UNDERWATER STRUCTURAL ASSESSMENT WITH MULTI-BEAM SONAR
Thew Associates was retained in January of 2019 by an engineering firm providing design services for the stabilization of a concrete flood wall on the northerly bank of Rondout Creek in Rosendale, New York. The engineer required bathymetric surveying and mapping of the Creek in the vicinity of the flood wall to facilitate their design. Thew Associates collected bathymetric data utilizing a Norbit iWBMS multi-beam echo sounder mounted on a remotely controlled EchoBoat. In addition to the bathymetry, the multi-beam sonar utilized for the survey was able to confirm areas of significant scouring and undermining of the flood wall footings.
The 1.8-meter EchoBoat USV was utilized due to restricted access (steep banks), swift currents, and variable water depths. The vessel was secured with a tether held by personnel on a bridge adjacent to the upstream end of the survey area. The Norbit iWBMS integrated multi-beam sonar system is compact and lightweight enough to be easily used with the USV. A simplified graphical user interface that communicates with the shore station and operator through a remote connection allows the user to navigate the USV and observe the data being collected. Additionally, the system has the ability to transmit the data in real-time to the office so the Project Manager can also view the data being collected.
Multi-beam sonar was chosen over vertical single-beam sonar because of the ability to provide complete coverage of the Creek bottom and the ability to collect bathymetric data under the flood wall to map the extent of the undermining caused by scour beneath the flood wall footings.
During the survey, raw position and motion data was logged on the survey vessel, while GPS/GNSS observations were logged by a local base station set up over a project control point. This data was combined and post-processed to provide very accurate positioning of each sounding, even in areas where the GPS/GNSS signals are interrupted, such as near the bridge or immediately adjacent to the flood wall. The data was all initially post-processed and validated utilizing Hypack hydrographic surveying processing software. The bathymetric data was subsequently exported as an XYZ file to create a representative bathymetric surface, as well as an LAS point cloud file of all soundings. The LAS file was imported into TopoDOT to extract detailed breakline data for the flood wall, footings, extents of undermining, and adjacent concrete apron. The breakline and surface area was brought into an Autodesk Civil 3D environment to prepare the final bathymetric mapping, including plan and cross-sectional views to illustrate the existing conditions and for use in the remedial design.
Figure 1: Visible undermining of the concrete footing to the left of the image. The vertical section of the flood wall is seen to the extreme left of the image.
Multi-beam sonar can be used to inspect the integrity of many underwater components including:
- Bridge piers/abutments
- Retaining walls (submerged installed sheet pile retaining wall on Buffalo River project)
- Dam structures
- Submerged intakes/outfalls
- Abandoned-in-place infrastructure (submerged roads/bridges, buildings, walls)
- Breakwater armor (stone, blocks: dolos, antifer, tetrapod, etc)
Figure 2: The flat horizontal green line is the exposed portion of the top of the concrete footing. The blue represents the Creek bottom.
Figure 3: The vertical flood and concrete footing is readily visible with the adjacent Creek bottom. Scour was not occurring in this area.