Revised Clear-Water and Live-Bed Contraction Scour Analysis (2021)

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5-1   Live-Bed Laboratory Testing Results By definition, live-bed conditions require that sediment is supplied to the contracted section from the upstream approach reach. In the CSU flume, live-bed conditions in the 8-ft-wide approach reach were achieved by setting the pump flow at discharges high enough to mobilize the 0.26-mm sand. Continuous sediment feed was supplied for the duration of each test by an auger-equipped sediment hopper and movable distribution board. Sediment feed rates were determined from the rating curve developed by Guy et al. (1966), relating sediment discharge to water discharge. This curve was developed using the same 8-ft flume the Research Team’s study uses and essentially the same sand (0.27 mm versus 0.26 mm). Figure 5-1 shows the sediment transport rating curve, and includes for comparison the rating curves suggested using two well-known methods for estimating bed-sediment transport: • Modified Laursen (Madden 1993) • Yang (1973), as available in HEC-RAS The curve associated with the data from Guy et al. (1966) begins at lower transport rates, likely because the fine sand involved quickly developed bedforms (ripples and dunes) that slowed the sediment discharge. As the water discharge increases, the sediment discharge increases more rapidly than indicated by the two methods listed. 5.1 Severe Contraction Ratio No live-bed testing was conducted at the Severe contraction ratio. The discharge required to produce live-bed conditions in the approach reach at this contraction ratio would have resulted in severely choked flow at the contraction entrance and overtopping of the flume walls. 5.2 Moderate Contraction Ratio Four live-bed contraction scour tests were conducted in the 8-ft-wide indoor flume at CSU with a contracted section 4 ft wide by 85 ft long, giving a contraction ratio B2/B1 = 4/8 = 0.50. Wing walls at 45° transition the flow from the 8-ft-wide approach section into the contracted reach, the same entrance angle used for all clear-water tests. Figures 5-2 (a) and (b) are photos before and after a live-bed test. Note that for the live-bed tests, bedforms are developed in the approach section upstream of the contraction, as well as in the contracted section itself. Under live-bed conditions, it was determined that equilibrium scour conditions were achieved in the CSU 8-ft flume approximately 7 hours after beginning each test. The live-bed tests conducted at the Moderate contraction ratio are summarized in Table 5-1. C H A P T E R 5

5-2 Revised Clear-Water and Live-Bed Contraction Scour Analysis Figure 5-1. Sediment rating curves for the CSU 8-ft flume. a. Initial conditions for a live-bed test b. At the completion of a live-bed test. Figure 5-2. Typical photos of a live-bed test at contraction ratio B2/B1 = 0.50. Test number Duration(hours) Contraction ratio B2/B1 Velocity ratio Vn1/Vc Discharge Q (ft3/s) y2 tailgate (ft) LB_0.50-1.2 7 0.50 1.2 4.89 0.50 LB_0.50-1.4 7 0.50 1.4 5.70 0.58 LB_0.50-1.65 7 0.50 1.65 6.72 0.58 LB_0.50-2.0 7 0.50 2.0 8.14 0.58 Table 5-1. Live-bed contraction scour tests at the Moderate contraction ratio.

