NUMERICAL ANALYSIS OF THE BED
MORPHOLOGY IN THE REACH BETWEENCABRUTA AND CAICARA IN ORINOCO RIVER.
Raul A CABRITA FMEE13634

Supervisor: Shinji EGASHIRA ABSTRACT
The present study aims to evaluate numerically the river bed scouring resulting from sand bars formationand migration, and channel plan forms such as a change of flow width, curved flows, in addition to local
scouring due to construction of bridge pier. To achieve the objective of this study, the numerical modelInternational River Interface Cooperative (iRIC) developed by Hiroshi Takebayashi et. al (2009) isemployed to simulate and predict the morphological changes in river bed, and empirical formulas areapplied to estimate the local scour depth at bridge piers.
(1) The river bed variation is predicted well in terms of numerical model proposed by Takebayashi etal.
The bed erosion along the outer bank increases with time in initial stage, and then oscillates temporally.
The maximum depth increases with flow discharge.
(2) The local scour depth resulting from construction of bridge pier is estimated in terms of several
empirical methods. The results suggest that Andru’s method is suitable for study area.
(3) Combining the results of bed deformation and local scour due to bridge piers can be useful for theestimation of the depth of bridge footing.
Keyword: Orinoco River, Bed morphology, Bridge, Local scours, Piers.
1. INTRODUCTION
The National Railway System of Venezuela (NRSV) is a main concern for the nation and is currently
under construction. The “Instituto de Ferrocarril del Estado” (IFE) is the authority on all issues and hasplanned the completion of the NRSV for 20 years. One of the line of the NRSV is Chaguaramas Cabruta – Caicara with a total length of 201 km that would facilitate communications between the central
and southern of Venezuela. The connection between Chaguaramas and Caicara depends on theconstruction of the third bridge over the Orinoco River.
The global statistics of bridges over waterways show that most bridges fail due to hydraulic reasons
such as erosion around piers or abutments. So, it is important to evaluate erosion depth resulting frommorphological changes and local scour at bridge piers.
This study aims to evaluate the river bed scouring resulting from sand bars formation and migration,and channel plan forms such as a change of flow width, curved flows, in addition to local scouring due
to construction of bridge pier. To achieve the main objective, the numerical model proposed by Hiroshi
Takebayashi et. al (2009) is employed to simulate and predict the morphological changes in river bed,
and empirical formulas are applied to estimate the local scour depth at bridge piers.
2. REVIEW OF LOCAL SCOUR DUE TO BRIDGE PIERS
The local scour at bridge piers is a subject that has concerned the attention of many researchers for manyyears. This is due to erosion resulting from the interaction between local flow and sedimenttransportation. Table 2.1 shows three methodologies which are composed of parameters associated with

Universidad Central de Venezuela, Venezuela.
ICHARM The International Centre for Water Hazard, Japan.

1flow discharge, sediment transportation and geometry of the bridge piers to estimate the maximum
possible local scour depth at piers.
Table 2.1 Methodologies for estimation maximum possible local scour depth at piers

ResearchersMazza Alvarezand Echavarria
AlfaroAndru

Nakagawa &Suzuki

Formulas(2.1)
𝑑𝑠 = 2.4𝐾𝑠 𝐾𝜃 𝑏
Where, 𝑑𝑠 is the maximum possible depth scours at piers [m], 𝐾𝑠 and𝐾𝜃 are the shape and alignment factor, respectively, and 𝑏 is the Piers
width [m]𝑑𝑠(2.2)= 0.80𝑦Where, 𝑑𝑠 is the maximum possible depth scours at piers [m] and 𝑦is the water depth [m]𝑑𝑠𝑏(2.3)= 3.4 − 0.9 log10 ( )𝑏𝑑50
Where, 𝑑𝑠 is the maximum possible depth scours at piers [m], 𝑏 is thepier diameter [m], and 𝑑50 is the size of the particle sediment.
3. CHARACTERISTIC OF CHANNEL MORPHOLOGY

