Browsing by Author "Ettema, Robert, advisor"
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Item Open Access Air concentration and bulked flow along a curved, converging stepped chute(Colorado State University. Libraries, 2020) Biethman, Blake W., author; Ettema, Robert, advisor; Thornton, Christopher, advisor; Wohl, Ellen, committee memberThis thesis focuses on the air-entrainment performance of a stepped spillway of unique form. The performance was determined using a hydraulic model constructed at a length scale (prototype length/model length) of 24. The new stepped spillway is part of the Gross Reservoir Expansion (GRE) project, which by 2025 is expected to raise the existing Gross Dam about a third of its current height. The stepped spillway will be the tallest stepped spillway in the United States. The model spillway consisted of a chute whose step dimensions, vertical to horizontal, were 0.051 m by 0.025 m, resulting in a chute slope (V:H) of 2.0 and a chute angle of 63.4°. Additionally, the chute conformed, in planform, to the curved planform of raised Gross Dam. At the spillway's crest, that radius of curvature, at model scale, was 22.2 m. The chute width converged by about 20% from the top of the chute to the stilling basin at the base of the chute. The chute's steepness, height, curvature and convergence made the chute's geometry unique among existing stepped spillways. The evaluation involved measurements of air entrainment and flow velocity along the stepped chute, for which the skimming flow regime prevailed for discharge larger than about 9% of the spillway's design discharge. To date, the effect on water flow and air entrainment of chute curvature in stepped spillways had not been investigated. The investigation was facilitated from measurements obtained using a dual-tip conductivity probe, which detected the instantaneous void fraction of the air-water mixture. The probe also enabled measurement of the velocity of the bulked flow along the chute. The study showed that, when the chute conveyed the design discharge (at model scale, 0.347 m3/s), streamwise values of air concentration and flow depth (bulked with entrained air) were basically constant near the bottom of the chute. Additionally, the chute's planform curvature resulted in non-uniform flow across the chute. At the design discharge, and near the bottom of the chute, the flow depth along the chute's centerline was nominally about 30% greater than the flow depth at the sidewall. When the chute's curvature was accounted for, the water surface along the centerline of the chute was approximately level with the water surface near the sidewall. Further, the depth-averaged concentration of entrained air near the bottom of the chute decreased with increasing water discharge. The chute's converging sidewalls mildly affected the flow near the sidewalls, causing slight increases in flow depth and reductions in flow velocity. These changes, though noticeable, were negligible in terms of spillway performance because of their magnitude.Item Open Access Application of large-scale particle image velocimetry at the Hydraulics Laboratory of Colorado State University(Colorado State University. Libraries, 2018) Chen, Kaiwei, author; Ettema, Robert, advisor; Thornton, Christopher, committee member; Nelson, Peter, committee member; Landers, Stuart, committee memberLarge-scale particle image velocimetry (LSPIV) is a nonintrusive technique used to measure free-surface velocities of water flow in a manner that produces a two-dimensional vector field of flow velocity. LSPIV is gradually becoming quite widely used as a technique for measuring flow velocities in a range of flow areas. This study used readily available material and devices, and software, to apply LSPIV to flow fields in two laboratory flumes at the Hydraulics Laboratory of Colorado State University; LSPIV had not been used in this laboratory before this study. The applications used pieces of paper as tracer floats in the flow field, and employed a standard iPhone 6s to record video of the tracers moving in the flow field. The video record of tracer movements was then analyzed using Fudaa LSPIV software and Tecplot 360 software to calculate and present the flow velocity data. The applications demonstrated the utility of the LSPIV technique for determining the free-surface flow patterns, and their variations, in experiments done at the Hydraulics Laboratory. Additionally, this study examined the relationship between the tracer size and LSPIV accuracy with the objective of identifying an optimal width of tracer relative to the width of the flow field and its features. Five sizes of tracer were used in measuring the water-surface flow field through a series of contractions and expansions. It was found that the best tracer size is about from 3.80% to 6.33% of the wide of the channel.Item Open Access Application of large-scale particle image velocimetry to entrance flows(Colorado State University. Libraries, 2021) Fakhri, Alireza, author; Ettema, Robert, advisor; Thornton, Christopher, committee