A study of local scour at bridge pier

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A STUDY OF LOCAL SCOUR AT BRIDGE PIERS
OF EL-MINIA
Dr. Gamal A. Sallam1
and Dr. Medhat Aziz2
ABSTRACT
Bridges are critical structures that require a substantial investment to
construct and serve an important role in economic development. For a
developing country such as Egypt, there is need to protect these assets by
continuous monitoring, maintenance, proposing any required repair works
and execute them with a very high quality control. El-Minia Bridge was
constructed in 1987. It consists of 15 vents each of 40.00 m wide, except
the navigation vent, which is 62.00 m wide. Recently, there have been
some difficulties regarding navigation underneath the bridge. Since some
submerged islands are formed near the navigational vent, some deposition
near the navigational vent of the bridge has raised the bed level. This has
resulted in an insufficient depth of water for navigation through the
navigational vent. In addition, river bed has changed around bridge piers.
Nile Research Institute (NRI) has conducted the analysis for this problem
to avoid any damages that could occur to the bridge. Nile Research Institute
research team were sent for collecting the field data for the site of El-Minia
Bridge Km. 205.050 upstream of El-Roda site along the Nile River, in
which pier size, sediment size and mean approach flow velocities are
recorded. A mathematical model was used to study the measured scour
hole values and to predict to predict the future maximum expected scour
hole. That was conducted to evaluate the causes and consequences of scour
holes on the bridge piers of El-Minia. In addition, some methods and tools
for minimize the effect of scour holes on the bridge piers were discussed.
KEYWORDS:
Nile River Bridges, Local scour, Bridge local scour.
Researcher, Nile Research Institute, Egypt
Assoc. Prof., Nile Research Institute, Egypt

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1. INTRODUCTION
El-Minia bridge constructed in 1987 to join El-Minia City with the
west main highway (2) (Egypt - Aswan) and with the main new highway
(Cairo/ Bani Suif/ El-Minia) East of the River Nile which was constructed
lately through the main Governorate plan to support the network roads and
to decrease the traffic density on the Upper Egypt highway. The new El-
Minia Bridge will create a new civilization on the East of River Nile, in
addition to gain the best use for stone quarries in this area. The total length
of El-Minia Bridge is about 1803 m, it consists of 15 vents, width of each is
40 m. While the navigational vents, the biggest is about 62 m and on each
side 2 vents with a width of about 50 m and the maximum navigational
height is 13 m from the highest water level of River Nile. The total road
width on the bridge is 21 m, the bridge foundations are penetrated piles
with a length of about 20 m, all these piles are supported on sand or stone
soils.
Yet every year more additions are made to the list of bridges that
have failed because of scour of the stream bed around the piers and
abutments. Considering the overall complexity of field conditions, it is not
surprising that there is no generally accepted principles for the prediction of
scour around bridge piers and abutments have evolved from field
experience alone, (Emmett M. Laursen and Arthur Toch, 1956). The
alignment, cross section, discharge, and slope of a stream must all be
correlated with the scour phenomenon, and this in turn must be correlated
with the characteristics of the bed material ranging from clays and fine silts
to gravels and boulders. Finally, the effect of the shape of the obstruction
itself - the pier or abutment - must be assessed.
The equilibrium of any river under natural flows is affected by any
interference. Some examples of the interfering structures are bridges,
groins and hydraulic structures. Study the effect of any of these structures
before and after construction is an important factor to avoid any unexpected
results.
One of the Nile Research Institute tasks is to monitor, study, and
analyze these effects and to propose the suitable measures to be taken to
protect these structures and avoid major changes to the river, (Ahmed
Fahmy, 2002). During this research local scour caused by bridges is