3.1. Description of the area of study

Location of the bridge

Figure 3.1 shows the study area in which the third bridge
will be constructed over Orinoco River. The reach
between Cabruta and Caircara in Orinoco River are
specified as follows: bed elevation ranges between 11.36
m and the 46.26 m; an average discharge is 32,970 m3/s
and velocity is 1.28 m/s; median grain size diameters are
0.26 mm to 0.54 mm; a mean concentration ofsuspended sediments is 38mg/l. In addition, the ratio offall velocity of the sediment and the shear stress velocity(w0/u*) is 0.61.
For the analysis of the channel morphology of the reach
between Cabruta and Caircara del Orinoco in Orinoco
River, we need river cross section data, discharge data,water level data, grain size distribution data, channelroughness, etc. For this study, the basic data wasobtained from different sources and organization

Figure 3.1 Location of the study area. (Source:Google Eath)3.2. Data analysis
Figure 3.2 shows the cross section of interest at which the 3rd bridge will be constructed, and Figure 3.3
shows the monthly flow discharge at Caicara gauge station of Orinoco River, respectively. The widthof this section is about 2,600 m and the elevations ranges from 8.70 m to 24.76 m. The maximum
discharge is 77,900 m3/s and the minimum is 2,135 m3/s in this reach. Usually, the maximum level takes
place from around the end of July to the beginning of August.

2

distribution curves in the area ofCabruta. The median diameters forthese samples were 0.26 mm atstation 3, 0.50 mm at station 5, and0.54 mm at station 9. The sample atstation 3 consisted mostly of fine tomedium sand; 100 percent of thematerial is finer than 1.00 mm andabout 3 percent of the material isfiner than sand (<0.062 mm).

Percent finer than indicated size [%]Figure 3.4 shows the particle size

110.00100.00
90.00
80.00
70.00
60.00
50.00
40.00
30.00
20.00
10.00
0.01

0.10
1.00
Diameter [mm]Station 1

Station 3

Station 5

Station 9

Station 13

Station 17

Station 21

Station 25

Figure 3.4 Particle size distribution of bed material Orinoco River

Elevation [m]

25.00
20.00
15.00
10.00
5.00
0.00
-2000.00 -1000.00 0.00 1000.00 2000.00
Distance [m]Figure 3.2 cross section of interest at which the3rd bridge will be constructed

100000.00
80000.00

Discharge [m3/s]

30.00

1926

1950

1960

1970

1980

1990

2000

2010

2012

60000.00
40000.00
20000.00
0.00

1 2 3 4 5 6 7 8 9 10 11 12Time [month]Figure 3.3 Monthly discharge of Orinoco River at Caicara
Gauge Station Jan -1926 to Dec -2012

4. NUMERICAL METHOD TO PREDICT MORPHOLOGICAL CHANGES
4.1. Governing equations for the numerical model
The governing equations, which are depth – integrated two dimensional forms are employed in thenumerical model. These are composed of mass conservation equation for water flow, suspended
sediment of flow body and bed sediment as well as of momentum conservation equation for water flow.
 Mass conservation equation for flow body (Continuity equation)
𝜕ℎ 𝜕
𝜕
(4.1)
(𝑣ℎ) = 0+ (𝑢ℎ) +𝜕𝑡 𝜕𝑥
𝜕𝑦 Momentum conservation equation for flow body𝜕𝑢ℎ 𝜕𝜕
𝜕𝜏𝑥
𝜕
𝜕
(4.2)
(𝑢𝑢ℎ) +(𝑢𝑣ℎ) = −𝑔ℎ (ℎ + 𝑧𝑏 ) − +(ℎ𝜎𝑥𝑥 ) ++(ℎ𝜏𝑦𝑥 )
𝜕𝑡𝜕𝑥
𝜕𝑦𝜕𝑥
𝜌 𝜕𝑥
𝜕𝑦𝜏𝑦𝜕𝑣ℎ 𝜕𝜕𝜕𝜕𝜕(𝑣𝑣ℎ) = −𝑔ℎ (ℎ + 𝑧𝑏 ) − +(4.3)+ (𝑢𝑣ℎ) +(ℎ𝜎𝑦𝑦 ) +(ℎ𝜏𝑦𝑥 )
𝜕𝑡
𝜕𝑥
𝜕𝑦𝜕𝑥
𝜌 𝜕𝑦
𝜕𝑥
𝑢(4.4)
𝜏𝑥 = 𝜏𝑏√𝑢2 + 𝑣 23𝜏𝑦 = 𝜏𝑏𝑣

