Category: Hydraulics & Waterways
The inability to see underwater has long been a problem for bridge owners. Divers have been used in the past to try to determine if a structure has issues but a lot of these inspections are done by “feel” due to the clarity of the water. The inspections will let a bridge owner know if there is a problem, but measuring the extent of the problem can be difficult. Sonar has been used in the past to document problems to structures in coastal areas, but little has been done in a riverine environment.
MnDOT recently obtained 3D sonar equipment to assist in underwater inspections. The equipment can generate a 3D point cloud of an underwater structure that can then be used to measure any extent of damage. We are finding a variety of uses for the equipment such as imaging underwater structures to see how extensive damage is, post construction scans to make sure the contractor didn’t leave debris in the water body, baseline measurements on structures in areas where scour has been an issue in the past and extremely detailed river bottom mapping. The 3D equipment will never replace divers as the sonar device has limitations but it can give you a quick assessment of a problem. We feel that imaging is a great first step in diagnosing an underwater problem with a structure and gives divers necessary information so they can concentrate on the area of concern identified with the scan.
Petra DeWall– Waterway Engineer, MNDot Bridge Office, Oakdale, Minnesota
MNDot Bridge Office
Petra DeWall has worked for MnDOT for 27 years and is currently the State Waterway Engineer. She attended The University of Minnesota where she received her Bachelors degree in Civil Engineering and a Masters degree in Infrastructure Systems Engineering. Petra heads up the Waterway Unit in Hydraulics which issues hydraulic reports on all new bridge projects, manages the scour program and works with the local units of government assisting with technical guidance on hydraulic and scour issues.