Hydro Review

Underwater Work: Sonar Scans Reveal Sedimentation Issues with Shoshone Powerplant Intake

A 20% capacity decrease at the 3-MW Shoshone Powerplant led Bureau of Reclamation personnel to commission an underwater sonar inspection of the penstock and river outlet intakes. Images revealed significant sediment accumulation around the intake that likely is responsible for the decreased generation at the plant.

By Tracy B. Vermeyen

Over the past several years, operators of Buffalo Bill Dam on the Shoshone River in Wyoming have observed a gradual reduction in electricity output from the associated 3-MW Shoshone Powerplant. In fact, as of March 2013, maximum capacity of the project had dropped to 2.4 MW. Because of the excessive head loss experienced through the intake structure, project operators are concerned that sedimentation may be impacting power generation.

In April 2012, the Shoshone Powerplant and river outlet were placed under a clearance to perform an inspection of the intakes using a remotely operated vehicle (ROV). The inspection was abandoned when the ROV became stuck on the left canyon wall. Divers were called in to retrieve the ROV.

The Wyoming Area Office of the U.S. Department of Interior's Bureau of Reclamation asked personnel at Reclamation's Technical Service Center's (TSC) Hydraulic Investigations and Laboratory Services Group to perform an underwater sonar inspection of the Shoshone penstock and river outlet intakes at Buffalo Bill Dam. The goal was to collect underwater images that could help document any sediment/debris built up around the intake structures. Once the situation was better-understood, Reclamation could determine steps needed to bring the Shoshone plant back to full operating capacity.

Furthermore, in July 2011, the Shoshone Powerplant was damaged when rocks passed through the penstock and impacted the wicket gates and turbine runner. It is suspected that rocks fall from the canyon walls and occasionally pass through the trashrack bars and move into the power plant.

Background

Buffalo Bill Dam is a concrete arch dam of constant radius located about 6 miles upstream from Cody, Wyo. The structural and hydraulic heights of this dam are 350 feet and 265 feet, respectively. The reservoir impounds 646,500 acre-feet of water. The Shoshone powerhouse began operating in 1922 with two units, and a third unit came on line in 1931. In 1980, the plant was shut down because of the deteriorated condition of the units. Units 1 and 2 remain in place in decommissioned status. Unit 3 was replaced with a 3-kW turbine-generator set in 1991.

The primary purpose of the Shoshone Powerplant, which is located at the base of Buffalo Bill Dam, is to maintain a minimum instream flow in the 1-mile-long river reach upstream from the 18-MW Buffalo Bill Powerplant. The river outlet works at Buffalo Bill Dam are used during periods of high river flows to maintain reservoir level behind the dam and to provide supplemental river flows to meet downstream irrigation demands when they are greater than the capacity of the Shoshone, Buffalo Bill and 5-MW Heart Mountain plants.

The Heart Mountain Powerplant is at the outlet of Shoshone Canyon Conduit, about 4 miles west of Cody. As a result of the modification of Buffalo Bill Dam, the Buffalo Bill and Shoshone plants were completed in 1992, and winter releases previously discharged from Heart Mountain are now made through Buffalo Bill and Shoshone. Heart Mountain has operated on a seasonal basis since 1992.

The intakes for the Shoshone Powerplant and river outlet are located near the upstream toe of Buffalo Bill Dam (see Figure 1 on page 18). Each intake consists of a 28-foot-high intake well with a cylindrical trashrack structure on top. The intake well was added to the intake in the late 1950s to raise the trashrack above the accumulated sediment.

The trashrack structure consists of a 3.5-foot-high collar, a 6-foot-tall trashrack and a 4-foot-high cone-shaped lid, for a total height of 13.5 feet. The trashrack diameter is 8.3 feet. The lower framework of the trashrack fits inside the 6.3-foot-diameter intake well. The trashrack is seated at Elevation 5158 feet. The top of the trashrack is at Elevation 5171.5 feet.

At the time the work was conducted, the reservoir water surface was at Elevation 5371 feet and the dam was releasing a flow of 1,350 cubic feet per second (cfs). The Shoshone and Buffalo Bill plants were releasing 142 cfs and 1,208 cfs, respectively, while no water was being released from the river outlet works. No power plant or river outlet outages were necessary for this sonar inspection because the instrumentation used does not have to be proximate to the intakes to collect detailed images.

Performing the sonar inspection

A Kongsberg Mesotech MS1000 scanning sonar system was used for acoustic image collection in the forebay. The MS1000 system included a 675 kHz scanning sonar transducer (Model 1171) used to collect an accurate two-dimensional representation of underwater structural features. The sonar has a range of 1.5 to 300 feet, with a resolution of about 0.06 foot.

A global positioning system (GPS) receiver was used to provide data to the MS1000 software to document sonar scanning locations. The GPS was set up with differential corrections to provide horizontal position accuracy of ±3-7 feet. The sonar depth was determined using a graduated line.

