Identifying Critical Factors Contributing to Soil Shear Strength at Stream Restoration Sites

Madison Wichmann

Co-Authors: T. Knappenberger, E. Brantley

Human land use like farming, mining, and development, has destabilized streams nationwide resulting in erosion, property loss, as well as poor water and habitat quality. The Clean Water Act requires mitigation of degraded streams, so billions of dollars are allocated annually for stream restoration. Motivations for restoration projects include protecting endangered species from habitat degradation, increased flooding, and diminished water quality.

Stream restoration mostly includes construction and restructuring of stream channels and banks. Vegetation must be removed temporarily to create space for construction and allow for vehicles to enter and leave the site. Consequently, soils that form banks can be either relatively loose and unconsolidated or compacted which destroys the naturally present aggregate structure. These conditions make the streambanks vulnerable to erosion and structural failure in cases of flooding or other minor weather events, especially right after construction. Over time, however, soils consolidate through natural wetting and drying cycles and vegetation growth holds soils in place. This natural stabilization of restored stream banks can require years and it is unclear which physical parameter is the most critical factor in stability development.

Stream restoration sites of different age ranging from 1 year to a decade since restoration were identified in the Piedmont region of Alabama. At each stream restoration site, disturbed and undisturbed soil samples were taken and physically analyzed. Some of the factors measured included aggregate stability, root mass, soil texture, saturated hydraulic conductivity, shear stress under saturation and at field capacity (~-30kPa), bulk density, and water retention characteristics. Random forest analysis of the measurements compared against field capacity shear strength indicate strong relevance of age, texture, canopy cover, and root fraction. This result validates the expected factors of critical importance in the development of soil shear strength. Specifically, p-value permutation of the variables show that clay and root fraction pose the strongest influence on shear strength. Findings from this study indicate which variables to pointedly analyze when documenting soil aggregation through time or diagnosing shear failure at restoration sites.

Please post comments and questions for the author below.

2 thoughts on “Identifying Critical Factors Contributing to Soil Shear Strength at Stream Restoration Sites

  1. I’m a bit confused about why you looked at clay content as a factor, not because it’s not a factor, but because I’m not sure why it would change significantly over time. Or are you making the argument that stability will increase over time in high clay soils regardless of vegetation?

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  2. Texture is a standard parameter to analyze when investigating soils and their structure. I did not specifically aim to look at clay, rather when statistically analyzing all key variables measured in this experiment, clay exhibited the strongest correlation with shear strength.

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