Design Methods for In-Stream Flow Control Structures (2014)

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2C H A P T E R 1 Efforts to stabilize and restore streams and rivers across the nation have grown dramatically in recent decades, with over $1 billion spent annually on projects with goals including stabilizing banks, improving water quality, and improving aquatic habitat (Bernhardt et al., 2005). In-stream structures, constructed of rock or wood in various configurations, are often used to limit lateral migration, reduce bank erosion, and create diverse aquatic habitat (Radspinner et al., 2010). In-stream structures can be classified into two fundamental categories: sills and single-arm structures (Figure 1-1). Sills are structures that span the entire channel width, while single- arm structures extend from one bank into the channel with- out reaching the opposite bank. Single-arm structures can be further subdivided into deflector, redirective, and retard types, depending on the function of each structure configuration [National Resources Conservation Service (NRCS), 2007]. Proper structure design and placement are necessary to avoid channel aggradation, local bed scour, and bank erosion, all of which can result in structure failure and cause significant harm to the stream and nearby property. Furthermore, failure of these structures will accelerate the adverse effects they were initially installed to prevent, such as lateral migration and infrastructure endangerment (see Table 1-1); however, stream restoration and river training measures today rely heavily on empirical methods that emphasize a prescribed design approach rather than the application of physically based hydraulic engineering principles to attain performance-based criteria (Simon et al., 2007; Slate et al., 2007). The lack of a comprehensive physics-based design approach can be attributed to the unsteady, three-dimensional (3-D) character of the flow in the vicinity of structures and the fact that the complex interactions of stream hydrodynam- ics in the water column with streambed sediments are poorly understood. Consequently, the effects of various site-specific conditions on the overall performance and long-term stability of in-stream structures can neither be quantified a priori nor be accounted for in the design process. In an effort to supplement existing guidelines with compre- hensive quantitative analysis of frequently used flow control structures, in this work an in-depth review of current struc- ture use (see Radspinner et al., 2010) is integrated with state- of-the-art physical and numerical modeling and field data collection (Figure 1-2). The result is a set of design guidelines that are based on a state-of-the-art understanding of how turbulence, mobile beds, and sediments interact with and are modified by streams and rivers with flow control rock struc- tures. This report summarizes the methodology employed in this work and presents design guidelines derived by applying this methodology to in-stream flow control structures. Background

Figure 1-1. Illustrations of typical single-arm deflector structure and typical sill rock structure (from Radspinner et al., 2010). Large-scale physical modeling Numerical modeling Small-scale physical modeling Field measurements Previous praconer experience Guidelines for installing, monitoring, and maintaining Figure 1-2. Linkages between components used to develop guidelines for the use of in-stream structures to prevent erosion and protect surface transportation. Channel Characteristics Failure Structure Type Aspect Ratio Sinuosity Slope Reasons Modes Single-arm structures Rock vane (RV) 7–33 n/a 0.003– 0.008 Not keyed properly; rock size and shape Lateral circumvention; winnowing; local scour; aggradation J-hook vane (JH) 8.8–36 1.1–1.5 0.003– 0.02 Not keyed properly; rock size and shape Lateral circumvention; winnowing; local scour; aggradation Bendway weir (BW) 8.6–33 1.3–1.5 <0.003 Footer size; build depth Local scour Sill structures Cross vane (CV and CVA) 7.3–19.6 1.1–1.5 0.001– 0.03 Faulty installation; rock size and shape Lateral circumvention; winnowing; local scour; aggradation; displacement W-weir (WW) n/a n/a n/a Faulty installation; rock size and shape Lateral circumvention; winnowing; local scour; aggradation; displacement Table 1-1. Reasons and modes of failure reported in the practitioner survey published in Radspinner et al. (2010).