member; Bond, Laurel, committee memberThis thesis presents the findings of the application of Large-Scale Image Velocimetry (LSPIV) to illuminate three entrance flows. LSPIV is an image-based method that non-invasively measures two-dimensional instantaneous free-surface velocities of water flow using video equipment. Three different applications used in this study are a flume study with three different contraction ratios, flow through and over spillways in hydraulic models for Gross Dam and Los Vaqueros Dams. For the first application, large-scale-particle velocimetry (LSPIV) was applied to estimate the top-width of the vena contracta formed by an approach open-channel flow entering a contraction of the channel. The experiments investigated the requisite dimensions of two essential LSPIV components: the Search Area and Interrogation Area, to establish the optimum range of these components for use in LSPIV application to contractions of open-channel flows. Of practical concern (e.g., bridge hydraulics) is flow contraction and contraction scour that can occur in the vena contracta region. The thesis showed that optimum values for the Search Area (SA) and Interrogation Area (IA) were 10 and 60 pixels, respectively. Also, the study produced a curve indicating a trend for vena-contracta width narrowing with a variable ratio of approach-channel and contracted-channel widths and varying bed shear stress of approach flow. For the other two applications, the hydraulic models of the spillways for Gross Dam and Los Vaqueros Dam, LSPIV was applied to assess the robustness of LSPIV on mapping the streamlines through and over spillways.Item Open Access Baffle-post structures for flow control in open channels(Colorado State University. Libraries, 2015) Ubing, Caroline, author; Thornton, Christopher, advisor; Ettema, Robert, advisor; Bledsoe, Brian, committee member; Wohl, Ellen, committee memberThis thesis presents theory and laboratory findings regarding the hydraulic performance of baffle-post structures used as a means for controlling flow in open channels. Such structures comprise one to two parallel rows of posts that extend slightly higher than the anticipated depth of flow, and offer a useful means for retarding flow in various channel situations where there is a need to reduce flow energy, possibly to reduce flow capacity to transport bed sediment and manage channel morphology. Observations and data regarding headloss and discharge coefficients and backwater flow profiles associated with varying structure geometry were obtained so as to determine the extent to which a baffle-post structure will retard an approach flow and reduces its capacity to convey bed sediment. The creation of a M₁ gradually varied flow profile in the upstream reach complicates the use of headloss to characterize hydraulic performance of the baffle-post structures. Instead, the parameter, y₁/y₀, offers a practical means for describing such performance; y₁= flow depth at the upstream face of the structure, and y₀= the depth of uniform flow prior to use of a structure. The most influential geometric variable was influencing structure performance was the lateral spacing between posts, s; it is expressed non-dimensionally as s/D, where D = post diameter. Qualitative results regarding sediment transport confirm a reduction in bed-sediment transport rate upstream of the structure. However, the turbulent flow structures at the baffle-post structures promote local scour at the base of such structures. Due to the flow acceleration between posts, baffle-posts structures could potentially obstruct fish and other aquatic life passage along the channel.Item Open Access Bed sediment transport and channel morphology in a braided channel: insights from a flume experiment(Colorado State University. Libraries, 2017) Armstrong, Dylan L., author; Ettema, Robert, advisor; Nelson, Peter, committee member; Falkowski, Michael, committee memberThis thesis presents the methods and findings from an experiment aimed at relating the rate of bed-sediment transport through a reach of a braided channel to the intensity of the braiding sub-channels (anabranches) along the reach. The experiment was conducted in a large flume located at Colorado State University's Hydraulics Laboratory in Fort Collins, Colorado. No similar flume experiments have been conducted involving braided channels in a wide alluvial plain. Such experiments involve several challenging considerations that greatly complicate such experiments: braided channels are characteristically wide and shallow; have relatively large bed-sediment loads that are difficult to measure, because they move in multiple sub-channels; and the sub-channels (often termed anabranches) are ephemeral. The self-forming nature of the anabranches means that there is little direct control over the exact morphology of the braided channel. The objectives set forth in this experiment overcame the challenges of braided river flume studies, and allowed a comprehensive data set to be obtained of both bed sediment transport data and morphologic braided