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17
CHAPTER 4
THE SRICOS-EFA METHOD FOR CYLINDRICAL PIERS IN DEEP WATER
4.1 SRICOS-EFA METHOD FOR CONSTANT kinematic viscosity of water (10-6m2/s at 20°C). The initial
VELOCITY AND UNIFORM SOIL rate of scour z versus curve from the EFA
i is read on the z
test at the value of max.
Because cohesive soils may scour so much more slowly
The maximum depth of scour z max was obtained by per-
than cohesionless soils, it is necessary to include the scour rate
forming a series of 43 model scale flume tests (36 tests on
in the calculations, and the SRICOS Method was developed
three different clays and 7 tests in sand) (Briaud et al., 1999).
for this purpose. The SRICOS Method was proposed in 1999
The results of these experiments, and a review of other work,
to predict the scour depth z versus time t curve at a cylindrical led to the following equation, which appears to be equally
bridge pier for a constant velocity flow, uniform soil, and water
valid for clays and sands:
depth greater than two times the pier diameter. The SRICOS
Method consists of the following (Briaud et al., 1999): zmax ( mm ) = 0.18R e 0.635 ( 4.2)
1. Collecting Shelby tube samples near the bridge pier, In Equation 4.2, Re has the same definition as in Equation
2. Testing them in the EFA (Erosion Function Apparatus, 4.1. The regression coefficient for Equation 4.2 was 0.74.
Briaud et al., 2002) (Figure 3.1) to obtain the erosion rate The equation that describes the shape of the scour depth z
(mm/hr) versus hydraulic shear stress (N/m2) curve,
z
versus time t curve is
3. Calculating the maximum hydraulic shear stress max
around the pier before scour starts, t
z = ( 4.3)
4. Reading the initial erosion rate z i (mm/hr) correspond-
1 t
+
ing to max on the z versus curve,
i
z zmax
5. Calculating the maximum depth of scour z max,
6. Constructing the scour depth z versus time t curve using Where z i and z
max have been previously defined, t is time
a hyperbolic model, and (hours). This hyperbolic equation was chosen because it fits
7. Reading the scour depth corresponding to the duration the curves obtained in the flume tests well. Once the duration
of the flood on the z versus t curve. t of the flood to be simulated is known, the corresponding z
value is calculated using Equation 4.3. If z i is large, as it is in
The maximum hydraulic shear stress max exerted by the clean fine sands, then z is close to z
max even for small t values.
water on the riverbed was obtained by performing a series of But if z i is small, as it can be in clays, then z
may only be a
three-dimensional numerical simulations of water flowing small fraction of z max. An example of the SRICOS Method is
past a cylindrical pier of diameter B on a flat river bottom and shown in Figure 4.1.
with a large water depth (water depth larger than 2B). The The method as described in the previous paragraphs is lim-
results of several runs lead to the following equation (Briaud ited to a constant velocity hydrograph (v = constant), a uni-
et al., 1999): form soil (one z versus curve) and a relatively deep water
depth. In reality, rivers create varying velocity hydrographs
and soils are layered. The following describes how SRICOS
max = 0.094 v 2 -
1 1
( 4.1)
log Re 10 was extended to include these two features. The case of shal-
low water flow (water depth over pier diameter < 2), non-
Where is the density of water (kg/m3); v is the depth aver- circular piers, and flow directions different from the pier
age velocity in the river at the location of the pier if the bridge main axis are not addressed in this chapter.
were not there (it is obtained by performing a hydrologic
analysis with a computer program such as the Hydrologic 4.2 SMALL FLOOD FOLLOWED BY BIG FLOOD
Engineering Center--River Analysis System [HEC-RAS],
vB For a river, the velocity versus time history over many
1997); and Re is where B is the pier diameter and the
years is very different from a constant velocity history. In