Theses and Dissertations

Permanent URI for this collectionhttps://hdl.handle.net/10217/100381

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Open Access
Moment stability analysis method for determining safety factors for articulated concrete blocks
(Colorado State University. Libraries, 2010) Cox, Amanda L., author; Thornton, Christopher I., advisor; Vlachos, Evan, committee member; Abt, Steven R., committee member; Watson, Chester C., committee member
Articulated concrete block (ACB) revetment systems are widely used for channel lining and embankment protection. Available information pertaining to testing and analysis of ACB systems was identified. Current approaches for prediction of ACB system stability are based on a moment stability analysis and utilize shear stress to account for all hydrodynamic forces. Assumptions utilized in the moment stability analysis derivations were identified and the applicability to channelized and steep-slope conditions was investigated. The assumption of equal lift and drag forces was determined to be non-conservative and the most influential to computed safety factors. A database of twenty-four tests encompassing both channelized and overtopping conditions was compiled from available data for three ACB systems. Safety factors were computed using the current state-of-the-practice design methodology for each test. The current design methodology proved accurate at predicting the point of instability for five out of the nine total tested ACB installations. A new safety factor design methodology was developed using a moment stability analysis coupled with the computation of hydrodynamic forces using both boundary shear stress and flow velocity. Lift coefficients were calibrated for each of the three ACB systems within the database. Safety factors were computed using the new safety factor method and the calibrated lift coefficients. The new safety factor design method proved accurate at predicting stability for eight of the nine total tested ACB installations.
Open Access
Numerical analysis of river spanning rock U-weirs: evaluating effects of structure geometry on local hydraulics
(Colorado State University. Libraries, 2011) Holmquist-Johnson, Christopher Lee, author; Watson, Chester C., advisor; Abt, Steven R., committee member; Thornton, Christopher I., committee member; Doe, William, committee member
River spanning rock weirs are being constructed for water delivery as well as to enable fish passage at barriers and provide or improve the aquatic habitat for endangered fish species. Many design methods are based upon anecdotal information applicable to narrow ranges of channel conditions and rely heavily on field experience and engineering judgment. Without an accurate understanding of physical processes associated with river spanning rock weirs, designers cannot address the failure mechanisms of these structures. This research examined the applicability of a Computational Fluid Dynamics (CFD) model, U2RANS, to simulate the complex flow patterns associated with numerous U-weir configurations. 3D numerical model simulations were used to examine the effects of variations in U-weir geometry on local hydraulics (upstream water surface elevations and downstream velocity and bed shear stress). Variations in structure geometry included: arm angle, arm slope, drop height, and throat width. Various combinations of each of these parameters were modeled at five flow rates: 1/10 bankfull discharge, 1/5 bankfull discharge, 1/3 bankfull discharge, 2/3 bankfull discharge and bankfull discharge. Numerical modeling results duplicated both field observations and laboratory results by quantifying high shear stress magnification near field and lab scour areas and low shear stress magnification near field and lab depositional areas. The results clearly showed that by altering the structure geometry associated with U-weirs, local flow patterns such as upstream flow depth, downstream velocity, and bed shear stress distributions could be altered significantly. With the range of parameters tested, the maximum increase in channel velocity ranged from 1.24 to 4.04 times the reference velocity in the channel with no structure present. Similarly, the maximum increase in bed shear stress caused by altering structure geometry ranged from 1.57 to 7.59 times the critical bed shear stress in the channel for a given bed material size. For the range of structure parameters and channel characteristics modeled, stage-discharge relationships were also developed utilizing output from the numerical model simulations. These relationships are useful in the design process when estimating the backwater effect from a structure for irrigation diversion as well as determining the spacing between structures when multiple structures are used in series. Recommendations were also made, based on the analysis and conclusions gathered from the current study, for further research. The analysis and results of the current study as well as laboratory studies conducted by Colorado State University and field reconnaissance by the Bureau of Reclamation provide a process-based method for understanding how structure geometry affects flow characteristics, scour development, fish passage, water delivery, and overall structure stability. Results of the numerical modeling allow designers to utilize the methods and results of the analysis to determine the appropriate U-weir geometry for generating desirable flow parameters (i.e. upstream flow depth and downstream velocity and bed shear stress magnification) to meet project specific goals. The end product of this research provides tools and guidelines for more robust structure design or retrofits based upon predictable engineering and hydraulic performance criteria.
