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57 L B2 B1 q Turbulence (a) H1 V1 V2 X ma Zuni Z ma (b) Figure 7.1. Concepts and definitions in contraction scour. Based on the above analysis, the flume tests were divided 2. Initial water surface elevation along the centerline of into two parts: the tests run to obtain the function f (V1, B2/B1, the channel (measured using a point gage); H1), called primary tests, and the tests run for the correction 3. Contraction scour profile along the centerline of the factors K and KL, called secondary tests The contraction bottom of the channel, as a function of time (measured geometries were shown in Section 5.3 and in Figure 5.4. The using a point gage); parameters for the tests performed are listed in Table 7.1 4. Two abutment scour measurements, as a function of (primary tests) and Table 7.2 (secondary tests). There were time (measured using a point gage); seven primary tests where the contraction scour was gener- 5. Final longitudinal velocity profile along the channel ated in a long, contracted channel with a 90-degree transition centerline (measured using the ADV); angle. Among them, the contraction ratio was varied in Tests 6. Final water surface elevation along the channel center- 1, 2, and 3; the water depth was varied in Tests 4, 5, and 6; line (measured using a point gage); and and the velocity was varied in Tests 2, 6, and 7. There were 7. Photos of the final scour hole shape (taken by a digital two groups of secondary tests: Tests 2, 9, 10, and 11 were camera). for the transition angle effect on contraction scour, and Tests 2, 12, 13, and 14 were for the contraction length effect 7.4 FLUME TESTS: FLOW OBSERVATIONS on contraction scour. AND RESULTS The following measurements were carried out for each flume test: The water surface profiles along the channel centerline at the beginning and at the end of Test 2 are plotted in Figure 7.2. 1. Initial velocity distribution by ADV: vertical velocity The approaching flow of Test 2 was in the subcritical flow profile in the middle of the channel at a location of 1.2 m regime (all tests were subcritical except Test 1, which was upstream of the contraction and the longitudinal profile supercritical at the beginning of the test); as a result, there was along the centerline of the channel; a drop in water surface elevation in the contracted section. This TABLE 7.1 Parameters and results for the primary contraction scour tests V1 V1 V H1 H1 H Test Zmax Zunif Xmax (before) (after) (Hec) B2/B1 (before) (after) (Hec) L/B1 No. () (mm) (mm) (mm) (cm/s) (cm/s) (cm/s) (mm) (mm) (mm) 1 13.8 34.1 103 0.25 297 164.77 170 90 2.932 357.143 227.273 80 2 29 31 67 0.5 171.15 162.03 150 90 3.868 116.279 70.423 285 3 45 45.9 79 0.75 121.6 106.4 100 90 3.38 72.993 47.847 620 4 20.5 20.5 53 0.5 108.2 108.22 100 90 3.38 28.653 11.862 210 5 20.5 20.7 41 0.5 251.4 251.4 240 90 3.38 37.736 19.881 210 6 20.5 20.5 46 0.5 171.76 171.76 160 90 3.38 36.101 13.021 210 7 39 39 84 0.5 174.19 174.19 160 90 3.38 142.857 142.857 210

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58 TABLE 7.2 Parameters and results for the secondary contraction scour tests V1 V1 V H1 H1 H Test Zmax Zunif Xmax (before) (after) (Hec) B2/B1 (before) (after) (Hec) L/B1 No. () (mm) (mm) (mm) (cm/s) (cm/s) (cm/s) (mm) (mm) (mm) 2 29 31 67 0.5 171.15 162.03 150 90 3.868 116.279 70.423 285 9 30 30.2 68 0.5 161 160.21 150 15 3.868 90.909 ----- 785 10 30 30.2 78 0.5 153.6 152 110 45 3.38 128.205 95.234 385 11 30 29.3 69 0.5 166.59 163.25 150 60 3.38 80 41.322 785 12 29 33 75 0.5 172.37 160.5 130 90 0.844 111.11 ----- 85 13 29.2 33 72 0.5 170.54 162.34 140 90 0.25 128.21 ----- 152 14 29.2 34.1 70 0.5 180 164.77 140 90 0.125 208.33 ----- 385 difference decreased as contraction scour developed. When decreased but did not become zero. Instead, it was observed the equilibrium scour depth profile was reached, the water sur- that the final value of the velocity in the contracted channel face elevation in the contracted section could be considered as reached the same approximate value for all flume tests on the level with the water surface elevation in the approach channel. Porcelain clay. This tends to indicate that contraction scour This observation is also mentioned by other researchers stops at the critical velocity in the contracted channel no mat- (Laursen 1960, Komura 1966). As a result, the contraction ter what the contraction geometry is. scour depth can be simply calculated by subtracting the upstream water depth H1 from the total water depth in the con- V2 (equi) = Vc (7.3) traction section H2. At equilibrium contraction scour, the con- traction scour depth is It also was observed that the highest velocity and the low- est water surface elevation in the contracted channel hap- Zmax = H2 (equi) - H1 (7.2) pened at the same approximate location behind the contrac- tion inlet, but this location was different from the maximum In the final profile of the water surface elevation, it was also contraction scour location as described later. noticed that the upstream water surface gradually lowered to HEC-RAS (Hydraulic Engineering Center--River Analy- the downstream water surface. In other words, the equilibrium sis System, 1997) is a widely used program in open channel water surface elevation was intermediate between the initial analysis. It was used with the flume cross-section profiles upstream and downstream elevations. before scour started to predict the quantities measured during The longitudinal velocity profiles along the channel center- the tests. The HEC-RAS outputs are listed in Tables 7.1 and line at the beginning and at the end of Test 2 are presented in 7.2 and compared with the measured water surface elevation Figure 7.3. The velocity was measured at a depth of 0.4 H and velocity profiles in Figures 7.2 and 7.3. It is found that from the water surface with the ADV. As expected, the veloc- HEC-RAS leads to relatively constant values of the water sur- ity increased in the contracted channel since the water eleva- face elevation and velocity before and after the contraction, tion decreased. As contraction scour deepened, the difference which is a significant simplification of the measured behavior. 90 180 Water Surface Elevation (mm) 60 Velocity (cm/s) 120 Before After Before HEC-RAS 30 After 60 HEC-RAS 0 0 -1000 -500 0 500 1000 1500 -1000 -500 0 500 1000 1500 X(mm) X(mm) Figure 7.2. Water surface elevations along the channel Figure 7.3. Velocity distribution along the channel centerline in Test 2. centerline in Test 2.