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Home arrow Archives   The American Surveyor     

A Superior Effort Print E-mail
Written by Larry Trojak   
Saturday, 08 September 2012

Engineering firm teams up GPS with hydrographic equipment to map out floor of bay to guide restoration

A 1.810Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

Even though it's been a valuable and beneficial technology for decades now, GPS--by nature of its accuracy and inherent versatility--continues to find new uses and make inroads into new markets. Companies faced with difficult or seemingly impossible tasks are discovering that GPS can often provide the means to get the answers they need, even in the unlikeliest of situations. No one understands that better than Barr Engineering. The Minneapolis-area engineering and environmental consulting firm has repeatedly been challenged by clients seeking solutions which are beyond the reach of traditional technologies but nicely fit into the realm of GPS. One of the latest examples is a project to map sediments in the St. Louis River estuary as a prelude to habitat enhancement. Combining the depth-finding capability of a hydrographic unit with the spatial strengths of GPS allowed them to map the floor, identify variability in the submerged surface, and, most importantly, meet the needs of their clients.

What Lies Below
A key component to the area from a geographic, social and economic standpoint, the lower St. Louis River provides critical habitat to bottom-feeding organisms, fish, and waterfowl species, while, at the same time, providing an economic venue for Great Lakes shipping and business in the Duluth-Superior Harbor. Over the past 130 years, economic development of this area, while certainly beneficial to the region, has contributed a mixture of contaminants to the waterway, including polycyclic aromatic hydrocarbons (PAHs), mercury and other metals, and various chlorinated organic compounds. Over time, some of these contaminants have accumulated in the sediments, raising concerns about their potential ecological and human health effects, and prompting an evaluation prior to restoring the habitat. One facet of the remedy evaluation process includes identifying variability within the sediment surface--an important characteristic for understanding the role of wind, waves and river currents in sediment depositional patterns, according to Jim Staberg, Barr's Senior Civil Technician.

"The St. Louis River, which empties into Lake Superior in Duluth, is a unique environment for sediments," he says. "It is a huge flooded estuary where sediments carried from the upper reaches of the river are deposited as the river loses its momentum. But once deposited, some sediment can be reworked by wind, waves and `seiches,' which act like tides and can cause flow in the estuary to reverse. In some areas, the estuary is more than three thousand feet wide, with a narrow main channel and the remainder of it being quite shallow. Accurate mapping of the shallow areas is essential for developing a precise model of the forces of nature on the sediment material that, despite the years that have passed, might still not be stable in some areas. So we were asked to go in, locate those shallow areas, and allow geologists and engineers to determine how to improve the habitat while maintaining the stability of the existing materials."

Barr has completed many sedimentrelated projects that have called for the use of hydrographic equipment to measure water depths, but the extent of the open spaces in the St. Louis River estuary still proved demanding. Using an Odom hydrographic system, however, they've been able to shoot soundings to gather depths to within 1/10 of a foot accuracy.

"In this case, though, we also had to locate the soundings with horizontal accuracies to 1/100," says Staberg. "That meant the sub-meter capable GPS we'd been using to that point would not suffice. Luckily for us, we had an alternative ready."

Complimentary Technologies
Barr Engineering's alternative approach to tackling sediment mapping in the St. Louis River estuary was to team up their Odom hydrographic unit with HiPer Lite+ GPS from Topcon Positioning Systems. Doing so, says Staberg, gave them the accuracies they and their clients needed.

Staberg says, "we were able to use a 16-foot jon boat to survey the river bottom to 1/10 of a foot elevation and at the same time, gather survey-grade location information to 1/100 of a foot. It was an outstanding pairing of the two technologies. After each day's readings, we processed the data using the Hypack Hydrographic Survey and Processing Software which we generally use with our Odom gear, then brought that data into AutoCAD's Civil 3D for the final product: a 3D surface model of the river bottom."

As mentioned, the key areas being focused upon were the shallower parts of the estuary and the bulk of the work was in water that ranged from three to six feet deep.

On Measuring Mud
GPS has been a big part of Barr Engineering's operation since about 1997 when their dealer rep, Fred Meyer from the Minneapolis branch of RDO Equipment, suggested they try out a receiver he had on hand. They used that model for about three years until RDO became the regional dealer for Topcon, at which point Barr made the switch. Today, between their Minneapolis and Duluth offices, they have a half-dozen systems to meet a growing, diverse, project base.

"It's amazing how the work almost seems to find the technology," says Staberg.

"Last year Topcon introduced the IS total station and Fred brought in a unit to demo it for us. At the time I really didn't see any immediate need for a tool like that, but not more than a month later, we got a call from an engineer asking if we could scan mud. It was one of the stranger requests we've had but it got me thinking about that Imaging Station."

The project in question did, indeed, involve mud, more specifically, mud that was a residual by-product (tailings) from Canadian mines, which has a difficult time drying out on its own. As a result, flocculants are being used to speed up that drying process. That's where Barr comes in; by monitoring the mud surface as the tailings dry during field demonstration projects to develop tailings management techniques.

"The field demonstration project involved depositing tailings in individual test cells-- each a quarter mile long by 600 feet wide--and the client needs to know how fast it is drying and if there is any shrinkage," says Staberg. They had no idea how to accurately do that, so they called us. We went in with a Topcon IS, but even we were unsure of how well it would work in that application."

All About the Satisfaction
That reluctance was due to the fact Staberg and his team didn't initially know how wet the mud was going to be and if they would be able to even get a return off it. A test run scanning a lake with lily pads proved that it worked quite well, so they moved forward.

"On the mud itself, wherever there was standing water, we didn't get a return, so we tried to keep our grid down to a meter in width and length. From the client's perspective, it was really a work in progress: they would fill one cell, then try different flocculants and chemical agents in an attempt to speed up the drying process. So we would take the Topcon unit out to the cell one day, scan it, then do it again the following day to see how quickly it was drying."

Because of the large size of the cells, Staberg says they were limited to doing no more than one per day. He and his crew were onsite for three months, gathering data, post-processing it, again through AutoCAD Civil 3D, and using the resultant models to compare scans and evaluate shrinkage.

"In our mind, that was a really successful project and I have to believe the client feels the same way," he says. "The Imaging Station played the key role out there and we've since started using it for a lot of work in the area in which we are monitoring monuments on dams, then periodically checking to make certain the structure isn't moving. The technology--whether it's GPS or a robotic total station--has served us extremely well and helped us grow our business in ways we never expected."

He adds that they feel very fortunate to have had the level of support they've gotten from RDO. "Fred and his team at the Integrated Controls division have helped us every step of the way as we've grown into these newer technologies. It's really been a nice fit for us."

Larry Trojak is a communications writer for his own firm, Trojak Communications, in the town of Ham Lake, Minnesota. He is a frequent contributor to construction and survey magazines.

A 1.810Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

 
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