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Closing the flood gates

By [email protected] - 1st May 2019 - 14:27

With survey projects, there’s complex, there’s very complex, and then there’s the Cedar River Flood Control System (CRFCS) project in Cedar Rapids, Iowa, in the US. Launched in 2015, the 20-year, US$663m project is building a combination of permanent and removable flood walls, levees and gates along 11km of the river’s banks.

A survey consultant for the CRFCS, Cedar Rapids-based Foth Infrastructure Environment was tasked with an incredibly complex assignment: acquire railway pier features for hydraulic modelling at one site accessible only by boat and capture a topographic survey for flood wall design and clearance verification at a second site that would be accessible only for four hours.Armed with advanced scanning technology and a solid plan, a small crew captured all the infrastructure elements needed and delivered more data than was initially requested. They also debunked an inaccurate assumption about the low point of a bridge, saving the city from a costly oversight.

To prepare for the project, Foth first tested traditional survey technologies. Selecting a small area of interest, they used total station and scanning equipment to capture a bridge arch along the riverbank.

“Although we acquired the data, it required extra steps to create an accurate, georeferenced point cloud,” says Jody Budde, a professional land surveyor with Foth. “We needed several trips to the site to establish control and targets and scan the bridge arch. Those additional set-ups would add time we didn’t have and range limitations of the scanning technology wouldn’t allow us to shoot across the river.”

Based on the exercise, Foth determined they needed a single instrument that could integrate total station measurements with high-speed, georeferenced 3D scans. After evaluating available technology, they acquired a Trimble SX10 scanning total station. The SX10 combines surveying, imaging and scanning (up to 26,600 points per second with a range of 600m).

Site one

Based on the CRFCS project schedule, Foth began with Site One, a 2.3km stretch of riverbank about 9.7km north of downtown. An operational railroad bridge crosses the river and several abandoned bridge piers dot the water. Foth needed to capture railroad track locations and shape and size of the active and inactive bridge piers to within 1.5cm.

In autumn 2017, a two-person crew launched their motorboat carrying a Trimble R10 GNSS rover, the SX10, a Trimble Yuma 2 tablet and a Trimble TSC3 controller. Criss-crossing the 150m-wide river, they used the R10 to establish control points in nine locations.

Next the team set up the SX10 and captured the scene from the railroad bridge all the way downstream to the old pier remnants, collecting features along and across the river at distances of nearly 300m. By using resection, all set-ups were georeferenced and the crew could acquire measurements to critical features from multiple locations. In total, it took two days to scan the tracks and 12 piers, collect 11.8 million 3D points and capture colour photos with the SX10 built-in camera.

In the office, they used Trimble Business Center (TBC) software to integrate the GNSS and scanning data into one project. In a few hours, they created and delivered a 3D point cloud that the engineering design firm could immediately use in its own hydraulic analysis software.

Site two

Foth returned to the Cedar River in spring 2018 to tackle Site Two. This 1.6km stretch of riverbank will have a permanent floodwall and Foth needed to collect a topographic survey and as-built data of ground-level and submerged infrastructure as well as three bridges.

Collecting that information, however, required the city to close an upstream dam for the first time in its history. The dam would be closed for only four hours.

Foth dispatched two two-person crews who set 10 control points with the R10 and started scanning with the SX10 while the water level was lowering. To produce the required vertical precision – one bridge had a 3.8cm clearance tolerance to the bottom of the bridge – the crews used a Trimble DiNi digital level at each control point to ensure elevations were within the project’s 3mm specification.

In parallel with the control crew, the scanning crew used project control to capture 3D points of the infrastructure. Using four separate locations on the east side of the river, they scanned the entire length of the west bank area of interest. They captured features up to 245m away, including bridges and bridge arches, flood structures and utilities. As the water level dropped, they captured submerged features like underground pipe networks. For the 12th Avenue Bridge, the structure with the tight 3.8cm clearance requirement, the Foth team made additional scans of its underside, capturing 3D points with 3mm to 6mm spacing. By the time the dam reopened, the crews had finished the job and collected 27.6 million points.

“Without the speed and scanning range of the SX10, we would not have been able to do this job,” says Ben Sullivan, a lead geospatial specialist with Foth. “Conventional survey technology would’ve required up to three times more man-hours and we would’ve collected only about 20% of the data detail we captured with the scanner.” Had Foth used a dedicated scanner, it would have incurred significant additional time setting targets visible from the multiple setups.

Similar to Site One, they used TBC to integrate data from GNSS, total station, scanning and digital level instruments allowing them to process and validate the data into one georeferenced project. Using TBC’s plane-definition and cross-section tools, the Foth team discovered a discrepancy on the 12th Avenue Bridge. Historical data indicated the lowest arch points were the lowest clearance point. The scanning data showed a support pipe under the bridge deck that was lower than the bridge arches, revealing the pipe as the true low point for vertical clearance. Had the city built the new wall based on the original assumption, it would’ve been a costly mistake.

With many more phases of the CRFCS ahead, it’s a good bet that Foth will face its share of complexity in the future. Based on the results of its SX10’s scanning debut, they’re confident they have the right tools to succeed.

Mary Jo Wagner is a freelance writer who has been writing about the geospatial industry for more than 25 years ([email protected])

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