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Suspended sediment distribution and residual transport in the coastal ocean off the Ganges-Brahmaputra river mouth
Abstract
An examination of suspended sediment dispersal in the coastal ocean off the Ganges-Brahmaputra river system using shipboard and satellite data reveal distinct patterns of sediment distribution and sediment motion which help explain the dynamics of the world's largest sediment dispersal system. During the low-discharge period, suspended sediment concentration distribution is mainly a function of tidal energy and the sediments are primarily transported as a suspension population. Satellite and sea-bed data indicate that transport of river sediments is largely restricted to an area landward of about the 20-isobath during the low-discharge period. A simplistic estimate of residual transport and its separation into tidal and non-tidal components indicate that non-tidal transport contributes about 47% of the total transport and has transport directions mostly southwestward during the northeast monsoon. The tidal residual transport, which contributes about 53% of the total transport, however, does not show any particular direction for most of the stations, although some landward stations show shoreward transport. Although few water-column measurements were made during the high-discharge period, data from this and previous studies suggest that the dominant transport direction is to the southwest for most of the year.
... Drainage has played a major role in the morphological evolution of any area [53][54][55][56][57]. Therefore, any changes in a channel may affect the morphology of the region as drainage plays the main role in water discharge and sediment transport [25,57]. More than 6000 km of waterways have been lost in southwest Bangladesh over the last 40 years due to siltation and drainage congestion [58,59]. The average width of the channels was about 250 m but now it has come down to less than 30 m [58]. ...
... More than 6000 km of waterways have been lost in southwest Bangladesh over the last 40 years due to siltation and drainage congestion [58,59]. The average width of the channels was about 250 m but now it has come down to less than 30 m [58]. Rashid et al. [51] stated that within the embankments, the rivers lost their widths of about 5-8 m/year in the southwest coast of the country. ...
... Rashid et al. [51] stated that within the embankments, the rivers lost their widths of about 5-8 m/year in the southwest coast of the country. Aurbach et al. [58] addressed that the embanked lands are currently placed 1 m below the mean level of high water due to a lack of sedimentation and compaction of sediments. Therefore, if an embankment fails at any point, either through channel migration or storm surges, the area comes into contact with saline tidal water inundation. ...
The Amphan, a super cyclone, hit the Bangladesh coast on May 20, 2020. This study conducted on the worst calamity-affected areas of southwestern coast of Bangladesh through field investigation, and Modification of Normalized Difference Water Index, and Normalized Difference Vegetation Index analysis. The study disclosed that most of the areas were submerged by tidal saline water during the cyclone. The people in the areas were taken refuge on embankments and in cyclone shelters. The vegetation was fully or partially damaged/dead due to undesired inundation. Domestic animals are completely absent in the area due to the unavailability of freshwater.
... However, at present, there is only one active fan valley system connected to the submarine canyon (Curray et al. 2003) that delivers sediment to the distal reaches of the fan. Sediment and OC that reach the Bay of Bengal are reworked by freshwater fluxes, longshore currents, storm waves and tidal fluxes (Barua et al. 1994;Kuehl et al. 1989;Kuehl et al. 1997;Michels et al. 1998;Michels et al. 2003;Segall and Kuehl 1992). Sediment from the river mouth is transported to the southwest by large freshwater inputs, modified by tidal fluxes (Barua et al. 1994;Kuehl et al. 1989;Michels et al. 1998). ...
... Sediment and OC that reach the Bay of Bengal are reworked by freshwater fluxes, longshore currents, storm waves and tidal fluxes (Barua et al. 1994;Kuehl et al. 1989;Kuehl et al. 1997;Michels et al. 1998;Michels et al. 2003;Segall and Kuehl 1992). Sediment from the river mouth is transported to the southwest by large freshwater inputs, modified by tidal fluxes (Barua et al. 1994;Kuehl et al. 1989;Michels et al. 1998). Transport in the central portion of the shelf, west of the river outlet, is dominated by tidal cyclicity and storm resuspension (Barua et al. 1994;Kudrass et al. 1998;Kuehl et al. 1989;Michels et al. 2003;Segall and Kuehl 1992). ...
... Sediment from the river mouth is transported to the southwest by large freshwater inputs, modified by tidal fluxes (Barua et al. 1994;Kuehl et al. 1989;Michels et al. 1998). Transport in the central portion of the shelf, west of the river outlet, is dominated by tidal cyclicity and storm resuspension (Barua et al. 1994;Kudrass et al. 1998;Kuehl et al. 1989;Michels et al. 2003;Segall and Kuehl 1992). Approximately 9% of the annual river sediment load is eventually reworked into the head of the SoNG (Michels et al. 2003). ...
This volume provides a state-of-the-art summary of biogeochemical dynamics at major river-coastal interfaces for advanced students and researchers. River systems play an important role (via the carbon cycle) in the natural self-regulation of Earth's surface conditions by serving as a major sink for anthropogenic CO2. Approximately 90 percent of global carbon burial occurs in ocean margins, with the majority of this thought to be buried in large delta-front estuaries (LDEs). This book provides information on how humans have altered carbon cycling, sediment dynamics, CO2 budgets, wetland dynamics, and nutrients and trace element cycling at the land-margin interface. Many of the globally important LDEs are discussed across a range of latitudes, elevation and climate in the drainage basin, coastal oceanographic setting, and nature and degree of human alteration. It is this breadth of examination that provides the reader with a comprehensive understanding of the overarching controls on major river biogeochemistry.