Open Access
Pressure flow effects on scour at bridges
(Colorado State University. Libraries, 2000) Robeson, Michael D., author; Thornton, Christopher I., advisor; Abt, Steven R., committee member; Arneson, Larry A., committee member; Doe, William W., committee member
Scour caused by the occurrence of pressure flow requires a comprehensive understanding. Pressure flow can be defined as flow in which the low chord of a bridge becomes inundated and the flow through the bridge opening transitions from free surface flow to a pressurized condition, leading to a submerged or partially submerged bridge deck condition. A pressure flow condition often occurs at a bridge during a flood, potentially leading to bridge failure. Scour of bridge foundations (piers and abutments) represents the largest single cause of bridge failure in the United States (ASCE, 1999). Methodical scour research began in 1949 with the research of E.M. Laursen. Unfortunately, the application of scour research to the design of bridges did not occur until several bridges failed due to local scour. Over the years, bridge scour research has focused on the study of free surface flow. During the past decade, research related to pressure flow scour has become increasingly important. A testing program was developed and performed at the Hydraulics Laboratory of Colorado State University to examine pressure flow effects on scour at and around bridges. Flume experiments were conducted incorporating a physical model of a generic bridge with supporting abutments constructed at an approximate scale of 8:1. In an effort to simulate varying magnitudes of a pressure flow condition, the model was constructed in a manner that permitted the bridge deck to be lowered into the flow. By lowering the bridge deck and holding the level of the approach flow constant, multiple levels of deck submergence could be examined. Six vertical bridge positions, three discharges, two abutment widths and two sediment sizes were incorporated into a matrix comprising 69 tests. Data collected included hydraulic parameters and topographic surveys. Analysis of data collected during the study resulted in the formulation of a set of multivariate linear regression equations enabling the user to estimate abutment, local and deck scour depths during a pressure flow condition. Results of a dimensional analysis indicate that the dominant variables in predicting scour depths for a pressure flow condition include; the critical velocity of a given sediment size, the average velocity under the bridge deck, the height of the bridge deck above the initial and final bed surface, the depth of flow upstream of the bridge and the Froude number of the approach flow. Coefficients of determination for the developed equations ranged from 0.82 to 0.95.
Open Access
Quantification of hydraulic effects from transverse instream structures in channel bends
(Colorado State University. Libraries, 2014) Scurlock, Stephen Michael, author; Thornton, Christopher I., advisor; Abt, Steven R., committee member; Venayagamoorthy, Subhas K., committee member; Wohl, Ellen E., committee member
Meandering river channels possess hydraulic and geomorphic characteristics that occasionally place anthropogenic interests at risk. Loss of valuable land holdings and infrastructure due to outer-bank channel encroachment from erosion processes and complications for channel-bend navigation have prompted development of techniques for reconfiguration of instream hydraulics. Transverse instream structures are one type of technique and have been implemented in channel bends to reduce outer-bank erosivity and improve navigability. Instream structures use less material and have ecological and habitat benefits over traditional revetment type bank protection. Structures are typically constructed in series, extend from the outer-bank into the channel center, and are designed with various crest heights and slopes. Current design recommendations for the structures in natural channels provide generalized ranges of geometric parameters only; no specific information pertaining to hydraulic reconfiguration is provided. Understanding specific hydraulic response to alteration of geometric structure parameters is requisite for educated structure design. Focusing on two types of transverse instream structures, the spur-dike and vane, a mathematical design tool was developed for the quantification and prediction of induced hydraulic response. A series of dimensionless groupings were formulated using parameters obtainable from field data of natural channels and grouped using dimensional analysis. Each dimensionless grouping had an identifiable hydraulic influence on induced hydraulics. A conglomerate mathematical expression was established as the framework for induced instream structure quantification. The mathematical model was tailored to produce twenty-four hydraulic relationships through regression analysis utilizing a robust physical model dataset collected within rigid-bed, trapezoidal channel bends. Average and maximum velocity and boundary shear-stress data were segmented into outer-bank, centerline, and inner-bank regions and then normalized by bend-averaged baseline conditions. Velocity equations were developed for an all-structure dataset, a spur-dike dataset, and a vane dataset. Boundary shear-stress equations were developed for spur-dike structures only. Regression equations quantified laboratory hydraulics to a high level of accuracy. Equation response to independent parameter alteration coincided with continuity principles and physical hydraulic expectations. Methods performed well in application to extraneous natural channel data from the literature. Developed methodologies from this research presented a fundamental addition to the current design procedures for the installation of structures in migrating channel bends. Quantification of the reduction of outer-bank erosive potential and increase at the shifted conveyance zone within natural channels was made possible using readily measured field data and the proposed methodology. Equations allow for previously unattainable investigation of configuration geometry combinations to meet installation objectives using simple mathematical formulas. Configuration geometry optimization to meet hydraulic design criteria using the proposed methods may hold substantial economic benefit over traditional design protocols.
