Coastal River Delta

1. Intro
I have chosen the title of this piece to describe the deltas created by rivers such as the Amazon, the Nile, the Mississippi and the Ganges-Brahmaputra-Meghna (GBM) river systems, each of which debouches into the coastal ocean. The purpose is to distinguish this type of delta from its cousins – the land deltas such as the Okavango Delta in Botswana, the lake deltas, and flood and ebb tidal deltas we see at coastal inlets along alluvial shores.
All deltas have one common feature though. Deltas are a net sediment depositional geomorphic area, and are built when the sediment carrying capacity of a constricted flow is lost in relatively wide open water. They are the typical planform areas where a flow forks outward into multiple distributary channels, cut through the fresh deposits of shoals and islands.   
Large river deltas are important for at least two important reasons. The first is their capacities to support many unique aquatic creatures, and flora and fauna that call deltas their homes. The second is the delta processes that bury dying plants and creatures. Through geologic time, this second process is responsible for accumulation and trapping of hydrocarbons released by decaying lives in sedimentary basins.  
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2. Delta Studies Attract Many Disciplines
A delta is the topic of studies by at least four different disciplines of physical and applied sciences. There are the geologists in search of finding the clues to understand ancient deltaic hydrocarbon deposits – in attempts to connect the present to the past; and the geographers in search of characterizing the patterns of processes – the present and the past. The oceanographers primarily focus on hydrodynamics with some attentions paid to sediments as far as the bed-resistance to flow is concerned. In the process they discover and propose many easily understandable behavioral models coining and defining different terms to describe a delta. They mainly conduct field works far and wide, and with supporting laboratory experiments, try to connect dots to help us understand the delta processes better – not only with the behavioral models but also with the continuing progress on process-based models.
The first two of the above disciplines delve into the long scales of space and time, while the oceanographers mainly focus on contemporaneous processes. The applied scientists or civil/hydraulic engineers primarily concentrate on short engineering time-scale in the order of 100 years or less. Standing on the foundations created by geologists, geographers and oceanographers, but with the safety, stability and effects of structures/interventions in mind, engineers’ methods are mostly based on investigations and scale modeling to derive process-based models in the controlled and manageable conditions of a laboratory. Supported by field works, they focus on hydrodynamics as well as on the processes of sediment transport and morphology.
While the above generalizations and differences hold in general, there are considerable overlaps among the disciplines cross-fertilizing one another. This is especially true in advanced studies – where it is difficult to distinguish a certain work belonging to one discipline or the other, if the authors’ affiliations are not revealed.
My professional involvement with studying coastal dynamics in a deltaic environment started with my works at the Land Reclamation Project – a Dutch project in the GBM coastal delta of Bangladesh – tasked to develop engineering plans to promote accretion, and for reclamation of the new delta landmasses. The involvement led to my Masters degree in coastal/hydraulic engineering at the UNESCO-IHE, Delft, the Netherlands; and later to my Ph.D. at the University of South Carolina (USC). Some of my works are published in journal and conference proceeding titles like, Elsevier, Taylor & Francis, Springer, Journal of Coastal Research (JCR), American Shore and Beach Preservation Association (ASBPA) and American Society of Civil Engineers (ASCE). The USC academic program gave me the opportunity to participate in a field trip to have a bird’s eye view by flying over the unique form of the Mississippi Delta.  
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3. Genesis of Delta Formation
With this little background, let us now try to have a glimpse of the processes that lead to the formation of deltas at a coastal river mouth. Some of the materials I will cover are taken from one of my discussion articles {Discussion of ‘Development and Geometric Similarity of Alluvial Deltas’, ASCE Journal of Hydraulic Engineering, 2002}.
From the perspectives of delta building, two distinct processes can be identified at the very outset – these are the relative magnitudes of constructive and the destructive processes. Let us first try to see them by highlighting some examples. One can say that the ideal or the most recognizable delta morphology of the Mississippi and the Nile rivers shows more of constructive influences than the less recognizable delta of the GBM system. The Mississippi and the Nile flows face very low tidal forcings from the Gulf of Mexico and the Mediterranean Sea, respectively. However, since the commissioning of the Aswan Dam in July 1970, and because of several works on the Mississippi River basin (see the 2012 USGS report: A Brief History and Summary of the Effects of River Engineering Dams on the Mississippi River System and Delta), the delta morphologies of these two rivers have been increasingly getting more exposed to the destructive processes than the constructive ones. Although I have mentioned these two examples, all the major river systems and their deltas have been getting affected by increasing interventions – like the Farakka Barrage constructed on the Ganges River in 1975.      
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3.1 Constructive and Destructive Processes
What are the constructive and destructive forces exactly? First let us try to see the types of the constructive processes. In simple terms these are the amount of sediment loads per a unit volume of flowing water, and the type of sediments a river transports – the larger the sediment load, the higher is its chance of forming a delta. And the rivers carrying higher percentage of sand than the fine fractions of silt and clay contribute more to the delta building processes.