... Map drawn with ArcMap 10.4.1 using World Ocean Base Map and General Bathymetric Chart of the Oceans (GEBCO), NOAA National Centers for Environmental Information (NCEI), Natural Earth Ganges-Brahmaputra located along the eastern coastal islands (Allison 1998;Rahman et al. 2011;Sarwar and Woodroffe 2013;Wilson and Goodbred 2015). A portion of the fluvial load is transported southwestward by tidal currents (Barua et al. 1994) and temporarily settles out on the topset beds of the submarine delta in shallow water of less than 20 m (Fig. 2) (Kuehl et al. 1989). The topset beds, composed of clayey fine sand and silt, extend up to a water depth of about 30 m (Kuehl et al. 2005). ...
... Two important parameters are not considered in the above ranking procedure: tidal currents and availability of sediment. The semidiurnal tides with residual currents setting in southwesterly directions (Barua et al. 1994) certainly modify the sediment transport direction temporarily on the inner shelf, but they probably have only a transient effect because the durations of the cyclonic impacts usually last longer than 24 h. The availability of clayey silt on the inner shelf may significantly increase during the high summer monsoon runoff and would thus increase the amount of transported sediment during postmonsoonal cyclones. ...
... During the calm winter period, the turbid water generally extends up to the 10-m isobath, whereas during the high-discharge period of the summer monsoon, in combination with more wind-generated waves and turbulence, the plumes can extend up to the 80-m isobath. The weak residual tidal currents are generally directed to the southwest (Barua et al. 1994). Consequently, most suspended sediment probably settles out on the shallow inner shelf to form a meter-thick ephemeral mud layer (Kuehl et al. 1997). ...
Frequent cyclones originating in the Bay of Bengal landfall on the delta coast of the Ganges and Brahmaputra rivers. The cyclones are well recorded in the sediments of a canyon that is deeply incised into the shelf offshore Bangladesh. The large mud supply by the two rivers forms temporary deposits on the innermost shelf, where they are mobilized by waves and currents during the passage of cyclones. The resulting, highly concentrated fine sand-silt-clay suspension is moved by wind-induced currents and eventually plunges into the shelf canyon. These gravity flows are deposited as graded beds on the broad canyon floor. In a 362-cm-long section of a dated sediment core covering the period from 2006 to 1985, nearly all 59 graded beds can be correlated with 42 cyclones observed in that period. The threefold decrease in the sedimentation rate of the last decade compared to the period from 1994 to 1954 is due to the decreased number and power of cyclones. Compared to the sediment transfer by cyclones, the input by local sediment slumps, tidal currents, and monsoonal floods is small. Thus, cyclones dominate the mobilization and distribution of sediment on the Bangladesh shelf. This sediment dispersal mechanism is probably also typical for other shelf areas crossed by tropical cyclones.
... About 10% (96*10 6 tons/yr) of the Ganges-Brahmaputra river load of suspended particulate matter accumulates in the Sundarbans (Rogers et al. 2013). Due to the low land surface gradient, the tidal influence reaches 100 km inland (Bricheno et al. 2016;Zaman et al. 2017), supplying this wetland region with saline water and a large amount of ocean-derived suspended sediment (Barua 1990;Barua et al. 1994;Wilson and Goodbred 2015;Hale et al. 2019). In addition, cyclone-driven ocean currents occasionally transport sediment from the mouth of the Ganges-Brahmaputra river system and the inner shelf in westward direction, and the associated coastal surges make this material available for coastal land accretion (Kuehl et al. 1989;Rogers et al. 2013;Kudrass et al. 2018). ...
... The elevation of the modern coastal land surface in the mangrove forest varies between +3.3 and +3.4 m with reference to the local geoid EGM96 datum, an elevation that corresponds to the winter spring high-tide level (Table 2). This surface is in equilibrium with the influx of suspended sediment supplied by tidal pumping, monsoonal flooding, (Allison and Kepple 2001;Rogers et al. 2013;Auerbach et al. 2015;Bomer et al. 2020), and cyclone storm surges (Barua et al. 1994;Kudrass et al. 2018). Bomer et al. (2020) recently discussed the complex interaction between tidal dynamics, channel morphology, and sediment supply with respect to local sediment accretion rates, which altogether determine the elevation of the land surface: This surface corresponds to an elevation near the mean high water level (MHWL; Fig. 3), which, in turn, decreases in landward direction due to friction. ...
