Analysis of riprap design methods using predictive equations for maximum and average velocities at the tips of transverse in-stream structures
Date
2014
Authors
Parker, Thomas Richard, author
Thornton, Christopher, advisor
Abt, Steven, advisor
Williams, John, committee member
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Abstract
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.
Description
Rights Access
Subject
tips of in-stream structures
in-stream structures
maximum velocity
predictive equations
riprap
riprap design methods