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

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4C H A P T E R 2 The objective of this research was to develop comprehensive engineering guidelines, design methods, and recommended specifications for in-stream structure installation, monitoring, and maintenance. Phase I consisted of background research into the current use and success of in-stream structures and their long-term durability and maintenance requirements (Appendix A). In addition to performing a comprehensive lit- erature review, the combined experience of transportation and natural resource departments was used to describe and evalu- ate existing low-flow structure applications and failure modes (Appendix B). In Phase II, the findings from Phase I were used to develop a comprehensive study using physical and numeri- cal models to explore the most promising structures. Structures that were selected for this project are described in the following. The Virtual StreamLab (VSL3D) model was used to extend detailed field (Appendix C) and laboratory (Appendix D) measurements to a wider range of channel configurations and flow rates. VSL3D is a state-of-the-art 3-D computational fluid dynamics model developed at St. Anthony Falls Laboratory (SAFL) by Fotis Sotiropoulos and coworkers that is capable of simulating real-life hydraulic engineering flows using advanced numerical techniques and turbulence models that can be cou- pled with turbulent-free surface and bed-morphodynamics modules. Following validation (Appendix E), this model was used to systematically explore a matrix of structure scenarios in representative gravel and sand channels (Chapter 3). Design guidelines (Appendix F) for in-stream flow control structures based on a combination of laboratory, field, and numerical experiments were developed to inform the choice of structure type, angle of orientation, spacing, and location based on site- specific channel characteristics. 2.1 Rock Vanes The term “rock vane” (RV) applies to single-arm rock structures that extend out from a stream bank into the flow. These structures gradually slope from the bank to the bed such that, even at low-flow conditions, the tips of the struc- ture remain submerged (Radspinner et al., 2010). RVs are installed with an upstream angle to minimize erosive flow patterns near the bank by diverting high-velocity flow away from the bank (Maryland Department of the Environment, 2000; Johnson et al., 2002a; see Figure 2-1). Often, RVs and other in-stream rock structures are installed with a second- ary goal of improving aquatic habitat by creating flow diver- sity through the formation of scour pools (Rosgen, 2006). A series of vanes installed for bank protection is intended to move scour to the middle of the channel and enhance deposi- tion along the bank (e.g., Johnson et al., 2001; Bhuiyan et al., 2010). RVs and other in-stream rock structures can reduce or eliminate the need for bank armoring on unstable banks and can improve the effectiveness of other erosion protec- tion measures such as vegetation restoration (McCullah and Gray, 2005). Current guidelines for placement and spacing of RVs are based primarily on practitioner experience (e.g., Maryland Department of the Environment, 2000; Doll et al., 2003; NRCS, 2007). The Maryland Department of the Environment’s Water Management Administration included design guidelines for rock vanes in its Maryland’s Waterway Construction Guidelines (MWCG) manual (Maryland Department of the Environment, 2000). Included in this document is a sum- mary of the uses of common restoration and stabilization practices. Within this summary, applications for which rock vanes are well suited (e.g., protecting bank toe and redirect- ing flows), moderately well suited (e.g., providing in-stream habitat), and not well suited (e.g., stabilizing bed and use in bedrock channels) are discussed. Based on the practitioner experience reported in the MWCG, caution should be used in steep stream reaches with gradients that exceed 3%. It is also important to note that the stream bank opposite the RV structures should be monitored closely after installation for any increase in erosion occurring due to the presence of the RV and its ability to direct flow away from the outer bank, Research Approach

5 toward the center of the channel—on occasion negatively affecting the opposite bank. Several structures use the rock vane as the key component and modify it for various situations. All structures from the rock vane family can be subjected to failure by lateral circum- vention, winnowing, local scour, aggradation, and displace- ment (Johnson et al., 2002b). In addition to Rosgen (2001; 2006) and the MWCG (Maryland Department of the Envi- ronment, 2000), rock vane design guidelines can be found in the United States Department of Agriculture (USDA) NRCS Stream Restoration Design National Engineering Handbook, Part 654 (NRCS, 2007). Chapter 11 of this design manual, “Rosgen Geomorphic Channel Design,” focuses on the design guidelines for CVs, J-hook (JH) vanes, and W-weir (WW) structures; however, all of these structures are rock vane variations. 2.2 J-Hook Vanes JH vanes are a variation of the single-arm RV that include a hook that extends from the tip of the vane approximately perpendicular to flow (Figure 2-2). Similar to the rock vane, the vane portion of these structures gradually slopes from the bank to the bed such that even at low-flow conditions, the tip of the structure remains submerged (Radspinner et al., 2010). JH vanes are also installed with an upstream angle to minimize erosive flow patterns near the bank by diverting high-velocity flow away from the bank ( Maryland Figure 2-1. Typical rock vane installation (after Rosgen, 2006 and Radspinner et al., 2010). Figure 2-2. Typical J-hook vane installation (after Rosgen, 2006).

