Pier and Contraction Scour in Cohesive Soils (2004) / Chapter Skim
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Pages 17-29
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From page 17... ...
Reading the scour depth corresponding to the duration of the flood on the z– versus t curve. The maximum hydraulic shear stress τmax exerted by the water on the riverbed was obtained by performing a series of three-dimensional numerical simulations of water flowing past a cylindrical pier of diameter B on a flat river bottom and with a large water depth (water depth larger than 2B)
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From page 18... ...
The scour depth z– versus time t curve is given by Equation 4.4 for Flood 1 and by Equation 4.5 for Flood 2. After a time t1, Flood 1 creates a scour depth z–1.
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From page 19... ...
The calculations for scour depth are performed by choosing an increment of time ∆t and breaking the complete velocity versus time history into a series of partial flood events, each lasting ∆t. The first two floods in the hydrograph are handled by using the procedure shown in Figure 4.2 or Figure 4.4, depending on the case.
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From page 20... ...
Figure 4.3. Multiflood flume experiment results: a)
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From page 21... ...
. Therefore, when Layer 2 starts being eroded, the scour depth versus time curve proceeds from Point B to Point C on Figure 4.5b.
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From page 22... ...
For each bridge site, soil samples were collected in Shelby tubes and tested in the EFA to obtain the erosion function z˙– versus τ; then the hydrograph was collected from the nearest gage station and the SRICOS program was used to calculate the scour depth. That scour depth was entered in Equation 4.3, together with the corresponding z˙–i and z˙–max to get te.
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From page 23... ...
Velocity hydrograph and scour depth versus time curve for Bent 3 of the Brazos River Bridge at US 90A. Figure 4.11.
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From page 24... ...
The foundation type is steel piling down to 5.5 m below the channel bed, which consists of silty and sandy clay down to the bottom of the piling according to existing borings. Between 1956 and 1996 the peak flood took place in 1992 and generated a measured flow of 1,600 m3/s, which corresponds to a HEC-RAS calculated mean approach flow velocity of 3.9 m/s at Bent 5 and 2.6 m/s at Bent 3.
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From page 25... ...
River bottom profiles exist for 1956 and 1996 and show 0.76 m of local scour at Bent 3 and 1.8 m of total scour at Bent 5. At Bent 5, the total scour was made up of 1.41 m of local scour and 0.39 m of contraction scour as explained later.
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From page 26... ...
River bottom profiles exist for 1965 and 1997 and show 4.43 m of total scour at Bent 3 made up of 2.87 m of local scour and 1.56 m of combined contraction and general scour as explained later. The San Jacinto River Bridge at US 90 was built in 1988.
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From page 27... ...
. For a given discharge Q, HEC-RAS gives the velocity distribution in the river cross section, including the mean approach velocity v at the selected pier TABLE 4.2 Full-scale bridges as case histories TABLE 4.3 Soil properties at the bridge sites
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From page 28... ...
The measured local scour depth was obtained for each case history by analyzing the two bottom profiles of the river crosssection (Figures 4.17 and 4.18)
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From page 29... ...
. E-SRICOS and S-SRICOS described above do not include correction factors for pier shape, skew angle between the flow direction and the pier main axis, shallow water depth effects, and multiple pier effect.
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