Live-Bed Laboratory Testing Results 5-3   Prior to each live-bed test, the bed was smoothed perfectly flat at a reference elevation of 0.00 ft. At the beginning of each test, water was gradually introduced into the flume. The tailgate setting and the pump discharge were adjusted until the target discharge was achieved, the flow was steady, and the depth of flow at the tailgate (y2 tailgate) was 7 in. (0.58 ft). This process took about 30 to 45 min, at which time the data collection began. 5.2.1 Bed and Water Surface Profiles Measurement of water surface and bed elevations at the same predetermined flume stations as were used for the clear-water tests (see Table 4-2) were taken periodically during the 7-hour duration of each live-bed test. As with previous tests, measurements were taken from the data collection carriage that traverses the length of the flume. During the initial stage of each test, measurements were taken at half-hour intervals, and at 1-hour intervals after that. The water surface was measured along the centerline of the flume; bed elevations were obtained at both the centerline and along the right wall at the predetermined measurement loca- tions. Figure 5-3 provides an example of water surface and bed evolution during a live-bed test (Test LB_0.50-2.0 shown). In this figure, the bed profile taken along the centerline is markedly different from that taken along the right wall primarily in the throat region, where scour along the wall is much more pronounced than that at the centerline. LiDAR scans of the bed are taken at the beginning and end of each test. Figure 5-4 shows the LiDAR profiles of the bed from Test LB_0.50-2.0 along the centerline and also along the right wall. It was observed that at the end of Test LB_0.50-2.0, the scour depth in the contraction throat at the walls came very close to exposing the rigid plywood bed of the flume, which is at elevation –1.50 ft. This can be seen from the wall profiles in both Figures 5-3 and 5-4. Bedforms for all four live-bed tests were clearly in the ripple-dune regime. Dunes are very clearly distinguished along the wall; they are present but less pronounced at the centerline. Figure 5-5 presents the bed and water surface elevations recorded from the data collection cart at each of the predetermined measurement stations at the end of Test LB_0.50-2.0. In this figure, the bed elevation at the centerline and along the right wall are both shown. 5.2.2 1D Modeling Analysis Optimal Manning’s n values for the four live-bed tests were obtained in the same manner as previously described in Section 3.5. The Manning’s n value used to represent the plywood walls was 0.013, based on previous rigid-bed plywood tests. The results from the final HEC-RAS calibrations for the four live-bed tests at the 0.50 contrac- tion ratio are provided in Figures 5-6 (a) through (d). Final bed elevations used in the 1D models correspond to the centerline bed elevations measured from the data collection carriage. Refer to Table 3-4 for the details of the HEC-RAS calibrations for all live-bed tests conducted at the Moderate contraction ratio. 5.2.3 Velocity and Shear-Stress Profiles Velocity profiles were developed from the calibrated HEC-RAS model for each of the four live-bed tests at the completion of each 7-hour run. The results are shown in Figure 5-7 for the centerline measurements. In this figure, the final (post-scour) velocities downstream of flume station 40 are seen to range from about 1.4 ft/s to 1.7 ft/s.

5-4 Revised Clear-Water and Live-Bed Contraction Scour Analysis LB_0.50-2.0 Water Surface Evolution-Centerline 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0 10 20 30 40 50 60 70 80 90 El ev ati on (ft ) Station (ft) 7-Hr 6-Hr 5-Hr 4-Hr 3-Hr 2-Hr 1.5-Hr 1-Hr 0.5-Hr LB_0.50-2.0 Bed Evolution-Centerline -1.50 -1.25 -1.00 -0.75 -0.50 -0.25 0.00 0.25 0 10 20 30 40 50 60 70 80 90 Be d El ev . ( ft ) Station (ft) 7 hr 6 hr 5 hr 4 hr 3 hr 2 hr 1.5 hr 1 hr 0.5 hr 0 hr LB_0.50-2.0 Bed Evolution-Wall -1.50 -1.25 -1.00 -0.75 -0.50 -0.25 0.00 0.25 0 10 20 30 40 50 60 70 80 90 Be d El ev . ( ft ) Station (ft) 7hr 6hr 5hr 4hr 3hr 2hr 1.5hr 1 hr 0.5 hr 0 hr Figure 5-3. Water surface and bed evolution, Test LB_0.50-2.0.

