Innovation in Global Power - Parsons Brinckerhoff

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Hydropower – New Technologies, New Considerations http://www.pbworld.com/news_events/publications/network/ Dam Safety: State-of-the-Art Methodology Demonstrates that Costly Remediation is Not Needed By Jay Greska, Boston, Massachusetts, 1-617-960-5021, greska@pbworld.com; and Bryce Mochrie, 1-617-960-4971, mochrie@pbworld.com. Updated federal regulations require owners of hydroelectric dams in the USA to evaluate the safety of their facilities under all potential failure modes. In the case of one of the Tapoco Project dams where there was concern about potential scour, the authors based their investigation on recent research that applies specifically to jet streams, versus gradually varied flow. This approach was used successfully on this project and is applicable to other scour impact areas subjected to free falling water. Figure 1: Santeetlah Dam Looking Upstream. Plunge jets from overflow spillways are capable of eroding the bedrock upon which many dams are founded. Over time, this process can lead to undermining and eventual dam failure, especially if the bedrock is weathered or of poor quality. Comparing the stream power of the plunging jet at the point of impact to the erodibility index of the bedrock enables us to determine the scour threshold of a dam’s foundation and the dam’s overall susceptibility to scour. Overview of Tapoco Project’s Santeetlah Dam Constructed in 1928, Santeetlah Dam is a concrete gravity and arch structure 321.3 m (1,054 feet) long with a maximum height of 60 m (197 feet) (Figure 1). It is one of four dams comprising the Tapoco Project, a 380 MW hydropower facility in North Carolina and Tennessee 1 . The dam is located roughly 72 km (45 miles) south of Knoxville,Tennessee, and includes: • An integral intake section • Left and right non-overflow gravity sections • Two 42.7-m (140-foot)-wide thrust blocks • A 97-m (318-foot)-long arch section connecting the two thrust blocks • An 8-km (5-mile)-long pipeline to the 45 MW generating station. Santeetlah’s six Tainter gates 2 in the two thrust blocks are used to pass floods. They have a combined capacity of approximately 532.4 m 3 /s (18,800 cubic feet per second, or ft 3 /s) at the 553.8-m (1,817-foot) full-pool elevation (Tapoco datum), which is the crest elevation. Historical flood flow discharges have generally been limited to the Tainter gates, with little or no flow overtopping the arch. At reservoir levels above full pool, flow is regulated by a combination of the Tainter gates and the arch, which acts as an ungated spillway during floods. Spills over the arch create a rectangular jet that plunges at low flows onto a concrete apron constructed in 1938 to absorb the energy of such spills. The apron, located just below the arch between the thrust blocks, extends downstream from the toe of the arch for a distance of 19.8 m (65 feet) at an average elevation of 496.8 m (1,630 feet). When total flows at the dam exceed approximately 849.5 m 3 /s (30,000 ft 3 /s), the plunge jet would land beyond the apron, potentially eroding the exposed bedrock in the riverbed (Figure 2). This flow level (849.5 m 3 /s) represents a relatively “small” overtopping flood event compared to the 5,776.7 m 3 /s (204,000 ft 3 /s) probable maximum flood (PMF). The wedge-shaped area just below the arch backs up the water and acts like a plunge pool during flood events, with the arch and thrust blocks forming the sides of the pool and the apron at the upstream portion being the bottom. The plunge pool provides some protection for the concrete and bedrock, with water depths increasing as the arch discharge increases. This is due to the 32.9-m (108-foot)- wide constriction between the two spillway toes, which controls tailwater depths as reservoir elevations exceed full pool and water is discharged over the arch. Figure 2: Arch Dam Elevation Showing Jet Trajectories. 1 The Tapoco Project is the focus of the two preceding articles, which are about its relicensing by the Federal Regulatory Energy Commission in 2005, a project for which PB served as owner’s engineer. 2 A Tainter gate is a type of radial-arm floodgate used in dams and canal locks to control water flow. See also the following article, “Deck Slot Cutting and Tainter Gate Remediation Extend Safe Operations of a Hydroelectric Dam” by Marc Buratto et al for more information about Tainter gates. PB Network #68 / August 2008 38
