Table of Contents
Introduction
The 100 km long Padma River in central Bangladesh runs southeast from the Jamuna (or Brahmaputra) confluence and the Ganges to join the upper Meghna River, below which it is known as the Lower Meghna. River Training works design for Padma Multipurpose Bridge Project was a huge challenge for this giant river.
The Padma Multipurpose Bridge is located at Mawa, about two-thirds of the way down the Padma and about 35 km southwest of the capital city Dhaka.
Since the late 19th century, such training works have been an essential feature of bridge design for the large alluvial rivers of the Indian sub-continent (Bell 1890, Spring 1903, Inglis 1949; Joglekar 1971).
The paper describes the principal characteristics of the Padma River as they affect the design of river training works, including design flood discharges, cross-sectional dimensions, flow velocities, depths of scour, sediments, and morphology. It then describes various alternatives leading to the selected layout of the proposed river training works. Proposed erosion protection methods are discussed briefly, along with geotechnical aspects, construction procedures, and maintenance requirements. Other recent large bridge projects in Bangladesh are the Jamuna Multipurpose Bridge (see, for example, Tappin et al. 1998), Paksey Bridge on the Ganges (see, for example, Mott MacDonald 1996), and the Bangladesh-UK Friendship Bridge on the Upper Meghna (see, for example, Collings et al. 2003). An older facility is Hardinge Bridge on the Ganges (Gales 1917, 1938). All those projects involved river training works of some type, and most required periodic maintenance or upgrading.
Padma River Characteristics:
Physical setting and flow variations:
The Padma River lies within a vast delta complex that has been growing for several million years and forms the more significant part of present-day Bangladesh. Both major tributaries, Jamuna and Ganges, originate in the Himalayas and have been subject to major historical shifts associated with earthquakes and land-use changes. Of the current mean flow of around 30 000 m3 /s in the Padma, about two-thirds is derived from the Jamuna and one-third from the Ganges. The seasonal flow pattern throughout the year is monsoon, with maxima in August-September and minima in February-March. Year-to-year variation is moderate.
Hydrologic design parameters for River Training Works:
Hydrologic parameters related to bridge design include low and high water levels and 100-year and 500-year flood discharges. Analyses of wind speeds, wave heights, and wave run-ups were also conducted. Maximum discharges of 100- and 500-year return periods are considered Design Flood and Check Flood, amounting to approximately 128 000 and 138 000 m3 /s based on the historical record.
Based on a severe IPCC scenario, researchers investigated the effects of climate change on hydrologic parameters, resulting in potential 100- and 500-year discharges of 148 000 and 160 000 m3 /s within the design life of the bridge. Although there are tidal effects at lower discharges. So, potential rises in ocean levels don’t affect high water levels significantly. A 14-year water level record versus discharge at the bridge site, 1994-2008, shows considerable variability. For example, during the bank’s entire stage, the discharge has varied from about 60 000 to 90 000 m3 /s with an average of about 75 000 m3 /s. This variability is mainly due to the shifting morphology of the river channel.
River morphology:
The present general location of the Padma River dates from about 1826. The platform is mainly of the partly multi-channel or anastomosing type. Although a central reach, including the bridge site, has been most frequently single-channel. Aerial and satellite images from 1967 to date show that the bridge site has remained relatively stable. A reach extending about 20 km upstream has migrated back and forth from a near-straight alignment to a pronounced meander loop with a period of around 30 years. The primary approach to the bridge is relatively straightforward. But it is likely to resume the meander alignment several times during the bridge’s life. In this alignment, the main flow approached the bridge from the southwest at a skew angle of about 45 degrees. This tends to erode floodplain land on the right bank towards the proposed south approach road.
The bed and bank materials of the Padma are generally fine sands. Relatively low river flows can mobilize it, except at a few “nodal points” where cohesive outcrops in the banks more or less fix the channel location. The north or left bank near the bridge site is somewhat erosion-resistant along substantial lengths due to clay soils. Still, the south or right bank generally consists of unconsolidated, noncohesive, fine-grained soils and has exhibited significant erosional and depositional changes within periods of a few years.
The overall width of the Padma between the tops of banks is typically 5 to 10 km.