Live-Bed Laboratory Testing Results 5-5   -1.50 -1.25 -1.00 -0.75 -0.50 -0.25 0.00 0.25 0 10 20 30 40 50 60 70 80 El ev ati on , ft Station, ft LB_0.50-2.0 Lidar Bed Profiles Initial bed: Centerline Initial bed: Wall Final bed at 7 hours: Centerline Final bed at 7 hours: Wall Figure 5-4. Initial and final bed profiles from LiDAR scans, Test LB_0.50-2.0. -1.5 -1.0 -0.5 0.0 0.5 1.0 0 10 20 30 40 50 60 70 80 90 Ve lo ci ty , ft /s Station, ft LB_0.50-2.0 Bed and Water Surface Profiles at Measurement Stations 7 hours - bed elevation - centerline 7 hours - bed elevation - right wall 7 hours - water surface elevation Figure 5-5. Bed and water surface profiles for Test LB_0.50-2.0 at final conditions from the predetermined measurement stations. Test LB_0.50-1.2 Centerline -1.0 -0.5 0.0 0.5 1.0 0 10 20 30 40 50 60 70 80 90 100 El ev ati on ( ft ) Station (ft) Initial WS (HEC-RAS) Final WS (HEC-RAS) Final WS (observed) Initial bed Final bed Initial bed Final bed Figure 5-6 (a). Initial and final (7 hour) bed and water surface profiles for Test LB_0.50-1.2 from HEC-RAS.

5-6 Revised Clear-Water and Live-Bed Contraction Scour Analysis Test LB_0.50-2.0 Centerline -1.0 -0.5 0.0 0.5 1.0 0 10 20 30 40 50 60 70 80 90 100 El ev ati on ( ft ) Station (ft) Initial WS (HEC-RAS) Final WS (HEC-RAS) Final WS (observed) Initial bed Final bed Initial bed Final bed Figure 5-6 (d). Initial and final (7 hour) bed and water surface profiles for Test LB_0.50-2.0 from HEC-RAS. Test LB_0.50-1.4 Centerline -1.0 -0.5 0.0 0.5 1.0 0 10 20 30 40 50 60 70 80 90 100 El ev ati on ( ft ) Station (ft) Initial WS (HEC-RAS) Final WS (HEC-RAS) Final WS (observed) Initial bed Final bed Initial bed Final bed Figure 5-6 (b). Initial and final (7 hour) bed and water surface profiles for Test LB_0.50-1.4 from HEC-RAS. Test LB_0.50-1.65 Centerline -1.0 -0.5 0.0 0.5 1.0 0 10 20 30 40 50 60 70 80 90 100 El ev ati on ( ft ) Station (ft) Initial WS (HEC-RAS) Final WS (HEC-RAS) Final WS (observed) Initial bed Final bed Initial bed Final bed Figure 5-6 (c). Initial and final (7 hour) bed and water surface profiles for Test LB_0.50-1.65 from HEC-RAS.

Live-Bed Laboratory Testing Results 5-7   Shear-stress profiles were developed from the calibrated HEC-RAS model for each of the four live-bed tests at the completion of each 7-hour run. The results are shown in Figure 5-8 for the centerline measurements. In this figure, the final (post-scour) shear stress downstream of flume station 40 ranges from about 0.02 lb/ft2 to 0.06 lb/ft2. 5.2.4 Scour Depth Summary Final (7-hr) scour depths at key locations for the four live-bed tests at the Moderate contraction ratio are summarized in Table 5-2. Scour depths along both the centerline and wall are listed. 0.00 0.50 1.00 1.50 2.00 2.50 3.00 0 10 20 30 40 50 60 70 80 90 Ve lo ci ty , ft /s Station, ft Velocity Profiles at Centerline: Live-Bed Tests at Moderate Contraction Ratio B2/B1 = 0.50 Initial conditions Final, V1/Vc = 1.2 Final, V1/Vc = 1.4 Final, V1/Vc = 1.65 Final, V1/Vc = 2 Figure 5-7. Initial and final velocity profiles for four live-bed tests at the Moderate contraction ratio B2/B1 = 0.50. 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0 10 20 30 40 50 60 70 80 90 Be d Sh ea r S tr es s, lb /ft 2 Station, ft Shear Stress Profiles at Centerline: Live-Bed Tests at Moderate Contraction Ratio B2/B1 = 0.50 Initial conditions Final, V1/Vc = 1.2 Final, V1/Vc = 1.4 Final, V1/Vc = 1.65 Final, V1/Vc = 2 Figure 5-8. Initial and final shear-stress profiles for four live-bed tests at the Moderate contraction ratio B2/B1 = 0.50. Test number Short Contraction: Maximum scour depth (ft), at throat Long Contraction: Average scour depth (ft), sta. 40-80 Centerline Wall Centerline Wall LB_0.50-1.2 0.47 0.85 0.16 0.19 LB_0.50-1.4 0.55 1.06 0.20 0.28 LB_0.50-1.65 0.67 1.09 0.31 0.30 LB_0.50-2.0 0.88 1.43 0.42 0.46 Table 5-2. Live-bed contraction scour depths, Moderate contraction ratio.

