It has long been established that the basic mechanism causing local scour at
bridge piers is the down-flow at the upstream face of the pier and formation
of vortices at the base (Muzzammil et al., 2004).
The flow decelerates as it approaches the pier coming to rest at the face of
the pier. The approach flow velocity, therefore, at the stagnation point on
the upstream side of the pier is reduced to zero, which results in a pressure
increase at the pier face. The associated stagnation pressures are highest near
the surface, where the deceleration is greatest and decrease downwards (Melville
and Raudkivi, 1977). Figure 1 shows the flow and scour
pattern at a circular pier. As shown in this Fig. 1, the strong
vortex motion caused by the existence of the pier entrains bed sediments within
the vicinity of the pier base (Lauchlan and Melville, 2001).
The downflow rolls up as it continues to create a hole and through interaction
with the oncoming flow, develops into a complex vortex system. The vortex then
extends downstream along the sides of the pier. This vortex is often referred
to as horseshoe vortex because of its great similarity to a horseshoe (Breusers
et al., 1977).
||Illustration of the flow and scour pattern at a circular pier
Thus, the horseshoe vortex developed as a result of separation of flow at the
upstream face of the scour hole excavated by the down-flow. The horseshoe vortex
itself is a lee eddy similar to the eddy or ground roller downstream of a dune
crest (Breusers and Raudkivi, 1991). The horseshow vortex is very effective
at transporting the dislodged particles away past the pier.
|| Some equilibrium scour depth prediction equations
The horseshoe vortex is as a result of scour but is not the cause of scour
(Breusers and Raudkivi, 1991). As the scour depth increases,
the horseshoe vortex strength diminishes, which automatically leads to a reduction
in the sediment transport rate from the base of the pier (Lagasse
and Richardson, 2001).
Numerous experimental and numerical studies have been carried out by researchers in an attempt to quantify the equilibrium depth of scour in various types of soil material. Moreover, while a lot of work has been done to develop equations for predicting the depth of scour, researchers have also worked extensively to understand the mechanism of scour.
Estimation of the depth of scour in the vicinity of bridge piers has been the main concern of engineers for years. Therefore, knowledge of the anticipated maximum depth of scour for a given discharge is a significant criterion for the proper design of a bridge pier foundation.
In current practice, the design scour depth is chosen to be the maximum equilibrium
scour depth achieved for steady flow under the design flow conditions (Gosselin
and Sheppard, 1995). A number of studies have been performed with a view
to determining the equilibrium scour depth for clear-water scour conditions
(Raudkivi and Ettema, 1983). In these studies, the maximum
scour depth under steady flow conditions is related to the hydrodynamic and
sediment parameters, pier shape and flow intensity. Empirical equations based
on the results of such studies are used in the design of bridge pier by way
of computing the expected maximum scour depth for a particular flow condition.
Some of the most common equilibrium scour depth predicting equations are shown
in Table 1. For a river system, the use of equilibrium scour
depths is reasonable since, in many cases, even though the flow is unsteady
during storm events, high velocities can persist for long periods of time (Gosselin
and Sheppard, 1995). The idea of bed protection and prevention of scour
at a pier has attracted a good deal of attention. Reduction of scour depth would
mean shallower foundations and reduced cost (Breusers and
Majority of researches on scour at spur dike are conducted at a straight flume.
In such a case the flow patterns which are mostly the cause of scour would not
be the same as the case of straight canal. Therefore, it is the principal objective
of this study was to carry out experimental tests on the effect of oblong pier
on scour depth at in different positions in a 180 degree river bend. The study
reported herein, is based on experiments carried out in the Hydraulic Laboratory
at the Islamic Azad University in Ahwaz using a model 180 degree flume bend.