intensity data. The intensity of braiding was characterized using a braiding index (Flow Width Ratio) developed during this experiment. A relationship was identified and a trend established – as FWR increased, the rate of bed-sediment transport decreased – but the stochastic nature of transport rates and morphology introduced much scatter in the relationship. It was found that local morphologic features have a large impact on the transport of sediment through braided systems, and that the features could help explain some of the scatter in the data.Item Open Access Computational fluid dynamics models of Rio Grande bends fitted with rock vanes or bendway weirs(Colorado State University. Libraries, 2019) Siefken, Seth, author; Ettema, Robert, advisor; Thornton, Christopher, committee member; McGrath, Daniel, committee memberRock vanes (also known as stream barbs) and bendway weirs are two types of transverse rock structures used to modify the flow field in river bends. This study examines the effectiveness of the two types of structures at reducing velocity along the outer bank of river bends to protect the bank from erosion. A numerical model using the commercially available FLOW-3D software was used to evaluate the effect of various rock vane and bendway weir configurations on the flow field through two river bends typical of the Middle Rio Grande. The model was calibrated and validated using data from a previous physical model study of rock vanes. 33 different rock vane configurations were tested in the numerical model to evaluate the effect of altering planform angle, crest slope, projected length, and structure spacing. In addition, 14 different bendway weir configurations were tested to provide a comparison of the relative performance of bendway weirs and rock vanes. The numerical modeling results indicate that rock vanes are more effective at reducing the velocity along the outer bank of a bend than bendway weirs. Modelling showed that the completely submerged crest of bendway weirs allows a substantial amount of flow to pass over the crest, limiting their effectiveness in reducing velocity along the bank. In contrast, rock vanes, with a sloped crest intersecting the waterline at the design flow rate, directed more flow around the tip of the structure rather than over the crest and were much more effective at reducing velocity along the bank. Based on the modeling results, it is recommended that bendway weirs not be installed for the purpose of erosion protection along riverbanks. The reduction in velocity along the bank produced by the various rock vane configurations varied considerably with the geometry of the configuration. Based on the results, the following conclusions are made regarding rock vane geometry: (1) Rock vanes should be installed at a planform angle between 45° and 90° to the river bank. (2) There exists an optimal projected length of rock vane, which lies in the neighborhood of 1/5 to 1/3 of the channel top-width. (3) Rock vanes with a 10% crest slope perform well, although decreasing the crest slope will decrease the velocity along the outer bank and vice versa. (4) Decreasing the spacing of rock vanes decreases the velocity along the outer bank, up to a limit. A design equation was developed to predict the velocity reduction along the bank of a river bend produced by a given configuration of rock vanes, based on the geometry of the rock vanes and the river channel. The equation provided good predictions for the range of configurations tested, having a coefficient of determination r2 = 0.83 and predicting the velocity reduction along the outer bank to within 15 percentage points for all of the tested configurations.Item Open Access Effect of spillway-ogee location on flow uniformity and turbulence at the crest of an ogee weir(Colorado State University. Libraries, 2023) Alsultan, Faisal Abdulaziz, author; Ettema, Robert, advisor; Thornton, Christopher, committee member; Conrad, Steve, committee memberThis study investigated the effect of spillway-crest location relative to spillway entrance, and spillway-abutment shape, on uniformity of flow distribution and turbulence intensity of flow, over the ogee crest of a spillway. These issues are especially of concern for spillways for which the approach flow to the ogee weir is relatively shallow. Circular and elliptical shapes of abutments were used. These abutment forms, and the bathymetry of the reservoir approach to the spillway, cause the approach flow to the ogee weir to be non-uniformly distributed and turbulent for part of the ogee weir. Turbulence can be generated by flow separation from an abutment or by the manner whereby the flow approaches the spillway. In the latter case, the reservoir bathymetry at the spillway entrance is important, as it affects flow distribution at the spillway entrance. The base spillway used for the study was a hydraulic model used to assist in the design of the new spillway for Los Vaqueros Dam on Kellogg Creek near Brentwood, California. The flow approach to this spillway is typical of many, relatively shallow