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35 C H A P T E R 4 Results from case studies and physical experiments (see Appendices C and D) were extended to typical gravel and sand-bed streams using the VSL3D for two channel-spanning or sill structures, cross vanes, and W-weirs. The goal of these investigations was to determine what structures are most appropriate, how they should be installed (including loca- tion of specific structures), how they should be monitored and maintained, and the likely failure mechanisms for each structure installation based on site-specific stream properties (e.g., curvature, slope, bed material). For each structure type, a systematic approach was used to determine the optimum angle of orientation relative to the bank, location, and the effect of multiple structures for typi- cal sand (VSL-S) and gravel (VSL-G) channels. For channel- spanning structures, the optimum angle of orientation was determined using quasi-equilibrium bed elevation (see Table 3-1 for structure geometry). The optimum angle was selected as the configuration that resulted in (1) the most cen- tral thalweg, and (2) the least erosion near either bank. Once the optimum angle was selected for each structure configura- tion for sill structures located at the upstream end of a straight reach, the effect of structure location in a meandering chan- nel was tested by shifting the structure to the downstream end of the straight reach (see Figure 4-1) approximately one channel width (B) downstream of the previous (upstream end) location. The performance of multiple sill structures in meandering sand and gravel channels was investigated. Two sill structures were installed upstream and downstream of a meander of interest bracketing the meander apex (Figure 4-2). Time-averaged quasi-equilibrium bed elevation (Zbed) was analyzed to investigate the influence of a second structure. 4.1 VSL3D Results for Cross Vanes Single Cross Vane Scour and Deposition Two configurations of a cross vane structure, a U-shaped cross vane (CV) and an A-shaped cross vane (CVA), were tested with the VSL-G and VSL-S bed-morphodynamics models. The VSL3D was applied to simulate the resulting bed morpho- dynamics for two different angles for each CV configuration placed just after a meander and before a straight section of chan- nel. These angles, 20° and 30°, were chosen within the range of angles that define CV and CVA structures (Rosgen, 2006; NRCS, 2007). The computed results are shown in Figures 4-3 and 4-4 for VSL-G and VSL-S, respectively. For each case, the time-averaged bed elevation and the difference between the time-averaged bed elevation and the quasi-equilibrium bed elevation with no structure (see Figure 3-2) are shown. The computed results for the VSL-G channel show very little effect of angle on scour patterns (Figure 4-3). The addition of a step to form a CVA structure results in a larger (longer) scour hole downstream of the structure in both VSL-G and VSL-S. For the VSL-S channel, the effect of angle was more evident, with the 20° structure resulting in greater scour downstream of both CV and CVA structures. Optimum Angle and Structure Shape Both CV and CVA structures have been used in the field, although current guidelines (i.e., Rosgen, 2006, NRCS, 2007) recommend the additional step present in the CVA. The field performance of both CV and CVA was evaluated by the U.S. Bureau of Reclamation (U.S. Department of the Interior, Bureau of Reclamation, 2009). The effect of the arm angle of orientation was tested with numerical simulation in the VSL-G and VSL-S channels (Figures 4-3 and 4-4). Differences in scour depth and pattern were small for different structure angles of orientation. However, for the VSL-G and VSL-S channels, the 30° angle of orientation for both A-shaped and U-shaped structures performed better. The criteria for selec- tion included the threat of scour on either bank. A more cen- tral scour hole with less scour adjacent to either bank would be preferred over a wide, deep scour hole that extends to the banks. The optimum cross vane shape, U- or A-shaped, Numerical Methodology for Developing Design Guidelines for Sill Structures
36 Figure 4-1. Schematic of meandering channel and sill structure locations at upstream and downstream of straight reach. Figure 4-2. Schematic of meander and two sill structure locations. The reach between the two blue lines on each side of the apex shows the straight reaches. The upstream and downstream structures are installed at the downstream end and upstream end of the straight reaches. depends on the project goals. CVA structures were successful in protecting the upper cross piece of the cross vane by pre- venting deep scour immediately downstream of these rocks. The scour hole downstream of the CVA structure extended to the banks in the VSL-G channel, creating greater risk to the banks. The following sections will investigate both types of cross vanes with the 30° angle of orientation. Evaluation of Structure Location To investigate the effect of cross vane location on the sedi- ment transport process, a single cross vane structure (CV or CVA) was placed at the downstream end of the straight reach of VSL-S and VSL-G (see Figure 4-1) approximately one chan- nel width (B) downstream of the previous (upstream