Browsing by Author "Thornton, Christopher I., advisor"

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Open Access
Bulking coefficients of aerated flow during wave overtopping simulation on protected land-side slopes
(Colorado State University. Libraries, 2016) Scholl, Bryan N., author; Thornton, Christopher I., advisor; Abt, Steven R., advisor; Hughes, Steven A., committee member; Venayagamoorthy, Subhas K., committee member; Kampf, Stephanie K., committee member
Post hurricane Katrina there has been more interest in erosion on the landward side of levees resulting from wave overtopping during storm events. The development of wave overtopping simulators has enabled more rigorous evaluation of levee armoring alternatives under controlled conditions similar to those on levees. Steady state overtopping studies have demonstrated a reduction in shear stress due to air entrainment in the flow. There has not been an evaluation of air entrainment during wave overtopping simulation. For this reason, a study was conducted to quantify flow bulking occurring during wave overtopping simulation. Testing was conducted at the Hydraulics Laboratory at Colorado State University at the Engineering Research Center using a wave overtopping simulator. The simulated levee was 6 ft wide. Levee geometry in the direction of flow was a 13.2 ft. horizontal crest, 30.5 ft levee face with 3:1 (horizontal:vertical) slope and 12.2 ft berm with 25:1 slope. Un-bulked flow thickness was measured with “surfboards” which hydroplane along the surface of flow. Bulked flow thicknesses were measured using visual observations of maximum flow thickness on eight staff gages along the wall of the simulated levee. Wave volumes ranged from 20 ft3/ft to 175 ft3/ft. Conservation of mass and testing repeatability is demonstrated. Bulking values range from zero for the smallest wave volumes to over 100% for the largest wave volumes. An empirical model is developed to estimate bulking on the 3:1 levee slope. A comparison is made to steady state flows with similar air entrainment. The effect of bulking on shear stress is a potential decrease in shear stress over 50% relative to un-bulked flow thickness. A method to incorporate wave overtopping bulking into design is proposed using a cumulative work approach.
Open Access
Hydraulic effects of biofilms on the design and operation of wastewater forcemains
(Colorado State University. Libraries, 2016) Michalos, Christopher T., author; Thornton, Christopher I., advisor; Grigg, Neil S., committee member; Julien, Pierre Y., committee member; Williams, John D., committee member
The impact of biofouling on wastewater forcemains is generally not accounted for in current design practice and little information is available in literature regarding the effect of wastewater biofilms on forcemain hydraulics. In practice, many engineers select a clean water, new pipe roughness factor, to perform hydraulic calculations which may lead to under-sizing wastewater lift station pumps. Forcemains have to cope with a particularly challenging task; they have to ensure that solids contained in the wastewater (sand, gravel, organics) are readily transported along with the wastewater. Forcemain design standards generally recommend a velocity of 2.0 ft/s (0.6 m/s) to prevent deposition of solids and a velocity of 3.5 ft/s (1.1 m/s) to re-suspend solids that may have settled. To further complicate forcemain design and operation; wastewater lift station pumps generally operate intermittently which requires remobilization of any material that may have settled while the pumps remain idle. Therefore, forcemains must be designed to be self-cleaning in order to prevent solids deposition which could cause increased sulfide production leading to corrosion and odor issues; loss of capacity through a reduction of cross sectional area; or even blockage at low points, or at the toe of an adversely sloped pipe leading to costly removal. The goal of this research is to identify short-comings in current forcemain design practice by 1) evaluating the hydraulic effect of biofilms on the absolute roughness (ks) of forcemains; 2) evaluating the hydraulic effect of biofilms on Hazen-Williams C factor; and 3) determine critical velocity required for sediment transport, air clearing, self-cleansing, and optimal diameter of forcemains, which are not identified in forcemain design standards. Operational data were collected and evaluated for 20 municipal wastewater forcemains located in the United States. Data from previous studies, academic research, reports, and published papers were used to supplement and support research findings. A total of 415 data points obtained from 68 forcemain systems ranging from 3- to 66 inches in diameter were evaluated as part of this research. Results of the hydraulic analysis determined that 44% of the systems evaluated were operating at velocities between 2- and 3.5 ft/s and 16% of systems were operating at velocities less than 2 ft/s; indicating that these systems are over designed and do not provide sufficient velocity to re-suspend solids promoting sedimentation. The hydraulic effect of biofilms on forcemain