Among the many force fields (see The Quantum World and Entropy – and Everything Else for a general description) that govern and dominate what we do and live with – are the substantial effects of two – the ubiquitous Gravitational Force Field (GFF) and the Electromagnetic Force Field (EMFF). With the proliferation of electronic communication and our overwhelming dependence and works in cyberspace (see aspects of it in Artificial Intelligence – the Tool of No Limit) – the EMFF of the past centuries is getting transformed into something totally different. The result is that it is taking an unprecedented bite on everyone’s life and livelihood – the longterm impacts of which are neither addressed nor understood.
This piece is not about the impact of this kind – but the processes of river sediments sequestered by floodplains – and of sediments delivered into the coastal ocean. Of these two sedimentary functions of an alluvial river – the first represents floodplain sedimentation – while the second is responsible for building Coastal River Delta. The ocean delivery comes as an impact/effect on the ambient coastal ocean – the LDFF (Land Drainage Force Field) – described in the Force Fields in a Coastal System. In that piece the effects of LDFF have been identified in three different seasonal aspects: sediment delivery, and freshwater discharge to – and density-driven circulation in the river-mouth coastal ocean. As can be understood, the larger the river – the larger is its LDFF – going beyond the river mouth coastal ocean into the deep ocean.
What I am going to discuss in this article – are part of the works that were submitted for publication in 1998 to Sedimentology – the Journal of the IAS. The paper was titled: Suspended Sediment Transport through the Ganges-Brahmaputra Fluvial System: Implications for Floodplain Sedimentation and Sediment Delivery to the Ocean. Despite having great reviews from reviewers and the editor, I got busy and could not manage time to submit the updated revised manuscript for publication (I feel like I owe an apology to IAS for not managing time to submit the revision). Since then, I forgot about it and almost lost the manuscript. Luckily, I was able to trace back the manuscript and thought of presenting some important aspects of it in this piece. It is about the Ganges-Jamuna River System (GJRS) – how this top ranking large alluvial system functions – in terms of its role in sediment delivery into the coastal ocean – and how much sediments are lost into or are sequestered by the floodplain. Let us attempt to understand these processes in simple terms with the help of the shown image – illustrating the defined GJRS and sediment flows.
Writing this piece is enriched by many of my works and publications prior to and after 1998 – and several of my WIDECANVAS articles. They include:
Investigative Works and Papers: my 1992 USC Ph.D. Dissertation; the 1994 Asia Pacific Journal, Environmental Controls of Bangladesh River Systems; the 1996 Asia Pacific Journal, Environmental Functions of an Alluvial River; the 1995 Oxford & IBH, Adaptation and Equilibrium of Bangladesh Rivers; the 1995 IEB, Sediment Transport in Suspension; the 1997 Water Nepal, Characteristics and Mobility; the 1998 Taylor & Francis, Turbulent Flow Structure; and the 2017 Encyclopedia, Seabed Roughness of Coastal Waters. Some short articles include: the 2001 ASCE, Field Techniques; the 2002 ASCE, Alluvial Deltas; the 2004 ASCE, Settling Velocity of Natural Sediments; the 2001 ASCE, Suspended-Sediment Measurement; the 2001 ASCE, Suspended Particles Fraser River; and the 2008 ASCE, Fluid Mud.
WIDECANVAS articles include: – Environmental Controls and Functions of a River; Common Sense Hydraulics; The Hydraulics of Sediment Transport; Resistance to Flow; Harbor Sedimentation; Force Fields in a Coastal System; Coastal River Delta; Managing Coastal Inlets; and Civil Engineering on our Seashore.
Ganges-Jamuna River System. Two large international rivers draining the slopes of the Himalayas and associated catchments – the Ganga – draining mostly the southern slope (in Bangladesh, the deltaic reach of the Ganga River is known by its British name the Ganges River), the Brahmaputra – draining mostly the northern and eastern slopes (the deltaic reach of the Brahmaputra River is known as the Jamuna River in Bangladesh) and a relatively small river, the Meghna – debouch into the northern Bay of Bengal as a combined force. In the water-flow and sediment transport processes of the three rivers – the contribution of the Meghna River is relatively minor (see my Water Nepal paper) – therefore not considered important for this study. The flow of the Ganges and Jamuna merge into one – as the Padma River at a confluence near Baruria. When the mighty Padma meets the smaller Meghna – the combined downstream flow is known as the Meghna River Estuary.