This study reconstructs the coastal subsidence over the past 1300 years in a mangrove region along the coast of the Ganges–Brahmaputra Delta, an area not affected by anthropogenic interference. The relative sea level (RSL) history is based on radiocarbon and luminescence ages measured on 108 submerged kilns and in situ mangrove stumps. While the regional, long-term average subsidence rate is calculated to be 2.7 ± 0.3 mm/yr, modern RSL (including eustacy, isostasy, ocean level, and subsidence) rises by 8.7 ± 0.4 mm/yr. This rate has been balanced by natural sediment accretion so far. A reduction in sediment supply by engineering projects along rivers and coasts may, however, accelerate coastal inundation and retreat. Subsidence has accelerated during specific episodes since the eighth century. A major land-sinking event happened in the eighteenth century, with a downward displacement of 1–2 m, depending on geographic area. We propose that the subduction-related 1762 Arakan earthquake caused this sudden lowering. Prior to this event, RSL was nearly stable for 900 years. An earlier major subsidence event occurred around 900 CE, when the land suddenly sank by about 1 m, which also coincided with a major earthquake along the Arakan coast. Event-driven, sudden, significant subsidence, thus, needs to be considered a potential major hazard for coastal Bangladesh.
... The bi-directional flow associated with tidal rise and fall, generating huge volume of water inland with amplitude reaching more than 4-7 m during spring tides, is called tidal prism. As the tidal oscillations conduct sediment from the shelf across the delta plain, up to 120 km towards hinterland the tidal prism retains substantial suspended sediment concentrations [30]. Suspended sediment amount may be estimated by the widely applied form: ...
... Sediment convergence takes place at the turbidity maximum zone (Figure 2) where sediments are trapped by outrushing freshwater discharge and onshore tidal deluge. The sediments typically precipitate as velocities decline at the end of the transport path along the upstream reaches of the tidal channels or on the intertidal mangrove platform and small creeks and the process is further sustained by relatively low flow velocity of the falling tide [30,35]. This tidal pumping [33] is imperative in facilitating vertical accretion in the plains which receive little or no sediment influx from upstream barring Hugli (India) and Baleshwari river (Bangladesh). ...
The deltaic landscape of the Ganga-Brahmaputra delta has evolved through a complex interplay of geomorphic processes and tidal dynamics coupled with the anthropogenic modifications brought over in course of the reclamation of the islands since the late 18th century. The reclamation process was characterized by clearing lands for paddy farms and fish ponds by building a mesh of earthen embankments along creek banks to restrict saltwater intrusion. The length of the embankments in the Indian Sundarbans alone is 3638 km (World Bank, 2014) which altered the tidal inundation regimes, sediment accretion and geomorphic character of the deltaic inlets. The mean annual sedimentation rate (2.3 cm y−1) in the central Ganga-Brahmaputra delta is over two times higher than sedimentation within the natural intertidal setting of the Sundarbans (Rogers et al., 2017). The tidal range has also increased inland due to polder construc¬tion, with high water levels within the polder zone increasing as much as 1.7 cm y−1 (Pethick and Orford, 2013). Embankments have impacted on the biodiversity and physiological adaptations of mangroves within the sphere of tidal ingression, habitat fragmentation and seedling establishment. The chapter attempts to reappraise the impact of dykes on the geomorphology of the deltaic landscape and on the functionalities of mangrove forests.
... A substantial amount of muddy sediment carried by these rivers is transported downstream and eventually temporarily settles in a shallow depth, typically less than 20 m on the bay. The central coastal island is accumulating fluvial sediments such as clay, silt, and fine sand at the mouth of the GBM estuary, and a portion of sediment has moved southwestward by coastal current ( Fig. 1; Barua et al. 1994;Allison 1998;Allison and Kepple 2001;Mikhailov and Dotsenko 2007;Wilson and Goodbred Jr. 2015;Mouyen et al. 2018;. The Payra, Bishkhali, Buriswar, Baleswar, and Tetulia Rivers are the principal rivers that traverse the region. ...
The study analyzes satellite imagery, beach morphology, grain size, river discharge and bathymetry, and tidal water level along the central Ganges-Brahmaputra-Meghna (GBM) delta coast and examines the processes of coastal development. Integrated remote sensing and geographic information system (GIS) techniques were used to monitor coastal dynamics in the GBM delta from 1978 to 2023. The analysis shows that the west and central coasts eroded about 0.03-2.40 km and lost 14.54 km 2 of land during the study period. In contrast, the east coast advanced 0.09-1.42 km and gained 2.80 km 2. Beach slopes varied with the west coast having slope between 1° and 2°, and consisting of bimodal, poorly sorted, very fine-grained sand. The central and eastern coastline have slopes between 1° and 4°, and 3° and 5°, respectively, with unimodal, well-sorted to moderately well-sorted fine-grained sand. Statistical analysis of tidal water levels shows an increasing trend (8-12 mm yr −1), while river discharge decreased by 22.57-55.94% during the period November to May due to the upstream dams, and channels were silted. On the east coast, characterized by weak longshore currents, there is minimal transfer of sediment to the sea, and the coastline advanced. Conversely, the current gets stronger when it moves to the west, and the coastlines experience the erosion. The coastal development of the area was influenced by unequal sediment supply and accommodation created at the river mouth due to the sea level rise (SLR). In addition to the natural factors, anthropogenic activities have also influenced the coastal sustainability in the study area.
... subcontinent, southwesterly winds blow from the BoB to the coast from June to September, causing continuous rainfall (Fig. 3b and c) and flooding all through the area. Barua et al. [69] found that in the BoB, the way of residual current and sedimentation is landward and westward. As a consequence, nearby tides and waves affect the internal turbulent water through a concentrated association of tidal channels, and distribute sediment across the low tide deltaic plain [44]. ...