over-flow spillways that involve an ogee crest. As often is the case for spillways associated with embankment dams, the spillway is built on an abutment of the dam itself and must deal with non-uniform and turbulent approach flow from the reservoir retained by the dam to the spillway's location at the side of the dam. Consequently, the approach flow was non uniform and turbulent, and design questions arise as to where to place the ogee crest for the spillway and what shape to use for a spillway. Though the investigation used the spillway just mentioned, the results have general application. The experiments were performed using a spillway flume with a rectangular cross-section, a circular abutment intake and a controllable spillway crest with changing the crest location to five locations (one downstream of the selected location and three upstream of that location). Measurements included water profiles, velocity across transects downstream of the spillway's entrance. These measurements were made for a circular spillway-abutment and for an elliptical spillway-abutment. The results show that changing the crest location significantly affects flow uniformity and possible shed-vortex formation from the intake abutment. Therefore, the results also indicate that spillway crest location has a direct impact on the hydraulic performance of the spillway. Suitable selection of crest location can be used to minimize non-uniformity and vortex-related problems in spillway design. The present study recommends that the ogee crest face be placed at least 1.5 crest widths from the entrance of the spillway. This position enables the flow to the crest to become suitably uniform and turbulence of flow within the entrance to extensively (though not entirely) decay. The findings from this study are significant for engineers and researchers involved in spillway design and generally in many aspects of hydraulic engineering design. The findings also demonstrate the importance of careful consideration of crest location in spillway design to mitigate problems related to vortex formation. Overall, this study adds to the knowledge base regarding spillways and their design. Spillways have been used for hundreds of years but there are many aspects of these hydraulic structures requiring continued research.Item Open Access Flow field at open-channel contractions: insights from a two-dimensional numerical model(Colorado State University. Libraries, 2017) Zey, Scott S., author; Ettema, Robert, advisor; Nelson, Peter A., committee member; Covino, Timothy P., committee memberA depth averaged, two-dimensional numerical model was used to investigate the hydraulics of flow passing through open-channel contractions. The investigation focused on the magnitude and location(s) of maximum velocity of flow entering a contraction. The effective flow width at the entrance of the contraction and the maximum lateral velocity at the contraction entrance were also investigated. The responses of these flow characteristics were studied as values of contraction ratio, channel roughness, bed slope, and transition geometry were varied. The numerical model produced significant new insights. The factors affecting the values and distribution of velocity in a contraction include: channel slope, bed roughness, and contraction shape. The magnitude and location of maximum velocity in the contraction varies with contraction ratio. For contraction ratios milder than approximately 0.5 the velocity maximum occurs at two locations, and at one location for tighter contractions. At a contraction ratio of 0.5 lateral velocity reached a maximum and effective flow width a minimum. Channel slope and bed roughness affect the values and distribution of velocity in a contraction, as did contraction shape. These findings have engineering significance for explaining and estimating scour of alluvial channels in contractions, such as at bridge waterways.Item Open Access Frequency of pressure fluctuations in the stilling basin for the spillway of raised Gross Dam, Colorado(Colorado State University. Libraries, 2021) Tasdelen, Selina, author; Ettema, Robert, advisor; Thornton, Christopher, advisor; Little, Ann M., committee memberGross Dam, Colorado, was constructed in 1954 to provide potable water to the city of Denver, Colorado. The location of Gross Dam is in Boulder County, Colorado. The dam itself is a high, curved concrete gravity-arch dam that retains Gross Reservoir, a reservoir capacity that of volume 51,573, 109.1 cubic meter. The Gross Reservoir Expansion (GRE) Project will increase the height of the Gross Dam from 39.93 m to an ultimate height of 143.56 m by 2025, thereby creating more storage behind the Gross Dam. The new stepped spillway required for GRE will be the highest stepped spillway in the U.S. Besides the height of the spillway, the steepness, the length, and the curved form of the chute will make the spillway stand out. This study focused on (1) determining how roller-rotation frequency varied with water discharge for the full range of the discharges expected for the