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63
and water depth after contraction scour has occurred. The X (mm)
difference between the V1 and H1 values before and after con- 0
traction scour is small in most cases and, except for in Test 1,
50
the correlations were good when using the values of V1 and
Scour Depth (mm)
H1 before the contraction scour occurred. Test 1 had a very 100
small contraction ratio (B2/B1 = 0.25) and an initial super-
critical flow, whereas all other tests had larger contraction 150
ratios and an initial subcritical flow. In the case of Test 1,
200
Equations 7.9 and 7.10 worked only when using the values B2/B1=0.25
of V1 and H1 after contraction scour had occurred. This use 250 B2/B1=0.5
of values after scour occurs is consistent with the approach B2/B1=0.75
300
taken by other researchers (Laursen, 1960,1963; Komura,
-500 0 500 1000 1500
1966; Gill, 1981; and Lim 1998).
The value of the factor is shown as a function of the con- Figure 7.16. Influence of the contraction ratio on the
traction ratio for all of the flume contraction tests on Figure longitudinal scour profile.
7.15. An average value of 1.38 was chosen for Zmax and 1.31
for Zunif. Since should be equal to 1 when B2/B1 is equal to 1
(no contraction), the best-fit line on Figure 7.15 should go
occurs close to and behind the opening of the contraction. For
through that point. It was decided to achieve that result as
the primary tests, the data listed in Tables 7.1 and 7.2 indi-
shown on Figure 5.15.
cate that Xmax is mostly controlled by the contracted channel
An attempt was made to find a relationship between Zunif
width B2 and the contraction ratio B2/B1. The wider the con-
and Zmax. The following equation gave a high R2 value:
traction opening is, the bigger the Xmax is (Figure 7.16). Fig-
ure 7.17 shows the relationship between Xmax/B2 versus B2/B1.
Z unif Zmax = 4 Fr where Fr = V1 ( gB1 )0.5 (7.11)
By regression, the best-fit equation for Xmax is
Xmax B
7.7 LOCATION OF MAXIMUM CONTRACTION = 2.25 2 + 0.15 (7.12)
DEPTH FOR THE REFERENCE CASES B2 B1
Knowledge of the location of the maximum contraction Equation 7.12 indicates that the location of the maximum
scour depth is very important for the design of a bridge. depth of contraction scour is independent of the velocity and,
Indeed, the bridge is usually a fairly narrow contraction and therefore, remains constant during the period associated with
the maximum contraction scour depth in this case can be a hydrograph. This means that an accumulation method such
downstream from the bridge site. Based on the flume test as the SRICOS Method can be used to predict the maximum
observations, the maximum contraction scour generally
2.5
1.6
Xm ax/B2= 2.2519(B2/B1) + 0.1457
1.5 2 R2 = 0.9999
1.4
1.5
Xmax/B2
B
1.3
1
1.2
B - max - Measurement
B - unif - Measurement 0.5
1.1
B - max - Propose
B - unif - Propose
1 0
1 1.5 2 2.5 3 3.5 4 4.5 0 0.2 0.4 0.6 0.8 1
B1/B2 B2/B1
Figure 7.15. The factor as a function of the contraction Figure 7.17. Relationship between Xmax and the
ratio. contraction geometry.