This article originally appeated in Storm Water Solutions March/April 2020 issue as "Bridging Flood Solutions"
The Cedar River Flood Control System (CRFCS) in Cedar Rapids, Iowa, did not start out as complicated, but it is. Launched in 2015, the 20-year, $750 million construction project involves building permanent flood walls, removable walls, levees and gates along 7 miles of the river’s banks.
Enter engineering firm Foth Infrastructure Environment, a survey consultant for the CRFCS providing data for the CRFCS. In 2017, the company was tasked with surveying one of the sites to acquire railroad pier features for hydraulic modeling and analysis and to perform an as-built, topographic survey at another site for flood wall design and clearance verification. Upon further inspection, complicated challenges arose. One of the sites was only accessible by boat and involved an active railroad bridge, and workers would only have four hours to capture 1 mile of as-built data for the other site.
Considering the setting, time constraints and required precision of the data, “we had to have extremely efficient and reliable technology that could give us long-range data capture without sacrificing accuracy,” said Jody Budde, a land surveyor with Foth, based in Cedar Rapids. “Conventional survey techniques would challenge us to meet those specs and timelines and would put our crews’ safety at risk.”
Armed with a tactical fieldwork plan, GNSS and advanced scanning, imaging and high-speed scanning technology, a small crew not only captured all the infrastructure elements needed, they also delivered more data than was initially requested and debunked a previous assumption about the true low point of a bridge arch, saving the city from a costly oversight.
First Things First
The CRFCS started planning while the last bit of water was receding from the “Flood of 2008,” the most devastating flood in the region’s history. In June, the Cedar River crested at 31 feet, surpassing the previous record of 11 feet. The water flooded 10 square miles, displaced 10,000 residents and swamped Cedar Rapids, Iowa’s second-largest city, leaving $6 billion in damages. Seven years later, the city adopted the CRFCS master plan and started redeveloping the riverfront and building the flood protection elements in earnest.
In addition to levees and permanent and removable flood walls, the CRFCS plans to add some eight pump stations, replace one bridge, create a basin to store excess rainwater, and construct an amphitheater to serve as a storage basin during high water.
So far, seven phases of the long-term plan have been completed and four projects, including the first permanent flood protection control on the west side of the river, are underway.
Key to designing the new flood structures was understanding the Cedar River bank topography and existing infrastructure along the bank and in the water to accurately model hydraulic flows and determine volume tolerances through specific sections of the river. When Foth was tasked with gathering this information, it became clear that they would move beyond conventional total station and scanning technology to provide precise detail both on spec and on time.
“We studied the areas of interest and the project specifications, and we scanned one of the bridge arches with our existing equipment as a test,” Budde said. “Although we acquired the data, it required several trips 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.”
Foth decided that they needed a single instrument that could integrate total station measurements with high-speed, geo-referenced 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 600 meters).
“The SX10 was not only going to give us the range we would need, it would allow us to eliminate the need for additional field work because it automatically integrates the project coordinate system and georeferences the points,” said Ben Sullivan, a lead geospatial specialist at Foth. “That would save us significant time both in the field and in processing the data in the office. It also offers a true survey workflow that our crews are already familiar with so we’d need little training, and it would provide a safer working environment.”
Foth began with the 1.4-mile-long stretch of river bank north of downtown where an operational railroad bridge crosses the river and several abandoned, 100-year-old piers sit in the water. They needed to capture railroad track and define the shape and size of the existing and inactive bridge piers to within 0.05 of a foot. That data would be used in a hydraulic model to understand the effects of water flow with those objects in the river.
A two-person crew launched their motorboat carrying a Trimble R10 GNSS rover, the SX10, a Trimble Yuma 2 tablet computer and a Trimble TSC3 data controller. Their first task was to find land to set project control, locations that would also provide good vantage points for scanning. Criss-crossing the 500-foot-wide river, they navigated to nine locations to establish control using the R10.
Once complete, they used the SX10 and followed the same path. Using the same control points, the team set up the robotic scanner and captured the scene from the railroad bridge piers all the way downstream to the old pier remnants, collecting features along the river and across the river at distances of nearly 1,000 feet. The crew performed resections between the control points to acquire several separate 3D measurements of the same feature, enabling them to better show the varied bridge piers’ characteristics. It took them two days to scan the tracks and 12 piers, collect 11.8 million 3D points and take photos with the built-in camera for coloring the point cloud.
“We were asked to define the size of the piers on one plane,” Sullivan said. “But the old piers had vertical size discrepancies between 4 and 6 feet and varied shapes. Accurately measuring those variations with a total station would have been difficult. Scanning was able to quickly and accurately capture the unique detail of each pier and in color, allowing the engineers to better evaluate and incorporate those structures into their model for a more precise analysis.”
They used the drag-and-drop feature of Trimble Business Center (TBC) software to take the data from the TSC3 and Yuma and integrate the GNSS and scanning data into one project. In a few hours, they created and delivered a 3D point cloud for the engineering design firm to integrate into its own hydraulic analysis software.
“The data deliverable request was for a standard CSV text file,” Budde said. “But we knew a point cloud would give them more data depth and much better feature definition for further analysis. After they realized how beneficial the point cloud information would be, they asked for the 3D data going forward, starting with Site Two.”
Foth returned to the Cedar River last year to tackle the second site. This particular 1-mile stretch of riverbank would have a permanent flood wall, so Foth needed to collect a topographic survey and as-built data of ground-level and submerged infrastructure and three bridges to get the design of the wall construction.
Gathering that information meant the city needed to close an upstream dam for the first time in its history. Because of the unknowns, the city was over-cautious and only allowed the dam to be closed for four hours.
Foth dispatched two, two-person crews to set 10 control points with the R10 and started scanning while the water was lowering. To accommodate the high-accuracy vertical precision required–one specific bridge had a 1.5 inch clearance tolerance to the bottom of the bridge–the crew used a Trimble DiNi digital level at each control point to ensure height elevations would be within the project’s 0.01 of a foot specification.
At the same time, the scanning crew used project control to capture 3D points of the infrastructure. There were four separate locations on the east side of the river, and they scanned the entire length of the west bank AOI, collecting up to 800 feet away, including bridges and bridge arches, existing flood structures, utilities and submerged features like underground pipe networks as the riverbank became more exposed. They also performed resections between control points to gain better perspective on objects, such as the bridge structures. By the time the dam was reopened, the crews had collected 27.6 million points.
“Without the speed and scanning range of the SX10, we would not have been able to do this job,” Sullivan said. “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.”
Again, they used the TBC software to integrate the diverse data streams from the R10, DiNi level and the SX10 to process and validate the optical, leveling and GNSS data into one georeferenced project. In processing the point cloud and using the TBC plane-definition and cross-section tools, the Foth team discovered an unexpected data discrepancy on one bridge. Historical data indicated that the lowest arch points were the lowest clearance point. The 3D scanning data clearly 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.
“The wall structure would be constructed using pre-cast concrete panels fabricated off-site,” Budde said. “If they’d designed and built the panels on the original assumption, they would’ve had a costly mistake.”
Instead, the engineering design firm has a precise 3D point cloud to integrate into its own modeling software, providing the information they need to create the most effective permanent flood wall for that part of the west bank.
There is much more to go until the CRFCS is complete, but Foth is confident it has the tools to succeed.