For decades, the U.S. Army Corps of Engineers' Portland District has managed and maintained Columbia River navigation channels, as well as dams on the Columbia River and in the Willamette River basin. This work includes hydraulic appurtenance inspections of dams, including stilling basins downstream of dam spillways.
Stilling basin hydraulic performance is important during major spill events to ensure dissipation of the energy from released water, thus minimizing the impact on the areas immediately below the dam. Without energy dissipation, scour damage could erode dam foundation bedrock. The Corps maintains a database of survey information, and this is updated periodically to check for changes, as well as after significant flooding events.
One structure that needed to undergo this evaluation is Green Peter Dam, which was constructed in 1962-1963 and is located on the Middle Santiam River, about 30 miles southeast of Albany, Ore. It is a 380-foot-high, 1,517-foot-long concrete gravity structure. It has two spillway control tainter gates, two regulating outlets below the spillway crest, and an 80-MW two-unit powerhouse adjacent to the stilling basin.
Rather than performing a traditional multi-beam hydrosurvey to gather this data, the Corps chose to use the BlueView system provided by BlueView Technologies in Seattle. The BlueView system, with its three-dimensional mechanical scanning sonar, was chosen as it is more compact than multi-beam systems, and offers finer resolution.
|A remotely-operated vehicle captured this image of scour damage at Green Peter Dam's stilling basin. Shown here is a section of exposed rebar that might have otherwise escaped detection for some time.|
Assessing options for the hydrosurvey
Several factors led the Corps to proceed with using BlueView technology for bathymetric survey of the Green Peter Dam stilling basin and surrounding areas.
Small-scale, remotely-controlled surface boats equipped with multi-beam survey equipment were not commercially available in local markets at the time the work needed to be performed. Because of space constraints in the stilling basin vicinity, a 70- to 80-ton mobile crane would be required to launch the conventional, available survey boats.
However, the deck in front of the powerhouse was too narrow and confined to accommodate such large lifting equipment and too small to accommodate available survey boats. In addition, there is no access in the narrow rock canyon on the left abutment.
Consequently, the smaller-scale BlueView/remotely operated vehicle equipment - less than 5 feet long and a little more than 2 feet wide and high - was identified as being suitable for the site conditions, requiring only a mobile truck-crane to launch the ROV into the stilling basin. The Corps determined that there was 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.
This hydrosurvey will enable comparisons with future surveys to detect any changes or accumulation of debris in the stilling basin. Circulating debris during flood events has been known to inflict damage to concrete stilling basin surfaces.
Understanding the technology
The BlueView BV5000 3D technology employed for this survey work is a mechanical scanning system based on sound sonar, as opposed to light detection and ranging (LiDAR), to create point cloud data. The BlueView system can gather data in murky or muddy water. The data can be processed using Leica Cyclone software. The BlueView system compiles data similar to topographic laser scanners but uses high-frequency sound beams to produce detailed three-dimensional composite depictions and imagery. Complex positioning data inputs are not required.
The Deep Ocean Engineering Phantom HD2 ROV used to carry this technology is equipped with a heading sensor, lights and a secondary navigation sonar (Tritech Super Seaking dual-frequency sonar). The ROV is fitted with video equipment to furnish close-up views of areas of interest, such as scour holes, exposed rebar and exposed aggregate on the surfaces of the floor slabs.
|This protective cage was used to carry the ROV in and out of the water during launch and retrieval. The stilling basin is shown in the background.|
Field operations at Green Peter Dam
The survey of Green Peter Dam was undertaken in March 2011 by David Evans and Associates' Marine Division of Vancouver, Wash. To provide still water conditions in the stilling basin, daytime powerhouse flows were reduced to a minimum during the hydrosurvey.
The ROV and its protective cage were lowered via a mobile crane-truck located on the powerhouse tailrace deck directly into the stilling basin. This approach exceeded expectations.
Once lowered into the stilling basin, the tethered ROV was released from the protective cage and navigated through the stilling basin by a controller located adjacent to the stilling basin. Real-time imaging was used to find and adjust ROV scanning locations; to detect visually damaged areas on which to focus efforts; and to make duplicate, overlapping scans without resorting to the use of divers and safety watch personnel. Scan locations were pre-determined based on stilling basin geometry and location of baffle blocks. After finishing each scan, the ROV was remotely piloted to the next scan location.
Once this scanning was complete, the ROV was retrieved. The survey was performed in two days, requiring about 41 scanning positions.
|A computer image of the Green Peter Dam stilling basin, with a close-up of damaged floor areas. The images show evidence of scour damage to the stilling basin.|
With the BlueView equipment, the extent of the stilling basin, as well as condition of baffle-blocks, walls, part of the downstream armored sill and riverbed bottom was captured within about a 0.5-inch to 1-inch accuracy. This is a significant improvement over the previously expected conventional hydrosurvey accuracies of 3 to 6 inches. The joints between stilling basin floor slabs were picked up, as well as armoring rock beyond the endsill.
Two things discovered during the inspection are:
-Scour damage areas on the upper (north) side of the stilling basin; and
-Damage on the left (north) side of the stilling basin, believed to be caused by intermittent releases from the northern regulating low-level outlet gate.
The Green Peter BlueView hydrosurvey appears to have captured early stages of erosion damage in the stilling basin, well before extensive damage could ensue.
This successful hydrosurvey confirmed the need for future preventative repairs and for possible interim operational modifications. This survey information allows the Corps to plan repairs in a timely, orderly manner to ensure continued proper functioning of the project. This includes assuring major flood flows can be safely passed and to avoid exacerbating existing erosion damage.
The documented information enables the Corps to go forward with repair and maintenance planning and is consistent with providing continued safe performance of the Green Peter project, as well as for power, flood and navigation water regulation and recreation. The lessons learned will be applied to other Willamette River Valley projects.
This methodology has other applications besides dam safety inspections. The Corps 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 Corps now views the BlueView system as another tool for underwater surveillance for a variety of needs in addition to dam safety.
Other potential applications from published sources include biological surveys such as those done by King County, Wash., Environmental Lab1 (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.
By Martin P. Hansen, P.E., a hydraulic engineer with the U.S. Army Corps of Engineers' Portland District.
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