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Title: Misconceptions about mechanical and fluvial erosional strength: implications to streambank stability

Author
item SUTARTO, TOMMY - SAMARINDA POLYTECHNIC STATE UNIVERSITY
item PAPANICOLAOU, ATHANASIOS - UNIVERSITY OF TENNESSEE
item WILSON, CHRISTOPHER - UNIVERSITY OF TENNESSEE
item Langendoen, Eddy

Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 4/26/2014
Publication Date: 6/3/2014
Citation: Sutarto, T.E., Papanicolaou, A.N., Wilson, C.G., Langendoen, E.J. 2014. Misconceptions about mechanical and fluvial erosional strength: implications to streambank stability. In: Proceedings of the 2nd International Seminar on Infrastructure Development (ISID 2014), June 3-4, 2014, Balikpapan, Indonesia. pp. 1-8.

Interpretive Summary: Streambank erosion can occur by two different mechanisms, mass failure by gravitational forces and fluvial erosion by flowing water. Mass failure represents the upper limit of streambank erosion and it occurs when a streambank is geotechnically unstable. Fluvial erosion is the lower limit of bank erosion and it occurs when the flow shear stress is larger than erosional strength of the streambank soils. Scientists at the USDA, ARS, National Sedimentation Laboratory in collaboration with researchers of the University of Iowa tested streambank soils from Clear Creek, IA, in the laboratory to determine their mechanical and fluvial erosional strengths. It was found that the mechanical strength was 3 to 4 orders of magnitude larger than the fluvial erosional strength. Using the measured data, bank erosion calculations with the USDA, ARS channel evolution computer model CONCEPTS showed that a geotechnical stability criterion alone is not sufficient to test the stability of streambanks at the study site since fluvial erosion can be a precursor to the collapse of streambank soils or mass failure. This study demonstrated that even though the factor of safety in term of mass failure indicated stable banks (i.e., no retreat), the factor of safety in terms of fluvial erosion could be lower than 1. This implied that fluvial erosion occurred which changes streambank geometry and ultimately leads to failure due to geotechnical instability. This information is useful for stream-restoration practitioners active in the design of stable stream banks for developing appropriate bank-soil property data collection strategies and for improving the accuracy of stream bank stability analyses.

Technical Abstract: The central objective of this study was to highlight the differences in magnitude between the mechanical and fluvial streambank erosional strength measures of with the purpose of developing a more comprehensive bank stability analysis. Mechanical erosion and ultimately failure signifies the general movement or collapse of large soil blocks due to geotechnical instability and is the upper limit of streambank erosion. Conversely, fluvial erosion is the detachment of individual particles or aggregates due to the shearing action of flow and is the lower limit of streambank erosion. A total of 18 streambank samples from a representative stream in the U.S. Midwest (i.e., Clear Creek, IA) with semi-cohesive soils were analyzed in terms of both mechanical and fluvial erosional strength. Mechanical strength was measured using a direct shear device and ranged from 1,500 to 15,000 Pa. Fluvial strength was measured using a conduit flume, which applied a shearing force to the sample, and averaged 2.12 Pa. Thus, mechanical strength was 3 to 4 orders of magnitude larger than fluvial erosional strength, which suggests that identifying the different modes of streambank erosion (e.g., mechanical or fluvial) during a hydrograph is needed to provide better design specifications for bank stabilization practices.