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54 C H A P T E R 6 6.1 Length of Bank Protection in Meandering Channels In meandering channels, the length of bank protection can be determined for approximately sinusoidal channels by determining the tangent points to the meander (Figure 6-1) and identifying a point approximately one channel width (B) upstream and 1.5B downstream from these points, as pre- sented in HEC 23 (Lagasse et al., 2009). Both the OSL and VSL3D meandering channel results confirm the guidance for the length of bank that needs to be protected in meandering channels, as shown in Figure 6-1 (Lagasse et al., 2009). All three channels that have been studied herein fall within the radius of curvature to width ratio, Rc/B, range of 2 to 3. This is expected to be the range of ratios at which meander migra- tion is fastest (Johnson et al., 2002b). 6.2 Spacing and Number of Structures Based on the results of the VSL3D numerical simula- tions, an interactive method is suggested to determine the optimum structure layout for in-stream flow control structures similar to the methodology presented in NRCS (2007) for BWs. This spacing methodology was based on a detailed analysis of the shear layer reattachment in a meandering channel. These results were compared to guidelines for spacing based on the equations in Rosgen (2006) (Figure 6-2). Based on the Rosgen guidelines, as the ratio of radius of curvature to channel width increases, structure spacing (Vs) should increase. Based on the analy- sis described previously, the opposite was found, although there was not a channel with an Rc/B equal to 5, and VSL-S had a Rc/B below the threshold of sharp meanders of 3.0, where flow and shear stress distributions behave differently (Kashyap et al., 2012). For example, as the ratio of radius of curvature to channel width increased (Rc/B), the spac- ing recommended by this study (Vs/B) increased, while the spacing recommended by Rosgen (2006) decreased. This indicates that understanding the flow paths around the site-specific channel meander configuration is essential to optimal placement of structure arrays. Also, as the angle of orientation increased, the spacing recommended in this study increased, while the spacing in the Rosgen (2006) guidelines decreased. For these reasons, a revised spacing methodology based on a vector analysis similar to that described in HEC 23 (Lagasse et al., 2009) for BWs and NRCS (2007) for stream barbs is suggested as an alterna- tive to the spacing described for JH and RV structures in Rosgen (2006) and NRCS (2007). Typical BW or stream barb spacing is described in HEC 23 (Lagasse et al., 2009) and NRCS (2007). Based on these guidelines, BW spacing should be between 4 and 10 times the effective length, Le, of the structure (ideally 4 to 5 times Le). Spacing can be calculated using the following equation. ( ) ( )= 1.5 0.8 0.3Vs Le RcB LeB And maximum spacing can be calculated by: ( )= â âï£ï£¬ ï£ï£¬ 1 1 2 0.5Vs Rc LeRc These equations produce a spacing of 17.4 m and 12.2 m and a maximum spacing of 34 m and 27 m for VSL-G and VSL-S, respectively. Based on the structure optimization described previously, the average spacing in VSL-G was 45 m (6.7 times Le), and the average spacing in VSL-S was 34 m (5 times Le). The use of high-resolution numerical model- ing determined an optimal structure spacing that was greater than the recommendations, thus reducing the number of structures and the cost. A vector analysis (e.g., NRCS, 2007) provides further guidance into BW structure spacing and Evaluation of Current Guidelines
55 (a) (b) (c) (d) Figure 6-1. Extent of protection required at a channel bend for (a) general guidelines (from HEC 23, Lagasse et al., 2009). The extent of protection applied to (b) the OSL channel with no structures, (c) VSL-G, and (d) VSL-S. Contours show the bed elevations at quasi-equilibrium. Blue and red zones are associated with the regions of significant scour and deposition, respectively. 0 0.5 1 1.5 2 2.5 3 3.5 0 10 20 30 40 50 Va ne Sp ac in g (V s/ B) Channel Width, B (m) Rc/B = 3; 20 degrees Rc/B = 3; 30 degrees Rc/B = 5; 20 degrees Rc/B = 5; 30 degrees VSL G; Rc/B = 3.3; RV; 30 degrees VSL G; Rc/B = 3.3; JH; 30 degrees VSL G; Rc/B = 3.3; JH_new; 30 degrees VSL S; Rc/B = 2.2; RV1 2; JH1 2; 20 degrees VSL S; Rc/B = 2.2; RV2 3; 20 degrees VSL S; Rc/B = 2.2; JH2 3; 20 degrees Figure 6-2. Structure spacing based on Rosgen (2006) guidelines and optimized spacing derived from the VSL3D described previously. Gray line indicates channel width of VSL-S and VSL-G.
56 location around a meander. In this type of analysis, lines are projected from the tip of the upstream structure to the bank to locate the second structure. Based on the vector analysis commonly used for BWs, the following guidelines are suggested to determine the optimum structure layout for each single-arm structure array described in Chapter 3. 1. Project a line tangent to the bank at the meander apex. Set this line at 0° (horizontal). 2. Place the first structure at the apex. The angle of ori- entation should be such that it intercepts flow from upstream. This angle is dependent on the sinuosity and curvature of the meander where the structures are being installed (see Figure 6-3). a. RVs and JHs: Greater angles protected more bank length in channels with a larger radius of curvature, and smaller angles protected more bank length in channels with a smaller radius of curvature. b. Bendway weirs/stream barbs: An angle of 50° was deter- mined to be the optimum between cost (length of struc- ture) and length of bank protected. 3. Draw a horizontal line parallel to the first line from the tip of the structure. Draw another line with an offset angle from the parallel line. Where this line intersects the bank, the next structure should be located. The offset angle is a function of stream radius of curvature and channel width (see Chapter 3). 6.3 Footer Rock Depths To inform the choice of footer rock depth, the maximum scour depth adjacent to each structure type was compared to the maximum scour depth, ScMAX, in the structure-free chan- nel, as defined in Figure 6-4 as the difference between the maximum scour depth and the average depth across a cross- section. Sill-type structures had the greatest scour, followed by JHs (which perform as a partial sill), followed by RVs and BWs. Dividing the maximum scour in each structure case by ScMAX from the baseline case resulted in a ratio of 0.6 to 0.7 for RVs, 1.1 to 1.6 for JHs, and 0.8 to 0.9 for BWs. Multiplying these values by a factor of safety of 1.3 results in footer rock depth of 1 to 1.5 ScMAX, for RVs and BWs, 1.5 to 2 for JHs, and 2 to 3.5 for sill structures. Figure 6-3. Example layout for first structure at apex and second (top) and third (bottom) structure in different meandering channels.
57 Figure 6-4. Sketch of definition of maximum scour depth, ScMAX. Dotted line is average bed elevation calculated from field data. ScMAX is the difference between the maximum scour depth and the average depth.
