Streambed degradation poses a continuing risk to bridge infrastructure. Often, degradation occurs as a series of abrupt drops called nickpoints that migrate upstream. The passage of a nickpoint may leave the foundations of bridge peirs and abutments dangerously exposed. We will study the flow behavior and geology associated with nickpoint migration and provide highway engineers with a set of tools that they can use to stem stream degradation.
The objective of this research by the Universities of Iowa (UI) and Nebraska â€“ Lincoln (UNL) is to examine the geologic and hydraulic mechanisms that most strongly control knickpoint erosion rates.
The primary benefit of this research is that it will allow highway engineers to more accurately predict knickpoint migration rates and more adequately prevent damage caused by streambed degradation.
The goal of the proposed research is to conduct laboratory and field investigations on knickpoint migration in western Iowa and eastern Nebraska streams to stabilize the streams and prevent 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, damaging critical bridge infrastructure. Preliminary studies in western Iowa indicate that geotechnical properties of the knickpoint bed stratigraphy control its migration rate. We propose state-of-the-art geotechnical analyses and continuous monitoring of knickpoint geometry and hydraulics 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. We will also 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 knickpoint migration and the related water velocity distribution, respectively, for several high flow events. This research will help local agencies and the U.S. DOT to better understand the principal factors that cause knickpoint propagation. Consequently, appropriate grade control structures (e.g., sheet-pile weirs and flumes) can be identified near bridge crossings to control knickpoint propagation and reduce infrastructure damage, thus improving the safety of the bridges and minimizing the risks to freight transport associated with bridge failure.