The destructive processes are the adverse ambient coastal environments, within which a river debouches – the submarine topography, tide, wave, and most importantly the episodic events such as tectonic activity, flood and storm surge. When a river debouches at a steep coast or a submarine canyon, the chances are that the river will only be able to build a submarine delta. A submarine delta or fan develops quietly without the disturbing effects of tide, wave and storm surge.
The half-day oscillating tidal forcing erodes and resuspends the river-deposited sediments and transports them back and forth. The tidal processes align the sand shoals and islets with the prevailing current direction, and winnow the sediments to make the residual transport of the fines in preferential landward directions. The GBM delta is the typical example of a tide-dominated delta.                                                        
Pounding waves work in a similar fashion in time scales in the order of 10 seconds – they winnow the deposits and transport the sand fraction in cross-shore and downdrift longshore directions to form depositional features like barrier islands. But the least understood episodes of tectonics, river flood and storm surge affect the delta evolution, perhaps more than any other.
Tectonic uplift or subsidence either in secular trend or in episodes, shifts the epicenter of the delta deposits from one location to another – in the process, flow-shares of distributaries are changed. And the effects of flood? Let us try to see them. When a distributary becomes too long, its flow becomes sluggish [the Common Sense Hydraulics piece on the SCIENCE & TECHNOLOGY page indicates how delta progradation causes reduction of flow velocity], and the river looks for opportunities to find a steeper slope to the sea. A river flood comes with the help, and when the conditions are right, it cuts through the shoals or moribund channels giving a new life to the delta flow-distribution dynamics.
Similar episodes happen when a storm surge forcing occurs on a delta landscape. Its power can erode and sweep away sand deposits out to the sea or could cut new channels redefining the delta dynamics. The historic Mississippi delta sequence during a portion of the Holocene Transgression period, shown in the image (credit: anon) gives an impression how deltas change their depo-centers over time. Similar shift happened in the GBM delta when the Ganges River shifted to the east to meet the Brahmaputra and the Meghna rivers, to reach out to the sea as a combined flow.       
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3.2 Distinct Delta Building Phases
Delta building processes go through three major hydraulic phases – the first two primarily represent the sequences of constructive forces, but the third phase is a vigorous showcase of interactions between the constructive and destructive forces. The first phase processes are better understood through a jet theory. This theory in simple terms, explains how a jet emanating from a river, a diffuser outlet or a ship propeller, expands into the wide-open water. The expansion decelerates the flow velocity, and the river has no option other than to depositing its sediment load – the coarser fraction within the immediate vicinity, and the finer fractions further into the sea.
Geologists have termed the depositional features as the seaward bottomset bed of the fines, and the coarse deposits of the topset bed. Like the submarine delta, the bottomset bed develops quietly with minimal disturbance from tide and wave. An episodic storm surge may affect the bottomset processes to some extent though. The topset bed or the delta proper, progrades forward into the sea through the avalanching of sand on the inclined foreset bed. In some instances when the foreset delta front becomes too steep, a sudden rapid submarine slope failure occurs causing tsunami. There are many examples of this type of tsunamis; some are often triggered by tectonic activities. Such a tsunami occurred in 1975 – in a little known delta of the Kitimat Arm at the head of the Douglas Channel fjord in British Columbia.
The second phase occurs when the decelerated jet faces further frictional resistance from the built-up deposits of the topset bed. The result is the erratic channel and depositional patterns occurring in lateral expansions. Scientists try to understand this phase by describing and formulating the processes mostly through behavioral morphometric models.
The third phase represents the most complex processes of delta evolution through time, showcasing the intensive actions and reactions of constructive and destructive forces. In this phase, delta deposits have already anchored themselves through colonization by plants and trees – therefore flows encounter more resistance. The destructive forces of tide and wave, and the episodes of extreme events when dominant, can completely redefine the delta planform evolution.
Understanding delta dynamics as complicated as they are, has been made much easier with increasing applications of numerical computational models – by the advanced models dynamically coupling hydrodynamics, wave mechanics and sediment-morphology modules. Let us try to talk about this important topic at some other time.               
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Here is an anecdote to ponder:
The disciple asked the master, “Sir, what do the delta processes tell us?”
The master smiled, “Umm! Let me see. A river matures on its journey to the sea overcoming resistances and obstacles, meeting something different and overwhelming in the end. The loads it carries on its shoulder appear burdensome. It finds the comfort to be in right place to unload to build something new – the delta. Now, you figure out what it says to life and social living of humans.”

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- by Dr. Dilip K. Barua, 15 September 2016

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