6 Department of the Environment, 2000; Johnson et al., 2002a). Often, rock vanes and other in-stream rock structures are installed with the secondary goal of improving aquatic habi- tat by creating flow diversity through the formation of scour pools (Rosgen, 2006), and J-hook vanes are expected to pro- vide additional in-stream habitat enhancement in the form of a mid-channel scour pool (Maryland Department of the Environment, 2000). Current guidelines for placement and spacing of JH vanes are similar to those developed for RVs, based primarily on practitioner experience (e.g., Maryland Department of the Environment, 2000; Doll et al., 2003; NRCS, 2007). Applications for JH vanes are similar to those for RVs, with the exception that JH vanes are expected to provide better in-stream habitat in the form of a deep mid-channel scour hole (Maryland Department of the Environment, 2000). Limitations of J-hook vanes are similar to those for rock vanes. These structures should not be used in steep stream reaches (greater than 3%). With any flow redirection structure, the stream bank opposite the structure should be monitored closely after installation for any increase in ero- sion occurring due to the presence of the structure. All struc- tures from the rock vane family can be subjected to failure by lateral circumvention, winnowing, local scour, aggradation, and displacement (Johnson et al., 2002b). J-hook design guidelines can be found in Rosgen (2006), the MWCG (Maryland Department of the Environment, 2000), and the USDA NRCS Stream Restoration Design National Engineering Handbook, Part 654 (NRCS, 2007). Chapter 11 of this design manual, “Rosgen Geomorphic Channel Design,” focuses on the design guidelines for cross vanes, J-hook vanes, and W-weir structures. 2.3 Bendway Weirs/Stream Barbs The terms “bendway weir” (BW) and “stream barb” refer to single-arm rock structures extending from the bank that are submerged in all but low flows and are designed to mitigate erosive flow patterns through weir mechanics (Derrick, 1998; Evans and Kinney, 2000; see Figure 2-3). Several state agencies have published technical notes and case studies for bendway weir use under a variety of stream characteristics (e.g., NRCS, 2010; NRCS, 2000). Stream barbs are designed to protect the bank by disrupting velocity gradients in the near-bed regions, deflecting currents toward the tip of the weirs (Matsuura and Townsend, 2004). Guidelines for bendway weir or stream barb design can be found in the U.S. Department of Transporta- tion (U.S. DOT) Federal Highway Administration (FHWA) Hydraulic Engineering Circular (HEC) No. 23, Volume 2 (Lagasse et al., 2009) and the USDA NRCS Stream Restoration Design National Engineering Handbook, Part 654, Technical Supplement 14H (NRCS, 2007). 2.4 Cross Vanes Cross vanes are low-profile, channel-spanning structures designed to provide grade control, divert flow away from unstable banks, and create scour pools for aquatic habitat (Maryland Department of the Environment, 2000). Cross vanes have also been installed upstream of bridge piers to reduce scour. There are two types of cross vane. The first is a U-shaped structure (CV) that has two arms angled upstream at 20° to 30° from the banks that slope downward to the streambed cross piece (Figure 2-4). The second type, an A-shaped structure (CVA), is a modified cross vane with a step located in the upper 1⁄3 to 1⁄2 of the arm. This step is Figure 2-3. Typical bendway weir/stream barb installation (after NRCS, 2007 and Radspinner et al., 2010).

7 designed to dissipate energy, thereby reducing footer scour and protecting the structure from failure (Rosgen, 2006). Cross vanes are well suited for use in moderate- and high- gradient streams and should be avoided in bedrock channels, streams with unstable bed substrates, and naturally well- developed pool–riffle sequences (Maryland Department of the Environment, 2000). Cross vane design guidelines can be found in Rosgen (2006), the MWCG (Maryland Department of the Environment, 2000), and the USDA NRCS Stream Restoration Design National Engineering Handbook, Part 654 (NRCS, 2007). Chapter 11 of this design manual, “Rosgen Geomorphic Channel Design,” focuses on the design guide- lines for cross vanes, J-hook vanes, and W-weir structures. 2.5 W-Weirs Like cross vanes, WWs are low-profile, channel-spanning structures designed to provide grade control, direct flow away from unstable banks, create scour pools for aquatic Figure 2-4. Typical cross vane installation (after Rosgen, 2006). habitat, and protect downstream bridge piers (Figure 2-5). WWs are similar to a double cross vane and are typically applicable in larger channels (>12 m or 40 ft in width). They are well suited to protect the bank toe, redirect flow, create flow diversity, and stabilize bed and lateral channel adjust- ments in channels with highly erodible and steep banks, high design velocity, flashy flows, and high-bedload transport. With proper support, WWs can be used with rigid or fixed banks with limited backwater effects. WWs are not suited for slow-flow or pooled reaches with silt or fine sand beds (Maryland Department of the Environment, 2000). W-weir design guidelines can be found in Rosgen (2006), the MWCG (Maryland Department of the Environment, 2000), and the USDA NRCS Stream Restoration Design National Engineering Handbook, Part 654 (NRCS, 2007). Chapter 11 of this design manual, “Rosgen Geomorphic Channel Design,” focuses on the design guidelines for cross vanes, J-hook vanes, and W-weir structures.

8Figure 2-5. Typical W-weir installation (after NRCS, 2007).