Waterlogging is one of the major global problems which affects agro-economic activities around
the world. In the coastal areas of Bangladesh, especially the southwestern coast, drainage
congestion and waterlogging are very common which makes the area uninhabitable. Therefore,
timely checking of drainage systems and surface water, and conveying data on the dynamics of
drainages and surface water are important for plan and supervisory processes. The present study
took an effort to illustrate the waterlogging and morphological change of the rivers in the
southwestern coast of Bangladesh through the Modification Normalized Difference Water Index
(MNDWI) values which are valuable indicators for monitoring the water area and land use pattern
change. Landsat images (Landsat L8 Oli TIRS, Landsat ETM+, Landsat TM) were used in the
research. The study reveals that from 1989 to 2020, the shallow water area (mostly covered with
rivers) decreased by ~14.30 km2 yr− 1
, whereas the wet-land area (mostly covered with beels and
water logging areas) increased by ~ 67.12 km2 yr− 1
. The bare land area also increased at a rate of ~ 36.90 km2 yr− 1
. On the other hand, the green vegetation decreased at a rate of ~166.1 km2
yr− 1
, whereas the moderate green vegetation area increased by ~ 69.77 km2 yr− 1 for the same
period. In the coastal zones of Bangladesh, the polders, embankments, upstream dams, etc.,
enhance more sedimentation within the channels rather than in the nearby tidal plains. As a
result, the shallow water area which is mostly covered by rivers is gradually decreasing. Moreover, due to increase in wet-land areas with salinity intrusions which affect the vegetation.
Therefore, the green vegetation area is regularly declining due to demolition or conversion to
moderate green vegetation. The findings of the research will be supportive for coastal scientists
worldwide, policy makers & planners, and finally supportive for sustainable management of the
coastal areas including Bangladesh.
... In the upper delta, the channels are predominantly braided and sustain rapid channel aggradation and migration, favoring the preservation of coarse-grained channel deposits (Pickering et al., 2014;. In the lower delta, almost all flow and sediments pass through bifurcating channels of the Meghna estuary and form a vast turbid river plume on the inner shelf (Atker et al., 2016;Barua et al., 1994). Flood-dominant tides then advect suspended river-plume sediments back onshore to form a vast muddy tidal delta plain sustained by a dense network of channels and intertidal platform (Allison et al., 1998;Rogers et al., 2013). ...
Groundwater is the primary source of water in the Bengal Delta but contamination threatens this vital resource. In deltaic environments, heterogeneous sedimentary architecture controls groundwater flow; therefore, characterizing subsurface structure is a critical step in predicting groundwater contamination. Here, we show that surface information can improve the characterization of the nature and geometry of subsurface features, thus improving the predictions of groundwater flow. We selected three locations in the Bengal Delta with distinct surface river network characteristics—the lower delta with straighter tidal channels, the mid‐delta with meandering and braided channels, and the inactive delta with transitional sinuous channels. We used surface information, including channel widths, depths, and sinuosity, to create models of the subsurface with object‐based geostatistical simulations. We collected an extensive set of lithologic data and filled in gaps with newly drilled boreholes. Our results show that densely distributed lithologic data from active lower and mid‐delta are consistent with the object‐based models generated from surface information. In the inactive delta, metrics from object‐based models derived from surface geometries are not consistent with subsurface data. We further simulated groundwater flow and solute transport through the object‐based models and compared these with simulated flow through lithologic models based only on variograms. Substantial differences in flow and transport through the different geologic models show that geometric structure derived from surface information strongly influences groundwater flow and solute transport. Land surface features in active deltas are therefore a valuable source of information for improving the evaluation of groundwater vulnerability to contamination.
... Whereas the western and central coasts exhibit a significant erosion/accretion scenario, the eastern coastal zone shows a lower dynamism. The flat and unbroken coast (Huq et al., 1999) and northerly transportation of sediments along this zone could be the reason behind this lower rate of accretion and erosion (Barua et al., 1994). Although the eastern zone is less dynamic compared to the other regions, erosion and shoreline changes have still been observed, which could be the cause of the anti-clockwise circulation of tidal current passing through the Sandwip channel (Ahmed et al., 2018) and the excessive amount of rainfall due to the increasing temperature (Krantz, 1999). ...
... SW winds blow onshore from the Bay of Bengal, triggering persistent rainfall and flooding throughout the Indian subcontinent from June through September. Integrated satellite imagery analyses and field observations on the inner continental shelf indicate that the direction of the residual current and sediment transport is landward and to the west (Barua et al., 1994). Nearshore waves and tides then advect the turbid water inland through a dense network of tidal channels, dispersing sediments throughout the lower tidal delta plain (Hale et al., 2019). ...
