Civil Engineering


Date of this Version

Spring 4-17-2013


Kephart, Clark. Flow and Geometry Measurments at an Active Knickpoint. MS Thesis. University of Nebraska, Lincoln, 2013.


A THESIS Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Master of Science, Major: Civil Engineering, Under the Supervision of Professor David Admiraal. Lincoln, Nebraska: May, 2013

Copyright (c) 2013 Clark Willis Kephart


Channel degradation is an area of growing concern in areas of the Midwestern United States where there are large deposits of loess soils. Channel degradation occurs as a result of disturbances to the sediment load, bed soil configuration, and hydraulic characteristics of the stream (i.e. discharge, channel slope and geometry). These changes may occur suddenly or slowly over time. A given disturbance may be due to a large rain event, channel adjustments (i.e. dredging and straightening), and changes in land use around the stream (Chen et al. 1999).

A knickpoint is an abrupt drop in stream bed elevation over which water plunges and scours the downstream bed. The plunging water may lead to intense bed degradation and subsequent upstream migration of the knickpoint, often causing stream banks to become unstable and unsafe. Knickpoint migration problems have been particularly prevalent in the loess soil regions of western Iowa and eastern Nebraska as a result of wide spread channel straightening projects in the region. The goal of this project is to monitor the migration of an active knickpoint located on Mud Creek in Mills County, Iowa. Analysis of the knickpoint consists of: (1) acquiring time lapse images of the knickpoint every thirty minutes from a camera installed at the site, (2) periodically collecting detailed survey data of the stream channel and knickpoint, (3) collecting Large-scale Particle Image Velocimetry (LPIV) videos of the flow for a variety of high and low flow conditions, and (4) estimating discharges in the channel using the LPIV results.

The time lapse images provide a frame-by-frame depiction of the upstream movement of the knickpoint. The images allow us to assess when the knickpoint has migrated and if its migration is associated with a particular storm event. The less frequently collected survey data provide a more accurate assessment of knickpoint position and can also be used in conjunction with LPIV videos to establish velocity distributions and a rating curve for the knickpoint. LPIV videos have been converted to bitmaps and rectified for analysis using Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV) software. They are then used to examine depth, discharge, and velocity conditions for different flow events.

Adviser: David M. Admiraal