Open Access
Quantification of shear stress in a meandering native topographic channel using a physical hydraulic model
(Colorado State University. Libraries, 2011) Ursic, Michael E., author; Thornton, Christopher I., advisor; Abt, Steven R., committee member; Williams, John D., committee member
Current guidelines for predicting increases in shear stress in open-channel bends were developed from investigations that were primarily prismatic in cross section. This study provides possible increases in shear stress relative to approach flow conditions resulting from planimetric and topographic geometric features. Boundary shear stress estimates were determined by several methods utilizing acoustic Doppler velocimeter (ADV) and Preston tube data in a physical model of a full meander representing native topographic features found in the Middle Rio Grande. Methods examined include: the law of the wall, Preston tube, turbulent Reynolds stress approximations, and a turbulent kinetic energy (TKE) proportionality constant approach. Results from each method were compared by magnitude and distribution and limitations were noted. Measured boundary shear stresses in the bend were, in some instances, nearly thirteen times the approach shear stress. Relationships were determined for the expected increase that may provide practical application. Measured bend velocities were four times greater than approach velocities and relationships were determined between velocity and bend geometry. Multipliers for shear stress and velocities were determined for one-dimensional model results.
Open Access
Three-dimensional computational modeling of curved channel flow
(Colorado State University. Libraries, 2014) Sin, Kyung-Seop, author; Venayagamoorthy, Subhas K., advisor; Thornton, Christopher I., advisor; Abt, Steven R., committee member; Julien, Pierre Y., committee member; Dasi, Lakshmi P., committee member
Investigating flow dynamics in curved channels is a challenging problem due to its complex three-dimensional flow structure. Despite the numerous investigations that have been performed on this important topic over the last several decades, there remains much to be understood. The focus of this dissertation is on flow around curved channel bends with an emphasis on the use of three-dimensional numerical simulations to provide insights on the flow dynamics in channel bends. In particular, the answers to the following two main questions are sought: 1) when is it appropriate to use the rigid lid assumption for simulating flow around bends?; and 2) what is (are) the most relevant parameters for quantifying the enhanced shear stress in channel bends from a practical standpoint? A computational fluid dynamics framework was developed using the ANSYS Fluent code and validated using experimental flume data. Following the validation study, a total of 26 simulations were performed and the results analysed in an attempt to answer the two main questions. In an attempt to answer the first question, a broad parametric study was conducted using both free surface resolving simulations as well as simulations that make use of the rigid lid assumption. It is shown that the two main parameters that appear to control the flow dynamics in a bend are the maximum bend angle, expressed as the ratio of the length of the channel bend Lc to its radius of curvature Rc, and the upstream Froude number. Analysis reveal when that Lc/Rc ≥ π/2, the curvature effects begin to dominate the dynamics and the error between the free surface model and the rigid lid model dramatically increases regardless of the value of the Froude number. The study calls for caution to be used when using the rigid lid assumption and indicates that this assumption should not be used for simulating flows when Lc/Rc ≥ π/2, especially for sharply curved channels with a radius of curvature to top width ratio Rc/Tw < 2. The increase in shear stress is commonly expressed as a Kb value, which is simply the ratio of shear stress in a bend of the channel to the averaged approach shear stress in a straight channel. The results from the parametric study show that the conventional approach for parameterizing Kb as a function of Rc/Tw, where Rc is the radius of curvature and Tw is the channel top width, appears to be inadequate because the distributions in the Kb values exhibit significant scatter for small changes in Rc/Tw i.e. for flow around sharply curved bends. Dimensional analysis reveals that for a given channel cross-section, constant flow rate, bed slope and channel bed roughness, Kb depends on both Lc/Rc and Rc/Tw. In this study, the combined effects of these two parameters were investigated. It is shown from the parametric study that the magnitude of the shear stress increases as a function of Lc/Rc and reaches an asymptotic limit as Lc/Rc > π/2, for Rc/Tw < 2. The study also highlights that the location of the maximum shear stress occurs in the inner (convex) side of the bend for Rc/Tw < 2 but shifts towards the outer bend for Rc/Tw > 2. While the emphasis (and in a sense a limitation) of this study has been mainly on sharp curved bends (Rc/Tw < 2), the analysis can be readily extended to curved bends with Rc/Tw > 2. It is envisaged that such an analysis will lead to a framework for parameterizing Kb in a comprehensive manner that would be useful for practical design guidelines.