spillway, (2) determining the main frequencies in the pressure fluctuations at selected locations along the stilling basin, and (3) relating frequency fluctuations of measured pressure to frequencies of features evident in the flow field too and through the stilling basin. This effort involved assessing the influence of flow discharge on the rotation frequency of a major roller formed immediately upstream of the row of baffle blocks for each discharge. The experimental investigation carried out at the Hydraulics Laboratory of Colorado State University, Engineering Research Center, for the current Gross Dam. The frequency of the rotation of the roller formed immediately upstream of the row of the baffle blocks determined approximately from the observation for every flow rate. The mean value of the rotation frequency of the roller formed for the PMF-equivalent discharge down the hydraulic model of the spillway (0.348 m3/s) was 2.45 Hz or 0.5 Hz at prototype scale. The plot of the roller-rotation frequency versus discharge showed that there was a proportional relationship between the rotation frequency and the discharge. The dynamic pressures were measured with the use of four pressure sensors which were positioned in front of the floor, behind the floor, at the face of the baffle block, and the behind the baffle block. The sampling rate of these sensors was 2,500 Hz. The maximum pressure (prototype scale) recorded at the front face of the baffle block when the model-scale flowrate was equivalent to the 1.0 PMF was 59.78 kPa. Low-pass filter applied to the original signal of pressures, and the pressure signal was filtered out at frequencies above 200 Hz (model scale). The cut off frequency of the filtered signal was chosen 200 Hz, as flow oscillations would not occur at this frequency. Then, Fast Fourier Transform (FFT) method was applied to both original and filtered signal. The result showed that filtered FFT gave about the same result as the FFT from the unfiltered data and there was no continuous low frequency or continuous high frequency pattern, indicating that the pressure signals oscillated irregularly, as did the roller formed in the front of the stilling basin. Therefore, FFT could not find the dominant frequency in the signal. The largest peak frequencies at prototype scale for the upstream floor, front face of the baffle block, downstream face of the baffle block, and downstream floor of the stilling basin were 0.496, 1.15, 1.396, and 1.544 Hz, respectively.Item Open Access Live-bed failure modes of bendway weirs and rock vanes in alluvial channels(Colorado State University. Libraries, 2022) Maddocks, Parker, author; Ettema, Robert, advisor; Thornton, Christopher, advisor; Wohl, Ellen, committee memberBendway weirs and rock vanes have been used and refined for decades to control thalweg location and alignment along alluvial channel-bends and decrease flow velocity along the outer bank of such channels. Since the early 2000s, Colorado State University's Hydraulics Lab has assisted the U.S. Bureau of Reclamation (USBR) in refining design guidelines for bendway weirs, rock vanes, and other in-stream rock structures. This effort has entailed optimizing the layout of configurations of bendway weirs and rock vanes. The present study, however, focuses on the failure modes of bendway weirs and rock vanes, and led to the development of refinements to the design recommendations for individual bendway weirs and rock vanes so that such structures can perform as intended, even though the structures have encountered scour. Live-bed conditions were selected for the experiments, as such conditions involve active bed-sediment transport and, thereby, pose more severe conditions than do clear-water conditions in which little bed-sediment transport occurs. To investigate live-bed failure modes at bendway weirs and rock vanes, two flumes were used: a straight flume and a curved flume. The experiments used different parameters suggested in technical literature as documents as affecting bendway-weir and rock-vane performance (e.g., structure geometry, spacing, flow condition, and angle relative to a channel's outer bank). The straight flume was chosen for its capacity to create the constrained flow conditions needed to illuminate the failure modes, which then were verified using the curved flume, which was wider and subject to the effects of flow curvature. Each experiment involved a series of three bendway weirs or rock vanes. Preliminary experiments indicated that three structures were needed, because of observed differences in the failure modes at the three structures in a series. Experimental results revealed that failure modes of bendway weirs and rock vanes were primarily driven by rock dislodgement due to contraction scour at the tip of such structures, and by dune-trough presence at the upstream face (the first rock structure) and flow impingement (against the second rock structure). Also, flow swept some rock from the crest of bendway weirs and rock vanes. The observed failure modes in the straight flume were confirmed by the