The average width increased after 1930 and particularly after 1990. Allegedly due to sediment inputs from a significant 1950 earthquake with landslides far upstream in Assam. Bankfull cross-sections are typically irregular, with a main deep channel and a wide shallower part. Average bank full depths are around 10 m. But maximum depths at a particular time and space can be 40 m or more.
River bathymetry and hydraulics:
A bathymetric survey of the 100-km Padma River was conducted as part of the bridge design project in August-October 2008. A length of 14 km near the bridge site was re-surveyed in August 2009. An Acoustic Doppler Current Profiler measured velocities and discharges at eight transects. The average slope of the river upstream of the bridge site is around 5 cm per km (0.00005). The overall hydraulic roughness (Manning n) decreases from about 0.040 at the lowest discharges to a minimum of about 0.013 at a bank full discharge of around 75 000 m3 /s and increases somewhat at overbank discharges.
A tentative explanation supported by limited field profiling is that primary bed forms at low discharges flatten with increasing discharges to reach a plane-bed condition under high flows. The fine sand of the bed is quickly mobilized for bed and suspended transport. The cross-sectional average velocity at a bank full discharge is only about 1.6 m/s. However, the more significant part of the flow is carried in a minor part of the width where maximum vertically-averaged velocities are up to nearly 3 m/s. Under 100-year design flood and 500-year check flood conditions, the researchers estimated the maximum vertically-averaged velocities as around 4.5 and 5 m/s.
Maximum Scoured Depths:
Two government agencies collected extensive bathymetric data in the general vicinity of the bridge site from the mid-1960s. Generally, the greatest depths occur near the relatively erosion-resistant north or left bank. Frequency analysis yields 100- and 500-year maximum scoured depths of about 52 and 60 m below the top of banks. If we exclude the data near the north bank, these values reduce to about 40 and 45 m.
These maximum natural scoured depths do not allow for local scour due to bridge piers and river training works. Physical model testing on bridge pier foundations indicates a potential additional depth of as much as 15 m below the ambient bed for the preferred design involving 8 raked tubular piles of 3 m diameter. Near the proposed river training works, total scoured depths below the top of banks may range from about 60 to 70 m, depending on flood frequency and location. (Although those depths are vast in relation to broad river experience, they amount to only about 1% of the 6-km river width.)
Geotechnical Aspects:
The main geotechnical issues affecting the river training works are:
1) The maximum allowable steepness of underwater slopes to resist static failure due to river action, scour, and construction operations
2) Potential liquefaction of constructed works under dynamic loading due to earthquake action. Flow slides in oversteepened river banks are not uncommon in Bangladesh. This was an experience during the construction of the Jamuna Bridge in the 1990s. Earthquake tremors are also reasonably typical. The experts conducted test hole drilling for the river training works during the 2009 monsoon and the following dry season. Geotechnical assessment for common static load cases was based on deriving safety factors using generalized data for various soil strata, accounting for the effect of mica content on the loose, fine, and poorly graded sands and silts. Triaxial extension tests using shear parameters assessed the obtained Potential failure in locally weaker zones also.
General conclusions regarding static stability were that underwater slopes should be no steeper than 1 vertical to 6 horizontal down to the lower limit of dredging at about 30 m below the top of banks, where a wide apron would locate to allow for deeper scour. An inner berm should retain at that level after the outer part of the apron launches to cover the lower slope exposed by scour. Concerning earthquakes, it concluded that the uppermost 10 m or so of south bank soils are susceptible to liquefaction under 50- to 100-year earthquake conditions. The relatively steep slope of a launched apron is also vulnerable. However, the combined risk of critical earthquake action concurrent with deep scour and an exposed launched apron was low.
Layouts Proposed in Previous Studies:
A Pre-feasibility Report preceded the current Padma Bridge design study (R.P.T. et al. 2000) and a Feasibility Study (Nippon Koei 2004). The Pre-feasibility Report showed no river training works on the north bank and only a relatively short (3 km) guide bund on the south bank. The Feasibility Study showed a 6 km length of revetment on the north bank, plus a 10 km length of continuous embankment and revetment on the south bank, extending far into the present minor south channel.
Considered alternative Layouts:
The experts presented the following three layout alternatives in an inception report of April 2009:
Alternative 1
A modified Feasibility Study layout, recognizing that the south bank at the bridge alignment receded by about 500 m between 2004 and 2009.
