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

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6-1   6.1 Overview Rigid-boundary hydraulics of contracted reaches have not been adequately addressed in previous laboratory studies. A series of rigid-bed tests at contraction ratio B2/B1 = 0.25 (Severe contraction) were performed in the 8-ft flume at CSU to better define contraction hydraulics without the complicating effects of scour. For these tests, the bed and walls of the flume were constructed of plywood and a series of flows were run at the Severe contraction ratio. For all rigid-bed (plywood) tests, Acoustic Doppler Velocimetry (ADV) measurements were made at predetermined points along two cross sections in the approach reach, and six more cross sections in the vicinity of the contraction throat. At each cross section, the measurement points were closely spaced in the lateral dimension to obtain a detailed picture of the local flow field. Vx, Vy, and Vz velocity components were recorded at six-tenths flow depth at a frequency of 40 Hz for approximately 25 seconds at each location. Thus, at each measurement location, there are approximately 1,000 readings for each of the velocity components (longitudinal, lateral, and vertical). Calculation of the mean and root mean square (RMS) velocities at each point allows determination of the turbulence intensity at each location. 6.2 Rigid-Bed Testing at a Severe Contraction Ratio Photographs of a typical rigid-bed test in progress are shown in Figures 6-1 (a) and (b). The rigid-bed tests conducted at the Severe contraction ratio are summarized in Table 6-1. Note that the contraction ratio, velocity ratio, discharge, and tailgate settings are identical to those used in the clear-water tests at the Severe contraction ratio. 6.2.1 1D Modeling Analysis Optimal Manning’s n values for the four rigid-bed tests were obtained in the same manner as described in Section 3.5. Results from HEC-RAS calibrations for these tests at the Severe con- traction ratio are provided in Figures 6-2 (a) through (d). Bed elevations used in the 1D models are referenced to elevation 0.00 as confirmed by measurements taken from the data collection carriage. Refer to Table 3-4 for details of the HEC-RAS calibrations for all rigid-bed tests con- ducted at the Severe contraction ratio. 6.2.2 ADV Data For all four rigid-bed (plywood) tests at this contraction ratio, ADV measurements were made at predetermined points along two cross sections in the approach reach, and six more C H A P T E R 6 Rigid-Bed Laboratory Testing Results

6-2 Revised Clear-Water and Live-Bed Contraction Scour Analysis a. Contraction entrance b. Rigid-bed test looking upstream Figure 6-1. Photographs of rigid-bed Test RB_0.25-0.75 in progress. Test number Duration(hours) Contraction ratio B2/B1 Velocity ratio Vn1/Vc Discharge Q (ft3/s) y2 tailgate (ft) RB_0.25-0.55 2 0.25 0.55 2.24 0.58 RB_0.25-0.65 2 0.25 0.65 2.65 0.58 RB_0.25-0.75 2 0.25 0.75 3.05 0.58 RB_0.25-0.80 2 0.25 0.80 3.26 0.58 Table 6-1. Rigid-bed contraction scour tests at the Severe contraction ratio. Test RB_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) Plywood (HEC-RAS) Plywood (observed) Initial bed Figure 6-2 (a). Bed and water surface profiles for Test RB_0.25-0.55 from HEC-RAS.

Rigid-Bed Laboratory Testing Results 6-3 Test RB_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) Plywood (HEC-RAS) Plywood (observed) Initial bed Figure 6-2 (b). Bed and water surface profiles for Test RB_0.25-0.65 from HEC-RAS. Test RB_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) Plywood (HEC-RAS) Plywood (observed) Initial bed Figure 6-2 (c). Bed and water surface profiles for Test RB_0.25-0.75 from HEC-RAS. Test RB_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) Plywood (HEC-RAS) Plywood (observed) Initial bed Figure 6-2 (d). Bed and water surface profiles for Test RB_0.25-0.80 from HEC-RAS.

6-4 Revised Clear-Water and Live-Bed Contraction Scour Analysis cross sections in the vicinity of the contraction throat. Vx, Vy, and Vz velocity components were recorded at six-tenths flow depth at a frequency of 40 Hz for approximately 25 seconds at each location. Figure 6-3 shows the location of the ADV measurements. The results of the ADV measurements show clear and consistent trends in the flow fields for all four discharges. The ADV measurements for each of the four rigid-bed tests are presented graphically as Figures 6-4 (a) through (d). Note the change in vertical scale needed to display the velocity magnitudes in the X (longitudinal), Y (lateral), and Z (vertical) directions. A summary of observations follows. (a) At all locations, the streamwise velocity Vx is greater than the lateral or vertical velocity components. (b) A pronounced wall effect on Vx is seen in the transition section (station 3) and the contracted reach (stations 4–6) but not in the approach reach. (c) Significant lateral velocities Vy are seen in the transition section (station 3) and the contracted reach (stations 4–6), and even at station 2 in the approach reach. Lateral velocities are positive on the left-hand side of the flume and negative on the right-hand side. (d) Lateral velocities Vy are most pronounced at station 4, which is at the end of the transition reach and the beginning of the fully contracted flow. (e) Significant downward, plunging flow is seen in the Vz velocity component at stations 4 and 4.5 in all tests, indicated by negative velocity values. Upwelling then occurs at stations 5 and 5.5 immediately downstream, as indicated by positive velocity values. (f) Vertical flow components Vz are essentially dampened out at downstream station 6, which is located 3.7 ft downstream of the contraction throat. The ADV data were also used to assist in calibrating 2D and 3D CFD models to selected tests using the models SRH-2D (Sedimentation and River Hydraulics-2D) and FLOW-3D, respectively (see Chapter 7). The objective was to better define the vena-contracta region in the vicinity of the contraction throat and also to examine the plunging flow/upwelling in this region previously described. Figure 6-5 is a photograph of a dye trace at the entrance to the contraction. -2.00 0.00 2.00 4.00 6.00 8.00 10.00 -5.0 0.0 5.0 10.0 15.0 20.0 25.0 La te ra l s ta tio n, ft Longitudinal station, ft ADV measurement locations for tests at B2/B1= 0.25 Sta. 1 2 3 4 4.5 5.5 65 Flow Figure 6-3. Locations of ADV measurements for four rigid-bed tests at the Severe contraction ratio (B2/B1 = 0.25).

