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Title: A stability analysis of semi-cohesive streambanks with CONCEPTS: Coupling field and laboratory investigations to quantify the onset of fluvial erosion and mass failure

Author
item SUTARTO, TOMMY - University Of Iowa
item PAPANICOLAOU, ANASTASIOS - University Of Iowa
item WILSON, CHRIS - University Of Iowa
item Langendoen, Eddy

Submitted to: Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/4/2014
Publication Date: 5/19/2014
Publication URL: https://handle.nal.usda.gov/10113/5695346
Citation: Sutarto, T., Papanicolaou, A.N., Wilson, C.G., Langendoen, E.J. 2014. A stability analysis of semi-cohesive streambanks with CONCEPTS: Coupling field and laboratory investigations to quantify the onset of fluvial erosion and mass failure. Journal of Hydraulic Engineering. doi:10.1061/(ASCE)HY.1943-7900.0000899.

Interpretive Summary: Currently, few established approaches to assess bank stability consider both mass failure and fluvial erosion processes, and in most cases, the stability analysis of a bank is only based on mass failure estimations. Further, few studies provide the distribution of bank erosion resistance parameters along the bank profile and its characteristic locations, or layers (that is, crest, mid-bank, and toe). Instead most studies typically assume homogeneous, well compacted soils along the bank profile. The nature of this study was twofold. First, field and experimental analyses were conducted to generate data for channel cross-section properties, soil index properties, and mechanical and erosional strengths at two sites along a mid-sized Midwestern stream in southeastern Iowa, as a necessary precursor for performing a bank stability analysis. Second, the channel surveys and data obtained from the field and laboratory analyses were used as input parameters for the USDA, ARS one-dimensional, channel evolution model CONCEPTS (Conservational Channel Evolution and Pollutant Transport System) to estimate the factor of safety for mass failure (FSm) and fluvial erosion (FSf), and simulate the bank retreat as a result of either fluvial erosion or mass failure or the interaction between the two modes of erosion. Simulations using CONCEPTS show that estimation of FSm must be complemented with the estimation of FSf for not underestimating mass failure. Otherwise, based on mass failure criteria alone, the stability analysis fails to consider the potential for interconnection between bank toe undercutting and mass failure. Simulation results also showed the importance of accounting for bank heterogeneity in the performed stability analysis. Assuming homogeneous soil properties (either those of the crest or toe) resulted in: (1) mostly erosion near the crest due to mass failure contrary to the heterogeneous simulations and the reported in-situ observations, where bank toe undercutting is predicted and observed, respectively; or (2) no significant retreat along the bank profile. 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 overarching goal of this study is to perform a comprehensive bank stability analysis that is phenomenologically sound by considering both mass failure and fluvial erosion. The nature of this study is twofold. First, field and experimental analysis is conducted to generate data for channel cross-section properties, soil index properties, and mechanical & erosional strengths at two sites in a representative, mid-size, Midwestern stream in south-eastern Iowa that is subjected to frequent flash floods and characterized by active fluvial erosion and cantilever failure. Second, the channel surveys and data obtained from the field and laboratory analyses are used as input parameters for an established 1D, channel evolution model, namely the Conservational Channel Evolution and Pollutant Transport System, CONCEPTS, to estimate the factor of safety for mass failure (FSm) and fluvial erosion (FSf), and simulate the bank retreat as a result of either fluvial erosion or mass failure or the interaction between the two modes of erosion. In CONCEPTS, a bank profile can be divided into several layers allowing the user to account for heterogeneity in soil properties. The results show that estimation of FSm must be complemented with the estimation of FSf for not underestimating mass failure. Otherwise, based on mass failure criteria alone, the stability analysis fails to consider the potential for the interconnection between bank toe undercutting and planar failure and may lead to the underestimation of mass failure over an interval. Second, bank soil heterogeneity plays an important role in bank stability analysis. The variability of mechanical and erosional strengths, shown in this study, along the downslope of the banks highlights the need to acquire both mechanical and erosional strengths for the three layers along a bank profile (namely crest, midbank, and toe) as is the case in the present study to improve the commonly adopted protocols that typically assume homogeneous, well compacted soils along a bank profile. The predicted bank profile obtained from the model compare favorably with cross-sectional measurements obtained for a period of two hydrological cycles.