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

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4-1   4.1 Severe Contraction Ratio Initially, four clear-water contraction scour tests were conducted in the 8-ft-wide indoor flume at CSU. For these tests, the contracted section was 2 ft wide by 85 ft long, giving a con- traction ratio B2/B1 = 2/8 = 0.25. Wing walls at 45° transitioned the flow from the approach section into the contracted reach. The naming convention for each test provides information on the sediment transport condi- tion, contraction ratio, and velocity ratio. For example, Test CW_0.25-0.55 indicates a clear- water test with a contraction ratio B2/B1 of 0.25 and a velocity ratio Vn1/Vc of 0.55, where Vn1 is the assumed velocity in the upstream 8-ft-wide approach channel associated with normal flow depth, and Vc is the critical velocity for sediment transport. The fine sand had a d50 particle size of 0.26 mm and a specific gravity of 2.68 (average of four measurements), and the critical velocity and shear stress for d50 particle movement are estimated from standard HEC-18 equations as Vc = 0.90 ft/s (0.27 m/s) (4.1) τc = 0.0040 lb/ft2 (0.19 Pa) (4.2) The tests conducted at the Severe contraction ratio are identified in Table 4-1. Prior to each 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 and the tailgate setting adjusted until the target discharge was achieved and the flow was steady. 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. 4.1.1 Bed and Water Surface Profiles Throughout each test, the centerline bed elevation and water surface elevation were measured from the data collection cart at predetermined stations along the flume. Initial measurements were made every 30 min, and then less frequently as the test progressed and the bed gradually eroded until an equilibrium condition was established. The first trial was Test CW_0.25-0.75, which was run for 30.5 hours total duration. From that test, it was deter- mined that 18 hours duration was sufficient to establish equilibrium conditions, so subsequent tests were run for 18 hours. Figure 4-1 provides photos of the bed surface at the completion of Test CW_0.25-0.75. C H A P T E R 4 Clear-Water Laboratory Testing Results

4-2 Revised Clear-Water and Live-Bed Contraction Scour Analysis Detailed LiDAR scans were taken at the beginning and end of each test. As an example, a plot of the centerline bed elevations from Test CW_0.25-0.75 is provided in Figure 4-2. During this test, LiDAR scans at 0, 12, 24, and 30.5 hours were taken. To obtain these scans, the test flow was temporarily stopped and the flume carefully drained to expose the bed surface. After each LiDAR scan was complete, the test was continued. The centerline bed elevation plots were developed from the LiDAR point cloud data and reported at intervals of 0.01 ft along the length of the flume. Table 4-2 provides the locations of the 13 designated monitoring stations used to collect bed and water surface elevations at intervals throughout each test. Figure 4-3 presents the bed and water surface elevations recorded at each of the stations at the end of Test CW_0.25-0.75. For comparison, the centerline bed elevation profile from the LiDAR scan at the end of the test is also shown in this figure. Figure 4-4 provides an example of the evolution bed and water surface elevations during the course of Test CW_0.25-0.75. 4.1.2 1D Modeling Analysis Each of the four clear-water contraction scour tests at the Severe contraction ratio of B2/B1 = 0.25 was calibrated to a 1D HEC-RAS model (see Section 3.5). Each model was set up to replicate the exact dimensions of the flume geometry, with model stations corresponding to the predetermined measurement locations in the flume. From rigid-bed (plywood) tests, a Manning’s n value of 0.013 was found to be representative of the relatively smooth, painted vertical side walls. The Manning’s n of the bed was varied until Test number Duration(hours) Contraction ratio B2/B1 Velocity ratio V1/Vc Discharge Q (ft3/s) y2 tailgate (ft) CW_0.25-0.55 18 0.25 0.55 2.24 0.58 CW_0.25-0.65 18 0.25 0.65 2.65 0.58 CW_0.25-0.75 30.5 0.25 0.75 3.05 0.58 CW_0.25-0.80 18 0.25 0.80 3.26 0.58 Table 4-1. Clear-water contraction scour tests at Severe contraction ratio B2/B1 = 0.25. a. Overview b. Close-up photo of throat region Figure 4-1. Bed condition at the completion of Test CW_0.25-0.75.