Hydropower – New Technologies, New Considerations The Need to Investigate Potential Foundation Erosion During Overtopping Floods Under FERC’s new Dam Safety Performance Monitoring Program, owners of hydroelectric dams must evaluate the safety of their facilities under all potential failure modes. As such, the hydraulic performance of high hazard dams, such as Santeetlah, whose failure could endanger lives or property, must be evaluated to the PMF level. Because spillway overtopping during a high-flow event had eroded more than 9.1 m (30 feet) of bedrock depth at Tapoco’s nearby Calderwood Dam during construction, FERC had become concerned about the potential for foundation erosion at Santeetlah Dam, stating: The consultant should provide an appraisal of potential foundation erosion during overtopping. The erodibility of the rock below the arch section should be specifically addressed to determine if it could be lost during an overtopping flood event.... http://www.pbworld.com/news_events/publications/network/ scour susceptibility based on the concrete’s properties. Our results were as follows: • Bedrock. A minimum K h was estimated at 3,411 with an equivalent applied power of 445 kW/m 2 . • Concrete. A minimum K h was estimated at 3,000 with an equivalent applied power of 405 kW/m 2 . Figure 3: Jet Power at the Base of Santeetlah Dam for Various Reservoir Elevations. Our Investigation Approach Foundation scour from plunging jets is the result of turbulent, fluctuating pressures. Under certain conditions, these fluctuating pressures are capable of loosening and eventually dislodging large blocks of rock. Using the methods in Scour Technology — Mechanics and Engineering Practice 3 and FERC’s Engineering Guidelines for the Evaluation of Hydropower Projects (see Related Web Sites on the following page) we quantified the bedrock’s susceptibility to scour and estimated the mean and fluctuating dynamic pressures at the bottom of the plunge pool for various arch discharges. Power values for the plunging jet in kW/m 2 were then determined for various reservoir elevations. We estimated the likelihood that a scour hole would form during any given event by comparing the jet power at the bottom of the plunge pool to the threshold value of the bedrock or concrete. This methodology differs from conventional scour analysis, which involves the estimation of shear stress based on flow depth and energy slope, and is more suited for laminar, gradually varied flow. Conventional scour analysis was not performed because flow conditions at the dam, i.e., the plunge jet, were not suitable for this type of analysis. Estimating the Susceptibility to Scour of Bedrock and Concrete We estimated the bedrock’s susceptibility to scour using its erodibility index, K h , (Figure 3). Then, based on a combination of geologic data, field observation and engineering judgment, we took a similar approach in quantifying the concrete apron’s 3 G. W. Annandale. 2006. Scour Technology: Mechanics and Engineering Practice. McGraw-Hill, New York, NY. Comparing the Power of the Plunge Jet to the Susceptibility to Scour Downstream hydraulics and plunge jet trajectories were investigated at reservoir elevations ranging from a small overtopping flood up to a major flood just below the PMF. Here we present two examples. Small Overtopping Flood. At a reservoir elevation of 554.7 m (1,819.8 feet), which was 0.9 m (2.8 feet) above the spillway crest, the total discharge at the site was 847.9 m 3 /s (29,943 ft 3 /s) with 119.7 m 3 /s (4,226 ft 3 /s) overtopping the arch dam (assuming all six gates were fully open). The wedge-shaped area at the toe of the arch would be filled to a depth of 1.3 m (4.3 feet). The resulting jet was approximately 0.2 m (0.7 feet) thick at issuance and 0.8 m (2.5 feet) thick at impact. It hit the tailwater near the downstream edge of the apron 19.0 m (62.3 feet) from the toe of the arch with an applied power (neglecting aeration) of 894 kW/m 2 . The jet began to break up about 6.4 m (21 feet) below the point of issuance, or approximately 48.8 m (160 feet) above the water surface, however, so the spray that actually hit the tailwater did so with only a fraction of the jet’s original power. By the time it reached the bottom of the pool, the applied power of the jet was reduced to 97 kW/m 2 , well below the 405 kW/m 2 minimum threshold value of the concrete (Figure 4 on the following page). 39 PB Network #68 / August 2008
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