Design Variable Development for Transverse Features in the Middle Rio
Transverse features include structures like bendway weirs, spur dikes, and rock vanes. These have been used on many streams and riverways to disrupt secondary current, improve navigation, and redirect flow away from the outer bank of a bend. Bendway weirs have a flat,lower crest with the weirs pointing upstream. Vanes are also angled upstream, but have a slope on the crest that extends from bankfull to the design elevation. Spur dikes are angled downstream and are flat crested structures set at the bankfull height. Typically transverse structures use less rock than a traditional riprap revetment, while still providing a measure of engineering support to the banklines. Because transverse features are not continuous bank protection there are more uncertainties in providing adequate engineering protection at the eroding bankline. While there has been some recent documentation providing design guidelines for transverse features (Brown, 1985; Maryland Department of the Environment, 2000; Johnson et al.; 2001; Julien and Duncan, 2003; McCullah and Gray, 2005; Lagasse et al., 2009; Adolfo M. de Almeida and P. MartĂÂn -Vide, 2010;), there is considerable variation in the design criteria and very little documentation of expected results for the field practitioner, such as expected reduction in velocity or shear stress. Knowledge of how the velocity and shear stress changes occur after field implementation of a design help enhance the management of water uses, whether through increased riverside protection of critical infrastructure like irrigation drains and canals or more efficient use of available water in the river to create habitat variability without impairing effective water delivery. Over the last decade, Reclamation has worked with Colorado State University (CSU) to create a physical scale model to investigate the hydraulics around transverse features (Heintz et al., 2002; Darrow et al., 2004; Kasper et al., 2005; Kinzli et al., 2005; Schmidt et al., 2005; Kinzli and Thornton, 2010; Sclafani et al., 2012; Scurlock et al., 2012A; Scurlock et al., 2012B; Scurlock et al., 2012D; Scurlock et al., 2012E; Sin et al., 2012; Thornton et al., 2012; Ursic et al., 2012; Youngblood et al., 2012). Two different physical scale models were developed at CSU, both based on collected field data from bends on the Rio Grande between Cochiti Dam and Angostura Diversion Dam. One was an idealized trapezoid geometry and the other used natural channel geometry. The collected data set has resulted in the ability to develop empirical equations for the idealized trapezoid geometry to describe the predicted changes in velocity or shear stress given explicit changes in the assessed design variables. During the physical model testing however, significant differences were observed in the performance of the idealized trapezoid geometry and the natural channel geometry and between structure types (bendway weir, spur-dike, and rock vanes) such that there are still limitations with the developed equations. The testing done to date has allowed the identification of other tests that would improve the statistical robustness and general applicability of these equations. Objective The goal of this cooperative agreement is to develop design equations and guidelines for transverse features (bendway weirs and rock vanes) that the field practitioner is able to use with a high degree of confidence in a wide variety of hydraulic conditions. This will help enhance management of water uses, whether through increased riverside protection of critical infrastructure like irrigation drains and canals, or more efficient use of available water in the river to create habitat variability without impairing effective water delivery. The objectives for these design equations and guidelines would be as follows: 1) Easy to determine variables 2) Rapid optimization of design variables 3) Ability to apply to a wide variety of hydraulic conditions 4) Ability to apply to a wide variety of geomorphic conditions 3 Reports at each phase of this proposal would be produced to document design guidance for the optimization of transverse feature design variables. The end result of this design guidance is to allow the field practitioner to achieve a desired hydraulic and geomorphic response given existing hydraulic and geomorphic conditions.
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