Abstract
The present work is an effort to expose the sedimentation rate of the Ganges-Brahmaputra (GB) tidal delta plain from the subsurface-dated sediments samples. The study reveals that the average sedimentation rates (about 2.12-4.59 mm yr−1) of the area are not as good to minimize as the estimated range of relative sea-level rise (RSLR) (1-9 mm yr−1). Moreover, the sediments cannot be distributed properly in the tidal and fluvio-tidal plains due to different anthropogenic activities like poldering and embankments. In addition, the upstream diversion of the Ganges and the Brahmaputra rivers water reduces the suspended sediment load to the downstream and consequently, the sedimentation rates. Finally, the GB tidal delta plain and its coastal area are in a vulnerable situation in consideration of sedimentation rates, RSLR and different anthropogenic activities. Therefore, regular monitoring and evaluation is essential about sedimentation, coastal and upstream anthropogenic practices, RSLR, etc. for the implementation of effective and efficient mitigation and adaptation measures in the deltaic plains.
... In the GBM delta system, most sediment from upstream accumulates along the central coastal island located at the mouth of the estuary and a portion is transported southwestward and temporarily stored offshore where the water depth is less than 20 m. This sediment has been resuspended and carried west by coastal current and directed in the tidal channels by tidal currents (Allison, 1998;Allison and Kepple, 2001;Barua et al., 1994;Mikhailov and Dotsenko, 2007;Mouyen et al., 2018;Rahman et al., 2018;Wilson and Goodbred Jr, 2015) (Figure 1). ...
Sea-level rise and sediment supply have influenced coastal morphology and sedimentation on Bangladesh’s southwestern Ganges‐Brahmaputra‐Meghna (GBM) delta coast. Satellite images and geological core from the Haringhata coastal region were analyzed to explain the morphological changes and to understand the influences on deposits. The results derived from satellite images indicate that the southern coastline experienced a retreat that ranges between 2.3 and 2.9 km. In contrast, the eastern and western coastline advanced. The erosion and accretion ratio was 0.29 from 1977 to 1989, while the ratio was higher 2.90–4.77 from 1989 to 2020. Two sedimentary facies were identified using 130 cm thick successions. A parallel to wavy laminated bluish gray mud facies of deeper part was deposited in a marine-influenced environment. A planar to hummocky cross stratified, gray to grayish white silty sand facies of storm overwash deposits overlies the mud facies with sharp contacts. Unimodal to bimodal grain distributions of sandy sediments suggest two sources: sand derived from the beach and mud carried by adjacent tidal rivers and resuspended offshore sediment. Coastline dynamics and sedimentation of the area were influenced by inequality of accommodation and sediment supply ratio in the river mouth. This occurs due to sea-level rise and deficit in upstream water and sediment discharge. Morphological change along the southwestern GBM delta coast was not only caused by wave energy, but also by rising sea levels which shifted sediment accommodation space landward.
... Different paleoclimatic proxies such as detrital and organic material, clay minerals, carbonates, metals, pollens have been studied in the Bay of Bengal sediments (Wei et al., 2003). Based on the study carried out on suspended sediment transport and dynamics Barua et al. (1994) revealed that sediment transport is a function of tidal energy within the Ganges-Brahmaputra river system. Chauhan et al. (2005) reported different dispersal patterns for the terrigenous flux of the Ganges-Brahmaputra rivers during SW and NE monsoons. ...
Understanding the source of sediments along with delineating the changes in climate and monsoons from sediment proxies in the western Bay of Bengal fed by huge river influx from peninsular rivers is the focus of this chapter. The distribution of predominant sediment components, clay minerals, and metals indicates material supply from the peninsular rivers to the bay and also from Hoogly river under varying climate conditions. More illite indicates its release under mechanical weathering in arid cold climatic conditions by the alteration of muscovite mica in the Himalayan region suggesting a strengthening of NE monsoon; while high smectite, Fe, Ti reveal mafic-rich source rocks and from the red beds supplied through Godavari and Krishna rivers during the increased southwest monsoon. Further, the variation of proxies along the length of the gravity core represents Holocene climatic oscillations. The bottom two zones (zones A and B) largely coincide with the sub-boreal climate periods during which dry conditions favored increased physical weathering. Zone C represents intermediate climate conditions and coincides with the Roman cold–warm period, while the upper two zones of the core represent warm periods of the Medieval Warm Period and recent global warming. During these periods there has been a significant increase in warm–wet conditions that facilitated the leaching of mafic rocks.
... Whereas the western and central coasts exhibit a significant erosion/accretion scenario, the eastern coastal zone shows a lower dynamism. The flat and unbroken coast (Huq et al., 1999) and northerly transportation of sediments along this zone could be the reason behind this lower rate of accretion and erosion (Barua et al., 1994). Although the eastern zone is less dynamic compared to the other regions, erosion and shoreline changes have still been observed, which could be the cause of the anti-clockwise circulation of tidal current passing through the Sandwip channel (Ahmed et al., 2018) and the excessive amount of rainfall due to the increasing temperature (Krantz, 1999). ...
... The fluvial sediments of clay, silt, and fine sand accumulate along the central coastal island located at the mouth of the estuary and a portion is transported southwestward by coastal current. The muddy sediment carried by GBM estuary temporarily settles on the bay where water depth is less than 20 m ( Fig. 1c; Kuehl et al., 1989;Barua et al., 1994). The southern coast of Bangladesh has a tidal range up to 2-4 m (meso tidal variation) where major cyclone trail with disastrous storm surges (Kudrass et al., 1998;Karim and Mimura, 2008). ...
