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OCR for page 36

36
1.6
0.34
H H B < 1.62
Kw = 0.8493(H/B) 0.3385 0.85 B
Kw =
1.4
R2 = 0.7753
(5.2)
1.2 1 H B < 1.62
1.0
0.8 5.9 SHALLOW WATER EFFECT ON INITIAL
Kw
Test SHEAR STRESS
Meville Equ(3.7)
0.6
Johns on 0.273/0.3, Equ(3.6A) For a given scour depth versus time curve, the initial
0.4 0.273/0.3 , Equ(3.6C) scour rate is the initial slope of that curve. It can be obtained
Johns on, 0.16/0.4, Equ(3.6A) by fitting a hyperbola to the data. These are the rates shown
0.2 0.16/0.4, Equ(3.6C) in Table 5.3. The two groups of initial scour rates are plot-
Best Fit of Test ted in Figure 5.12. Test Sh-8 gave a much higher initial
0.0
scour rate than the other tests (11 mm/hr), so the large pic-
0.0 0.5 1.0 1.5 2.0 2.5 3.0
ture does not show its value, but the inset one does. The inset
H/B
indicates that the initial scour rate tends to increase as the
Figure 5.11. Correction factor for shallow water effect water depth decreases and that the increase becomes partic-
on maximum pier scour depth. ularly significant when H/B< 0.5. The figure also shows that
the scour rates for the larger pier (B = 0.273 m) are smaller
than the rates for the smaller pier (B = 0.160 m). Since the
correction factor can be derived. These values correspond to initial scour rate is directly tied to the initial shear stress
the curves labeled "0.273/0.3, Equ(3.6C)" and "0.160/0.4, through the erosion function, it can be stated that the initial
Equ(3.6C)" with H/B = 2.5 as the reference cases for B = shear stress increases when the water depth decreases and
0.273 m, V = 0.3 m/s and B = 0.160, V = 0.4 m/s, respectively. decreases when the pier diameter increases. These trends are
Figure 5.11 shows that the shallow water effect factor the opposite of the trends for the maximum scour depth.
obtained in this study is close to the correction factors for This means that a pier in shallow water subjected to a con-
cohesionless soils. stant velocity will scour faster at the beginning but will end
By regression, the expression for the proposed cohesive up scouring to a shallower maximum depth than the same
soil correction factor Kw is pier in deep water (Figure 5.13).
5
12
Erosion Rate (mm/hr)
10
8
4
6
4
Erosion Rate (mm/hr)
2
3
0
0 1 2 3
H/B
2
1
V=0.3m/s, B=273mm
V=0.4m/s, B=160mm
0
0.0 0.5 1.0 1.5 2.0 2.5 3.0
H/B
Figure 5.12. Initial scour rate as a function of water depth.