As scouring around foundations is the most common cause of bridge failures, one of the most pressing questions of this research is to determine whether or not it is possible to predict the critical shear stress of different soil types using only soil property information. This report shows that it is possible to predict critical shear stress and determines the soil properties that are required to predict the critical shear stress based on soils from Georgia. Multiple methods to predict soil erodibility categories are developed based on the amount of soil information available to the researcher. The report shows how the methods to predict soil erodibility can be integrated with HYRISK, a scour risk assessment tool. In particular, the probabilities of bridge failures and expected economic losses are calculated for approximately 40 bridges in Georgia; soil erodibility characteristics for these bridges are calculated using the methods developed in this thesis. The goal of this thesis is to provide a faster and more cost-effective approach to calculate critical shear stress ranges likely to be encountered at a bridge foundation. Implementation of theses methodologies will help balance funding for new and existing bridges while simultaneously ensuring safe bridge foundation and minimizing economic consequences associated with overbuilding a bridge and/or having to retrofit or replace a bridge that has scour damage due to underbuilding it to withstand a major storm event.

Erosion of a river bed has important implications with respect to scour around river structures such as bridges, transport of contaminants attached to the sediment, and disruption or destruction of aquatic habitats. Erosion occurs when the resistive strength of the sediment is overcome by the hydrodynamic forces produced by the flow of water. This resistance to erosion in a sediment originates from gravity or interparticle forces for coarse sediment (sand and gravel) and fine sediment (silt and clay), respectively. Since the erosion of fine sediment depends on the combination of many interparticle forces, and this combination fluctuates widely amongst different fine sediments, past studies have had difficulty finding a consistent method to estimate fine sediment erosion. This study analyzes sediments that fall in the transition size range between fine and coarse sediments and compares the findings with those from fine sediments (Wang 2013) and sandy coarse sediments (Navarro 2004, Hobson 2008), in order to correlate the erosion rates of both sediment types to their physical characteristics. In this study, kaolin-sand mixtures were prepared by mixing various percentages of Georgia kaolin by weight ranging from 30% to 100% with industrial fine sand and tap water. Geotechnical and other tests of sediment properties were performed to measure water content, bulk density, grain size distribution, temperature, pH, and conductivity of these mixtures. Hydraulic flume experiments measured the erosion rates of each sediment and these rates were used to estimate the critical shear stress correlating to that mixture. Relationships between the physical properties of the sediment and critical shear stress were developed by multiple regression analysis. An alternative option of estimating the critical shear stress by a weighted equation, which uses the combination of fine sediment erosion and coarse sediment erosion equations separately, was explored and found to be a viable and accurate option to estimating both coarse and fine sediment erosion from the same parameters and equation. The results from this study can be used to estimate sediment erodibility and thus river bed stability based on simple tests of physical properties of the river bed sediment and will help predict scour around bridges and other flow obstructions.

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 amount and type of cohesive sediment found in gravel river beds can have important implications for the health of aquatic biota, surface/groundwater interactions and water quality. Due to landscape disturbances in the Elbow River watershed, increased sediment fluxes have negatively impacted fish habitat, water quality and water supply to the City of Calgary. However, little is known about the source of cohesive sediment and its interaction with gravel deposits in the Elbow River. This research was designed to: 1) quantify the transport properties (critical shear stress for erosion, deposition, porosity, settling velocity, density) of cohesive sediment and 2) evaluate the potential for coarse gravel to entrap cohesive sediment in the Elbow River. A 5m annular flume was used to conduct erosion and deposition experiments using plane and coarse bed conditions. The critical shear stress for deposition and erosion of the Elbow River cohesive sediments was 0.115Pa and 0.212Pa, respectively. The settling velocity of the cohesive sediment had an inverse relationship between floc size and settling velocity for larger flocs, due to a decrease in floc density with increased size. Cohesive sediment moved from the water column into the gravel bed via the coupling of surface and pore water flow. Once in the gravel bed, cohesive sediments were not mobilized from the bed because the shear produced by the flume was less than the critical shear to mobilize the gravel bed. Using a model developed by Krishnappan and Engel (2006), an entrapment coefficient of 0.2 was determined for the gravel bed. Entrapment coefficients were plotted against substrate size, porosity and hydraulic conductivity, demonstrating a relationship between entrapment coefficient and these variables. It was estimated that 864kg of cohesive sediment is stored in the upper 0.08m of a partially submerged point bar in the Elbow River. Accordingly, when flow conditions are sufficient to mobilize the gravel bed and disturb the amour layer, cohesive materials may be entrained and transported into the Glenmore Reservoir, where it will reduce reservoir capacity and may pose treatment challenges to the drinking water supply.

Doctor of Philosophy / Department of Civil Engineering / Stacey E. Tucker-Kulesza / Surface soil erosion is a widespread problem that impacts the natural and built environment. Many disciplines, such as hydrology, soil science, agriculture, and geotechnical engineering, have investigated soil erosion. Although empirical equations to predict soil erosion exist, they are typically inaccurate, so several devices have been developed to quantify the erodibility of soil. The erosion function apparatus (EFA) was developed to predict the erosion potential of soil for evaluating bridge scour; however, it has been used for several other geotechnical applications. The main disadvantages of the EFA are that it is unable to directly measure the shear stress, it remains operator dependent, and it is time consuming as a standard test requires at least eight hours. Moreover, as erosion occurs, it affects the water quality and makes it difficult to observe the soil sample surface during the test, affecting the operator judgement. The research objective of this project is to instrument the EFA to address the limitations of the device. A stereo-photogrammetry system was developed to measure the soil surface roughness following an EFA test and reduce operator dependency. Turbidity sensors were added to provide a secondary measurement of erosion. The newly instrumented EFA was used to develop a new methodology for interpreting erosion results. Lastly, the new methodology and instrumentation were used to explore the influence of natural and engineered soil properties on soil erosion.

