Stream Bank Stability in Eastern Nebraska

Abstract

This report describes a study to analyze bank stability at selected sites in eastern Nebraska and develop a simplified method for estimating the stability of banks at future study sites. Bank cross sections along straight reaches of channel and geotechnical data were collected at approximately 150 sites. The sites were categorized into three groups based on mapped soil permeability. With increasing permeability of the soil groups, the median cohesion values decreased and the median friction angles increased. Three analytical methods (Culmann, Agricultural Research Service and an indirect method) were used to determine if banks were stable, at risk, or unstable. The maximum angle from horizontal at which the bank would be stable for the given soil and bank height conditions also was computed with the indirect method. A comparison of the expected stable bank angles for saturated conditions and the surveyed bank angles indicated that many of the surveyed bank angles were considerably less than the maximum expected stable bank angles despite the banks being classified as at risk or unstable. For severely degraded channels along straight reaches this was not expected. Several possible explanations are discussed. At least for some bank sections, it appears that the summary results of bank stability for this study are overly optimistic. Although some individual bank sections may be accurately portrayed, it is not known which or how many bank sections are accurately classified without more extensive data to determine variability in soil shear strength. If the variability of soil parameters, especially cohesion, can be determined for a site, and if the variability is small so that average or weakest values can be used to represent the banks, any of the methods demonstrated in this report can be used to make preliminary assessments of channel bank stability at future study sites. An electronic spreadsheet has been developed to make preliminary assessments of existing bank stability or to assess future bank stability. The spreadsheet also can automatically compute the maximum uniform angle at which a bank section would be expected to be stable.