The study was confined to uniform cohesionless material and clear-water flow
MATERIALS AND METHODS
Experimental apparatus: This experiment was conducted in a laboratory flume at Hydraulic Laboratory of Islamic Azad University of Ahwaz. The study was conducted using in a 180 degree laboratory flume bend with a relative radius (Rc/b) was 4.7. The bottom of the flume is made up of an aluminum bottom and plexiglass sidewalls along one side for most of its length to facilitate visual observations. At the end of this flume a control gate was designed to adjust the water surface height at the desired levels (Fig. 2). The study conducted at Hydraulic Laboratory of Islamic Azad University of Ahwaz during September 2007 to September 2008.
In this study, the size of pier was defined to meet the criteria which have
been defined by other investigators. Pier diameter should not be more than 10%
of flume width to avoid wall effect on scouring (Chiew and
Melville, 1987). Melville and Sutherland (1988)
defined L/B (L = length of pier and B = width of pier) should not be more than
1/3. So, one oblong model pier of width 60 mm and length 180 mm were used for
the study (Fig. 3).
The effects of the grain size and the density of the sediment material are
often expressed as a function of the critical flow velocity for the initiation
of sediment motion. It was concluded by Breusers and Raudkivi
(1991) that ripples usually developed at shear velocities u*
above 0.6 u*c for sediment of size, d50 < 0.7 mm. In this work,
the bed sediment consisted of uniform sand, with median diameter d50 = 2 mm
and geometric standard deviation σg = 1.7.
|| Schematic illustration of the experimental setup (plan)
|| Schematic illustration of oblong pier
In this study, the experiments were performed under clear-water conditions at three different flow intensities (u*/u*c) of 0.70, 0.80 and 0.89 (u* is approach velocity and u*c is critical velocity for sediment movement), corresponding to a shear stress levels of 49, 67 and 80% of the critical shear stress level based on shields stress, respectively. Three different Froude numbers of 0.36, 0.38 and 0.41 were applied in order to investigate the effect of flow conditions on the scouring. All the experimental tests were conducted under the same flow depth. The pier positions were at 0, 30, 60, 90, 120, 150 and 180 degree. A 60 degree triangular weir was used at the upstream section of the flume for flow measuring.
Initially the bed surface was leveled by a plate attached to the carriage mounted on the channel. Then, inlet valve was opened slowly, the discharge increased to a predetermined value so that no scour occurs at the straight reaches of flume. At the end of the experiments, the topography of bed was measured at grids of 2x2 cm around pier oblond nose and at grids of 2x14 cm in the other sections.
Duration of scour test: In this study, to obtain equilibrium time of
the experiments, a long time experiment was conducted at Froude number of 0.36,
0.38 and 0.41 and the position of 60 degree. Considering the curves of Fig.
4, approximately 95% of scouring occurs during the first 3 h.
|| Equilibrium time in the position of 60 degree
Therefore in all our experiments, equilibrium time of 3 h was selected for
each test (Fig. 4).
RESULTS AND DISCUSSION
In all experiments, after adjusting the water depth and flow discharge, immediately
eddy flows around the piers formed and scouring began with the high rate at
upstream. Arisen sediments from scouring cavity, moved toward down stream and
after while they reach to the region where the pier effect diminished and the
eddy flow effect cannot feel more. Under these conditions, due to the secondary
effects transported sediments from scouring cavity moves toward the inner wall
and two or more small dune formed near the inner wall. Figure
5a and b show also in this region depends on hydraulic
conditions, deposition occurs near the external wall with minimum scouring.
Because of the flow deviation due to cylindrical pier, water hole formed in upstream which observed in all experiments, but its size vary depends on different pier positions. Some times in downstream this water hole collide with inner wall and cause scouring near the wall bend. If no colliding happens no scouring occurs but sediments deposit. Clearly affecting parameters on water hole size are cylindrical pier position in the bend and the flow rate.
|| Scour hole: (a) sedimentation at the downstream and (b) erosion
at a location far upstream of the pier
||(a-c) Transverse section of scour profile across the plain
pier centre in different positions and for three different flow conditions
Transverse scour profile in different positions for a plain pier: Figure
6a-c show the lateral profile of the scour hole through
the centerline of the pier in the positions of 0, 30, 60, 90, 120, 150 and 180
degree for different Froude number conditions. The (0, 0) reference point is
at the original bed level at the centerline of the pier. As shown in Fig.