Doctor of Philosophy / Department of Biological & Agricultural Engineering / Aleksey Y. Sheshukov / As the world faces an increasing demand for food due to the growing global population and the pernicious effects of land degradation, there is a need to overcome this challenge by using sustainable management practices for agricultural productions. One of the problems, which sustainable agriculture seeks to address, is the loss of topsoil due to soil erosion. Changing weather patterns also contribute to the average annual rainfall across the globe with an excess precipitation, which creates runoff and causes soil erosion. One of the significant yet less studied types of soil erosion is ephemeral gully erosion. Formed by the concentrated overland flow during intensive rainfall events, ephemeral gullies are channels on agricultural fields that can be removed by tillage operations but appear at the same location every year. Even though simplified ephemeral gully models estimate soil losses, they do not account for complicated hydrological and soil erosion processes of channel formations. The purpose of this research work is to investigate sediment sources and develop tools that can predict ephemeral gully erosion more efficiently. To achieve this goal, an experimental study was conducted on an agricultural field in central Kansas by tracking channel development, monitoring soil moisture content, and recording the amount of rainfall. Runoff and sediment loads from contributing catchment and critical and actual shear stresses were estimated by the computer model, and conclusions were made on the effect of saturation dynamics on the erosion processes. Furthermore, a two-dimensional subsurface water flow and soil erosion model was developed with the variable soil erodibility parameters which account for the subsurface fluxes and the effects on the soil detachment process. The model was applied to study the impacts of variable soil erodibility parameters on the erosion process for different soils and various antecedent soil moisture conditions. Also developed to estimate the soil losses at the field scale was an integrated spatially-distributed ephemeral gully model with dynamic time-dependent channel development. The model showed good fit by matching the experimental data. The results from this work can be used to advance the research of soil erosion prediction from concentrated flow channels and ephemeral gullies formed on agricultural fields.

Estimates of bank erosion typically require field measurements to determine the soil erodibility since soil characteristics are highly variable between sites, especially for cohesive soils. The submerged jet test device is an in situ method of determining the critical shear stress and soil erodibility of cohesive soils. A constant velocity jet, applied perpendicular to the soil surface, creates a scour hole which is measured at discrete time intervals. While the results of these tests are able to provide values of critical shear stress and soil erodibility, the results are often highly variable and do not consider certain aspects of scour phenomena found in cohesive soils. Jet test measurements taken on the lower Roanoke River showed that the results varied for samples from similar sites and bulk failures of large areas of soil were common on the clay banks. Computational Fluid Dynamics (CFD) can be used to determine the effect of scour hole shape changes on the applied shear stress. Previous calculation methods assumed that the depth of the scour hole was the only parameter that affected the applied shear stress. The analysis of the CFD models showed that depth did heavily influence the maximum shear stress applied to the soil boundary. However, the scour hole shape had an impact on the flow conditions near the jet centerline and within the scour hole. Wide, shallow holes yielded results that were similar to the flat plate, therefore it is recommended that field studies only use jet test results from wide, shallow holes to determine the coefficient of erodibility and the critical shear stress of cohesive soils. / Master of Science

No description available.

Previous studies have shown that soil erodibility and critical shear stress are highly influenced by weathering processes such as freeze-thaw cycling and wet-dry cycling. Despite over forty years of research attributing changes in soil properties over time to climate-dependent variables, little quantitative information is available on the relationships between streambank erodibility and critical shear stress and environmental conditions and processes that enhance streambank erosion potential. The goal of this study was to investigate temporal changes in streambank erodibility and critical shear stress due to surface weathering. Soil erodibility and critical shear stress were measured monthly in situ using a multi-angle submerged jet test device. Environmental and soil data were also collected directly at the streambank surface to determine freeze-thaw cycles, soil moisture, soil temperature, bulk density, soil erodibility, critical shear stress, and other atmospheric conditions that could impact bank erosion potential. Statistical tests, including a nonparametric alternative to ANOVA and multiple comparison tests, were used to determine if temporal changes in soil erosion potential were greater than spatial differences. Regression analyses were also utilized to identify the factors contributing to possible changes in soil erodibility, critical shear stress, and bulk density. The nonparametric alternative to ANOVA in combination with Dunn's nonparametric multiple comparison test showed soil erodibility was significantly higher (p=0.024) during the winter (November - March) and the spring/fall (April - May, September - October). Regression analyses showed 70 percent of soil erodibility variance was attributed to freeze-thaw cycling alone. Study results also indicated that bulk density is highly influenced by climate changes since gravimetric water content and freeze-thaw cycles combined explain as much as 86 percent of the variance in bulk density measurements. Results of this study show significant amounts of variation in the resistance of streambank soils to fluvial erosion can be attributed to subaerial processes, specifically changes in soil moisture and temperature. These results have potential implications for streambank modeling and restoration projects that assume constant values for soil erodibility. Watershed models and restoration designs should consider the implications of changing soil erodibility during the year in model development and stream restoration designs. / Master of Science

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