Using tree ring data to determine past streamflow
Tree-ring records are proving useful in estimating streamflow history, according to two Canadian hydro project owners and a U.S. university.
The pattern of a tree’s growth rings reflects the environmental conditions experienced during each year. This data can be interpreted to improve forecasts of future flow conditions that might affect the decision to invest in or design a hydroelectric project.
To gather tree-ring data, researchers select tree-ring sites close to important streamflow gages. To form a chronology, researchers take samples from a group of trees in a single location, determine the growth trend and ring widths, and standardize them to a common scale.
Canadian provincial utilities Manitoba Hydro and Hydro-Québec are both funding tree ring studies, undertaken by scientists at Natural Resources Canada.
Manitoba Hydro’s work is taking place in northwestern Ontario, the most important runoff-generating component of the utility’s hydroelectric system. Manitoba Hydro wants to extend its records on the Winnipeg River system, as streamflow records in that region typically are less than 80 years old.
Hydro-Québec is sponsoring research in northern Québec. The intention is to provide relevant information on natural variability of the precipitation regime in the Canadian Boreal forest, which is a key region for hydroelectric production. The provincial utility then plans to develop models and scenarios for future water supply.
Meanwhile, in the U.S., researchers at the University of Arizona’s Laboratory of Tree-Ring Research have been collecting tree-ring data for more than 30 years and using the information to develop a streamflow history for the Colorado River. Researchers hope to use this data to improve predictions of drought in the Upper Colorado Basin, both for hydroelectric projects and for consumptive water use.
Researchers take samples of trees close to important streamflow gages and use tree-ring records to estimate streamflow history.
– For details on Natural Resources Canada’s tree-ring research, visit http:// ess.nrcan.gc.ca/2002_2006/rcvcc/j27/ 2_e.php. For details on research at the University of Arizona, visit www.ltrr. arizona.edu and http://ag.arizona.edu/ AZWATER/EWSR.
Researchers seek to understand seepage barrier problems in dams
Researchers at Virginia Polytechnic Institute and State University investigating seepage barriers in embankment dams are finding that while most barriers appear to be performing as expected, there are several instances of unsatisfactory performance. They attribute this finding to buildup of water pressure behind the seepage barrier and the associated increase in hydraulic gradient beneath, around, and through the barrier. The increased gradient can lead to internal erosion and piping in the dam and foundation.
Researchers in the Department of Civil Engineering at Virginia Tech collected performance data from nearly 40 dams built between 1916 and 1992 where seepage barriers have been in place for more than ten years. Barriers being studied include walls made of slurry, concrete, secant pile, jet grouted, deep soil mixed, and sheetpile.
Data collected for each dam includes original dam design and construction documentation, reports of seepage incidents, design basis or justification for the seepage barrier, seepage barrier design and construction, and long-term performance data.
In addition to identifying situations of seepage barrier success and failure, researchers hope to gain a further understanding of the mechanisms that lead to distress or unsatisfactory performance. To do this, they are numerically modeling deformations and changes in hydraulics that occur as a result of seepage barrier construction.
The eventual objective of the research is to develop guidelines for the design and assessment of seepage barriers as well as for monitoring and instrumentation programs for dams with seepage barriers.
The research is expected to be complete in August 2007.
– For more information, contact John D. Rice, P.E., a PhD candidate in the Department of Civil Engineering at Virginia Tech, at E-mail: [email protected]
EPRI releases report on how flow fluctuations affect fish
EPRI announces availability of a free report, Simulation Analysis of Within-Day Flow Fluctuation Effects on Trout below Flaming Gorge Dam. This report demonstrates the use of an individual-based model for studying the effects of river flow fluctuations from hydroelectric project operation on a tailwater trout fishery.
The study was conducted using data from the 151.5-MW Flaming Gorge project on the Green River in Utah, which is operated by the U.S. Department of the Interior’s Bureau of Reclamation. This project engages in load following (i.e., storing water during low demand periods and releasing it for generation during high demand periods). This operational mode results in river flow fluctuations in the project’s tailwaters and downstream reaches.
To determine the effects of within-day flow fluctuations on fish populations, EPRI performed computer simulation experiments using inSTREAM-SD, an individual-based river trout model developed by Lang, Railsback & Associates and Humboldt State University. This model represents each individual trout in the site’s population. A computer simulation experiment allows control of important variables and inclusion of natural mortality. Four study sites were simulated.
One clear result of this study is that flow fluctuations are not predicted to have drastic effects on trout populations under any of the simulated conditions at Flaming Gorge Dam. Trout habitat is not severely reduced during either on- or off-peak flows. The simulation analysis indicates that fluctuations have generally small, usually negative effects. Effects vary among downstream sites and among wet and dry flow years.
This report was funded in part by the Western Area Power Administration and was co-produced by the Argonne National Laboratory.
– To download a copy of the report, visit www.epri.com and search for 1012855.
Notching inflatable dam will improve fish passage
Sonoma County Water Agency in California plans to deform the crest of Mirabel Dam – an inflatable rubber bladder dam – into a notched configuration during fish out-migration periods to increase the likelihood of safe and efficient passage. This decision was based on the results of a three-year research project that showed smolt movement improved when a notch is placed in the crest of the dam to increase flow velocity.
Each year during the low-flow season, the agency operates Mirabel Dam on the Russian River to provide drinking water for area residents. This dam is a 45-meter-wide, 4-meter-high air- and water-filled rubber bladder supplied by Bridgestone Industrial Products America Inc.
Researchers with Sonoma County Water Agency and the National Marine Fisheries Service performed the study during three years (2001, 2002, and 2004). They studied a 4.5-kilometer-long free-flowing reach and an adjacent 5.1-kilometer-long impoundment created by this inflatable rubber bladder dam.
To perform this study, researchers implanted radio-tags supplied by Lotek Engineering Inc. in 110 hatchery-raised steelhead smolts and released them upstream of the study reaches. They then measured travel rates in the free-flowing and reservoir reaches. Fish traveled 0.6 to 0.8 kilometer per hour in the free-flowing section and 1 to 1.4 kilometers per hour in the reservoir section. However, the smolts slowed significantly in the forebay of the dam, dropping to travel rates of 0.02 to 0.1 kilometer per hour.
Measurements of fish passage indicated that more than 75 percent of the fish detected in the forebay successfully passed the dam. More than 80 percent of the passing fish traveled over the dam crest as opposed to through the two fish ladders or the flow bypasses.
To determine whether increased spill depth and velocity would reduce forebay delay, in 2004 researchers deformed the crest of the inflatable dam into a notched configuration. This involved releasing water from the bladder to lower the crest elevation by 0.4 meter, then injecting air to raise the bladder on both sides of the notch. The resulting notch was 6.7 meters wide and 0.91 meter deep.
The notched configuration increased crest depth and velocity about 20-fold over the fully inflated condition. As a result, it took smolts a median of 2.4 hours to move through the forebay when the dam was notched, compared with 6.3 hours when the dam was fully inflated in 2001.