end) location. Figures 4-5 and 4-6 show the quasi-equilibrium bed morphology with a CV or a CVA at the downstream end of the straight reach for VSL-G and VSL-S, respectively. The compari- sons between this case and the previous case (Figures 4-3 and 4-4) show that shifting the structureâs location to the down- stream end of the straight reach leads to a shift in the location of the scour hole and point bar within the meander. Shifting the sill structure downstream caused deeper scour near the outer bank around the meander apex (Figures 4-5 and 4-6); however, the point bar was also pushed downstream of the apex as a result of the highly turbulent flow downstream of the sill structure. Comparing Figures 4-3 and 4-4 (upstream) with Figures 4-5 and 4-6 (downstream) indicates that shifting CVs or CVAs downstream by approximately B moves the scour pat- tern to approximately 0.5B downstream. Hence, for meander- ing streams, the turbulent flow downstream of the sill structure is a potential threat to the stability of the outer bank (especially at regions near the apex); therefore, a single cross vane should be installed at the upstream end of a straight reach in a mean- dering channel (similar to suggestions by Doll et al., 2003). Evaluation of Multiple Sill Structures The performance of multiple sill structures in VSL-G and VSL-S was investigated. Two CV structures were installed upstream and downstream of a meander bracketing the mean- der apex. Figure 4-7 illustrates the quasi-equilibrium bed mor- phology of VSL-G with CVs or CVAs bracketing the meander. As shown in this figure, the presence of the second sill struc- ture leads to sediment deposition immediately upstream of the structure; however, the general morphological pattern at the apex region is very similar to that in the previous case with one sill structure (Figure 4-5). The downstream sill structure (for both CV and CVA structures) only influenced the morphologi- cal pattern immediately upstream. For example, in Figure 4-5, with one sill structure upstream of the apex, the near-bank scour depth at the apex is comparable with the results with two sill structures in Figure 4-7. This is because the CV con- figuration with sloping arms allows sediment movement to be transported over the second sill structure. Once the depth of deposited sediment upstream of the second CV was equal to the structure height, the sediment material was easily entrained and moved downstream. Adding a second sill structure resulted in two scour pools, one downstream of each structure. The scour hole downstream of the first structure has similar character- istics to the scour pool downstream of a single CV structure (Figure 4-5) that can cause some structural stability issues downstream of the apex for the inner bank (see Figure 4-7). In Figure 4-8 the quasi-equilibrium bed morphology of VSL-S with two cross vanes is shown. Like VSL-G, the sec- ond sill structure caused sediment deposition immediately upstream of the structure; however, the general morphologi- cal pattern at the apex region is similar to that in the previ- ous case with one sill structure (Figure 4-6). The use of the second sill structure did not influence the quasi-equilibrium morphological pattern at the meanderâs apex and, like VSL-G, the main reason could be the passage of the sediment material over the second sill structure. For VSL-S, the scour hole down- stream of the first structure also had similar characteristics to the scour hole in the single sill structure case (Figure 4-6). A second scour hole can cause some structural stability issues downstream of the apex for the inner bank (see Figure 4-8).
37 Figure 4-3. Zbed for a standard or U-shaped cross vane and a stepped, A-shaped cross vane located at the upstream end of the straight reach of the VSL-G. The DZ represents the Zbed of the baseline case (with no rock structure; Figure 3-2) subtracted from the Zbed of this case. Flow is from left to right.
38 Figure 4-4. Zbed for a U-shaped cross vane and a stepped, A-shaped cross vane located at the upstream end of the straight reach of the VSL-S. The DZ represents the Zbed of the baseline case (with no rock structure; Figure 3-2) subtracted from the Zbed of this case. Flow is from left to right.
Figure 4-5. Zbed for a U-shaped cross vane and a stepped, A-shaped cross vane located at the downstream end of the straight reach of the VSL-G. The DZ represents the Zbed of the baseline case (with no rock structure; Figure 3-2) subtracted from the Zbed of this case. Flow is from left to right. Figure 4-6. Zbed for a U-shaped cross vane and a stepped, A-shaped cross vane located at the downstream end of the straight reach of the VSL-S. The DZ represents the Zbed of the baseline case (with no rock structure; Figure 3-2) subtracted from the Zbed of this case. Flow is from left to right.
Figure 4-8. Zbed for two U-shaped cross vanes and two stepped, A-shaped cross vanes in VSL-S. The DZ represents the Zbed of the baseline case (with no rock structure; Figure 3-2) subtracted from the Zbed of this case. Flow is from left to right. Figure 4-7. Zbed for two U-shaped cross vanes and two stepped, A-shaped cross vanes in VSL-G. The DZ represents the Zbed of the baseline case (with no rock structure; Figure 3-2) subtracted from the Zbed of this case. Flow is from left to right.
