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Hydro Review

Tech Briefs

Seismic method for estimating depth of concrete, masonry dams

An innovative down-hole seismic method provides an accurate measure of the depth of concrete or stone masonry dams with soil foundations. Consultants developed this method to determine the depth of a concrete retaining wall that supports the upstream face of the 15-foot-tall Gunstock Dam in New Hampshire. The method was adapted from one used to determine the depth of steel sheet piling in soil.

The method involves drilling a hole with steel or polyvinyl chloride (PVC) casing left in the ground, to provide access for a geophone. Vibrations are induced by striking the concrete structure and recorded by the geophone. The arrival times of these vibrations at various depths above the bottom of the structure are relatively constant because the distance that vibrations must travel through the soil between the structure and the geophone does not change. However, the arrival time of vibrations measured below the bottom of the structure increase with depth because of the increasing distance the vibrations must travel through the soil. The depth to the bottom of the structure then can be estimated by plotting compression wave arrival times versus the depth of the geophone and determining the depth at which vibration arrival times begin to increase.

This method is applicable in cases where determination of structural depth using conventional explorations – such as drilling through the structure or excavating test pits – is not practical. For example, core drilling can be expensive and time consuming and can weaken the structure. Excavation of test pits to determine structural depth can only be performed for small dams with relatively shallow structures, and this method usually requires draining the impoundment.

Using the down-hole seismic method at Gunstock Dam, consultants estimated the depth to the bottom of the structure. As a result, the dam owner, Gunstock Mountain Ski Area, was able to plan rehabilitation of the dam. The owner decided to construct a new concrete facing wall over the existing retaining wall. During construction, engineers determined the actual elevation of the base of the existing wall was within 0.5 foot of the elevation estimated using the down-hole seismic method.

– By Grace Levergood, P.E., New Hampshire Department of Environmental Services Dam Safety Bureau; Craig Ward, P.E., Ward Geotechnical Consulting; and John Kick, PhD, Kick Geoexploration. For more information, contact Grace Levergood, 29 Hazen Drive, P.O. Box 95, Concord, NH 03302; (1) 603-271-1971; E-mail: glevergood@ des.state.nh.us.

American Fisheries Society forms emerging technology committee

The bioengineering section of the American Fisheries Society (AFS) announces formation of an ad hoc emerging technology committee. The purpose of this committee is to provide strategic support and technical guidance for those who are pursuing either the development of new fish passage and intake protection technologies or the use of existing technologies in unusual or innovative conditions.

The new committee will: assist innovators, review concepts, foster communication, provide a forum for highlighting new technologies, and identify potential funding sources, says Lynn A. Reese, P.E., chairman of the committee. Reese is a hydraulic engineer with the U.S. Army Corps of Engineers.

A work group has been formed to draft bylaws describing the emerging technology committee’s mission, objectives, operating structure, and technology evaluation criteria. Other members of the work group are: Ned Taft with Alden Research Laboratory, Jock Conyngham of the Corps’ Engineer Research and Development Center, Doug Dixon with EPRI, Larry Swenson with the National Marine Fisheries Service, and Marcin Whitman with the California Department of Fish and Game. The work group will serve as the initial ad hoc committee.

The committee plans to hold quarterly meetings by conference call, with a once-a-year in-person meeting during the AFS annual meeting. The 2007 meeting was held September 2 to 6 in San Francisco, Calif.

Committee membership will rotate beginning in 2008.

– For more information, contact Lynn Reese at (1) 509-527-7531; E-mail: lynn.a.reese@usace.army.mil.

Reclamation’s approach to analyzing concrete dams

To assess the condition of its concrete dams, the U.S. Department of the Interior’s Bureau of Reclamation is turning to nonlinear analysis. This type of analysis is different from linear structural analysis or limit equilibrium analysis because it can account for damage and subsequent loss of load-carrying capability, says Barbara Mills-Bria, P.E., a structural engineer with Reclamation.

Nonlinear structural analysis incorporates geometric and material nonlinearities inherent in dams and foundations. If modeled correctly, nonlinear analysis can provide increased understanding of the response and failure mechanisms of a structure and can show how load is redistributed under nonlinear behavior, she says.

Hydropower Generation Report
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Reclamation’s policy calls for first assessing a concrete dam using a linear elastic analysis, Mills-Bria says. Simplifying assumptions made include: massless foundations; monolithic foundations, neglecting contraction joints and weak lift lines; monolithic foundations, neglecting joints and discontinuities; added mass for hydrodynamic interaction; and linear-elastic material models. Because of these assumptions, linear analyses may not accurately depict the behavior of the structure, particularly under large loadings.

The next step is performance of a risk analysis, to determine if nonlinear analysis is justified. Reclamation also may use risk analysis to determine when specific field and material testing programs are justified to obtain detailed properties for nonlinear analysis.

Nonlinear analysis eliminates many of the simplifying assumptions, leading to potentially better estimates of the likelihood of structural failure. However, additional post-processing is required for nonlinear analyses in comparison with linear analyses, to verify the models of the dam developed to facilitate this analysis are performing correctly.

