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64
depth of contraction scour after the bridge site has been sub- 150
jected to a long-term hydrograph.
120
7.8 CORRECTION FACTORS FOR TRANSITION
ANGLE AND CONTRACTION LENGTH
Zmax (mm)
90
There are seven secondary tests listed in Table 7.2. Tests
2, 9,10, and 11 are for the transition angle effect, and Tests 60
2, 12, 13, and 14 are for the contraction length effect. Test 2
is the reference case for both groups and comparisons to 30
Test 2 are used to derive the correction factors for the equations
that give Zmax, Zunif, and Xmax.
0
A smooth transition angle is generally built to ease the
0 15 30 45 60 75 90
effect of the approaching flow (Figure 7.18). As can be seen,
the approaching flow runs against the abutments and then is Transition Angle (Degree)
guided toward the contracted channel at an angle related to the Figure 7.19. Transition angle effect on maximum
transition angle . The location of the maximum contraction contraction scour depth.
scour depth will be pushed further back from the contraction
inlet when the transition angle becomes smoother. Even
though the transition angle can change the local flow pattern Xmax ()
around the contraction inlet, this influence decreases into the = 0.4793 tan + 0.9475 (7.13)
Xmax (90°)
contraction channel where a uniform flow develops. This
indicates that the transition angle may affect the maximum
Figure 7.18 indicates that Xmax is at (B2/2)/tan if the flow
contraction scour but not the uniform contraction scour.
follows the prolongation of the transition sides, but Equation
Figure 7.19 shows the influence of the transition angle on
7.13 shows that the real location of the maximum contraction
the maximum contraction scour depth Zmax. It can be seen that
scour is farther downstream of this estimated point.
does not have a clear impact on Zmax. This observation is con-
In summary, if Test 2, the test with the 90-degree transition
sistent with Komura's (1977) observation on sands where
angle, is selected as the reference case for the transition angle
he stated that a smooth transition angle was not helpful in
reducing the scour depth around the abutment inlets. If Test effect, and the correction factors are calculated as the ratios
10 is ignored due to its odd scour profile, the uniform con- between the value for over the value for 90 degrees, the tran-
traction scour depth Zunif is practically independent of the tran- sition angle effect for contraction scour, in three aspects of
sition angle (Figure 7.20), as expected. However, the Zmax, Zunif, and Xmax, is
transition angle has a significant impact on the location of the
maximum contraction scour Xmax, as shown in Figure 7.21. K Zmax = 1.0
Regression analysis of the data leads to the following rela-
tionship between the Xmax and : K Zunif = 1.0 (7.14)
K Xmax = 0.48 tan + 0.95
Distance X (mm)
-400 0 400 800 1200 1600
0
Relative Scour Depth (Z/Zmax)
Angle=15 Angle=45
Angle=60 Angle=90
0.5
1
Figure 7.20. Transition angle effect on scour profile and
Figure 7.18. Flow around contraction inlets. uniform contraction scour.