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Browsing by Author "Abt, Steven, advisor"

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    Analysis of riprap design methods using predictive equations for maximum and average velocities at the tips of transverse in-stream structures
    (Colorado State University. Libraries, 2014) Parker, Thomas Richard, author; Thornton, Christopher, advisor; Abt, Steven, advisor; Williams, John, committee member
    Transverse in-stream structures are used to enhance navigation, improve flood control, and reduce stream bank erosion. These structures are defined as elongated obstructions having one end along the bank of a channel and the other projecting into the channel center and offer protection of erodible banks by deflecting flow from the bank to the channel center. Redirection of the flow moves erosive forces away from the bank, which enhances bank stability. The design, effectiveness, and performance of transverse in-stream structures have not been well documented, but recent efforts have begun to study the flow fields and profiles around and over transverse in-stream structures. It is essential for channel flow characteristics to be quantified and correlated to geometric structure parameters in order for proposed in-stream structure designs to perform effectively. Areas adjacent to the tips of in-stream transverse structures are particularly susceptible to strong approach flows, and an increase in shear stress can cause instability in the in-stream structure. As a result, the tips of the structures are a major focus in design and must be protected. Riprap size is a significant component of the design and stability of transverse in-stream structures, and guidance is needed to select the appropriate size such that the structure remains stable throughout its design life. The U.S. Bureau of Reclamation contracted the Engineering Research Center at Colorado State University to construct an undistorted 1:12 Froude scale, fixed bed, physical model of two channel bend geometries that are characteristic of a reach of the Rio Grande River south of the Cochiti Dam in central New Mexico. A series of factors including the construction of the Cochiti Dam and control levees has caused the historically braided river to meander and become more sinuous. Bank erosion threatens farmlands, irrigation systems, levee function, aquatic habitat, and riparian vegetation. The purpose of the model was to determine the effectiveness of in-stream structures in diffusing the magnitude of forces related to bank erosion. Multiple configurations of transverse in-stream structures with varying x, y, and z parameters were installed in the model, and velocity and shear stress data were collected. A series of twenty-two different configurations of transverse in-stream structures were tested. An analysis of the average and maximum velocities at the tips of the transverse in-stream structures was performed. Utilizing a channel bend approach velocity, average and maximum velocity ratios were calculated using physical model data. A set of dimensionless parameters consisting of influential structure design parameters was organized and arranged for regression analysis. Predictive equations were developed that describe the ratios of maximum and average velocity at the tips of the in-stream structures to bend-averaged velocities. The predictive equations for maximum and average velocity ratios function as a first approximation of in-stream structure riprap design for configurations that are within the range of tested data. Velocity data were used to assess the suitability of current riprap sizing techniques for transverse in-stream structures. Bank revetment design methodologies were found to be dependable methods for in-stream structure riprap design. Methodologies developed by the United States Army Corps (USACE) and the United States Bureau of Reclamation (USBR) were recommended for the sizing of riprap for in-stream structures. Velocity adjustment procedures were created for use in the USACE and USBR methods. The velocity adjustment procedures include a velocity factor for the determination of a riprap sizing design velocity. The riprap sizing design velocity produces a conservative riprap size for bank revetment, but an appropriate riprap size for in-stream transverse structures. Two velocity factors are provided: one for natural channels and the one for uniform, trapezoidal channels. Limitations and recommendations of the proposed tip velocity ratios and riprap sizing techniques are provided.
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    Design of converging stepped spillways
    (Colorado State University. Libraries, 2008) Hunt, Sherry Lynn, author; Abt, Steven, advisor
    Roller compacted concrete (RCC) stepped spillways are growing in popularity for providing overtopping protection for aging watershed dams with inadequate auxiliary spillway capacity and for the construction of new dams. Unobtainable land rights, topographic features, and land use changes caused by urbanization limit the ability to construct new dams or modify the dimensions of existing embankments and spillways. The advantages of stepped spillways are (1) they can be placed over the top of an existing embankment without causing significant changes to the dam or spillway dimensions, (2) they provide considerable energy dissipation in the chute, potentially reducing the size of the stilling basin, and (3) they permit shorter, more efficient, and feasible construction schedules than other design options. Currently, limited design guidelines are available in the literature for the design of stepped spillways constructed on flat slopes (&thetas; < 30°). Auxiliary spillways are designed to safely pass exceptionally large flood events to the downstream channel. In structural auxiliary spillways, spillway chute and stilling basin training walls are typically designed to prevent overtopping. However, the aspect of converging training walls increases the flow depth in the chute near the walls, and it changes the hydraulic parameters for designing the stilling basin. To assist with the design of converging stepped spillways having similar design parameters (i.e. chute slope, step height, etc.), a study utilizing a three-dimensional, 1:22 scale physical model was conducted to evaluate the flow characteristics in the spillway. This study is the first known attempt at developing generalized design criteria for converging stepped spillways having vertical training walls. Conclusions drawn from this study are that as the convergence of the training wall increases the flow depth near the wall also increases. A simplified control volume momentum analysis was used to predict the minimum training wall height necessary to prevent overtopping. The equation developed slightly under-predicted the results. This under-prediction may be a result of the assumptions made in the development of the prediction equation. Other design aids for determining training wall height were developed based on observations with the data. The results of the study will be discussed further herein.