Clear-Water Laboratory Testing Results 4-3   -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0 10 20 30 40 50 60 70 80 90 El ev ati on , f ee t Station, feet Initial 12 hours 24 hours 30.5 hours Figure 4-2. Bed centerline profiles from LiDAR scans at 0, 12, 24, and 30.5 hours during Test CW_0.25-0.75. Station number Distance along flume (ft) Comment 1 0 Upstream approach channel 8-ft wide 2 10 “ “ 3 13 In transition zone 4 14.3 At throat; beginning of contracted reach 5 16 Contracted reach, 2 ft wide 6 18 “ “ 7 20 “ “ 8 30 “ “ 9 40 “ “ 10 50 “ “ 11 60 “ “ 12 70 “ “ 13 80 “ “ Tailgate 99 Sluice gate, 2 ft wide; water surface held constant at a depth of 0.58 ft for each test. Table 4-2. Designated monitoring station locations. -1.5 -1.0 -0.5 0.0 0.5 1.0 0 10 20 30 40 50 60 70 80 90 El ev ati on , f ee t Station, feet 30.5 hours - bed elevation from lidar 30.5 hours - bed elevation from data collection stations 30.5 hours - water surface elevation from data collection stations Figure 4-3. Bed and water surface profiles for Test CW_0.25-0.75 at final conditions.

4-4 Revised Clear-Water and Live-Bed Contraction Scour Analysis -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0 10 20 30 40 50 60 70 80 90 Be d El ev . ( ft ) Station (ft) CW_0.25-0.75 Bed Evolution - Centerline 30.5 hr 24.5 20.5 16.5 12.5 8.5 6.5 4.5 3.5 2.5 1.5 1.0 0.5 0 hr Station Figure 4-4. Water surface and bed evolution, Test CW_0.25-0.75. a good match was achieved between the observed water surface elevations at the end of each test versus the water surface elevations predicted by HEC-RAS. Strickler’s approximation of Manning’s n based solely on grain size is n 0.034 d where d is the median particle size in ft (4.3)50 1 6 50( )= n 0.041 d where d is the median particle size in m (4.4)50 1 6 50( )= For a median particle size of 0.26 mm (0.00026 m or 0.000853 ft), a Manning’s n grain rough- ness of 0.010 would be expected. As indicated by the flume tests, however, ripple-dune bedforms

Clear-Water Laboratory Testing Results 4-5   are immediately established in the contracted section once the flow is introduced. The bedforms are irregular, with an amplitude on the order of 0.10 ft to 0.15 ft and a wavelength of 2 ft to 3 ft, resulting in increased flow resistance due to form roughness. Best-fit calibration to the observed final water surface resulted in Manning’s n values ranging from 0.023 to 0.028 for the four tests at the Severe contraction ratio (refer to Table 4-4). The initial (pre-scour) conditions were then estimated using HEC-RAS. Figures  4-5 (a) through (d) show the results of the HEC-RAS simulations for each of the four clear-water scour tests with the Severe contraction ratio B2/B1 = 0.25. Refer to Table 3-4 for the details of the HEC-RAS calibrations for all clear-water tests conducted at the Severe contraction ratio. 4.1.3 Velocity and Shear-Stress Profiles The initial (pre-scour) and final (equilibrium scour) conditions predicted by HEC-RAS are presented as velocity and shear-stress profiles in Figures 4-6 and 4-7, respectively. For all four tests, the region of long-contraction equilibrium scour extends downstream of flume station 40, where the bed elevation, velocity, and shear stress tend to be nearly constant. From Figures 4-6 and 4-7, it can be seen that at equilibrium scour conditions, the velocity downstream of station 40 consistently ranges from about V = 1.4 ft/s to 1.6 ft/s for all tests at the Severe contraction ratio. Likewise, the bed shear stress consistently ranges from about Test CW_0.25-0.55 -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 Figure 4-5 (a). Initial and final (18 hour) bed and water surface profiles for Test CW_0.25-0.55 from HEC-RAS. Test CW_0.25-0.65 -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 Figure 4-5 (b). Initial and final (18 hour) bed and water surface profiles for Test CW_0.25-0.65 from HEC-RAS.