Among many of world’s large rivers debouching into the ocean – the GJRS ranks first in terms of sediment delivery (JD Milliman and RH Meade 1983) and third in terms of water-flow (JM Coleman 1969). These characterizations immediately indicate the higher sediment/water ratio of the GJRS compared to others. Because of the high ratio of such a large system, there have been considerable interests among researchers and Government sponsored studies to explore sediment-water dynamics of GJRS. The most notable among the sponsored studies – under the auspices of Master Plan Organization (MPO) – were those conducted by the French Engineering Consortium, China-Bangladesh Joint Expert Team, the European Union River Survey Project, and the US Environmental GIS Project. This contribution is part of the last two efforts - where I worked.
As presented in the shown image, the GJRS is defined by – the inflow boundaries at Bahadurabad in the Jamuna River and at Hardinge Bridge in the Ganges River. Draining the main combined flows – the outflow boundary is located at Baruria in the Padma River. Two distributaries – the Gorai (measuring station at Kushtia) and Dhaleswari (measuring station at Jagir and Taraghat) – take a small share out of the combined flow to drain individually to build floodplains and finally debouching into the ocean.
Following the Environmental Controls and Characteristics and Mobility papers and referring to the measuring stations described above and in the image – a brief on the main fluvial characteristics of the GJRS reads as follows:
Floodplains. Before moving further, let me first present a brief on some floodplain basics – because knowing it is very important to understand the dynamics of large rivers in their sluggish lower deltaic reaches. To describe it, I will mostly depend on some summaries presented in FAO (1988) and Alam et al (1990). A floodplain is usually divided into different units by looking into its fluvial geomorphology and age. It has three distinct idealized geomorphic units – from nearest to the furthest distance off the river bank – are the natural levees, seasonally drained back-swamps, and marshes of poorly drained standing water bodies.
Of these three, natural levees are a strip of land on both sides of a river – the width of which could be as wide as 4 times river size. The strip has a relatively higher elevation than other floodplain areas, experience higher flow velocities than the rest, and mostly consists of deposits of coarse sediments. They are utilized to locate engineered flood-control-and-mitigation dikes. Natural levees usually have discontinuities along their length – intersected by crevasse splays – the shallow channels that feed and empty the floodplains.
The back-swamps are seasonally drained, experience less flow velocities and are characterized by deposits of fine sands and silts. The marshes face very negligible flow velocities and are characterized by paludal deposits of clay and peat.
In terms of age, a floodplain is distinguished as active, young and old. An active floodplain is reworked seasonally by migrating rivers. They are comprised of braid-bars, meander scroll-bars, natural levees and crevasse splays. Inland from this, lies the broad young floodplain – a relatively inactive region shielded from the dynamic effects of erosion, scour or localized burial deposits. Compared to these two, the old floodplains are stable without any appreciable sediment deposit or erosion. However, this idealized characterization of floodplains are often broken by a river system such as the GJRS – where bank erosion protection measures are minimal or absent – with the system enjoying the freedom to move swiftly and dramatically during the flood stage.
Further, as the name suggests a floodplain belongs to the river regime – built by the river on its banks to store and convey water when the river is on high seasonal flood stage. Its important functions include: the modulation and alleviation downstream flooding problems – and refurbishing the fertility of the floodplains. Indiscriminate installation of levees (see aspects of it in Flood Barrier Systems) separating the plains from the river have been reported to have adversely affected both the river (e.g. the Mississippi River in USA, the Yangtze River in China) and the associated floodplains. They come in the form of the pondage or sinking of floodplains, and raising the elevation of the embanked river bed.
Reviews of Transport Estimates – Prior Researches
Before moving further, it is important to stress that all transports (presented in this piece) – relevant to floodplain sequestering and ocean delivery – represent suspended sediment load (SSL). SSL comprises of fine sediments that include both suspended bed-material load and wash load (see more on The Hydraulics of Sediment Transport). There have been two fundamentally different approaches of estimating SSL for large rivers. The first represents estimates based on looking at things at a high level, and the second is on analysis of actual observations.
High Level Holistic Approach – was a hybrid method based on sediment yield estimates using data comprising of basin-wide information of topographical relief, hydro-meteorology, geology and vegetative covers – and on information derived from some secondary sources. Using the data of 280 large rivers, such researches have shown that SSL is a long-linear function of basin size and elevation (the works of Fournier 1960, JN Holeman 1968, JD Milliman and RH Meade 1983 and JD Milliman and PM Syvitski 1991). Using notations (million tons in US or short tons as MT, and million tonnes or metric tons as MTE) their estimates showed total annual SSL of GJRS varies from 1670 MT (or 1515 MTE) by Milliman and Meade to 2400 MT (or 2177 MTE) by Holeman.
