During and after flood events, inspectors monitor scour at bridges to ensure that public safety is not compromised. The effect of scour on structure stability is difficult to assess during events as the extent of scour is not visible and cannot be easily verified by probing. Thus, inspectors make stability determinations based on experience, visual cues, flow rates, and other factors that hint at potentially unsafe conditions.
To help, the Michigan Department of Transportation (MDOT) researched several platforms for collecting scour-related data during and after flood events. To meet MDOT’s needs, the platforms needed to be stable in swift-moving water, able to collect data using sonar technology, and easily and safely deployable throughout the State.
The platforms researched included monohull and catamaran (multihull) unmanned surface vessels (tethered and untethered) as well as stationary and fixed-position systems. MDOT evaluated both multibeam and single beam sonars for use with the platforms.
Assessing Vessel Design
Early on, the research team dismissed tethered and stationary or fixed-position systems as viable alternatives because of their inherent limitations. Tethered systems prove hard to deploy when there is limited clearance, or freeboard, between the waterline and a structure’s low chord–the lowest portion of the superstructure (bottom of the lowest beam or girder)–which hinders the inspector’s ability to collect scour-related data. Stationary or fixed-position systems are difficult to use when flood events create unsteady or irregular flows and are unable to collect data rapidly at multiple locations (such as structures and substructure units).
A catamaran hull design provides a stable platform; however, keeping the vessel as small as possible means limited internal hull storage, requiring more of the instrumentation and equipment to be mounted on the exterior of the hull. Exterior mounting exposes the equipment to environmental hazards and snagging by trees or debris within the waterway or channel.
MDOT chose a monohull design because it offers protection for instrumentation and equipment within the enclosed hull, reduces the potential for hooking obstructions within the waterway or channel, and is durable enough to be deployed from riverbanks during flood events. The untethered vessel may be safely operated from shore, ensuring that the inspector is out of harm’s way.
Selecting Sonar
The research for appropriate sonar equipment focused on an inspector’s ability to collect, interpret, and evaluate the sonar data to aid the decisionmaking process while on site.
Multibeam sonar provided more accurate data than single beam sonar; however, the multibeam sonar equipment was cost prohibitive and required extensive knowledge to obtain and process the data. Researchers also expressed concern about maintaining the GPS signal lock adjacent to or below structures to provide correct spatial orientation.
Single beam sonar has its own limitations. Sonar imagery is collected and displayed relative to the position of the vessel and the fixed markers (such as piers and abutments), requiring the operator to maintain a relatively straight parallel path to the fixed marker to ensure data quality. The heaving, swaying, or surging motion caused by whirlpools and eddies formed during an event also affects data quality. As translational motion increases, data quality is reduced, and images may become distorted.
Despite these limitations, by testing the capabilities of single beam sonar, the research team found it to be advantageous for field use during flood events. Data can be collected and interpreted quickly by inspectors with varying levels of experience, ensuring that safety-related decisions are sound and data driven.
Proof of Principle
MDOT verified the sonar capabilities of the recommended unmanned surface vehicle (USV) platform (monohull vessel with single beam sonar) using underwater diver inspection data collected during a scour inspection on Michigan’s Upper Peninsula. The researchers first plotted the inspection data on a drawing of the plan and the elevation of the submerged pier, then overlaid sonar imagery on the drawing using known points of reference. The results from sonar imagery proved comparable to inspection findings.
During and after the spring storms experienced throughout mid-Michigan in 2020, MDOT used the platform to monitor and assess conditions at scour-susceptible structures. The results further confirmed the capabilities of the selected platform.
“MDOT’s USV platforms are an excellent tool for bridge inspectors to safely assess the condition of underwater elements and monitor scour sensitive issues safely, without putting inspectors at risk in the waterway itself,” says Matt Chynoweth, the director of MDOT’s Bureau of Bridges and Structures. “The platform was used extensively immediately following the flooding from the dam breaches along the Tittabawassee and Tobacco Rivers in May 2020 on many trunkline and local agency-owned structures to ensure safety and stability prior to reopening.”
Additional Value
Rising water levels throughout Michigan have reduced the available distance between the waterline and a structure’s low chord, limiting inspector access to the underside of the structures. USVs, when outfitted with video cameras and advanced imaging equipment, can gather photos and data to supplement routine visual inspections when access may be difficult or unsafe.
USVs can also be used to verify probing (wade and probe or boat and probe) and waterway information gathered during routine and construction inspection, which is crucial for managing long-term risks associated with scour. Information gathered before, during, and after construction of scour countermeasures ensures that measures are installed in accordance with design specifications and plans and will provide adequate protection during the design flood event.
The USV platform has also proven to be a valuable tool when performing underwater inspections. The detailed imagery can be used to identify underwater hazards and to verify inspection findings when underwater visibility is limited.
“We have integrated the use of this USV platform into our routine and high-risk dive operations to identify hazards, to monitor and assist divers during the dives, and for additional data that are not attainable during the dive,” says Casey Collings, a project manager with a private engineering group. “The platform has made our higher risk dives safer.”
For more information, see www.michigan.gov/mdot/0,4616,7-151-9622_11045_24249-487023-,00.html or contact Chad Skrocki at skrockic@michigan.gov or Ralph Pauly at ralph.pauly@dot.gov.