4-6 Revised Clear-Water and Live-Bed Contraction Scour Analysis Figure 4-5 (c). Initial and final (30.5 hour) bed and water surface profiles for Test CW_0.25-0.75 from HEC-RAS. Test CW_0.25-0.75 -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 Test CW_0.25-0.80 -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 Figure 4-5 (d). Initial and final (18 hour) bed and water surface profiles for Test CW_0.25-0.80 from HEC-RAS. 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0 10 20 30 40 50 60 70 80 90 Ve lo ci ty ( ft /s ) Station (ft) Initial Conditions Final, V1/Vc = 0.55 Final, V1/Vc = 0.65 Final, V1/Vc = 0.75 Final, V1/Vc = 0.80 0.80 0.75 0.65 0.55 Velocity Profiles at Centerline: Clear-Water Tests at Severe Contraction Ratio B2/B1 = 0.25 Vn1/Vc Figure 4-6. Initial and final velocity profiles from HEC-RAS for four clear-water scour tests at the Severe contraction ratio B2/B1 = 0.25.

Clear-Water Laboratory Testing Results 4-7   τ0 = 0.03 lb/ft2 to 0.06 lb/ft2. These values suggest that the critical values Vc and τc for the threshold of particle motion are substantially greater than would otherwise be predicted from the standard HEC-18 equations. Figure 4-8 shows the entire point cloud image from a typical LiDAR scan (Test CW_0.25-0.75 final condition is shown in this example). The bed, flume walls, bracing, and other laboratory features are depicted. Figure 4-9 provides a montage of the LiDAR data for just the sand bed at the end of Test CW_0.25-0.75. In this figure, a plan view, side view, and orthographic view of the bed are shown for illustration purposes. From the LiDAR bed elevation data, the regions of short-, intermediate-, and long-contraction scour depths can be identified. Moreover, the LiDAR point cloud resolution is sufficient to distinguish individual bedforms, such that the bedform amplitude and wavelength can be estimated. For Test CW_0.25-0.75, Figure 4-10 indicates the three regions of contraction scour and shows that the amplitude of the bedforms (crest-to-trough) is about 0.14 ft (1.7 in.). The wavelength is estimated to be about 2.9 ft. For the four clear-water tests, the bedforms appear to represent the transition region between ripples and dunes. 0.000 0.025 0.050 0.075 0.100 0.125 0.150 0.175 0.200 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: Clear-Water Tests at Severe Contraction Ratio B2/B1 = 0.25 Initial conditions Final, V1/Vc = 0.55 Final, V1/Vc = 0.65 Final, V1/Vc = 0.75 Final, V1/Vc = 0.80 Vn1/Vc 0.80 0.75 0.65 0.55 Figure 4-7. Initial and final bed shear-stress profiles from HEC-RAS for four clear-water scour tests at the Severe contraction ratio B2/B1 = 0.25. Figure 4-8. Typical LiDAR point cloud scan.

4-8 Revised Clear-Water and Live-Bed Contraction Scour Analysis 4.1.4 Scour Depth Summary Observed clear-water equilibrium contraction scour depths and flow depths are summarized in Table 4-3 for the four clear-water contraction scour tests performed with the Severe contrac- tion ratio. 4.2 Moderate Contraction Ratio Following completion of the Severe contraction ratio tests, the flume was reconfigured to a new contraction ratio B2/B1 = 0.50. The new contracted section was 4 ft wide. Important construction considerations included special attention to the forming of the entrance corners, Plan view Side view Ortho view showing close-up of throat region. Figure 4-9. LiDAR scan of the bed surface at the completion of Test CW_0.25-0.75 from different viewpoints. -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0 10 20 30 40 50 60 70 80 90 El ev ati on , f ee t Station, feet 30.5 hours - bed elevation from lidar troughs: -0.34 crests: -0.20 short intermediate long Figure 4-10. Test CW_0.25-0.75 bed profile interpretation showing regions of short-, intermediate-, and long-contraction scour. Bedform characteristics are also shown.