Approach Relying on Observed Database – used actual observations derived from secondary sources, which are most often sparsely covered in space and time. To cope with the missing data, researchers then used different analytic techniques to make a reasonable estimate. JM Coleman (1969) got his estimates by using sparsely covered EPWAPDA (East Pakistan Water and Power Development Authority) data (1958-1962). During this period an elutriation of 5 min was used (compared to the later lime of 100 s, used in our estimate). His estimates showed total annual SSL as 1087 MT or 986 MTE. The comparison of both the methods immediately shows that the High-Level approach grossly overestimated the transports. Elaborated as follows – the estimates presented in this piece are based on this second approach – but pursuing a systematic analysis of 25-years of data of Bangladesh Water Development Board, BWDB (reincarnation of erstwhile EPWAPDA) – and by looking into uncertainty of them.
FLOODPLAIN SEDIMENTATION AND OCEAN DELIVERY
The presented works in this piece were conducted under the auspices of Bangladesh Environmental GIS Project and River Survey Project. In contrast to earlier efforts, this work presents estimates in terms of fractional distributions of sand, and silt-clay. Here are some gists of the methodology and findings.
The method that prevailed during drawing up of this paper – was systematized by the consultants of FAO (1966) and UNESCO (1974 and 1979) – with some refinements following the general guidelines of WMO. The following procedure has been in practice since 1965.
SSL System Balance – Sequestering and Ocean Delivery. The presented estimates made use of 25 years (1966 – 1991; with a 1-year gap in 1971) of data collected by BWDB. This length of time is considered adequate for meaningful statistical analyses.
I like to stop at this – only to outline briefly that collecting data and managing the described efforts for a large alluvial river system such as the GJRS – is a challenging undertaking. But with the help from international partners, BWDB has maintained a laudably long record of sediment transport data – and the effort continues. However, many lapses and limitations do exist – in particular in the domain of quality assurance of the collected data. As identified during drawing up of the paper in 1998, they include:
Despite all these limitations – it can be argued that the estimates provide a defensible transport in orders of magnitude – albeit with some degrees of uncertainty.
Let me finish this piece with a sweet poem ‘Unbeaten by Rain’ by Miyazawa Kenji (1896 – 1933) – a Japanese poet, a novelist and a devout Buddhist. Revered and loved to the likeness of a national poem in Japan – it echoes the sound and rhythm of the Buddha (563 – 483 BCE) - The Tathagata's Karniya Metta Sutta (the 10-verse discourse on Loving Kindness).
Unbeaten by rain
Unbeaten by wind
Unbowed by the snow and summer heat
Strong in body
Free from greed
Without any anger
In the East, if there is a sick child
Go there and take care of him
In the West, if there is an exhausted mother
Go there and relieve her of her burden
In the South, if there is a man near death
Go there and comfort him, tell him ‘Don’t be afraid’
In the North, if there is an argument and a legal dispute
Go there and persuade them it’s not worth it
In a drought, shed tears
In a cold summer, carry on
. . . . .
Paying homage to those who worked for, and dedicated their lives for the right to use mother language in daily and state businesses in their native land – on this day of International Mother Language on 21st February (the day represents and commemorates Bangladesh language movement day) – this piece is a tribute in memoriam of Vietnamese Buddhist monk Thich Nhat Hanh (1926 – 2022) who passed away on 22 January 2022 at the age of 95. A veteran of the worldwide Vietnam War Resistance movement and a nominee of the Nobel Peace Prize, he left an enlightened legacy of compassionate accomplishments that transcended religious and sectarian boundaries across the globe. In the footsteps of the Buddha – and in the framework of Engaged Buddhism, he gave a forceful impetus to the practice of mindfulness (Samma Sati) meditation. It is hard not to love and cherish his words: the best gift one can give to oneself, to his or her loved ones, works, co-workers or to others is mindful ATTENTION. With this gift, the receiver blooms like a flower – like a Lotus. In another, he explained very lucidly the meaning of Emptiness – that when one sees a tree in meditation, he or she not only sees the tree itself – but every other interconnected factors contributing to the existence of the tree. This realization of the truth of Dependent-Origination makes one humble and respectful to others.
. . . . .
- by Dr. Dilip K. Barua, 21 February 2022