(4.5)
√𝑢2 + 𝑣 2
 Mass conservation equation of suspended sediment.
𝜕𝑐ℎ 𝜕𝑟𝑢𝑐ℎ 𝜕𝑟𝑣𝑐ℎ𝜕𝜕𝑐𝜕𝜕𝑐(4.6)++=(ℎ𝜖𝑥 ) +(ℎ𝜖𝑦 ) + 𝐸 − 𝐷
𝜕𝑡
𝜕𝑥
𝜕𝑦
𝜕𝑥
𝜕𝑥
𝜕𝑦𝜕𝑦
 Mass conservation of bed sediment𝜕𝑧𝑏1
𝜕𝑞𝑏𝑥 𝜕𝑞𝑏𝑦(4.7)+++ 𝐸 − 𝐷) = 0(𝜕𝑡 1 − 𝜆 𝜕𝑥
𝜕𝑦Where, ℎ is the flow depth. 𝑢, are depth- averaged flow velocity along the longitudinal and transversal
direction, respectively, 𝑡 is the time, 𝑔 is the acceleration due to gravity, 𝑧𝑏, and 𝜌 are the bed elevationand mass density of water, respectively. 𝜎𝑥𝑥, 𝜎𝑦𝑦, 𝜏𝑦𝑥, and 𝜏𝑥𝑦 are the Reynolds stresses, 𝜏𝑏 is bed shear
stress, and 𝐷 are erosion and deposition rate of suspended sediment, respectively. 𝑞𝑏, 𝑐, 𝑟, and 𝜆 are bed
load rate, concentration of sediment, factor correction, and porosity of bed sediment, respectively. 𝜖𝑥,and 𝜖𝑦 are the x and y components of dispersion coefficient4.2. Grid generation for numerical computations and boundary conditions.
Figure 4.1 shows the study area in terms of grids generated in general coordinate system. A uniform
artificial channel, which is 10 km long and constant bed slope (I= 0.04‰), is attach to both side of thestudy reach in order to conduct a numerical computation stable. Numerical computations are conductedwith the following boundary conditions. At upstream boundary condition is considered a uniform steadyflow, the equilibrium sediment transport rate for both bed load Qb and suspended load Qs. At downstreamof the study area there is no information about water surface elevation. So, for downstream boundary
condition is considered the normal depth.

Upstream

Study area

Downstrea
mFigure 4.1 Physical plane of study area in terms of the generated grids

5. MORPHOLOGICAL CHANGES IN STUDY REACH
In this chapter shows the results and discussion of the case where is considered a constant discharge of32,920 m3/s that is corresponding to the average discharge of all the historical data since 1926. Figure
5.1 and Figure 5.2 shows the bed shear stress distribution and the bed elevation change of the river after
20 days of computation, respectively. In this figures suggest as the bed shear stress increase, the bed
erosion happens, on the contrary, when the shear stress decrease the sediment deposition take place. In
this channel the maximum bed shear stress occurs: reach between the sections 29 and 26 near to theright bank, expansion (between sections 20 and 18) and contraction in the channel (sections 12 to 10).
In those areas also take place the maximum erosion. On the other hand, the maximum deposition takeplace in areas as: left bank between sections 29 and 26, right bank between sections 25 and 23; also, in
this areas occurs the minimum bed shear stress in the channel. Moreover, the value obtained for thecumulative transported suspended sediment is 3.96 x 107 ton, value that is bigger than the value obtainedin previous study. The reason of this differences are the condition established in the numerical model
as: bed material type (uniform) and flow discharge (steady flow).

