Author(s): J. T. Thomas; A. N. Papanicolaou; C. Wilson; D. C. Dermisis; M. Elhakeem
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Abstract: Most studies of knickpoints to date have been performed in the laboratory where scaling effects hinder prediction of the migratory velocity of knickpoints. Knowledge of knickpoint migration rates would allow prevention of further channel bed incision in western Iowa streams. This study involved detailed field investigations over three years in order to monitor the upstream migration of a knickpoint on Mud Creek in Mills County, IA and identify the key mechanisms triggering knickpoint propagation. A pressure transducer located approximately 20 m upstream of the truss provided continuous measurements of flow depth. ADV and LSPIV technologies were used to measure flow properties like velocity and discharge. Semi-annual surveys of the knickpoint measured bed elevations and headward migration through time. A laser system mounted on a movable truss provided measurements of the knickpoint front for different flow conditions. Correlation of distinct erosional events at the knickpoint with discharge data will help to illuminate the conditions at which knickpoint migration commences. The stream has eroded through the weaker Roberts Creek Member and into the more resistant upper Gunder Member, causing lower knickpoint migration rates than reported in two other western Iowa studies. In highly unstable streams, the surface of current knickpoints may be the bed scar of past knickpoints. Water level proved a reliable surrogate measure for knickpoint advancement. Over the 3-year study period, a finger-like projection advanced upstream at a rate of 5.6 m/yr.
Year: 2009