When a rock slide occurred without warning and closed Wabasha Street in St. Paul, Minnesota, the area was deemed unsafe and closed to the public until the site could be assessed by engineers and a plan to clean up and stabilize the bluff could be prepared.
Since the slide area was potentially unstable, the data collection had to be performed without accessing the immediate slide area. The City of St. Paul decided that data collection with a drone could be an effective way to accomplish the project goals with minimal risk to the public and to field staff.
The City contacted Collins Engineers, Inc. on May 2nd to map the site with an unmanned aircraft system (UAS), or drone, and gather information needed to start the remediation effort. Collins’ field crew—two professional engineers certified as small UAS pilots—was mobilized and on-site the following morning. The team used the senseFly albris drone, which was designed for commercial inspection and mapping purposes.
The deliverables were high-resolution images including an orthomosaic map of the area, a 3-D point cloud, and a triangular mesh model of the site. Even with a few challenges including proximity to Holman Filed that is considered restricted airspace, the site’s large elevation change, and wind, the data was successfully collected in less than four hours including setup.
While the goal of the project was to collect data to help technical staff with the repair project, one lesson that emerged was that the 3-D modeling also became an effective tool to communicate the impacts to the City Council, senior City staff, and affected property owners. By viewing the 3-D model and area map, the extent and impact of the slide could be quickly understood without making a site visit.