experiments using the curved flume, though the curved flume's curvature of flow and greater width partially obscured the failure modes. The failure modes led to refinements regarding the design recommendations for the structure of bendway weirs and rock vanes. The recommendations essentially specify the widening and lengthening of the crest of bendway weirs and rock vanes, so that these rock structures may experience controlled failure to accommodate scour but preserve their main dimensions.Item Open Access Live-bed failure modes of bendway weirs and rock vanes in alluvial channels(Colorado State University. Libraries, 2022) Wittmershaus, Alex, author; Ettema, Robert, advisor; Thornton, Christopher, advisor; Kampf, Stephanie, committee memberBendway weirs and rock vanes are instream rock structures primarily used for managing the alignment of a channel's thalweg. Built from rock, bendway weirs and rock vanes are intended to function by directing flow away from a channel's outer bank and thereby reducing flow velocity along the outer bank. The present study investigated how bendway weirs and rock vanes placed in curved, alluvial channels subject to live-bed flow conditions (active bed-sediment transport) may fail. Further, the experiments then sought to recommend design dimensions so that bendway weirs and rock vanes accommodate failure (and loss of rock), thereby enabling them to continue performing as intended. A curved flume was constructed in Colorado State University's Hydraulics Laboratory to conduct experiments that illuminated the failure modes and to confirm (or modify) preliminary design recommendations obtained from experiments using a straight flume fitted with three bendway weirs or rock vanes. The curved flume experiments involved a series of six bendway weirs or rock vanes and used a hydrograph procedure to simulate the rising limb of a hydrograph of flow along a medium sized river like the Middle Rio Grande; the proportions of the flume were like selected bends in that river. Six bendway weirs or rock vanes were needed to direct flow around the curved flume, as opposed to the need for three bendway weirs or rock vanes in the experiments in the straight flume. Two sizes of non-uniform bed sediment also were used (a medium sand and very coarse sand) for the experiments. The two sands were used to see if bed sediment size affected the failure modes. The experimental results showed that bendway weirs and rock vanes experienced rock dislodgement primarily via contraction scour, which undermines the end, or tip, of these instream structures. Destabilized rock then tumbles into the scour zone along the channel's shifted thalweg, armoring the bed. This observation was observed for both the beds comprised of medium sand and very coarse sand. As flow depth increased above the mean elevation of the bendway weirs or rock vanes, contraction of flow reduced as more flow passed over the structures. The flow field at each bendway weir or rock vane changed. The hydrograph procedure yielded similar changes in bed bathymetry for beds of medium sand and very coarse sand over the rising limb of the hydrograph. When (Δy+H)/H = 0.75, a deep scour hole formed in between the first two structures in the configuration within about 15 minutes. Then, when (Δy+H)/H = 1.25, the scour hole was partially filled with sediment and extended downstream largely along the series of bendway weirs or rock vanes. Further, when (Δy+H)/H = 2.0, the scour hole was again partially filled with sediment, but scour extended along the entire configuration of bendway weirs or rock vanes, thereby delineating a defined thalweg. As the flow depth increased, the maximum scour depth along the thalweg decreased for the experiments. The bendway weirs and rock vanes experienced structural deformation due to rock dislodgement primarily from contraction scour. Less rock dislodgement occurred for these instream structures placed on the medium sand than when on the very coarse sand. Also, the rock vanes experienced less rock dislodgement than did the bendway weirs in general. This finding is attributed to upwards slope of the crest of rock vanes; the sloped crest directed more flow around each rock vane and over the already armored bed. The results from using the hydrograph procedure in a curved flume confirmed the preliminary design recommendations from the straight flume. The design recommendations required that bendway weirs or rock vanes be lengthened by 2d100 and their crests be widened by d100; here d100 is the diameter of the largest rock used to build bendway weirs or rock vanes. This lengthening and widening accounts for the shortening and narrowing of bendway weirs or rock vanes subject to scour. A prior study recommended the size of rock chosen in design to form bendway weirs or rock vanes.Item Open Access Observations from a series of flume experiments on contraction scour along a rectangular channel(Colorado State University. Libraries, 2020) Nowroozpour, Alireza, author; Ettema, Robert, advisor; Julien, Pierre, committee member; Nelson, Peter, committee