flow resistance was evaluated and determined that ks and C factor varied with forcemain velocity. Calculated values of ks ranged from approximately 35 mm to 0.01 mm, with larger values occurring at velocities less than 1 m/s (3.3 ft/s). The upper range of ks values are orders of magnitude larger than the standard clean water, new pipe ks value found in literature. C factor results ranged from approximately 30 to 150; approximately 60% of forcemain systems evaluated are operating at C factors less than 100, which is much lower than the recommended values of 130 – 150, depending on pipe material. Results suggest that biofilms effect forcemains in a similar manner regardless of pipe diameter, material, or age. Although velocity was determined to be the principle factor affecting ks and C factor; a comparison of the C factor results to ks results show that C factor is dependent upon both velocity and diameter. Equations were developed to estimate ks and C factor and should be utilized along with the Colebrook-White / Darcy-Weisbach and Hazen-Williams equations to estimate the friction headloss for forcemains. The required design velocity for self-cleansing, sediment transport, air clearing, and economical diameter ranges from approximately 4- to 11 ft/s, depending on diameter. Selecting a design velocity between 2 ft/s (0.6 m/s) and 3.5 ft/s (1.1 m/s) may not be appropriate and the minimum design velocity should be selected upon either the self-cleansing velocity or economical pipe sizing. Although each system should be evaluated to determine the correct minimum design velocity based upon the proposed system properties, these results indicate that the minimum forcemain design velocity should be at least 5 ft/s (1.5 m/s).
Open Access
Methodology for calculating shear stress in a meandering channel
(Colorado State University. Libraries, 2010) Sin, Kyung-Seop, author; Thornton, Christopher I., advisor; Julien, Pierre Y., committee member; Wohl, Ellen E., 1962-, committee member
Shear stress in meandering channels is the key parameter to predict bank erosion and bend migration. A representative study reach of the Rio Grande River in central New Mexico has been modeled in the Hydraulics Laboratory at CSU. To determine the shear stress distribution in a meandering channel, the large scale (1:12) physical modeling study was conducted in the following phases: 1) model construction 2) data collection 3) data analysis, and 4) conclusion and technical recommendations. Data of flow depth, flow velocity in three velocity components (Vx, Vy and Vz) and bed shear stress using a Preston tube were collected in the laboratory. According to the laboratory data analysis, shear stress from a Preston tube is the most appropriate shear stress calculation method. In case of the Preston tube, data collection was performed directly on the surface of the channel. Other shear stress calculation methods were based on ADV (Acoustic Doppler Velocity) data that were not collected directly on the bed surface. Therefore, the shear stress determined from ADV measurements was underestimated. Additionally, Kb (the ratio of maximum shear stress to average shear stress) plots were generated. Finally, the envelope equation for Kb from the Preston tube measurements was selected as the most appropriate equation to design meandering channels.
Open Access
Methodology for predicting maximum velocity and shear stress in a sinuous channel with bendway weirs using 1-D HEC-RAS modeling results
(Colorado State University. Libraries, 2010) Sclafani, Paul, author; Thornton, Christopher I., advisor; Watson, Chester C., committee member; Wohl, Ellen E., 1962-, committee member
The Middle Rio Grande is a 29-mi reach of the Rio Grande River in central New Mexico that extends from downstream of Cochiti Dam to Bernalillo, New Mexico. A series of anthropogenic factors including the construction of flood control levees and Cochiti Dam have altered the historically-braided morphology of the Middle Rio Grande to a more sinuous, degrading reach, with less overall channel migration within a natural floodplain area. Concentration of flow within an incised channel has caused areas of bank erosion and threatened riverside infrastructure, farmland productivity, irrigation systems, levee function, aquatic habitat, and riparian vegetation. Colorado State University (CSU) constructed an undistorted 1:12 Froude scale, fixed bed, physical model consisting of two channel bend geometries that are characteristic of the Middle Rio Grande reach below Cochiti Dam. Small rock structures extending from the outer bank of the bend into the main channel, referred to as bendway weirs, were constructed within each bend to research methods of stabilizing the outer bank with minimal disruption of sensitive habitat and riparian vegetation. Bendway weirs deflect current from the bank in which they are installed to the center of the channel, thus, moving erosive forces away from a degrading bank, establishing a stable channel, and providing or maintaining aquatic habitat between weir structures. Placement of bendway weirs along a river bank effectively creates two zones of flow: 1) the