Clear-Water Laboratory Testing Results 4-9   and sealing the wall to the floor and the joints between wall panels. Filter fabric was placed below the sand bed and partially up the side walls. Three clear-water 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 ft = 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 the previous series of tests. The naming convention for each test is the same as previously established, and provides information on the sediment transport condition, contraction ratio, and approach velocity ratio. For example, Test CW_0.50-0.55 indicates a clear-water test with a contraction ratio of 0.50 and a velocity ratio of 0.55. Note that Vn1 is the assumed velocity in the upstream 8-ft- wide approach channel associated with normal flow depth, and Vc is the critical velocity for sediment transport. The clear-water tests conducted at the Moderate contraction ratio are summarized in Table 4-4. Prior to each 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. 4.2.1 Bed and Water Surface Profiles Measurement of water surface and bed elevations at predetermined flume stations were taken periodically during the 18-hour duration of each clear-water test. 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 and 1-hour intervals; during the latter stages of the test, measurements were taken at 3-hour intervals. The water surface was Test number Duration(hours) Contraction ratio B2/B1 Velocity ratio Vn1/Vc Discharge Q (ft3/s) y2 tailgate (ft) CW_0.50-0.55 18 0.50 0.55 2.26 0.58 CW_0.50-0.65 18 0.50 0.65 2.67 0.58 CW_0.50-0.75 18 0.50 0.75 3.09 0.58 Table 4-4. Clear-water contraction scour tests at Moderate contraction ratio B2/B1 = 0.50. Test number Observed short- contraction scour Ys-Max (ft) Observed short- contraction flow depth YMax (ft) Observed long- contraction scour Ys (ft) Observed long- contraction flow depth Y2 (ft) CW_0.25-0.55 0.74 1.48 0.20 0.88 CW_0.25-0.65 0.95 1.71 0.28 0.97 CW_0.25-0.75 1.06 1.84 0.34 1.07 CW_0.25-0.80 1.04 1.84 0.36 1.09 Table 4-3. Equilibrium clear-water contraction scour results for the Severe contraction ratio.

4-10 Revised Clear-Water and Live-Bed Contraction Scour Analysis measured along the centerline of the flume; bed elevations were obtained at both the centerline and along the right wall. Figures 4-11 (a) and (b) are typical photographs of a clear-water test at the 0.50 contraction ratio. In Figure (a), note the beginning of bedform development in the contracted section even before the target discharge is achieved. Figure (b) shows the ripple bed in the contracted section compared with the plane bed in the approach section at the completion of a clear-water test. For all tests at the Moderate contraction ratio, it was observed that the bed elevation along the wall was significantly different from that along the centerline. As an example, Figure 4-12 pre- sents the bed and water surface elevations from the LiDAR scan at the end of Test CW_0.50-0.75. In this figure, the bed elevation at the centerline and along the right wall are both shown to illustrate this observation. Figure 4-13 presents the bed and water surface elevations recorded from the data collection carriage at each of the predetermined measurement stations at the end of Test CW_0.50-0.75. In this figure, the bed elevation at the centerline and along the right wall are both shown. Figure 4-14 provides an example of water surface and bed evolution during a clear-water test (Test CW_0.50-0.75 shown). In this figure, note that the water surface changes only by a few hundredths of a foot during an 18-hour clear-water test at the Moderate contraction ratio, whereas bed elevations change by about 0.5 ft. Also, note that the bed profile taken along the centerline is markedly different from that taken along the right wall, an aspect not experienced in the previous tests at the Severe contraction ratio (B2/B1 = 0.25, described in Section 4.1). 4.2.2 1D Modeling Analysis Each of the three clear-water contraction scour tests at the Moderate contraction ratio B2/B1 = 0.50 was calibrated to a 1D HEC-RAS model (see Section 3.5). Each model was set up a. Initiation of a clear-water test during the gradual start-up period. b. At the completion of a clear-water test. Figure 4-11. Typical photos of a clear-water test at Moderate contraction ratio B2/B1 = 0.50.