4Figure 5.1 Bed shear stress of the Study Area for20days computation

Figure 5.2 Elevation change of the Study Area for20days computation

Figure 5.3 shows the results of the maximum possible local scour at bridge pier in meters for sevendifferent methodolgies. It can be appreciate that Andru’s method provided the maximum scour depth atpier and only depend the water depth. The minimum scour depth is obtained with Yaroslavtziev’smethod. The values obtained using the methodologies of FHWA and Breuser still are low in one caseand in the other only depend of the geometry of the pier. Laursen and Toch, Mazza, and Nakagawa andSusuki present similar values
14.00
12.00
10.00
8.00
6.00
4.00
2.00
-13.02

7.20
5.32

5.25
3.50
1.98
0.44

Figure 5.3 Maximum possible local scour depth at piers in meters

For each case that was considered in this research is important to understand the variation in time of thelocal scour depth at bridge piers. Figure 5.4 shows the variation in time obtained after applied theequation (2.2) corresponding to Andru’s method. When is considered the flow discharge of 8,400 m3/s
for 20 days of computation, the maximum local scour (6.89 m) occurs after five days and then start todecrease to a depth of 4.89 m. For a flow discharge of 32,920 m3/s the local scour is around 13 m anddo not present great variation along the time. In the case of 77,909 m3/s only was obtained computationfor 3 days and the behavior is similar as the two other cases.
According to the previous explanation the main reason of the variation of this local scour in each caseis due to the change in the bed elevation of the river. In the case with a flow discharge of 8,400 m3/s iswhere presented more variation in time.

5Scour depth (ds) [m]

20.00
15.00

14.22
11.44

10.00
5.00

15.32

13.27

13.39

13.40

13.02

6.89

6.24

5.77

4.89

4.36

0

5

10

15

2025

Time [day]ds [m] (Q=8,400m3/s)

ds [m] (Q=32,920 m3/s)

ds [m] (Q=77,909 m3/s)

Figure 5.4 Variation in time of the local scour depth at bridge pier6. CONCLUSIONS
The depth of bridge pier footing should be determined taking the local scouring and river bed
evolution into consideration. The results obtained from( present study are summarized as follow:(1) The river bed variation is predicted well in terms of numerical model proposed by Takebayashi
etal. The bed erosion along the outer bank increases with time in initial stage, and thenoscillates temporally. The maximum depth increases with flow discharge.
(2) The local scour depth resulting from construction of bridge pier is estimated in terms of several
empirical methods. The results suggest that Andru’s method is suitable for study area.
(3) Combining the results of bed deformation and local scour due to bridge piers can be useful forthe estimation of the depth of bridge footing. .
ACKNOWLEDGEMENTS
I would like to express my appreciation and gratitude to my Sensei Shinji EGASHIRA for his valuableadvice to complete this study. Also, I wish to express my special gratitude to Ms. Yuri MEDINA andMr. Jacinto ARTIGAS, for their support.
REFERENCES(1) Egashira, Shinji (2009). Mechanics of sediment transportation and river changes, Lectures Note(2) Zanabria, Luis (2011). Hidraulica fluvial, Lectures Notes.
(3) Nordin, C. and Perez, D. Sand Waves, Bars, and Wind – Blown Sand of the Orinoco River,Venezuela and Colombia.United States Geological Survey Water Supply with collaboration of
Ministerio del Ambiente y Recursos Renovables of Venezuela. 1989.
(4) Ven Te Chow (1994). Open Channel Hydraulics, Mc Graw Hill.
(5) Takebayashi, H. (2011), Morpho 2D. Solver Manual. http://i-ric.org/en/software/?c=4(6) Instituto de Ferrocarriles del Estado. Sistema Nacional de Ferrocarril de Venezuela.
http://www.ife.gob.ve/index.php/8. Dic 2013
(7) Ashida, K. and Michiue, M., 1972, Study on hydraulic resistance and bed-load transport rate inalluvial streams, Proc. of JSCE, No. 206, pp.59-69.
(8) Ashida, K. and Michiue, M., 1972, Study on hydraulic resistance and bed-load transport rate inalluvial streams, Proc. of JSCE, No. 206, pp.59-69.
(9) B. Y. Liu, 1991, Study on Sediment Transport and Bed Evolution in Compound Channels.
Thesis presented to Kyoto University.

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