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analyzed, a case study, El-Minia Bridge, is considered for this analysis.
Figure (1) shows El-Minia Bridge along the River Nile.
Figure (1) El-Minia bridge location along the River Nile
2. SCOUR PHENOMENON
Scour is the removal of bed and bank material as a result of the
erosive action of flowing water. The three components of total scour are
general scour, contraction scour, and local scour.
The contraction scour is the scour that results from the acceleration
of the flow due to a contraction, such as a bridge. This type of scour also
occurs in areas where revetments are placed such that they reduce the
overall width of the stream segment. Contraction scour is generally limited
to the length of the contraction, and perhaps a short distance up and
downstream, whereas general scour tends to occur over longer reaches.
The local scour is the scour that occurs at a pier, abutment, erosion
control device, or other structure obstructing the flow. These obstructions
cause flow acceleration and create vortexes that remove the surrounding
sediments. Generally, depths of local scour are much larger than general or
contraction scour depths, often by a factor of ten. Local scour can affect the

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stability of structures such as riprap revetments and lead to failures if
measures are not taken to address the scour.
Factors that affect local scour include:
1. width of the obstruction
2. length of the obstruction
3. depth of flow
4. velocity of the approach flow
5. size of the bed material
6. angle of the approach flow (angle of attack)
7. shape of the obstruction
8. bed configuration
9. debris
3. FIELD INVESTIGATIONS
A field trip has been carried out to El-Minia Bridge; the bridge is
located in El-Minia Governorate, kilometer 242.050 along the River Nile
south of Cairo. This field trip was conducted to observe, measure, and
monitor the different factors affecting bridge local scour. This field trip has
included the water levels at the site, bridge pier dimensions and geometry.
Figure (2) shows one of the El-Minia Bridge piers as an example. A
hydrographic survey of the bed nearby and around bridge piers was
performed along the riverbed covering 1.00 Km (about 500 meter upstream
and 500 meter downstream the bridge), and 21 surveyed cross sections.
Velocity measurements were taken at five locations, two cross-sections
upstream the bridge, two cross-sections downstream bridge and at the
centerline of bridge. Samples of bed material were taken at the same cross-
sections. Table (1) shows surface bed grain size analysis at the bridge site.
It is noted from the grain size distribution that most of the bed samples are
medium sand with D50 ranging from 0.25 to 0.50 mm (medium sand is
ranging from 0.2 to 0.60 mm). Downstream cross sections have some
coarse sand ratio. It has to be noticed also that this is typical configuration
for the Nile River bed, and the existence of fine sand indicates the
possibility of further scour. Figure (3) shows the cross section changes at
the bridge for years 1982 and 2002. It is observed the major cross section
changes due to bridge existence. It can be noticed that a maximum local
scour depth of about 4.60 m has occurred during the period 1982 to 2001.

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Figure (2) One of the El-Minia bridge piers
Table (1) Surface bed grain size analysis at the bridge site
X-S
KM.
from
Aswan
High
Dam
LOCATION D 50
(mm)
D mean
(mm)
SOIL
TYPE
UPSTREAM
THE BRIDGE
684.910 Point 1
East
0.40 0.44 Medium sand
Point 2 0.25 0.28 Medium sand
Point 3 0.29 0.31 Medium sand
Point 4 0.42 0.47 Medium sand
Point 5
West
0.40 0.43 Medium sand
CENTER LINE
OF BRIDGE
684.940 Point 1 0.27 0.27 Medium sand
Point 2 0.32 0.32 Medium sand
DOWNSTREAM
THE BRIDGE
684.960 Point 1
East
0.50 0.55 Medium to
coarse sand
Point 2 0.42 0.47 Medium sand
Point 3 0.24 0.27 Medium to
fine sand
Point 4 0.42 0.50 Medium to
coarse sand
Point 5 0.25 0.24 Medium to
fine sand