Open Access
Time and scale effects in laboratory permeability testing of compacted clay soil
(Colorado State University. Libraries, 1989) Javed, Farhat, author; Shackelford, Charles D., advisor; Jameson, Donald A., advisor; Doehring, Donald O., committee member; Abt, Steven R., committee member
Permeability (hydraulic conductivity) testing of clays in the laboratory typically requires a significant amount of time. It is hypothesized that the time required for clay permeability test can be reduced substantially through a statistical modelling technique known as "time series analysis". In order to test this hypothesis, permeability tests were performed on compacted samples of a silty clay soil in a standard Proctor mold (9.4 x 10-4 m3). The soil was separated into five different fractions representing five ranges in precompaction clod sizes. Constant-head permeability tests were performed on each of these five fractions. Tests were replicated five times for the time series analysis. The results of analysis indicate that time series modelling can significantly reduce statistical error associated with permeability data. It is demonstrated that the time required for clay permeability test can be reduced appreciably through time series modelling. Permeability tests also were performed on four soil fractions in a large-scale (0.914 m x 0.914 m x 0.457 m) double-ring, rigid-wall permeameter. The results of small-scale (Proctor mold) permeability tests indicate that the soil permeability does not vary much with a change in the precompaction clod size. Presence of large clods (> 25 mm), however, may result in side-wall leakage. The large-scale tests indicated that permeability is strongly related to the precompaction clod sizes. Permeability of the soil increased more than two orders-of-magnitude as the maximum precompaction clod size increased from 4.75 mm to 75 mm. Comparison of the results from the small-scale and the large-scale tests indicated that, for all soil fractions, the large-scale permeability was higher by more than an order-of-magnitude. As a result, there appears to be a scale-effect associated with laboratory permeability testing. This scale effect is more significant when soil contains considerable quantity of clods that are large relative to the size of permeameter. These results imply that the large-scale test is more capable of accounting for the hydraulic defects resulting from large clods. A more realistic evaluation of the field permeability of a compacted clay, therefore, may be possible in the laboratory if the permeameter is fairly large relative to the maximum precompaction size of clods present under field conditions.
Open Access
Unification of large-scale laboratory rainfall erosion testing
(Colorado State University. Libraries, 2014) Robeson, Michael D., author; Thornton, Christopher I., advisor; Abt, Steven R., committee member; Watson, Chester C., committee member; Williams, John D., committee member
Water pollution degrades surface waters making them unsafe for drinking, fishing, swimming, and other activities. The movement of sediment and pollutants carried by sediment over land surfaces and into water bodies is of increasing concern with regards to clean waters, pollution control, and environmental protection. Due to increasing environmental concerns about sediment in water bodies derived from construction sites, along with increasingly stringent United States Environmental Protection Agency (USEPA) regulations, it is imperative to be able to have a uniform means to compute soil loss determined at large-scale laboratory rainfall-induced erosion facilities that can eventually be applied to construction sites. This dissertation utilized bare-soil data from the most commonly-utilized large-scale rainfall testing laboratories in the erosion-control industry to develop a unifying prediction equation that can be utilized to provide a proper foundation for determining design parameters to meet USEPA stabilization requirements. The developed equation was determined to be a function of the following key parameters: rainfall intensity, plot area, duration, slope gradient, median raindrop size, raindrop kinetic energy, percentage of clay in the soil, and compacted soil percentage. The developed equation for the prediction of rainfall-induced soil loss, developed from sixty-eight data points collected for this study, had a coefficient of determination (R2) of 0.88. The prediction equation unifies large-scale laboratory rainfall erosion testing and provides a means to determine the appropriate design parameters for USEPA stabilization requirements.

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