5-8 Revised Clear-Water and Live-Bed Contraction Scour Analysis 5.3 Mild Contraction Ratio Five live-bed contraction scour tests were conducted in the 8-ft-wide indoor flume at CSU with a contracted section 6 ft wide by 85 ft long, giving a contraction ratio B2/B1 = 6/8 = 0.75. Wing walls at 45° transition the flow from the 8-ft-wide approach section into the contracted reach, the same entrance angle used for all clear-water tests. Figure 5-9 is a photo after a live-bed test of the Mild contraction ratio. Note that for the live-bed tests, bedforms are developed in the approach section upstream of the contraction as well as in the contracted section itself. Under live-bed conditions, it was determined that equilibrium scour conditions were achieved in the CSU 8-ft flume approximately 7 hours after beginning each test. The live-bed tests con- ducted at the Mild contraction ratio are summarized in Table 5-3. Prior to each live-bed test, the bed was smoothed perfectly flat at a reference elevation of 0.00 ft. At the beginning of each test, water was gradually introduced into the flume. The tailgate setting and the pump discharge were adjusted until the target discharge was achieved, the flow was steady, and the depth of flow at the tailgate (y2 tailgate) was 7 in. (0.58 ft). This process took about 30 to 45 min, at which time the data collection began. Figure 5-9. Typical photo of a live-bed test at contraction ratio B2/B1 = 0.75. Test number Duration(hours) Contraction ratio B2/B1 Velocity ratio Vn1/Vc Discharge Q (ft3/s) y2 tailgate (ft) LB_0.75-1.2 7 0.75 1.2 4.89 0.58 LB_0.75-1.4 7 0.75 1.4 5.70 0.58 LB_0.75-1.65 7 0.75 1.65 6.72 0.58 LB_0.75-2.0 7 0.75 2.0 8.14 0.58 LB_0.75-2.5 7 0.75 2.5 10.18 0.58 Table 5-3. Live-bed contraction scour tests at the Mild contraction ratio B2/B1 = 0.75.

