USGS-VDOT Bridge Scour Pilot Study

Science Center Objects

Cost effective and safe highway bridge designs are required to ensure the long-term sustainability of Virginia’s road systems. The water flows that, over time, scour streambed sediments from bridge piers inherently affect bridge safety and design costs. To ensure safety, bridge designs must anticipate streambed scour at bridge piers over the lifespan of a bridge. Until recently Federal Highway Administration (FHWA) guidance provided only for scour estimates of granular, non-cohesive, highly erosive material – yielding overestimates of scour potential in instances when streambed materials offer some resistance to scour.

As research into properties of streambed materials has advanced, methods of evaluating scour that incorporate scour resistance are now included in new FHWA publications. Bridges with piers established in, or on, cohesive soil or erodible rock may now be designed so that the scour resistance of these materials is taken into account. These new and more complex design evaluations require knowledge of the duration of exposure of bed materials to scouring flows over the expected life of a bridge. Currently the established standards for evaluating hydrologic processes do not provide this type of information. As a step toward meeting this need, this pilot study seeks to estimate (1) duration of specific streamflows and (2) potential cumulative stream power over the design lifespan of a bridge. This information may then be used to calculate comprehensive projections of anticipated scour rates specific to individual bridge piers. This new knowledge of cumulative scour may provide significant bridge construction cost savings while ensuring design and construction of safe highway bridges.

The USGS Virginia Water Science Center (VWSC) is providing compilation, calculation, and summation of the hydrologic properties needed to determine potential rates of streambed scour at study test sites, along with hydrologic statistics (fig. 1), statistical methods (fig. 2), and hydrologic modeling methods (fig. 3) to assist in estimating future cumulative streambed scour at specific bridge pier locations over their projected design lifespan. The Virginia Department of Transportation (VDOT) is providing non-hydrologic elements essential to determining potential rates of streambed scour, such as bridge location and design specifications, attributes of cohesive soils associated with bridge piers, geotechnical attributes of bridge scour computations, and preferred specifications and methods for estimating soil scour.

Study deliverables include:

  • Analysis and evaluation of daily and instantaneous stream discharge data.
  • Hydrologic design products.
  • Hydraulic geometry characteristics.
  • Evaluation of discharge probabilities.
  • Simulation tools for probabilistic and deterministic analyses of discharge–duration scenarios.