Alternative 2
A north bank revetment as in the Feasibility Study, plus an 8-km long guide bund on the south bank extending mainly upstream of the bridge centreline and curving into the present minor south channel.
Alternative 3
Continuous revetments or embankments on both sides of the river for 20 km or more upstream of the bridge, are intended to maintain the present favorable alignment throughout the bridge’s life.
The experts rejected Alternative 3 as impractical due to construction time, cost, and uncertainties over maintenance. Then comes Alternative 2 which would have required a substantial part of the south guide bund to be placed on loose char deposits and would have afforded only limited protection for the south approach road. Therefore the experts preferred Alternative 1. For the erosion-resistant north bank, further studies mainly involved shorter lengths of revetment than the Feasibility Study. For the south bank, the following three additional options, partly based on suggestions by the project’s Panel of Experts, were studied:
Option 1
The experts modified protection on the present central south bank into the shape of a guide bund, and they replaced the continuous revetment in the current minor south channel with two or more hard points shaped like short guide bunds.
Option 2
They shifted the south guide bund offshore onto a char, enabling a shorter length of the main bridge.
Option 3‘s second guide bund was located some 10 km upstream to keep the river aligned.
The potential impacts on river flows and erosion patterns of all considered layouts were examined. Morphological studies, numerical modeling, and physical modeling were considered using natural and low-density sediments.
Morphological studies involved selecting “worst case” historical meander loops from elsewhere on the Padma River. Then superimposing them on the training works layouts.
Numerical modeling involved examining velocity magnitudes and directions during flood flows in near-straight and meandering river configurations. The data gained from bathymetries surveyed in 2009 and 1996 were used.
Physical modeling included:
- A comprehensive, vertically distorted model.
- A limited-area “sectional” model.
- A flume model to examine bridge pier scour.
Together with multi-criteria assessments, these studies generally confirmed that Alternative 1 (modified Feasibility Study layout) was likely to be the most constructive, durable, and effective in the long run, given appropriate materials for erosion protection. As a result of physical model testing, the plan geometry of the south side work was further modified to soften the curvature.
Training works cross-section, and erosion protection:
The information provided here is basically for the preferred Alternative 1 layout (modified Feasibility Study).
Details are partly novel and based on previous Bangladesh practices at Jamuna Bridge. The essential components of embankment and revetment protection are as follows:
Upper slope wave protection mainly above low water level, consisting of side-by-side concrete blocks underlain by a geotextile filter. The lower end of this component takes the form of a 5 m wide horizontal berm.
Lower (underwater) slope protection on a dredged slope of 1:6. This consists of rock riprap resting on several layers of sand-filled geotextile bags (bags) acting as a filter. The toe level of this slope is variable. But it cannot be below the practical dredging limit at about 30 m below the bank’s top.
The median rock size is about 450 mm. The experts considered replacing the rock over part of the revetment length with geo bags. It utilizes local materials and labor, which are less expensive than imported rock and permits faster construction.
Toe apron consists of several layers of rock or geo bags placed horizontally.
The designers designed the outer part to “launch” on a slope of 1: 2 . The more downward slope is undermined by scour, leaving behind the inner part as a horizontal berm. Geotechnical stability considerations indicate that in areas subject to deep scour, apron placement and launching should be a multi-stage process whereby a new apron will take place once scour and launching have proceeded to a certain depth. As the timing of such a process is indefinite, this concept has important implications for monitoring and maintenance.
Where required to prevent outflanking of the bridge ends by overbank flow, the training works would be
raised to above maximum flood levels (embankment). Elsewhere, they would terminate at the top of the natural bank (revetment). Special features, including minor bridges, would be required to maintain access to existing south bank distributaries and floodplain channels used for local navigation.
Construction and Maintenance
In Padma Bridge, cutter suction dredgers were excavated and filled for the construction of the training works and erosion protection. Construction proceeded upstream from the downstream end of the works.