Live-Bed Laboratory Testing Results 5-9   5.3.1 Bed and Water Surface Profiles Measurement of water surface and bed elevations at the same predetermined flume stations as were used for the clear-water tests (see Table 4-2) were taken periodically during the 7-hour duration of each live-bed test. As with previous tests, measurements were taken from the data collection carriage that traverses the length of the flume. During the initial stage of each test, measurements were taken at half-hour intervals, and at 1-hour intervals after that. The water surface was measured along the centerline of the flume; bed elevations were obtained at both the centerline and along the right wall at the predetermined measurement locations. Figure 5-10 provides an example of water surface and bed evolution during a live- bed test (Test LB_0.75-2.5 shown). In this figure, the bed profile taken along the centerline is markedly different from that taken along the right wall primarily in the throat region, where scour along the wall is much more pronounced than that at the centerline. LiDAR scans of the bed are taken at the beginning and end of each test. Figure 5-11 shows the LiDAR profiles of the bed from Test LB_0.75-2.5 along the centerline and also along the right wall. Bedforms for all five live-bed tests were clearly in the ripple-dune regime. Dunes are clearly distinguished along the wall; they are present but less pronounced at the centerline. Figure 5-12 presents the bed and water surface elevations recorded from the data collection cart at each of the predetermined measurement stations at the end of Test LB_0.75-2.5. In this figure, the bed elevation at the centerline and along the right wall are both shown. 5.3.2 1D Modeling Analysis Optimal Manning’s n values for the four live-bed tests were obtained in the same manner as described in Section 3.5. The Manning’s n value used to represent the plywood walls was 0.013, based on previous rigid-bed plywood tests. The results from the final HEC-RAS calibrations for the five live-bed tests at the 0.75 con- traction ratio are provided in Figures 5-13 (a) through (e). Final bed elevations used in the 1D models correspond to the centerline bed elevations measured from the data collection carriage. Refer to Table 3-4 for the details of the HEC-RAS calibrations for all live-bed tests conducted at the Moderate contraction ratio. 5.3.3 Velocity and Shear-Stress Profiles Velocity profiles were developed from the calibrated HEC-RAS model for each of the five live-bed tests at the completion of each 7-hour run. The results are shown in Figure 5-14 for the centerline measurements. In this figure, the final (post-scour) velocities downstream of flume station 40 are seen to range from about 1.4 ft/s to 1.7 ft/s. Shear-stress profiles were developed from the calibrated HEC-RAS model for each of the five live-bed tests at the completion of each 7-hour run. The results are shown in Figure 5-15 for the centerline measurements. In this figure the final (post-scour) shear stress downstream of flume station 40 ranges from about 0.02 lb/ft2 to 0.06 lb/ft2. 5.3.4 Scour Depth Summary Final (7-hour) scour depths at key locations for the five live-bed tests at the Mild contraction ratio are summarized in Table 5-4. Scour depths along both the centerline and wall are listed.

5-10 Revised Clear-Water and Live-Bed Contraction Scour Analysis Figure 5-10. Water surface and bed evolution, Test LB_0.75-2.5.

Live-Bed Laboratory Testing Results 5-11   Figure 5-11. Initial and final bed profiles from LiDAR scan, Test LB_0.75-2.5. Figure 5-12. Bed and water surface profiles for Test LB_0.75-2.5 at final conditions from the predetermined measurement stations.

5-12 Revised Clear-Water and Live-Bed Contraction Scour Analysis Figure 5-13 (a). Initial and final (7 hour) bed and water surface profiles for Test LB_0.75-1.2 from HEC-RAS. Figure 5-13 (b). Initial and final (7 hour) bed and water surface profiles for Test LB_0.75-1.4 from HEC-RAS. Figure 5-13 (c). Initial and final (7 hour) bed and water surface profiles for Test LB_0.75-1.65 from HEC-RAS.

Live-Bed Laboratory Testing Results 5-13   Figure 5-13 (d). Initial and final (7 hour) bed and water surface profiles for Test LB_0.75-2.0 from HEC-RAS. Figure 5-13 (e). Initial and final (7 hour) bed and water surface profiles for Test LB_0.75-2.5 from HEC-RAS. Figure 5-14. Initial and final velocity profiles for five live-bed tests at the Mild contraction ratio B2/B1 = 0.75.

5-14 Revised Clear-Water and Live-Bed Contraction Scour Analysis Figure 5-15. Initial and final shear-stress profiles for five live-bed tests at the Mild contraction ratio B2 /B1 = 0.75. Test number Short Contraction: Maximum scour depth (ft), at throat Long Contraction: Average scour depth (ft), sta. 40-80 Centerline Wall Centerline Wall LB_0.75-1.2 0.29 0.55 0.17 0.16 LB_0.75-1.4 0.26 0.69 0.22 0.21 LB_0.75-1.65 0.29 0.81 0.21 0.26 LB_0.75-2.0 0.46 0.83 0.46 0.63 LB_0.75-2.5 0.49 1.11 0.49 0.60 Table 5-4. Live-bed contraction scour depths, Mild contraction ratio B2/B1 = 0.75.