main or constricted flow where the velocity, shear stresses, and potential for channel degradation are increased, and 2) the area between weirs where velocities and shear stresses are greatly reduced and sediment deposition is encouraged. Design criterion to predict increases in velocity and shear stress caused by placement of bendway weirs in a channel bend has not yet been established. Two-dimensional and three-dimensional computer models have been utilized to describe complex flow phenomena associated with bendway weirs in channel bends; however, such computer models may not be practical for typical design projects (Jia et al., 2005; Molls, et al., 1995; Abad et al., 2008; Seed, 1997). Because of historic precedence, continual development, and prevalence in the engineering community, many engineers use one-dimensional (1-D) computer modeling tools, such as Hydrologic Engineering Center's River Analysis System (HEC-RAS), as a first choice in modeling channel flow. 1-D computer models were developed for the trapezoidal channel geometry present in the physical model and for fifteen weir configurations constructed during testing at CSU. Computed results from the 1-D models were compared to data collected from the Middle Rio Grande physical model. Regression relationships were developed to predict velocities and shear stresses in the trapezoidal channel constructed for physical testing at CSU, at the tips of the constructed bendway weirs, and along the inner bank opposite the constructed bendway weirs. From predictive regression relationships for the velocity and shear stress in channel bends, with and without bendway weirs, a four-step design process was developed to provide practicing engineers with guidance that can be used to design bendway weir fields.
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 model of sediment transport in sediment bypass tunnels: influence of transverse slope in tunnel bend
(Colorado State University. Libraries, 2025) Brown, Jesse, author; Thornton, Christopher I., advisor; Ettema, Robert, advisor; Dumitache, Ciprian, committee member
Sediment Bypass Tunnels (SBTs) convey sediment around reservoirs, increasing reservoir lifespan by greatly reducing reservoir sedimentation and, thereby, mitigating consequent loss of reservoir water-storage capacity. To keep SBTs small and economical in cross-section, SBTs convey super-critical flows. Consequently, SBTs convey super-critical flows with large sediment loads, typically containing high concentrations of coarse particles of sediment that can cause abrasion of SBT liners. Especially vulnerable are SBT reaches where secondary currents develop, notably SBT bends. The sediment abrasion that occurs along the invert of a bend requires expensive, frequent replacement of the invert's concrete liner. Consequently, the abrasion rate of inverts and, therefore, bend flow fields are of interest to SBT designers. SBT design variables such as sediment-size distribution, invert-liner type (usually concrete), flow cross-section dimensions, tunnel slope and bend radius can affect sediment abrasion in an SBT, doing so by influencing flow field, secondary currents, and patterns of sediment abrasion. This study focuses on sediment abrasion of SBTs:1. The flow field generating secondary currents associated with free-surface flows along SBT bends; and 2. Banking of an SBT invert to reduce sediment abrasion. The concept of invert-banking was proposed in personal communications with Dr. Ismail Albayrak of the Federal Institute of Technology (ETH), Zurich, Switzerland. The concept was floated during a SBT site inspection in April 2024. The problem of sediment abrasion is a problem for hydraulic structures in mountainous regions such as Switzerland and parts of the United States (e.g., Muller-Hagermann et al. 2020; Melesse et al. 2023). The present study uses the Computational Fluid Dynamics (CFD) code OpenFOAM to create a numerical model of an existing SBT for which hydraulic-model and field data and observations exist. The numerical model was used with the solver interFoam, and the renormalization group (RNG) k-ε turbulence flow assumption, the volume of fluid (VOF) method, and a Discrete Element Model (DEM) coupling. Of focal interest in the modeling was the pattern of secondary flow in a bend whose invert had variable transverse sloping. The prototype bend selected for this study is Switzerland's Solis SBT. The pattern of secondary flow in the bend affected the distribution of sediment across the bend's invert and, therefore, the sediment abrasion experienced by the bend. The Solis SBT, part of the sediment control system used for Solis Reservoir, was chosen for this study because of data and observations availability. Built in 2012, the invert of a bend in the Solis SBT has experienced severe abrasion owing to sediment. This study recommends a small amount of banking in the Solis SBT and other tunnels with similar hydraulic properties. Even a 1% to 2% slope appears to have a substantial effect in distributing the sediment evenly.
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
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.