member; Zevenbergen, Lyle, committee member; Gallen, Sean, committee memberTo view the abstract, please see the full text of the document.Item Open Access The effects of bend radius on flow around a configuration of bendway weirs: insight from a numerical model(Colorado State University. Libraries, 2019) Hogan, Taylor, author; Thornton, Christopher, advisor; Ettema, Robert, advisor; Williams, John, committee memberBendway weirs have been used and refined for decades by hydraulic engineers to control thalweg location within alluvial rivers and to decrease flow velocity along the outer bank of channel bends. Although these structures have been used in a variety of applications, there are still a wide range of acceptable design parameters that vary in accordance with the specific design methodology being used. Since the early 2000s, Colorado State University's Hydraulics Lab has assisted The U.S Bureau of Reclamation (USBR) in refining the design of bendway weirs and similar in-stream rock structures. During this period of time, Colorado State University and The USBR have utilized hydraulic and numerical models to develop systematic design guidelines for bendway weirs and other in-stream rock structures. Hydraulic modeling has also provided a large database of velocity and water surface measurements that have been used to calibrate and validate subsequent numerical models. The partnership between Colorado State University and the USBR has led to design recommendations and equations in which the effect of many variables and their sensitivity in overall bendway weir design has been identified. This study investigates the parameter radius of curvature over channel top width, Rc/Tw, and its effect on the flow field around bendway weirs, as its significance in bendway weir design is not well known. To investigate the effects of Rc/Tw on the bendway weir flow field, the 2D numerical model SRH-2D was used in conjunction with AutoCAD Civil3D software. The SRH-2D model was created using the bathymetry of the hydraulic model and then also calibrated and validated using data collected in the hydraulic model. AutoCAD Civil3D was used to create four different bend radii while holding Tw constant, representing Rc/Tw values between 3.0 and 8.0 which are typical of the Middle Rio Grande that the hydraulic model represented. Two additional trapezoidal channel models were also created to isolate the possible effects from specific channel geometry on the bendway weir flow field comparisons. 2D numerical modeling results revealed that the bend radius of curvature had negligible effect on the bendway weir flow field. Velocity patterns in the trapezoidal and native bathymetry channels changed negligibly in location and magnitude across varying bend radii. Cross-sectional velocity distributions were also evaluated and showed that the inner and middle third lateral sections of the channel showed the same (within fractions of a percent) velocity increase after the installation of bendway weirs. The outer fifth of the channel resulted in 6% velocity decrease only varying approximately 0.1% between bend radii. Overall numerical modeling results showed that the bendway weir flow field was negligibly affected by the bend radius of curvature, Rc.Item Open Access The effects of scour on the flow field at a bendway weir(Colorado State University. Libraries, 2019) Garfield, Mason, author; Ettema, Robert, advisor; Thornton, Christopher, committee member; Wohl, Ellen, committee memberBendway weirs (BWs) are rock structures commonly used for controlling the thalweg location in alluvial channels, especially bends. Although there are many studies analyzing the effect of BWs on bend flow fields, there is little known about the effects of scour on the modified flow field. Many physical and numerical models of BWs use a fixed bed with the existing river bathymetry, assuming that the effects of scour are negligible. This study analyzed the effects of the scour on the flow field using SRH-2D, a 2-dimensional numerical model solving the full St. Venant equation (also termed the dynamic wave equation). In addition, the study used small scale and large scale hydraulic models and a 3-dimensional numerical model, FLOW-3D, to validate and calibrate the 2-D model. Using FLOW-3D, the 3-dimensional flow field was used to identify areas where SRH-2D was accurate and where it was lacking. The small scale hydraulic model was used to determine the effect of streamwise blockage on scour using plates. Results from the small scale hydraulic model indicate that when the degree of streamwise blockage (L/B where L is the width of the plate and B is the width of the flume) and submergence (Y/H where Y is the flow depth and H is the height of the plate) remained the same, but the acute angle to the flume wall varied (α), the maximum scour depth did not change significantly. Accordingly, the scour morphology from the tests with α = 90° could be applied to tests with α = 30° and 60° using the 2-D model. When comparing the flow field using SRH-2D to FLOW-3D, it was found