This study examines the sedimentary characteristics, provenance, and depositional processes of the storm deposits from 2007 Cyclone Sidr that have been identified on the southern coast of Bangladesh. Three sedimentary units up to 70 cm thick have been identified in fifteen geological core collected at a distance of 135–277 m from the coast. Laboratory analyses of grain size, TOC, TN, δ13C, and diatom assemblages were carried out to characterize the deposits. Massive to parallel laminated bluish gray mud underlies the storm overwash deposits. White to light gray, massive to parallel laminated, normal graded 1–17 cm thick sand overlies the mud unit with sharp to erosional contact. The mean grain size and thickness of this sand unit decrease landward with increasing sorting value, where grain size distribution of sand is comparable with modern beach sand. The unimodal sand that dominated the base of the storm deposits grades into bimodal olive-gray sandy silt in the upper part of the deposits. The grain size implies that the sand carried from the beaches and mud likely sourced from the suspended, nearshore sediment of the bay and adjacent rivers. TOC/TN, δ13C values and the presence of brackish mudflat origin diatoms with silty sand laminae in bluish gray mud indicate a tide-influenced environment. Post-storm sand and sandy silt sediments unit contain freshwater and marine-brackish diatoms that have likely been deposited from flooding caused by storm surge water from the bay and overbank flooded river water due to subsequent heavy rainfall. These sediments were sorted according to storm waves and settled over the study area. This study indicates that the low-lying delta coast gets the influences of high water levels both from the bay and the river during a storm. The modern storm deposit will help to characterize the paleo/prehistoric tropical cyclone in geological time for future studies in this area.
... Hence, a complete description of sediment transport should comprise a detailed study on coastal hydrodynamics, bottom stresses, and sediment transport processes. In the near-shore waters, high suspended sediment concentration (SSC) within the surf zone can also result from river discharge and littoral drift (Barua et al. 1994). In this context, precise knowledge on bottom shear stress is an essential prerequisite. ...
... There, distributive mouth-bar development is fed by suspended fine sand from the two main rivers (Rogers & Overeem, 2017), accounting for 12-18 km 2 /yr of net land growth (Sarwar & Woodroffe, 2013). Much of the river's silt-dominated mud fraction bypasses the river mouth estuary to the inner shelf (Barua et al., 1994), where wave, tide, and storm transport disperse the sediment plume to the subaqueous delta-front and canyon (Kuehl et al., 1997;Michels et al., 1998;Rogers et al., 2015). About 40% of total fluvial sediment discharge can be accounted for through deposition in the river mouth estuary (20%) and subaqueous delta (20%). ...
Abstract Arising from the non‐uniform dispersal of sediment and water that build deltaic landscapes, morphological change is a fundamental characteristic of river delta behavior. Thus, sustainable deltas require mobility of their channel networks and attendant shifts in landforms. Both behaviors can be misrepresented as degradation, particularly in context of the “stability” that is generally necessitated by human infrastructure and economies. Taking the Ganges‐Brahmaputra‐Meghna Delta as an example, contrary to public perception, this delta system appears to be sustainable at a system scale with high sediment delivery and long‐term net gain in land area. However, many areas of the delta exhibit local dynamics and instability at the scale at which households and communities experience environmental change. Such local landscape “instability” is often cited as evidence that the delta is in decline, whereas much of this change simply reflects the morphodynamics typical of an energetic fluvial‐delta system and do not provide an accurate reflection of overall system health. Here we argue that this disparity between unit‐scale sustainability and local morphodynamic change may be typical of deltaic systems with well‐developed distributary networks and strong spatial gradients in sediment supply and transport energy. Such non‐uniformity and the important connections between network sub‐units (i.e., fluvial, tidal, shelf) suggest that delta risk assessments must integrate local dynamics and sub‐unit connections with unit‐scale behaviors. Structure and dynamics of an integrated deltaic network control the dispersal of water, solids, and solutes to the delta sub‐environment and thus the local to unit‐scale sustainability of the system over time.
... Examples include the Georgia Bight (St. Helena Sound) on the U.S. Atlantic coastline (Hayes, 1976;Hayes and Sexton, 1989), and the estuarine system of the Ganges-Brahmaputra Delta (Barua et al., 1994;Allison et al., 2003;Kuehl et al., 2005). ...
The Midway to Carrizo succession in the southeastern Texas Gulf Coast: Evolution of a tidally-influenced coastline
... The annual deluge is largely responsible for delivering 1270 km 3 of water and 1060 Mt of sediment to the G-B delta and Indian Ocean each year, representing one of the largest fluvial sediment loads in the world (Milliman and Farnsworth, 2011 and references therein). Integrated satellite imagery analyses and field observations on the inner continental shelf indicate that the direction of the residual current and sediment transport is landward and to the west (Barua et al., 1994). Nearshore waves and tides then advect these turbid waters inland through a dense network of tidal channels, dispersing sediments throughout the lower tidal delta plain (Hale et al., 2019a). ...