Clear-Water Laboratory Testing Results 4-11   -1.20 -1.00 -0.80 -0.60 -0.40 -0.20 0.00 0.20 0 10 20 30 40 50 60 70 80 90 El ev ati on , ft Station, ft CW_0.50-0.75 Lidar Bed Profiles Initial bed: Centerline Initial bed: Wall Final bed at 18 hours: Centerline Final bed at 18 hours: Wall Figure 4-12. Initial and final bed profiles from LiDAR scan, Test CW_0.50-0.75. -0.60 -0.40 -0.20 0.00 0.20 0.40 0.60 0.80 0 10 20 30 40 50 60 70 80 90 El ev ati on , ft Station, ft CW_0.50-0.75 Bed and Water Surface Profiles at Measurement Stations 18 hours - bed elevation - centerline 18 hours - bed elevation - right wall 18 hours - water surface elevation Figure 4-13. Bed and water surface profiles for Test CW_0.50-0.75 at final conditions from the predetermined measurement stations. to replicate the exact dimensions of the flume geometry, with model stations corresponding to the predetermined measurement locations in the flume. From previous rigid-bed tests, a Manning’s n value of 0.013 was again used to represent the relatively smooth, painted vertical side walls. The Manning’s n of the bed was varied until a good match was achieved between the observed water surface elevations at the end of each test versus the water surface predicted by HEC-RAS. The results from the final HEC-RAS calibrations for the three clear-water tests at the Moderate contraction ratio are provided in Figure 4-15 (a) through (c). Final bed elevations

4-12 Revised Clear-Water and Live-Bed Contraction Scour Analysis CW_0.50-0.75 Bed Evolution-Centerline CW_0.50-0.75 Bed Evolution-Wall CW_0.50-0.75 Water Surface Evolution-Centerline 0.50 0.55 0.60 0.65 0.70 0.75 0 10 20 30 40 50 60 70 80 90 El ev ati on (ft ) Station (ft) 18-Hr 15-Hr 12-Hr 9-Hr 6-Hr 4-Hr 3-Hr 2-Hr 1-Hr 0.5-Hr -0.50 -0.45 -0.40 -0.35 -0.30 -0.25 -0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10 0 10 20 30 40 50 60 70 80 90 Be d El ev . ( ft ) Station (ft) 18hr 15 hr 12 hr 9 hr 6 hr 4 hr 3 hr 2 hr 1 hr 0.5 hr 0 hr -0.50 -0.45 -0.40 -0.35 -0.30 -0.25 -0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10 0 10 20 30 40 50 60 70 80 90 Be d El ev . ( ft ) Station (ft) 18hr 15 hr 12 hr 9 hr 6 hr 4 hr 3 hr 2 hr 1 hr 0.5 hr 0 hr Figure 4-14. Water surface and bed evolution, Test CW_0.50-0.75.

Clear-Water Laboratory Testing Results 4-13   Test CW_0.50-0.55 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 Figure 4-15 (a). Initial and final (18 hour) bed and water surface profiles for Test CW_0.50-0.55 from HEC-RAS. Test CW_0.50-0.65 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 Figure 4-15 (b). Initial and final (18 hour) bed and water surface profiles for Test CW_0.50-0.65 from HEC-RAS. Test CW_0.50-0.75 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 Figure 4-15 (c). Initial and final (18 hour) bed and water surface profiles for Test CW_0.50-0.75 from HEC-RAS.

4-14 Revised Clear-Water and Live-Bed Contraction Scour Analysis 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 clear-water tests conducted at the Moderate contraction ratio. 4.2.3 Velocity and Shear-Stress Profiles Velocity profiles were developed from the calibrated HEC-RAS model for each of the three clear-water tests at the completion of each 18-hour run. The results are shown in Figure 4-16 for the centerline measurements. In this figure, the final (post-scour) velocities downstream of flume station 40 are seen to consistently range from about 0.75 ft/s to 1.15 ft/s. These velocity values compare with the final (post-scour) velocities from the previous series of clear-water tests at the Severe contraction ratio (B2/B1 = 0.25), where the final velocities ranged from 1.4 ft/s to 1.6 ft/s. Shear-stress profiles were developed from the calibrated HEC-RAS model for each of the three clear-water tests at the completion of each 18-hour run. The results are shown in Figure 4-17 for the centerline measurements. In this figure, the final (post-scour) shear stress downstream of 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 0 10 20 30 40 50 60 70 80 90 Ve lo ci ty , ft /s Station, ft Velocity Profiles at Centerline: Clear-Water Tests at Moderate Contraction Ratio B2/B1 = 0.50 Initial conditions Final, V1/Vc = 0.55 Final, V1/Vc = 0.65 Final, V1/Vc = 0.75 Figure 4-16. Initial and final velocity profiles for three clear-water tests at the Moderate contraction ratio B2/B1 = 0.50. 0.00 0.02 0.04 0.06 0.08 0.10 0 10 20 30 40 50 60 70 80 90 Be d Sh ea r S tr es s, lb /ft 2 Station, ft Initial conditions Final, V1/Vc = 0.55 Final, V1/Vc = 0.65 Final, V1/Vc = 0.75 Shear Stress Profiles at Centerline: Clear-Water Tests at Moderate Contraction Ratio B2/B1 = 0.50 Figure 4-17. Initial and final shear-stress profiles for three clear-water tests at the Moderate contraction ratio B2/B1 = 0.50.

