, and a fully stochastic, Monte-Carlo approach.

The resulting HHCs are estimates of peak inflow rates for a range of annual exceedance probabilities (AEPs) from 0.01 to less than 1×10 7 for both cool-season (rain on snow) and warm-season (rain only) events.

The SSEFM approach is the cornerstone of this RIDM study, allowing all other aspects of the study to relate important loading characteristics (peak water level, hydrostatic pressure, auxiliary spillway flow duration, etc.) to AEPs and, therefore, a proper estimate of the risk.
The instructor will share the steps of the approach, the results of the SSEFM, and the lessons learned throughout the process.

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