In the Ganges-Brahmaputra (G-B) delta, periodic flooding of the land surface during the tidal cycle coupled with enormous sediment delivery during the monsoon promotes sediment accretion and surface elevation gain through time. However, over the past several decades, widespread embankment (“polder”) construction in the G-B tidal delta plain has led to numerous environmental disturbances, including channel siltation and tide range amplification. While previous research indicates that rates of sediment accretion are relatively high in the G-B tidal delta plain, it remains unclear if and how surface elevation is maintaining pace with relative sea-level rise (RSLR) in this region. In this study, we utilize an array of surface elevation tables, sediment traps, and groundwater piezometers to provide longitudinal trends of sedimentation and elevation dynamics with respect to local platform elevation and associated hydroperiod. Two hydro-geomorphic settings of the Sundarbans mangrove forest are compared: higher elevation stream-bank and lower elevation interior. Seasonal measurements over a time span of 5 years reveal that elevation gain is occurring in all settings, with the highest rates observed at elevated stream-bank zones. Elevation gain occurs primarily in response to sediment accretion, with possible minor contributions from pore-water storage and swelling of clay minerals during the monsoon season (i.e., belowground biomass and organic contribution is minimal). As a result, elevation loss and shallow subsidence in the G-B delta is unlikely to be caused by compaction of organic-rich soils, but rather appears to be controlled by seasonal lowering of the groundwater table and compaction of clay minerals. Rates of surface elevation gain in the Sundarbans greatly exceed rates of RSLR and more closely follow rates of RSLR augmented from tide range amplification, indicating that this landscape is adapting to human-induced environmental change. The proceedings of this study underscore the adaptability of the natural G-B tidal delta plain to local environmental disturbances, with the caveat that these defenses may be lost to future upstream reductions in sediment supply.
... Therefore, the oscillatory velocities are significantly larger than mean current velocity and involves relatively high turbid in the inner shelf waters than open ocean waters that moves shore to outer self 8 . In the nearshore waters, high suspended sediment concentration (SSC) within the surf zone can also result from river discharge and littoral drift 9 . The degree of interchange between the opposing water masses appeared to determine regional turbidity patterns. ...
The suspended sediment concentration (SSC) in the coastal ocean of the Bay of Bengal (BOB) is retrieved with high-resolution ocean colour data of the Ocean Colour Monitor (OCM) on-board satellite Oceansat-I. The SSC distribution in the coastal waters of the BOB has a regional gradient from north to south that follows the general monsoon circulation of the east coast of India. However, factors like tidal, fluvial and long
shore currents play a major role in the regional sediment dynamics with understandable source, sink and pathway. In addition, other influences like geo-strophic currents in the coastal sediment transport and dynamics are evident from the ocean colour data.
... The Ganges distributaries have undergone siltation in historical times which has necessitated the dredging to maintain water flow during the dry season. This reduction in water and sediment discharge through older distributaries, which dissect the lower delta plain suggests that much of the sediment reaching the lower delta plain as suspended sediment is discharged through the GB river mouth and carried westwards by residual (non-tidal and tidal) coastal currents for most of the year (Barua et al., 1994). The lower delta plain may receive sediment from off-shore during the tidal and storm high-water events, ameliorating wetland loss through subsidence (Allison and Kepple, 2001). ...
Bangabandhu Island (BBI) is a newly emerged offshore island in the western shelf zone of Bangladesh. It covers an area of 8 sq. km and shows the records of significant changes in its morphology during the last four decades. In this study, attempt has been made to explore the lithological sequences and land stability of the island, and also to show how the emergence and formation of the island is related to sediments accumulation and sediment dynamics in this part of the shelf. Besides, the DCP (Dynamic Cone Penetrometer) data along two transactions and one sediment core of 1203 cm long collected from the field, 14 subsamples at different depths were considered in the lab for grain size analysis, heavy mineral separation and microfossil study. The study shows that the Bangabandhu Island has gone through at least four phases of its formation. Initially the area was a typical shelf zone under strong marine influence. Due to an accelerated sediment accumulation, particularly around the head of the SNG, the area tuned into a shallow-water zone (phase 2) with frequent and strong influence of fluvio-marine process. During the next phase (3 rd phase) of its formation, the island was a blurred shape landfill (shoals)/ submersible island, which occasionally emerged out during the low tide. Only since last four decades it has emerged as an offshore island (final stage) and shows the evidence of fast sediment accumulation, land stabilization and colonization of mangrove vegetation. The higher ZTR index near to surface than the sub-surface layers shows the higher tendency of recent deposition of sediments in the upper layers from Himalayan origin, whereas the bottom layers represent the low energy environment of the shelf zone, mostly deposited by sediments of shallow marine sources.
... Water and sediment are primarily delivered to this region by a tidal channel extending from the Sutarkhali River, called the Sorbathkhali, which lies between two primary conduit channels that connect this region to the broader Ganges-Brahmaputra-Meghna river system: the Pussur and Shibsa Rivers (Figure 1). The majority of freshwater and sediment are delivered to the region by inland advection from the Bay of Bengal during the southwest monsoon season (July to September; e.g., Barua, 1990;Barua et al., 1994;Allison and Kepple, 2001;Rogers et al., 2013;Shaha and Cho, 2016;Hale et al., 2019). As a result, the entire SNF is characterized by fresh water (∼0 PSU) from July to late November, and brackish to saline waters (∼20-30 PSU) for the remainder of the year (Shaha and Cho, 2016;Ayers et al., 2017). ...