5, the scour profile is symmetrical about the pier. For instance, it is
shown that the scour hole extended to a distance of either side of the pier.
To see the effects of different pier position in scour depth, (Fig.
7) was plotted. In this Fig. 8 the maximum measured scour
depth has been plotted at different position. As, it can be seen the maximum
scour depth at the beginning of bend is lowest and then increases to reach its
maximum value at 60 degree position. From this position the scour depth is decreases
up to end of bend (180 degree position) which reaches at the same scour depth
as it was at the beginning of bend (0 degree position). Experimental results
of Melville and Coleman (2000), about flow pattern in
180 degree bend show that the maximum velocity distribution occurs in the angle
60 degrees due to the high power of secondary flow and consequently causes maximum
scouring depth in this position.
||Maximum scouring depth in different Froude number and positions
||Transverse section of scour profile across the plain pier
centre in different Froude number
The effect of froude number at maximum scour: Figure 8
show, longitudinal and transversal profiles of oblong bridge pier in the position
60 degree with maximum scouring for three different Froude numbers of 0.36,
0.38 and 0.41. Considering Fig. 8 can conclude that overall
bed topographical schema is approximately as the same as each other for three
froude number but size and depth of the scouring cavity decrease by decreasing
the froude number. Also, the sediment pile length in downstream of oblong bridge
pier increases rapidly by increasing Froude number due to high power of arising
vortex. Furthermore transversal profile shows that there is a direct relation
between scouring depth and Froude number which of course is in agreement with
previous studies (Melville and Coleman, 2000).
Comparison of measured and predicted scour depth: Several of the common
equilibrium scour depth prediction equations as published in the literature
were used to compute the equilibrium scour depth to be expected for the test
conditions applicable to the 115 mm pier. The reason for the analysis is to
evaluate the usefulness of some of the renowned equations in the literature
with a view of testing how reasonably they can predict the equilibrium scour
depth based on the flow and sediment conditions used in this study.
|| Measured maximum scour depth in mm
The results of measured scour depth at various positions and different Froude
numbers are shown in Table 2.
Figure 9 shows comparison of regression analysis of the present
study data with results of study of Shen and Schneider (1969)
and Breusers et al. (1977) and Colorado State
University (CSU). For this comparison, a good agreement can be found between
the results of this study with work of Breusers et al.
(1977) with R2 = 0.96. The adjusted equation was re-written as:
||Maximum of scour depth
||Coefficient of position in bend with clear-water scour (Table
|| Coefficient of position in bend with clear-water scour
||Comparison of measured and predicted scour depth
with regression coefficient of 0.94. Figure 10 shows the comparison of calculated values with use to Eq. 1 and tested values of relative maximum scour depth. It is evident that Eq. 1 predicts the maximum scour depth with acceptable accuracy.
In this study, the temporal development of scour at oblong pier was experimentally studied using a physical hydraulic model. The study was performed under clear-water conditions using a uniform cohesionless bed material and oblong pier.
The results of the model study indicated that the maximum depth of scour is highly dependent on the experimental duration. The depth of the scour hole increases as the duration of the increased flow that initiates the scour increases. The extent of scour observed at the pier also increases as the duration of the tests increases.
The results of this study showed that, while oblong pier is placed in the bend, the maximum scouring depth is alternatively and maximum scouring occurs in position of 60 degrees. Also, it was observed that in all positions, increasing the Froude number increases the scouring depth.
The results of this study showed that, Breusers et al.
(1977) equation made the modified equation fit well to the data from
the present study when compared with the original equation.
Author thankfully acknowledge the financial support provided by Islamic Azad University Ahwaz Branch.