Monitoring the Effects of Knickpoint Erosion on Bridge Pier and Abutment Structural Damage Due to Scour.

摘要

The goal of this jointly proposed research (led by the University of Iowa, in collaboration with the University of Nebraska Lincoln) is to conduct laboratory and field research on knickpoint migration in western Iowa and eastern Nebraska streams to stabilize the streams and prevent future damage to bridge infrastructure. Knickpoints are abrupt drops in the streambed over which flow plunges and scours the downstream bed. Streambed downcutting increases bank height, which facilitates bank failures and stream widening, and damages critical bridge infrastructure. Preliminary studies in western Iowa indicate that geotechnical properties of the knickpoint bed stratigraphy control its migration rate. We propose to conduct state-of-the-art geotechnical analyses and continuous monitoring of knickpoint geometry and hydraulics in order to determine the presence of specific layers of weakness along which the streambed will fail. We believe that seepage is a primary contributor to knickpoint erosion in the Midwest, either through aggregate detachment or static liquefaction, which creates layers of weakness. Seepage reduces the effective stress within the soil, facilitating failure of subsurface structure. Initially, we will extract cores from a knickpoint in Mud Creek of Mills County, IA and characterize the cores using X-ray Computed Tomography and Gamma Spectroscopy. In addition, we will perform in-situ seepage measurements using automated tensiometers concomitantly with water level measurements for relating seepage fluxes to hydraulic variables. Finally, we will use an automated laser system and time lapse images with Particle Image Velocimetry to monitor continuously knickpoint migration and the water velocity distribution, respectively, at the knickpoint for several high flow events. This research will assist local government agencies and the U.S. DOT in better understanding the principal factors that cause knickpoint propagation and identifying appropriate grade control structures (e.g., sheet-pile weirs and flumes) near bridge crossings to control knickpoint propagation and reduce infrastructure damage.