Estimating knickpoint migration in the Deep Loess Region of western Iowa

Knickpoint migration is a common geomorphic hazard in many streams of the Midwestern United States that is partly due to management practices, including stream channelization, which occurred during the first half of the 20th century. These practices caused the stream channels to become unstable, thereby facilitating the formation of knickpoints. Knickpoints are forms of bed degradation characterized by a sharp change in the channel slope that migrate upstream and cause bank instability, as well as the deterioration of fish habitats. Over the last forty years, local governmental agencies have attempted to halt knickpoint migration by constructing a series of weirs, flumes, and other types of grade control structures. Despite these attempts, the problem still persists. In this study, a field evaluation of a representative channel reach containing a knickpoint in the Deep Loess Region of western Iowa was used to test current theoretical and numerical models of knickpoint migration. The recent upstream migration rate of the studied knickpoint was determined through repeated topographic surveys at 3.40 m / yr. Additionally, continuous stage and periodic flow measurements were recorded, soil samples from the river bed and banks were collected and analyzed for their geotechnical properties (including soil strength tests), and the bathymetry was measured in order to fully characterize the stream reach. The data were used to estimate the parameters of various models for quantifying the knickpoint migration rate, which were compared with the field measurements. Preliminary results indicated that the models based on local flow disturbances, such as jet-like phenomena, performed better than models based on global flow assumptions, including the Saint-Venant equations. The final goal of this research is to identify the most suited modeling approach for estimating the rate of migration of knickpoints found in Midwestern streams. The results of the study can then be used by federal or state agencies to define better plans for the future control of bed degradation.