Rigid-Bed Laboratory Testing Results 6-5 0.0 0.5 1.0 1.5 2.0 2.5 0.00 2.00 4.00 6.00 8.00 Vx , ft /s Lateral location, ft CW_0.25-0.55 rigid bed Vx sta 1 sta 2 sta 3 sta 4 sta 4.5 sta 5 sta 5.5 sta 6 -1.00 -0.75 -0.50 -0.25 0.00 0.25 0.50 0.75 1.00 0.00 2.00 4.00 6.00 8.00 Vy , ft /s Lateral location, ft CW_0.25-0.55 rigid bed Vy sta 1 sta 2 sta 3 sta 4 sta 4.5 sta 5 sta 5.5 sta 6 -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.00 2.00 4.00 6.00 8.00 Vz , ft /s Lateral location, ft CW_0.25-0.55 rigid bed Vz sta 1 sta 2 sta 3 sta 4 sta 4.5 sta 5 sta 5.5 sta 6 Figure 6-4 (a). ADV measurements for rigid-bed Test RB_0.25-0.55.

6-6 Revised Clear-Water and Live-Bed Contraction Scour Analysis 0.0 0.5 1.0 1.5 2.0 2.5 0.00 2.00 4.00 6.00 8.00 Vx , ft /s Lateral location, ft CW_0.25-0.65 rigid bed Vx sta 1 sta 2 sta 3 sta 4 sta 4.5 sta 5 sta 5.5 sta 6 -1.00 -0.75 -0.50 -0.25 0.00 0.25 0.50 0.75 1.00 0.00 2.00 4.00 6.00 8.00 Vy , ft /s Lateral location, ft CW_0.25-0.65 rigid bed Vy sta 1 sta 2 sta 3 sta 4 sta 4.5 sta 5 sta 5.5 sta 6 -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.00 2.00 4.00 6.00 8.00 Vz , ft /s Lateral location, ft CW_0.25-0.65 rigid bed Vz sta 1 sta 2 sta 3 sta 4 sta 4.5 sta 5 sta 5.5 sta 6 Figure 6-4 (b). ADV measurements for rigid-bed Test RB_0.25-0.65.

Rigid-Bed Laboratory Testing Results 6-7 0.0 0.5 1.0 1.5 2.0 2.5 0.00 2.00 4.00 6.00 8.00 Vx , ft /s Lateral location, ft sta 1 sta 2 sta 3 sta 4 sta 4.5 sta 5 sta 5.5 sta 6 -1.00 -0.75 -0.50 -0.25 0.00 0.25 0.50 0.75 1.00 0.00 2.00 4.00 6.00 8.00 Vy , ft /s Lateral location, ft sta 1 sta 2 sta 3 sta 4 sta 4.5 sta 5 sta 5.5 sta 6 -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.00 2.00 4.00 6.00 8.00 Vz , ft /s Lateral location, ft sta 1 sta 2 sta 3 sta 4 sta 4.5 sta 5 sta 5.5 sta 6 CW_0.25-0.75 rigid bed Vx CW_0.25-0.75 rigid bed Vy CW_0.25-0.75 rigid bed Vz Figure 6-4 (c). ADV measurements for rigid-bed Test RB_0.25-0.75.

6-8 Revised Clear-Water and Live-Bed Contraction Scour Analysis sta 1 sta 2 sta 3 sta 4 sta 4.5 sta 5 sta 5.5 sta 6 0.0 0.5 1.0 1.5 2.0 2.5 0.00 2.00 4.00 6.00 8.00 Vx , ft /s Lateral location, ft CW_0.25-0.80 rigid bed Vx -1.00 -0.75 -0.50 -0.25 0.00 0.25 0.50 0.75 1.00 0.00 2.00 4.00 6.00 8.00 Vy , ft /s Lateral location, ft sta 1 sta 2 sta 3 sta 4 sta 4.5 sta 5 sta 5.5 sta 6 CW_0.25-0.80 rigid bed Vy -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.00 2.00 4.00 6.00 8.00 Vz , ft /s Lateral location, ft sta 1 sta 2 sta 3 sta 4 sta 4.5 sta 5 sta 5.5 sta 6 CW_0.25-0.80 rigid bed Vz Figure 6-4 (d). ADV measurements for rigid-bed Test RB_0.25-0.80.

Rigid-Bed Laboratory Testing Results 6-9 Figure 6-5. Dye trace at the entrance to the contraction during a rigid-bed test.