that the maximum depth-averaged velocity was closer to FLOW-3D's when Y/H = 1.25 than when Y/H = 2.0. This finding likely results from the greater effect of blockage directing the flow around the BW than the deeper case, where the flow has a higher vertical velocity over the top of the BW. The 2-D model cannot take this into account, leading to error. The pre- and post-scour maximum velocity magnitude and locations were compared to determine the effect of scour on the flow field using the 2-D model. A deep flow depth (Y/H = 2.0) and a shallower flow depth (Y/H = 1.25) were run for different BW orientations, (α=30°,60°,and 90°) to determine the applicability of the effect of scour on the flow field. The analysis found that, when Y/H = 2.0, the difference in the maximum velocity magnitude and location between pre-and post-scour were fairly negligible for all orientations, whereas when Y/H =1.25, the effects of the scour on the flow field were more prominent. Overall it was found that the scour morphology is important to take into consideration for a fixed bed numerical or physical model when analyzing flow depths slightly overtopping the BW, but has little effect when the flow is twice the size of the BW.Item Open Access The role of dynamic ice-breakup on bank erosion and lateral migration of the Middle Susitna River, Alaska(Colorado State University. Libraries, 2018) Vandermause, Renee A., author; Ettema, Robert, advisor; Zevenbergen, Lyle, advisor; Rathburn, Sara, committee memberRivers in northern, boreal regions experience frigid winters and ice formation that influence several geomorphic processes including bank erosion and lateral channel migration. Not only are the ice-driven processes complex and highly variable in time and space, but they are difficult to observe due to the logistical challenges of conducting fieldwork when the rivers are frozen in the winter and during ice breakup in the spring. Yet, characterizing and quantifying the processes that drive bank erosion, whether during the summer open-water period or when ice is in the channel, is important for predicting channel dynamics in the boreal rivers where there is a mixed ice-fluvial regime. Of particular importance is understanding the erosional processes that form or maintain complex channel and riparian systems which in turn provide diverse aquatic habitat for a range of salmonid species. This study capitalized on an extensive archive of field data and observations, of which the author was involved, to quantify the extent to which channel change is driven by ice and fluvial processes on the Susitna River, a large gravel-cobble bed river in south-central Alaska. As bank erosion is a key element of channel change, this study focused on bank erosion along the middle reach of Susitna River located downstream of a proposed dam site. Using aerial photography and videography over two one-year periods, 2011 to 2012 (included a thermal ice-breakup) and 2012 to 2013 (included a dynamic ice-breakup), the study identified short-term erosion rates for distinct geomorphic reaches, determined the amount of erosion by the type of geomorphic surface, and quantified when the most bank erosion occurred annually; whether during the open-water season or when ice was in the channel, particularly during ice-breakup. The aerial imagery was supplemented by observations and data collected along the Middle Susitna River over two field seasons. The study also used a synthesis of observations and 2-D depth averaged fluvial modeling using SRH-2D to characterize processes that drive bank erosion. This study found that the majority of bank erosion, 54 to 61 percent by sub-reach, occurs or is initiated over a short period of time during dynamic breakup of the river's ice cover. The dominant erosion process is attributable to the combination of relatively high-water discharge and the presence of ice floes and ice rubble. Vegetated bars and terrace margins were the most susceptible to bank erosion, notably by impacting ice floes. Erosion of banks adjoining floodplain surfaces, partly protected by vegetation rootmats and by shear walls of smaller ice rubble, accounted for less overall erosion than vegetated bars or terraces. Wide sub-reaches with multiple channels were prone to ice-jamming, diversion of flow conveying ice into side channels, and localized pockets of subsequent bank erosion. Bank erosion occurred less in predominantly single-channel reaches than in the predominantly multi-channel reaches. Rates of bank erosion along the Middle Susitna River are relatively low in comparison to bank erosion rates measured along gravel-bed rivers generally. This may partly be explained by the low frequency of dynamic, ice-breakup events that initiate large-scale bank erosion. It may also be due to protective effects of ice that inhibit bank erosion such as formation of gravel-cobble pavements at bank toes, or protective effects from vegetation such as root-reinforced top of bank layers that slump over cantilevered banks effectively providing vegetated rip-rap to an exposed bank face.