Master of Science / Department of Biological & Agricultural Engineering / Aleksey Y. Sheshukov / Soil erosion is one of the main reasons for agricultural land degradation in the world. Losses of land because of high soil erosion rates and rapidly expanding population result in significant reduction of cultivated land area per capita, and shortage of food on the global scale. Soil erosion can be a major source of sediment in the aquatic systems leading to reduction of organism population and poor water quality. Many factors affect soil erodibility, such as, soil properties, rainfall, topographic features, land use, and management practices, among others. The impacts of soil moisture content, however, are not well understood and. therefore, the primary goal of this study was to quantify two soil erodibility parameters, the erodibility coefficient and critical shear stress, under different soil moisture conditions using the jet erosion test (JET). The JET test uses the apparatus (called mini-JET) that creates an impinging jet of water into the soil and records the resulting scour depth over time. The scour depth time series are then fitted into a non-linear soil erosion equation, yielding the sought values of erodibility parameters. For this study, more than 40 soil samples were collected from several sites in Kansas, processed, and prepared to conduct JET tests in the lab setting. The effects of tillage and soil moisture content were of interest to this study. The results showed varied effects of soil type and sample soil moisture condition on the scour depth development and parameters sensitivity. The critical shear stress decreased and the erodibility coefficient increased with the increase of initial moisture content for clay loam soil, while critical shear stress did not change for sandy loam soil. The study also revealed higher erosive properties of soil collected from the tilled field compared to the no-till field.

The multiangle submerged jet test device (JTD) provides a simple in situ method of measuring streambank critical shear stress (Ï c) and soil erodibility (kd). Previous research showed streambank kd and Ï c can vary by up to four orders of magnitude at a single site; therefore, it is essential to determine if the large range is due to natural variability in soil properties or errors due to the test method. The study objectives were to evaluate the repeatability of the JTD and determine how it compares to traditional flume studies. To evaluate the repeatability, a total of 21 jet tests were conducted on two remolded soils, a clay loam and clay, compacted at uniform moisture content to a bulk density of 1.53 g/cm^3 and 1.46 g/cm^3, respectively. To determine the similarity between JTD and a traditional measurement method, JTD Ï c and kd measurements were compared with measurements determined from flume tests. The JTD kd and Ï c ranged from 1.68-2.81 cm³/N-s and 0.28-0.79 Pa, respectively, for the clay loam and 1.36-2.69 cm³/N-s and 0.30-2.72 Pa, respectively, for the clay. The modest variation of kd and Ï c for the remolded soils suggests the JTD is repeatable, indicating the wide range of parameters measured in the field was a result of natural soil variability. The JTD median kd and Ï c, except clay loam kd (clay loam kd = 2.31 cm^3/N-s, Ï c = 0.45 Pa; clay kd = 2.18 cm^3/N-s, Ï c = 1.10 Pa) were significantly different than the flume values (clay loam kd = 2.43 cm³/N-s, Ï c = 0.23 Pa; clay kd = 4.59 cm³/N-s, Ï c = 0.16 Pa); however, considering the range of potential errors in both test methods, the findings indicate the multiangle submerged jet test provides reasonable measurement of erosion parameters in a field setting. / Master of Science