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A study of local scour at bridge piers of El-Minia bridge
20
22
24
26
28
30
32
34
36
38
0 100 200 300 400 500 600
Distance (m)
Bedlevel(m)
1982 2002
Figure (3) Cross section changes at the bridge (1982 and 2002)
4. COMPUTING SCOUR
Because of complexity of evaluating flow pattern around piers and
shear forces generated by flow pattern, most of the estimates of scour depth
have been obtained by experimental work. An example of these estimates
is the Colorado State University (CSU) equation for predicting maximum
pier scour depths, the equation is (ASCE, 1975):
Ys/Y1 = 2.0 K1 K2 K3 K4 (A/Y1)0.65
Fr1
0.43
for round nose piers aligned with the flow.
where:
Ys = Scour depth, m
Y1 = Flow depth directly upstream of the pier, m
K1, K2, K3, and K4 = Correction factors for pier nose, angle of attack,
bed condition, and armoring by bed material size
respectively.
A = Pier width, m

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L = Length of pier, m
Fr1 = Froude Number directly upstream of the pier.
Ys ≤ 2.4 times the pier width (A) for Fr ≤ 0.8
Ys ≤ 3.0 times the pier width (A) for Fr > 0.8
5. COMPUTER MODEL
A Computer Model: The computer model HEC-RAS Model was
used during this analysis to evaluate local scour around El-Minia Bridge
piers. This model is developed by the US Army Corps of Engineers (US
Army Corps of Engineers, 2001). It is a one-dimensional model able to
simulate steady, unsteady and sediment transport for movable boundary
conditions. It has a special module to compute bridge piers and abutment
local scour. To be able to simulate the actual flow conditions, the whole
reach (Reach 4 - from downstream Assiut Barrage to upstream Delta
Barrage) has to be modeled and analyzed.
Reach 4: It extends downstream Assiut Barrage to upstream Delta
Barrage with a total length of about 410.00 km. It is the longest reach all
over the Nile River reaches. The surveyed cross sections for this reach in
both years 1982, 1997 and 2002 were used for this analysis. Figure (4)
shows the location of Reach 4 along the River Nile, and Table (2) shows
the cross section locations along the reach used during this study.

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Figure (4) Location of reach 4 along the River Nile
Model Calibration Process: The actual water level readings for
gauging stations along the fourth reach were used for the calibration
analysis. Two actual flow conditions were considered for the calibration
analysis; 170 and 350 million cubic meters per day. Figure (6) shows the
calibration process for the flow of 170 m.m3
/day and Figure (7) shows the
calibration process for the flow of 350 m.m3
/day. From these two figures,
we can conclude that there is a close agreement between measured and
predicted water levels. This indicates that the selected roughness values are
suitable for the simulation process.
Reach 4

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Table (2) Cross Section Locations
Number Cross
Section
Km from
Aswan
Number Cross
Section
Km from
Aswan
1 93 555 21 135 767
2 94 556.5 22 137 778
3 96 572.85 23 139 787
4 98 582 24 140 795
5 100 591 25 141 798
6 102 602 26 143 806.4
7 104 612 27 145 830.8
8 106 623.5 28 146 835
9 108 633.5 29 147 843.2
10 110 645 30 149 851.3
11 112 654 31 151 862.4
12 114 664 32 153 873.3
13 116 671 33 154 879
14 118 681 34 155 887.3
15 120 691 35 157 901.1
16 122 701 36 159 912.7
17 125 719.5 37 161 915.2
18 127 727 38 162 927
19 129 738 39 163 947.6
20 131 747
6. INPUT DATA
The data that are provided to the program is as follows:
1. Bed material grain size distribution, Table (1).
2. Reach 4 cross sections 5 Km a part, Table (2).
3. Water level for reach 4 gauging stations.
4. Bridge geometry pier dimensions, Figure (2) shows one of these
piers, where the pier width is 1.50 meter, and the pier length is
10.00 meter.
7. LOCAL SCOUR SIMULATION
Figure (5) shows the model output for the simulated flow conditions.
The simulated flow, considered during this analysis, is 180 m.m3
/day. This
flow is considered to simulate flow conditions causing local scour around
the bridge piers. This flow is almost the largest flow discharge passed along
this reach during the past period (1982-2002) and it is close to the

