Date of this Version
Report # MATC-UI-UNL: 471/424 Final Report 25-1121-0001-471, 25-1121-0001-424
The goal of this study was to conduct a field-oriented evaluation, coupled with advanced laboratory techniques, of channel degradation in a stream of the Deep Loess Region of western Iowa, namely, Mud Creek. The Midwestern United States is an ideal place for such a study, considering that approximately $1 billion of infrastructure and farmland has been lost recently to channel degradation. A common form of channel degradation in this region is associated with the formation of knickpoints, which naturally manifest as short waterfalls within the channel that migrate upstream. As flow plunges over a knickpoint face, scouring of the downstream bed creates a plunge pool. This downcutting increases bank height, facilitating bank failure, stream widening, and damage to critical bridge infrastructure. We conducted a state-of-the-art geotechnical analysis of the sediments from the knickpoint face, plunge pool, and adjacent stream banks to determine the areas of the streambed near the bridge infrastructure that favor knickpoint propagation. Soil characterization using particle size distributions and Gamma Spectroscopy identified a stratigraphic discontinuity at the elevation where the knickpoint forms. An automated surveillance camera was established to monitor the location of the knickpoint face relative to a fixed datum, and to provide a first-order approximation of its migration rate, which was approximately 0.9 m over a 248-day study period. Surveys conducted of the stream reach also facilitated information about knickpoint migration. Flow measurements using Large-scale Particle Image Velocimetry were conducted during the study to understand the hydrodynamic conditions at the site. The results of this research will assist local and federal transportation agencies in better understanding the following: (1) principal geotechnical and hydrodynamic factors that control knickpoint propagation, (2) identification of necessary data for extraction and analysis to predict knickpoint formation, (3) provision of mitigating measures, such as grade control structures (e.g., sheet-pile weirs, bank stabilization measures), near bridge crossings to control the propagation of knickpoints and prevent further damage to downstream infrastructure.