Date of this Version
Dredged and straightened channels in eastern Nebraska have experienced degradation leading to channel widening by bank failure. Degradation has progressed headward and affected the drainage systems upstream from the modified reaches. This report describes a study that was undertaken 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 in 26 counties of eastern Nebraska. 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 were used to determine if banks were stable (should not fail even when saturated), at risk (should not fail unless saturated), or unstable (should have already failed). The Culmann and Agricultural Research Service methods were based on the Coulomb equation and planar failure; an indirect method was developed that was based on Bishop’s simplified method of slices and rotational failure. 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. Because of few soil shear-strength data, all analyses were based on the assumption of homogeneous banks, which was later shown to be atypical, at least for some banks.
Using the Culmann method and assuming no soil tension cracks, 67 percent of all 908 bank sections were identified as stable, 32 percent were at risk, and 1 percent were unstable; when tension cracks were assumed, the results changed to 58 percent stable, 40 percent at risk, and 1 percent unstable. Using the Agricultural Research Service method, 67 percent of all bank sections were identified as stable and 33 percent were at risk. Using the indirect method, 62 percent of all bank sections were identified as stable and 31 percent were at risk; 3 percent were unstable, and 3 percent were outside of the range of the tables developed for the method. For each of the methods that were used, the largest percentage of stable banks and the smallest percentage of at risk banks was for the soil group with the lowest soil permeability and highest median cohesion values.
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 maxi-mum 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. It was expected that they would have angles close to the maximum stable angle as they should have been failing from an oversteepened condition. Several explanations are possible. The channel reaches of some study sites have not yet been affected to a significant degree by degradation; study sites were selected through-out individual basins and severe degradation has not yet extended to some sites along upper reaches; and some reaches have experienced aggradation as degradation progresses upstream. Another possibility is that some bank sections have been affected by lateral migration processes, which typically result in shallow bank angles on the inside bend of the channel.
Another possibility is that the maximum expected stable bank angles are too steep. The stability methods used were well established and in essential agreement with each other, and there was no reason to question the geometry data. This left non-representative soil data as a probable reason for computed stable bank angles that, at least in some cases, are overly steep. Based on an examination of the cohesion data, to which the stable bank-angle calculations were most sensitive, both vertical and horizontal variability in soil properties appeared likely for many of the sites. Because a weak soil area or an interface of two differing soil areas in a bank can determine where the failure plane will be and what the factor of safety might be, it is not likely that the few soil tests done at each of the sites identified the critical soil parameters needed to accurately assess bank stability or to determine the expected stable bank angle for each bank section. 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, developed for use with the indirect method, is included on a compact disk at the back of this report and can be used to make preliminary assessments of existing bank stability at study sites or to assess future bank stability under assumptions of degradation or aggradation by imposing projected changes on the existing bank geometry. The user needs cross-section data and estimates of the soil parameters—cohesion, friction angle, and ambient and saturated unit weight—to make the assessments. In addition, the spreadsheet can automatically compute the maximum uniform angle at which a bank section would be expected to be stable, for a given factor of safety, bank height, and soil parameters. For a bank with extensive variability in soil shear-strength, a method needs to be used that can account for multiple soil areas with differing parameters.