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calibration discharges. The computed water levels along the reach are
shown in this figure.
0 100000 200000 300000 400000
0
10
20
30
40
50
60
reach 4 brdg Plan: Plan 06
Main Channel Distance (ft)
Elevation(ft)
Legend
EG PF 2
WS PF 2
Crit PF 2
Ground
4
Figure (5) The model output for the computed water level

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Figure (6) The calibration results for 170 m.m3
/day
Figure (7) The calibration results for 350 m.m3
/day
Model Calibration (350 m.m3/day)
0
10
20
30
40
50
60
500 550 600 650 700 750 800 850 900 950 1000
Km (from Aswan)
Level(m)
Predicted Measured
Model Calibration (170 m.m3/day)
0
5
10
15
20
25
30
35
40
45
50
500 550 600 650 700 750 800 850 900 950 1000
Km (from Aswan)
Level(m)
Predicted Measured

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9. MODEL RESULTS
The model results represent the local scour occurred due to passing a
discharge of 180 m.m3
/day along El-Minia Bridge. Bed material gradations
were included for this analysis to compute the local scour. The CSU
equation is used by the model for local scour computations. The K terms
are taken as follows:
K1 correction factor for pier nose is taken as 1.00
K2 angle of attack of flow 90.0
K3 correction factor for bed conditions 1.1
(clear water scour)
K4 armoring effect coefficient 0.7
The computed scour represents both the contraction and the piers
scour occurred for this flow condition. The abutment local scour is not
applicable for this case because there were no interfering abutments to the
flow. Table (3) shows the results of the local scour for the bridge piers. A
maximum value of 4.9 meters was computed for pier scour. It has to be
mentioned that the value obtained from comparing the cross section (1982-
2002) is about 4.60 m of local scour and degradation for the bridge cross
section. Figure (3), shows cross section change at the bridge (1982-2002).
Also, Table (3) shows the computed local scour for 180 m.m3
/day and a
maximum flow of 350 m.m3
/day. The differences between the measured
and the computed local scour depths could be attributed to the variation of
bed grain size distribution and model simplification but the model gives a
good indication for the local scour values. However, the maximum
computed expected local scour is 5.60 m, which should be taken into
consideration during future programs.
Table (3) Computed local scour for a flow of 180 m.m3
/day
and 350 m.m3
/day
Piers Local scour of
180 m. m3
/ day
Local scour of
350 m. m3
/ day
All 4.9 meter 5.6 meter

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10. CONCLUSION AND RECOMMENDATIONS
- Local scour monitoring is very important to avoid major damages
that may occur.
- The monitoring of El-Minia bridge local scour shows a local scour
depth of about 4.60 m since its construction up till now.
- The mathematical simulation of the bridge local scour shows a local
scour of a depth 4.90 m. that means the difference between the
measured scour and calculated scour about 6.5 %.
- The HEC-RAS model is a good tool for predicting local scour for
bridge piers. Hence, it could be used to determine the type of bridge
pier foundations before constructing.
- The expected local scour of passing a discharge of 350 m.m3
/day is
about 5.60 m.
- Since the local scour was found 4.60 m during the last 10 years and
the bridge pier foundations can tolerate this scour. It is recommended
to continue local scour monitoring every 5 years minimum.
REFERENCES
Ahmed Fahmy, “A study of local scour around El-Minia Bridge”, Report
No 341, Nile Research Institute, National Water Research Center, Egypt
(2002).
ASCE Manuals and Reports on Engineering Practice, “Sedimentation
Engineering”, ASCE, No.54, 1975.
Bruce W. Melville, “Live-Bed Sour at Bridge Piers”, J., Hyd. Div., ASCE,
Vol. 110, No. 9, (1984).
Emmett M. Laursen and Arthur Toch, “Scour around Bridge Piers and
Abutments”, Iowa Institute of Hydraulic Research, State University of
Iowa, (1956).

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US Army Corps of Engineers, “HEC-RAS River Analysis System
Computer Model”, The Hydrologic Center, Davis, California, 2001.