41 4.2 VSL3D Results for W-Weirs Single W-Weir Scour and Deposition A WW was placed in the VSL-G and VSL-S bed- morphodynamics models just after a meander and before a straight section of channel (see Figure 4-1). VSL3D was applied to simulate the resulting bed morphodynamics for two different angles for each configuration. These angles, 20° and 30°, were chosen within the range of angles in WW guide- lines (Rosgen, 2006 and NRCS, 2007). The computed results are shown in Figures 4-9 and 4-10 for VSL-G and VSL-S, respectively. For each case, the time-averaged bed elevation and the difference between the time-averaged bed elevation and the quasi-equilibrium bed elevation with no structure are shown. The computed results for the VSL-G and VSL-S channel show very little effect of angle on scour patterns. Optimum Angle The optimum angle of orientation for WWs in the VSL-S and VSL-G channels was determined to be 30° from the banks. This evaluation was based on similar performance criteria as in the CV numerical experiments. The 30° arm angle provided a deep scour hole downstream of the struc- ture with less risk of bank erosion downstream. In addition, significantly less rock material is required to install a 30° arm angle relative to one with 20° arm angle. Evaluation of Structure Location In order to investigate the effect of the location of a WW on the final bed topography, the WW was relocated at the down- stream end of the straight reach for each meandering channel (see Figure 4-1). The WW structure was moved downstream with a length of approximately B. Figures 4-11 and 4-12 show the quasi-equilibrium bed morphology for VSL-G and VSL-S, respectively, with a WW located at the downstream end of the straight reach. Figures 4-11 and 4-12 are comparable with Figures 4-9 and 4-10 for 30° WWs. As shown in these figures, shifting the WW structure downstream results in a downstream shift of the scour hole pattern. A downstream shift in structure location of B resulted in a shift in the scour pool of 0.5B downstream. This finding is valid for both VSL-G and VSL-S and also for all other types of sill structures tested. Placement of the WW structure at the upstream end of the straight reach can cause less bank erosion near the apex of the downstream meander. Evaluation of Multiple Sill Structures The performance a WW structure array was evaluated in VSL-G and VSL-S. WW structures were located upstream and downstream of a meander bracketing the meander apex (see Figure 4-2). Figure 4-13 illustrates the quasi-equilibrium bed morphology of VSL-G. As shown in this figure, the pres- ence of the second sill structure leads to sediment deposition Figure 4-9. Zbed for a single WW located at the upstream end of the straight reach of the VSL-G. The DZ represents the Zbed of the baseline case (with no rock structure; Figure 3-2) subtracted from the Zbed of this case. Flow is from left to right.
42 Figure 4-10. Zbed for a single WW located at the upstream end of the straight reach of the VSL-S. The DZ represents the Zbed of the baseline case (with no rock structure; Figure 3-2) subtracted from the Zbed of this case. Flow is from left to right. Figure 4-11. Zbed for a single WW located at the downstream end of the straight reach of the VSL-G. The DZ represents the Zbed of the baseline case (with no rock structure; Figure 3-2) subtracted from the Zbed of this case. Flow is from left to right. immediately upstream of the structure; however, the general morphological pattern at the apex region is similar to that in the previous case with one sill structure. The scour hole downstream of the first structure has similar characteristics to the one in the single WW structure case. Figure 4-14 shows the quasi-equilibrium bed morphology of VSL-S with multiple WWs. The presence of the second WW structure causes sediment material deposition imme- diately upstream of the structure; however, the general mor- phological pattern at the apex region is similar to that in the previous case with one sill structure (Figure 4-12). Like the other two sill structures, use of the second sill structure does not significantly influence the quasi-equilibrium morpho- logical pattern at the meanderâs apex.
43 Figure 4-12. Zbed for a single WW located at the downstream end of the straight reach of the VSL-S. The DZ represents the Zbed of the baseline case (with no rock structure; Figure 3-2) subtracted from the Zbed of this case. Flow is from left to right. Figure 4-13. Zbed for two WWs in VSL-G. The DZ represents the Zbed of the baseline case (with no rock structure; Figure 3-2) subtracted from the Zbed of this case. Flow is from left to right. Figure 4-14. Zbed for two WWs in VSL-S. The DZ represents the Zbed of the baseline case (with no rock structure; Figure 3-2) subtracted from the Zbed of this case. Flow is from left to right.