Clear-Water Laboratory Testing Results 4-15   flume station 40 ranges from about 0.02 lb/ft2 to 0.07 lb/ft2. These values compare with the final (post-scour) shear stresses from the previous series of clear-water tests at the Severe contraction ratio (B2/B1 = 0.25), where the final shear stresses ranged from 0.03 lb/ft2 to 0.06 lb/ft2. 4.2.4 Scour Depth Summary Observed clear-water equilibrium contraction scour depths are summarized in Table 4-5 for the three clear-water contraction scour tests performed with the Moderate contraction ratio. Scour depths along both the centerline and wall are listed. 4.3 Mild Contraction Ratio Following completion of the Moderate contraction ratio tests, the flume was reconfigured to a new contraction ratio B2/B1 = 0.75. The new contracted section was 6 ft wide. Three clear- water 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 ft = 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 the previous series of tests. The naming convention for each test is the same as previously established, and provides information on the sediment transport condition, contraction ratio, and approach velocity ratio. For example, Test CW_0.75-0.75 indicates a clear-water test with a contraction ratio of 0.75 and a velocity ratio of 0.75. Note that Vn1 is the assumed velocity in the upstream 8-ft- wide approach channel associated with normal flow depth, and Vc is the critical velocity for sediment transport. Clear-water tests conducted at the Mild contraction ratio are summarized in Table 4-6. Prior to each 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, Test number Short Contraction: Maximum scour depth (ft), at throat Long Contraction: Average scour depth (ft), sta. 40-80 Centerline Wall Centerline Wall CW_0.50-0.55 0.24 0.25 0.07 0.05 CW_0.50-0.65 0.22 0.33 0.05 0.09 CW_0.50-0.75 0.34 0.42 0.07 0.10 Table 4-5. Clear-water contraction scour depths, Moderate 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) CW_0.75-0.75 18 0.75 0.75 3.09 0.58 CW_0.75-0.85 18 0.75 0.85 3.50 0.58 CW_0.75-0.95 18 0.75 0.95 3.91 0.58 Table 4-6. Clear-water contraction scour tests at Mild contraction ratio B2/B1 = 0.75.

4-16 Revised Clear-Water and Live-Bed Contraction Scour Analysis 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. 4.3.1 Bed and Water Surface Profiles Measurement of water surface and bed elevations at predetermined flume stations were taken periodically during the 18-hour duration of each clear-water test. 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 and 1-hour intervals; during the latter stages of the test, measurements were taken at 3-hour intervals. The water surface was measured along the centerline of the flume; bed elevations were obtained at both the centerline and along the right wall. Figure 4-18 is a typical photograph of a clear-water test at the 0.75 contraction ratio. In Figure 4-18, note the beginning of bedform development in the contracted section even before the target discharge is achieved. Also, note the plane bed in the approach section. For all tests at the Mild contraction ratio, it was observed that the bed elevation along the wall was significantly different from that along the centerline. As an example, Figure 4-19 presents the bed and water surface elevations from the LiDAR scan at the end of Test CW_0.75-0.95. In this figure, the bed elevation at the centerline and along the right wall are both shown to illustrate this observation. Figure 4-20 presents the bed and water surface elevations recorded from the data collection carriage at each of the predetermined measurement stations at the end of Test CW_0.75-0.95. In this figure, the bed elevation at the centerline and along the right wall are both shown. Figure 4-21 provides an example of water surface and bed evolution during a clear-water test at a Mild contraction with the highest velocity ratio (water surface and bed elevation changes Figure 4-18. Typical photo of a clear-water test at Mild contraction ratio B2/B1 = 0.75.

