Bathymetry in Tight Spaces

Limited access conditions lead a marine survey division to use acoustic sonar on an ROV to survey the stilling basin of Oregon’s Green Peter Dam.

by Martin Hansen, PE

For decades, the Portland, Oregon District of the U.S. Army Corps of Engineers (USACE) has managed and maintained Columbia River navigation channels, as well as multi-purpose dams on the Columbia River and in Oregon’s Willamette River basin.  The effort includes collecting and evaluating periodic underwater surveys of the stilling basins, including Green Peter Dam, one of 13 major dams in the Willamette River Basin.

Built between 1962 and 1963 and located on the Middle Santiam River in the northwestern portion of the state, the Green Peter Dam is  380 feet high and 1,517 feet long, constructed to generate hydroelectricity, provide irrigation, prevent flood damage, and improve water quality downstream. It has two spillway control tainter gates, two regulating outlets exiting below the spillway crest, and a separate two-unit powerhouse adjacent to the stilling basin.

The most recent underwater survey of the dam was performed by the marine division of David Evans & Associates, of Vancouver, Washington, for the USACE in March, 2011. Their greatest challenge was boat access in a narrow rock canyon below the dam’s spillway, even with daytime powerhouse flows being reduced to a minimum during the hydrosurvey to provide quiescent water conditions in the adjacent stilling basin.

These difficult site conditions led the team to use acoustic sonar from BlueView Technology's new generation of underwater survey equipment, mounted on a tethered and guided remotely operated vehicle (ROV). The ROV also has video capability. Ultimately, the fine resolution of digital data from the sonar scanning system revealed relatively minor, but unexpected, scour damage in the Green Peter Dam stilling basin.
 

Technology Options

The hydraulic appurtenance inspections of dams, including stilling basins downstream of dam spillways, are part of the USACE dam safety monitoring and surveillance program. The program also maintains and updates a database of survey information to check for changes periodically and after significant flooding events.

For dam safety, stilling basin hydraulic performance is important during major spill events to ensure dissipation of the energy of the released water on the areas immediately below the dam. Without energy dissipation, scour damage could result and erode dam foundation bedrock, which could eventually compromise the stability of the dam and related structures. 

Several factors led to proceeding with BlueView’s acoustic sonar technology for bathymetric survey of the stilling basin and surrounding areas. For one, small-scale, remotely controlled surface boats with multi-beam survey equipment were not commercially available in local markets at the time of the hydrosurvey.

Also, conventional and available survey boats were of a size that necessitated a large, 70- to 80-ton mobile crane with sufficient reach and capacity for launching such survey craft, given space constraints in the stilling basin vicinity on the right abutment. The deck in front of the powerhouse was simply too narrow and confined for lifting equipment with spread outrigger footings or to accommodate available survey boats.  Access in the narrow rock canyon on the left abutment does not exist at all.  

Consequently, the smaller-scale ROV equipment was selected as the most suitable for the site conditions, requiring only a mobile truck-crane to introduce it into the stilling basin.  The team also determined that there is less safety risk in using this lighter-weight equipment, and there would be no need for extensive cribbage to distribute crane loads on the concrete powerhouse deck area.
 

Field Operations

First, a relatively small, mobile crane-truck lowered the ROV and protective cage directly into the stilling basin. This approach exceeded expectations for ease of deployment. No significant problems resulted from using the powerhouse tailrace deck as an access platform for lifting the ROV and protective cage from the launching area, over the safety railing, and into the stilling basin.

Once the survey team lowered the ROV into an underwater position within the stilling basin, they released it from the protective cage and piloted it across the basin, scanning the pre-selected locations. Scan locations were pre-determined based on stilling basin geometry and location of baffle-blocks (some scans were of the same location but at different tilt angles).

Safety meetings were conducted on-site with project operators, who were also in radio contact. The David Evans & Associates operators, stationed on the powerhouse tailrace deck next to the stilling basin, used real-time imaging to find and adjust ROV scanning locations to detect visually damaged areas on which to focus efforts and make duplicate, overlapping scans without resorting to the use of divers and safety watch personnel.
 

The Scanner and Data

BlueView’s BV5000 3D is a mechanical scanning system based on sonar, as opposed to lidar, to create point cloud data. The system compiles data similar to topographic laser scanners, but it uses high-frequency sound beams to produce detailed 3D composite depictions and imagery. Therefore, it can gather data in murky or even muddy water, which then can be processed with Leica’s Cyclone software. Complex positioning data inputs are not required.

For this project, the Phantom HD2 ROV—equipped with a heading sensor, lights, and a secondary navigation sonar—was additionally outfitted with mounted video equipment that furnished close-up images in areas of interest: scour holes, plus exposed rebar and exposed aggregate on the surfaces of the floor slabs.

The extent of the stilling basin, as well as the condition of baffle-blocks, walls, part of the downstream armored sill, and riverbed bottom, were all captured within about  half-inch to one-inch accuracy.  This is a significant improvement over the previously expected conventional hydrosurvey accuracies of three inches to six inches or so. The joints between stilling basin floor slabs were also picked up, as well as armoring rock beyond the endsill.

The Green Peter hydrosurvey appears to have captured early stages of erosion damage in the stilling basin, well before extensive damage could ensue.  Such relatively minor damage is not unexpected given the dam’s age. This successful hydrosurvey confirmed the need for continued monitoring to aid in evaluation of the need for possible future repairs and for possible interim operational modifications. 

This survey information will also allow for the planning of such repairs if needed in a timely, orderly manner, ensuring the continued proper functioning of the dam. This includes making sure major flood flows can be safely passed while avoiding exacerbating existing erosion damage. 
 

Future Applications

This methodology has applications beyond periodic inspections for dam safety.  For example, the USACE is evaluating its use for post-flooding structure and appurtenance inspections in the midwest regions. The BlueView equipment is especially useful for defining undercut areas of structures and foundations that otherwise may be problematic. The system is now viewed as another tool for underwater surveillance for a variety of needs besides dam safety.  
 
Other potential applications from published sources include biological surveys such as was done by King County, Washington’s Environmental Lab (in eel-grass with some visual confirmation), water treatment plant inspections, outfall inspections, search and retrieval of lost equipment underwater, and underwater real-time guidance for clam buckets during rock-removal below dam spillways.

Martin Hansen, PE is employed by the US Army Corps of Engineers in Portland, Oregon. His previous experience includes dam safety monitoring at a public utility.

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