Sonar imaging was performed on Sept. 17 and 18, 2012. On Sept. 17, sonar images were collected at several depths and locations on the face of the dam. On Sept. 18, additional sonar images were collected from a modified mount in an effort to image the upstream face of the penstock intake trashrack.

An aluminum cage was used to provide a modified mount for the sonar imaging system, allowing collection of data to image the upstream face of the penstock intake trashrack.
An aluminum cage was used to provide a modified mount for the sonar imaging system, allowing collection of data to image the upstream face of the penstock intake trashrack.

For sonar imaging, the MS1000 was deployed using a tubular aluminum cage with wheels. The sonar was mounted to the cage so that its long axis was perpendicular to the dam face. The images were produced using a fan-beam transducer with a 30-degree field of view (FOV). The cage was lowered to several elevations while collecting images of the dam face and reservoir bottom near the toe of the dam.

For the first set of sonar scans, the transducer was offset about 1.5 feet from the upstream face of the dam. A second and third set of sonar images were collected with the transducer offset 4 and 8.5 feet upstream from the dam face, respectively. A modified mount was used to offset the sonar 8.5 feet from the dam face. The reason for relocating the sonar was to image the upstream face of the penstock intake trashrack. Sonar images were collected at depths of 40, 70, 110, 140, 160 and 190 feet below the water surface.

Sonar images collected

Several sonar images were collected to obtain detailed information on the river outlet and penstock intakes at the base of the dam. The sonar images are two-dimensional representations of a plane that is parallel to the dam face at the station where the sonar was positioned (because the dam is curved). The sonar images were collected near the mid-point of the dam.

Three sonar locations were used for this survey:

- Directly over the river outlet intake;

- Over the power penstock intake; and

- About midway between the two intake structures.

It is important to note that sonar images are not photographs and are open to interpretation by an experienced operator.

Figure 2 is an image with the sonar equipment located at Elevation 5228 feet, which is about 57 feet above the top of the river outlet trashrack. This image was collected using a scanning range of 150 feet to capture the entire cross-section of the forebay. The image is oriented as if looking at the dam from upstream.

The major features seen in the sonar image are the canyon walls, dam face, river outlet trashrack structure, two cables (reported to be attached to the trashrack structures), and sediment that has accumulated near the base of the dam. There appears to be a strong acoustic reflection from the river outlet intake foundation at Elevation 5132 ft. It is possible to image the foundation floor because acoustic energy can pass through the trashrack bars, strike the foundation, and be reflected back to the sonar.

Figure 3 is a short-range image with the sonar located at Elevation 5197.5 feet, which is 27 feet above the top of the river outlet trashrack. This image was collected using a scanning range of 45 feet, which covers a smaller area but has a higher image resolution.

The major features shown in the image are the dam face, river outlet trashrack, two cables, and the sediment accumulated around the penstock intake trashrack. There are no signs in the sonar image of structural components of the penstock intake trashrack. This image was collected with the sonar equipment located between the dam face and the downstream side of the trashrack. As a result, it is possible there are some portions of the upstream side of the trashrack that are not covered by sediment and/or debris. These sonar images support the observation of reduced generating capacity at the Shoshone Powerplant, which is consistent with head loss caused by blockage of the trashrack structure.

Conclusions

Careful review of the sonar images and the design drawings provided a high level of confidence in the sonar records, which showed significant sediment accumulation around the Shoshone Powerplant intake structure. This sediment accumulation observation supports the reduced power generation capacity at the plant.

Sonar images of the river outlet intake showed it is relatively clear of sediment accumulation. According to the project operational records, the river outlet has been used recently, which has maintained a scour hole in the vicinity of the intake. However, this intake will eventually be inundated with sediment if the sources of sediment and rock are not curtailed.

A detailed forebay bathymetry survey is required to formulate a plan to address sediment accumulation around the penstock and river outlet intakes at Buffalo Bill Dam.

A bathymetry survey of the forebay using a boat-mounted sonar and a high-precision GPS system will be completed in 2013 to create a detailed contour map of the forebay bathymetry for several hundred feet upstream from Buffalo Bill Dam. This data will be used to estimate the volume of sediment that has accumulated in the forebay since the dam was completed in 1910 as well as estimate the volume of material to be removed by some as-yet-undetermined method of dredging.

Reference

Vermeyen, Tracy B., "Scanning Sonar Survey Report - Buffalo Bill Dam River Outlet and Powerplant Intakes," Hydraulic Laboratory Technical Memorandum PAP-1072, published by the Bureau of Reclamation, U.S. Department of Interior, Denver, Colorado, 2013.

Tracy Vermeyen, P.E., is a hydraulic engineer in the hydraulic investigations and laboratory services group of the Technical Service Center run by the U.S. Department of Interior's Bureau of Reclamation.

For more information on underwater work at dams and hydro facilities, click here.

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