United States Department of Transportation
i
Monitoring the effects of knickpoint erosion on bridge pier and abutment structural damage due to scour.
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2012-04-01
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[PDF-3.21 MB]
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NTL Classification:NTL-HIGHWAY/ROAD TRANSPORTATION-Bridges and Structures
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Abstract: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 ~$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 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 Largescale
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) identify necessary data for
extraction and analysis to predict knickpoint formation, (3) provide mitigation 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.
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