To aid in performing nonlinear structural analyses, Reclamation published a guide, State-of-Practice for the Nonlinear Analysis of Concrete Dams at the Bureau of Reclamation.

The guide contains sections on:

– The three main types of concrete dams (arch, gravity, and buttress);

– Material properties, including typical property values and laboratory methods used to determine these properties;

– Load and load application, including static, reservoir, uplift, temperature, and seismic loads;

– Finite element analysis, with a comparison of implicit and explicit solution methods; and

– Interpretation, checking, and accuracy of results.

–  To obtain a copy of this guide, contact Barbara Mills-Bria at (1) 303-445-3229; E-mail: bmills@do.usbr.gov.

Stream trace modeling analysis used to track fish passage

Stream trace modeling analysis, which allows users to predict movement of fish, has proven useful in analyzing swimming patterns of fish at hydro projects. To illustrate the predictive paths of sub-yearling chinook and sockeye smolts, researchers with Hydroacoustic Technology Inc. (HTI) tested this technique at 1,312-MW Rocky Reach Dam on the Columbia River in Washington.

To test this technology, researchers with HTI implanted sub-yearling chinook and sockeye salmon smolts with micro-acoustic tags. They instrumented the dam with 66 hydrophones monitored by six receivers. The acoustic tags were set to transmit one ping every four to eight seconds.

One of the main objectives of this work at the dam over the past four years has been to evaluate the behavior of downstream migrating salmon smolts as they approach the dam, says Tracey Steig, senior project engineer with HTI. As a result of research performed from 2004 through 2007, the paths of about 450 sub-yearling chinook and 2,700 sockeye smolts have been determined.

To better characterize the behavior of the smolts, the forebay area was divided into individual cells, each measuring 50 feet wide by 50 feet long by 50 feet deep. The results of the fish passing within each cell were summarized for average velocity and average direction of movement. In addition, fish density was calculated for each of these cells, independent of how long each fish remained in a cell.

HTI input the data collected into the three-dimensional analysis software Tecplot, supplied by Tecplot Inc. Using this software to perform stream trace anlysis allows the researchers to release virtual fish and, using the average velocity and direction of each cell, move the fish from cell to cell within the forebay.

For the dam, researchers performed an analysis of virtual fish released at three depths equally spaced across the forebay. This analysis showed the virtual fish that were surface-oriented and near the center of the forebay tended to travel toward the project’s surface collector bypass. Deeper fish followed a more circuitous route toward the turbine units. The analysis also showed two areas of milling – in front of units 5 and 6 and just west of the entrance to the surface collector.

Two-volume handbook covers electric generators

CRC Press (Taylor & Francis Group) offers The Electric Generators Handbook, a two-volume set written by Ion Boldea. Boldea is a professor at Politehnica University in Romania. Volume 1 covers synchronous generators; Volume 2 covers variable-speed generators.

As the title implies, these books are not textbooks, but rather references for working electric power engineers and for those in other fields involved in electromechanical energy conversion.

The first three chapters of Volume 1 introduce the reader to the basics involving the principles of electric generators and prime movers. The author addresses the latest technologies and applications involving electromechanical conversion. Topics include doubly fed machines, wind turbines and associated controls and systems, and combined-cycle gas turbines and their controls.

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The book covers not only the generator, but also the prime mover and associated controls. Many books do not go into such depth regarding the prime mover, and in such a clear-cut and easy-to-understand way.

Other chapters in Volume 1 cover classic generator theory, parameter measurements, classical machine modeling and associated problems, transient behavior, equivalent circuits, saturation, standstill frequency and time domain, and electromechanical transients. A chapter on the d-Q model (used to convert equations for three-phase generator or motor into a two-axis stationary set of equations) includes a section on d-Q per unit per base selection and magnetic saturation. This is something many books do not adequately address.

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In addition, Volume 1 includes chapters on design, testing, and control of synchronous generators. Again, the detail provided regarding control of synchronous generators is excellent, with information that is left out of most books on generators.

Volume 1 is quite all-encompassing on the design, operation, control, and testing of synchronous generators. The author includes sufficient explanations that allow the reader to easily understand the subject matter.

Volume 2 covers variable-speed generators. The first five chapters address the design, testing, operation, modeling, control, and transient topics involving the wound-rotor induction generator.

In the second part of Volume 2, there is an excellent chapter on claw-pole-rotor generators for automobiles and a chapter on linear motion machines. However, the majority of the second part of the book focuses on induction and permanent-magnet-assisted reluctance starters/alternators for electric hybrid vehicles and their controls.

As in the first volume, Volume 2 is clearly and concisely written, enabling easy reading of complex subjects.

– By Bert Milano, manager of the Hydroelectric Research and Technical Services Group, U.S. Department of the Interior’s Bureau of Reclamation, P.O. Box 25007, Mail Stop D-8450, Denver, CO 80225; (1) 303-445-2300; E-mail: bmilano@do.usbr.gov. To purchase the set for 98.1 pounds (US$199) or an individual volume for 56.69 pounds (US$115), visit the Internet: www. crcpress.co.uk and search for “Elec- tric Generators Handbook.”

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