This study is the first to consider how the combination of root physical effects, microbial production of EPS, and root effects on the hydrodynamic boundary layer could influence streambank soil erodibility. Specifically, the goal of this research was to quantify the physical and biological effects of roots on streambank fluvial erosion. A series of laboratory-scale erosion tests were conducted using a mini jet erosion testing device and a recirculating flume channel to address this goal. Several soil and vegetation factors that influence fluvial entrainment, like extracellular polymeric substances (EPS), soil aggregate stability and root length density, were measured following erosion testing. For flume experiments, three streambank boundary conditions were constructed to simulate unvegetated streambanks, as well as streambanks with herbaceous and woody roots. Soil treatments were also created to represent unamended and organic matter (OM) amended soil either without roots (bare soil), with synthetic roots, or with living roots (Panicum virgatum). Median soil erosion rates along the simulated rooted boundaries were two to ten times higher compared to the unvegetated boundary due to protruding root impacts on the boundary layer. In flume experiments, median erosion rates were 30% to 72% lower for unamended soils containing compacted synthetic root fibers as compared to bare soil samples. Adding both OM and fibers to the soil had a greater effect; the median erosion rate reductions of live rooted treatments (95% to 100%) and synthetic rooted + OM treatments (86% to 100%) were similar and statistically lower than bare soil controls. Stimulated microbial production of EPS proteins were significantly correlated with increased erosion resistance in OM-amended treatments while OM treatments had significantly lower EPS carbohydrates compared to unamended treatments. In summary, while sparsely spaced roots exposed on streambanks may increase soil erosion rates due to impacts on the hydrodynamic boundary layer, overall results highlight how the synergistic relationship between root fibers and soil microbes can significantly reduce streambank soil erodibility due to fiber reinforcement and EPS production. / Doctor of Philosophy / Plant roots are known to protect streambank soils from erosion by water; however, exactly how roots provide this protection has remained unclear. Among other things, roots can influence streambank soil erosion by holding soil together through a thick root network, interacting with soil microorganisms to stimulate the release of "sticky" organic compounds called extracellular polymeric substances (EPS), and altering the force of the water against the streambank. This research aimed to quantify and compare the relative importance of these three mechanisms on streambank soil erosion using a mini jet erosion testing device and an indoor recirculating flume channel. To do this in the flume, three walls were constructed to simulate unvegetated streambanks, as well as streambanks with herbaceous and woody roots. In greenhouse settings, soil treatments were also created to represent unamended and organic matter (OM) amended soil either without roots (bare soil), with artificial roots, or with living roots (Panicum virgatum). While roots protruding out of streambanks appeared to increase median soil erosion rates due to the impact of roots on near-bank flow, artificial roots in the soil and OM amended soils reduced soil erosion rates. Specifically, OM amendments stimulated the production of EPS proteins, leading to improved soil stability and increased soil resistance to erosion by water. Overall results highlight how the synergistic relationship between root fibers (living roots and artificial roots) and soil microbes can significantly reduce streambank soil erodibility due to root binding and microbial production of EPS. While plant roots naturally provide both fibers and EPS to soils, these materials could be incorporated into fill soils during construction to rapidly increase soil erosion resistance following levee construction and stream restoration projects.

Riparian buffers are promoted for water quality improvement, habitat restoration, and stream bank stabilization. While considerable research has been conducted on the effects of riparian buffers on water quality and aquatic habitat, little is known about the influence of riparian vegetation on stream bank erosion. The overall goal of this research was to evaluate the effects of woody and herbaceous riparian buffers on stream bank erosion. This goal was addressed by measuring the erodibility and critical shear stress of rooted bank soils in situ using a submerged jet test device. Additionally, several soil, vegetation, and stream chemistry factors that could potentially impact the fluvial entrainment of soils were measured. A total of 25 field sites in the Blacksburg, Virginia area were tested. Each field site consisted of a 2nd-4th order stream with a relatively homogeneous vegetated riparian buffer over a 30 m reach. Riparian vegetation ranged from short turfgrass to mature riparian forest. Multiple linear regression analysis was conducted to determine those factors that most influence stream bank erodibility and the relative impact of riparian vegetation. Results of this research indicated woody riparian vegetation reduced the susceptibility of stream bank soils to erosion by fluvial entrainment. Riparian forests had a greater density of larger diameter roots, particularly at the bank toe where the hydraulic stresses are the greatest. These larger roots (diameters > 0.5 mm) provided more resistance to erosion than the very fine roots of herbaceous plants. Due to limitations in the root sampling methodology, these results are primarily applicable to steep banks with little herbaceous vegetation on the bank face, such as those found on the outside of meander bends. In addition to reinforcing the stream banks, riparian vegetation also affected soil moisture and altered the local microclimate. While summer soil desiccation was reduced under deciduous riparian forests, as compared to herbaceous vegetation, winter freeze-thaw cycling was greater. As a result, in silty soils that were susceptible to freeze-thaw cycling, the beneficial effects of root reinforcement by woody vegetation were offset by increased freeze-thaw cycling. Using the study results in an example application, it was shown that converting a predominately herbaceous riparian buffer to a forested buffer could reduce soil erodibility by as much as 39% in soils with low silt contents. Conversely, for a stream composed primarily of silt soils that are prone to freeze-thaw cycling, afforestation could lead to localized increases in soil erodibility of as much as 38%. It should be emphasized that the riparian forests in this study were deciduous; similar results would not be expected under coniferous forests that maintain a dense canopy throughout the year. Additionally, because dense herbaceous vegetation would likely not develop in the outside of meander bends where hydraulic shear stresses are greatest, the reductions in soil erodibility afforded by the herbaceous vegetation would be limited to areas of low shear stress, such as on gently sloping banks along the inside of meander bends. As the first testing of this type, this study provided quantitative information on the effects of vegetation on subaerial processes and stream bank erosion. It also represents the first measurements of the soil erosion parameters, soil erodibility and critical shear stress, for vegetated stream banks. These parameters are crucial for modeling the effects of riparian vegetation for stream restoration design and for water quality simulation modeling. / Ph. D.