Clear-Water Laboratory Testing Results 4-17   Figure 4-19. Initial and final bed profiles from LiDAR scan, Test CW_0.75-0.95. Figure 4-20. Bed and water surface profiles for Test CW_0.75-0.95 at final conditions from the predetermined measurement stations.

Figure 4-21. Water surface and bed evolution, Test CW_0.75-0.95.

Clear-Water Laboratory Testing Results 4-19   at lower velocity ratios were less significant) (Test CW_0.75-0.95 shown). In this figure, note that the water surface changes only by a few hundredths of a foot during an 18-hour clear-water test at the Mild contraction ratio, whereas bed elevations change by about 0.5 ft. Also, note that the bed profile taken along the centerline is markedly different from that taken along the right wall, an aspect not experienced in the previous tests at the Severe contraction ratio (B2/B1 = 0.25, described in Section 4.1). 4.3.2 1D Modeling Analysis Each of the three clear-water contraction scour tests at the Mild contraction ratio B2/B1 = 0.75 was calibrated to a 1D HEC-RAS model (see Section 3.5). Each model was set up to replicate the exact dimensions of the flume geometry, with model stations corresponding to the pre- determined measurement locations in the flume. From previous rigid-bed tests, a Manning’s n value of 0.013 was again used to represent the relatively smooth, painted vertical side walls. The Manning’s n of the bed was varied until a good match was achieved between the observed water surface elevations at the end of each test versus the water surface predicted by HEC-RAS. The results from the final HEC-RAS calibrations for the three clear-water tests at the Moderate contraction ratio are provided in Figure 4-22 (a) through (c). 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 clear-water tests conducted at the Mild contraction ratio. 4.3.3 Velocity and Shear-Stress Profiles Velocity profiles were developed from the calibrated HEC-RAS model for each of the three clear-water tests at the completion of each 18-hour run. The results are shown in Figure 4-23 for the centerline measurements. In this figure, the final (post-scour) velocities downstream of flume station 40 are seen to consistently range from about 0.75 ft/s to 1.15 ft/s. These velocity values compare with the final (post-scour) velocities from the series of clear-water tests at the Severe contraction ratio (B2/B1 = 0.25), where the final velocities ranged from 1.4 ft/s to 1.6 ft/s. Figure 4-22 (a). Initial and final (18 hour) bed and water surface profiles for Test CW_0.75-0.95 from HEC-RAS.

4-20 Revised Clear-Water and Live-Bed Contraction Scour Analysis Figure 4-22 (c). Initial and final (18 hour) bed and water surface profiles for Test CW_0.75-0.95 from HEC-RAS. Figure 4-22 (b). Initial and final (18 hour) bed and water surface profiles for Test CW_0.75-0.95 from HEC-RAS. Figure 4-23. Initial and final velocity profiles for three clear-water tests at the Mild contraction ratio B2/B1 = 0.75.

Clear-Water Laboratory Testing Results 4-21   Shear-stress profiles were developed from the calibrated HEC-RAS model for each of the three clear-water tests at the completion of each 18-hour run. The results are shown in Figure 4-24 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 0.07 lb/ft2. These values compare with the final (post-scour) shear stresses from the series of clear-water tests at the Severe contraction ratio (B2/B1 = 0.25), where the final shear stresses ranged from 0.03 lb/ft2 to 0.06 lb/ft2. 4.3.4 Scour Depth Summary Observed clear-water equilibrium contraction scour depths are summarized in Table 4-7 for the three clear-water contraction scour tests performed with the Mild contraction ratio. Scour depths along both the centerline and wall are listed. Figure 4-24. Initial and final velocity profiles for three clear-water 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 CW_0.75-0.75 0.12 0.25 0.06 0.06 CW_0.75-0.85 0.21 0.25 0.08 0.14 CW_0.75-0.95 0.12 0.41 0.07 0.14 Table 4-7. Clear-water contraction scour depths, Mild contraction ratio B2/B1 = 0.75.