The estimated total volume of dredged sand was 50 million m3, which required more than three construction seasons with three dredgers. The authority used the dredged material for local fill and to form temporary chars in the river. They should continue monitoring and maintenance throughout the bridge’s life for such extensive training in a shifting river environment. A multi-step process is envisaged, consisting of regular monitoring, evaluation, adaptation, and maintenance. Monitoring and assessment involved applying and interpreting general and local surveys, diving, and instrumental inspections. The transformation involved mainly the upgrading of launched aprons on scoured slopes. Maintenance consists of the repair of damage and gradual degradation.
Conclusion:
The Padma Bridges river training design presents serious river engineering challenges, similar in type but likely more severe than those encountered in other large bridge projects in Bangladesh. The river’s vast scale and periodic shifting, excellent noncohesive boundary materials, large scour depth, geotechnical instability, and the very costly erosion protection materials that must be imported from abroad, are among these issues.
After analyzing a series of alternative layouts using various techniques, including morphologic analysis and numerical and physical modeling, experts adopted a design somewhat similar to that proposed in the Feasibility Study of 2004. Rather than attempting to maintain the river in its present relatively straight alignment towards the bridge, this layout allows the main river to re-occupy the present minor south channel from time to time, permits the south bank training works to construct on top of the slightly more consolidated sediments of the current river bank, and leaves room for the river to adjust to possible future developments like climate change.
Those works then consist of a continuous embankment and revetment that starts downstream of the bridge and continues upstream for about 10 km, turning into the right bank of the south channel.
Experts designed this extension to prevent the outflanking of the south bridge abutment and eroding of floodplain land towards the south approach road. The work on the relatively stable north bank will consist of a relatively short length of embankment and revetment. The typical embankment or revetment cross-section has three main components: upper-slope wave protection using concrete blocks, a dredged underwater slope with erosion protection using rock riprap and geo bags, and a launching apron at the toe of this slope using rock riprap or geo-bags. Because potential maximum scoured depths far exceed the feasible maximum depth of the dredged underwater slope, the launching apron is critical for long-term stability. New ones may need to be placed at lower levels as scour deepens. This will require a long-term capability for in-river operations that goes beyond the common understanding of maintenance requirements.
Definitions:
In this study, we used the following terms to describe elements of river training works:
Revetment – erosion protection placed directly on a river bank or exposed slope.
Embankment – a dike or similar feature extending above a high water level to contain or exclude water; it may form an upward extension of a revetment.
Guide Bund (or Guide Bank) – a dike or dam, more or less at right angles to the bridge, intended to guide river flows smoothly into the bridge waterway opening; it may be constructed on the floodplain or in the river channel.
Hardpoint – a local erosion-resistant feature constructed on a river bank or projecting out into the river.
References:
-C. R. Neill, K. Oberhagemann & D. McLean 2010. River training works for Padma multipurpose bridge, Bangladesh
-Bell, J.R. 1890. Technical Paper No. 2B, Simla, India.
-Collings, D., D.Mizon & P.Swift 2003. Proc.Inst.Civ.Engrs (London), Bridge Engineering 156, Issue BE4, Dec.2003, pp.181-190.
-Gales, R.R. 1917. The Hardinge Bridge over the River Ganges at Sara. Paper no.4200, Inst.Civ. Engrs, London.
-Gales, R.R. 1938. Paper no. 5167, Inst.Civ. Engrs, London.
-Inglis, C.C. Research Publication 13, Central Waterpower Irrigation and Navigation Research Station, Poona, India.
-Joglekar, D. 1971. Manual on river behavior, control, and training. Central Board of Irrigation and Power, India. Mott MacDonald Ltd.1996.
-Paksey Bridge Construction Project, detailed engineering services, design report to Roads and Highways Department, Government of Bangladesh. Nippon Koei Ltd. in association with Construction Project Consultants 2004.
-The feasibility study of Padma Bridge. Report to Japan International Cooperation Agency, Tokyo, and Jamuna Multipurpose Bridge Authority, Dhaka. R.P.T. (Rendel Palmer & Tritton), NEDECO and Bangladesh Consultants 2000.
-Padma Bridge Study, pre-feasibility report to Jamuna Multipurpose Bridge Authority, Dhaka. Spring, F.J.E. 1903 (reprinted 1948).
-River training and control – the guide bank system. Railway Board, New Delhi, India. Tappin, R.G.R., J.van Duivendijk and M.Haque 1998. (London) 126, 11, pp.150-162.
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