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38
4 .0
A .J .R a ud k ivi
3 .5 S te rling J o ne s
K e ith R . E llio t
V e lo c ity ra tio
3 .0
P ro je c tio n W id th
P ro je c tio n W id th(K w )
2 .5 M e a s ure m e nt
K sp
2 .0
1 .5
1 .0
0 .5
1 3 5 7
C /B
Figure 5.15. Correction factor for pier spacing effect.
Jones (1998) was used by adding the widths of all of the piers 6
in the row. This was done by using Equation 5.1 with the
equivalent width. The Ksp curve obtained in such a way is
Initial S c our R ate (mm)
shown under the label "Projection Width" in Figure 5.15. That 5
curve does not fit the measured data well (too conservative).
Even after accounting for the water depth effect, the Ksp curve
is still too high as shown under the label "Projection Width 4
(Kw)" in Figure 5.15. This indicates that the single equivalent
pier model would overestimate the pier spacing effect at least
for piers installed in a row. It was found that the ratio of the 3
width of the channel without the piers over the unobstructed
width of the channel with the piers fit the data quite well. This
2
equation is shown in Figure 5.15 under the label "Velocity 1 2 3 4 5
Ratio" because the velocity also can be estimated through that C /B
ratio. For example, if the flume width is B1, the approaching
velocity V1, and there are n piers installed in a row with same Figure 5.16. Initial scour rate for the group pier tests.
diameter B, then the velocity with n piers can be estimated by:
Vn = (V1B1)/(B1 - nB), and the velocity ratio is: Vn/V1 =
B1/(B1 - nB). The equation proposed for Ksp is: 180
B1 150
K sp = (5.3)
( B1 - nB)
Scour Depth (mm)
120
5.12 PIER SPACING EFFECT ON INITIAL 90
SCOUR RATE
A For C/B, A>B>C
60 B
The initial scour rate for the pier spacing flume tests is pre-
C
sented in Table 5.4 and plotted in Figure 5.16 where C is the
center-to-center distance and B the pier width. It shows that 30
the initial scour rate tends to increase as the piers become
more closely spaced. In summary, both the maximum pier 0
scour depth and the initial scour rate will increase as the piers 0 50 100 150 200
Time (hr)
become more closely spaced. This means that curves such as
those presented in Figure 5.17 can be expected. Figure 5.17. Scour curves for groups of piers in a line.