Southwest Bangladesh, located on the Ganges-Brahmaputra-Meghna delta, is experiencing the impacts of sea level rise (SLR) due to processes at both the local and global scale. In particular, regional alterations of the hydrodynamic network, due to embankment construction, have drastically altered effective SLR, placing millions of inhabitants at risk of prolonged inundation, and threatening the world’s largest continuous mangrove stand, the Sundarbans National Forest (SNF). In order to effectively employ landscape recovery solutions, an understanding of local sediment transport and deposition is critical. This field-based study investigates the sediment dynamics between the mangrove platform and tidal channels of the SNF using data from a variety of instruments and sediment samples collected within a forested sub-basin (∼20 km²) fed by a major tidal channel. We observe profound seasonal variability within the sub-basin, with the wet season exhibiting a deeper and longer inundation of the mangrove platform and greater suspended sediment concentrations (SSC). Further, there exists a trend of decreasing SSC and median grain size from the perimeter of the SNF to the interior, and decreasing SSC from the tidal channel to the platform at both locations. We project seasonal platform sedimentation rates ranging from 0.17 ± 0.16 cm in the dry season to 1.8 ± 0.35 cm in the wet season. Importantly, the annual deposition rate measured at either location is sufficiently rapid to keep pace with observed rates of effective SLR published in other studies (∼1.0–1.7 cm/year). Based on our results, it appears that many controls on sedimentation are both covariant and of similar importance to land aggradation in the SNF. While inundation depth and frequency will likely increase under future SLR scenarios, sediment supply is threatened by India’s proposed River Linking Project, which could decrease the sediment loads of the Ganges and Brahmaputra Rivers by as much as 75 and 25%, respectively. These rivers provide the sediment for the entire delta, and we predict that with decreasing SSC, some regions—particularly interior sediment-depleted regions—may begin to deteriorate and become submerged, including within the SNF.
... Finally, it is worth noting that in field case studies, tidal channels downdrift of distributary channels with high suspended sediment concentrations have been shown to deliver fines upstream if tidal currents are flood-dominant, like in several of the north-south trending tidal channels west of the Ganges-Brahmaputra-Meghna river mouth in the Bay of Bengal (Barua, 1990;Barua et al., 1994;Wilson and Goodbred Jr, 2015). The ebb-dominated character of our experiments resulted in a lack of significant landward component of suspended sediment transport, though we propose that fine sediment import in the delta plaincombined with the presence of vegetated intertidal areas, which promotes both organic and inorganic sediment deposition (D'Alpaos et al., 2011;Goodbred and Saito, 2012)might enhance the capability of tide-influenced deltas to more efficiently keep pace with rising RSL. ...
Tide‐influenced deltas are among the largest depositional features on Earth and are ecologically and economically important as they support large populations. However, the continued rise in relative sea‐level threatens the sustainability of these landscapes and calls for new insights on their morphological response. While field studies of ancient deposits allow for insight into delta evolution during times of eustatic adjustment, tide‐influenced deltas are notoriously hard to identify in the rock record. We present a suite of physical experiments aimed at investigating the morphological response of tide‐influenced deltas subjected to relative sea‐level rise. We show that increasing relative tidal energy changes the response of the delta because tides effectively act to remove fluvially deposited sediment from the delta topset. This leads to enhanced transgression, which we quantify via a new methodology for comparing shoreline transgression rates based on the concept of a ‘transgression anomaly’ relative to a simple reference case. We also show that stronger tidal forcing can create composite deltas where distinct land forming processes dominate different areas of the delta plain, shaping characteristic morphological features. The net effect of tidal action is to enhance seaward transfer of bedload sediment, resulting in greater shoreline transgression as compared to identical, yet purely fluvial, deltaic systems that exhibit static or even regressive shorelines.
... By the end of observations in October 2015, SSC began to drop to levels similar to those observed in mid-August (0.01-1.0 g L −1 ; Fig. 3), but on average remained well above those of the dry season. For comparison, the mean annual SSC of the main-stem Ganges-Brahmaputra River is ∼ 1 g L −1 , and depth-averaged values in the main estuary mouth and on the inner shelf commonly range 2-5 g L −1 during high river discharge (Barua et al., 1994;Ali et al., 2007). In total, SSC values well in excess of 1 g L −1 are regularly observed during the wet season from the main-stem river to the inner shelf and into the tidal channels of the lower delta plain. ...
The landscape of southwest Bangladesh, a region constructed primarily by fluvial processes associated with the Ganges River and Brahmaputra River, is now maintained almost exclusively by tidal processes as the fluvial system has migrated east and eliminated the most direct fluvial input. In natural areas such as the Sundarbans National Forest, year-round inundation during spring high tides delivers sufficient sediment that enables vertical accretion to keep pace with relative sea-level rise. However, recent human modification of the landscape in the form of embankment construction has terminated this pathway of sediment delivery for much of the region, resulting in a startling elevation imbalance, with inhabited areas often sitting >1 m below mean high water. Restoring this landscape, or preventing land loss in the natural system, requires an understanding of how rates of water and sediment flux vary across timescales ranging from hours to months. In this study, we combine time series observations of water level, salinity, and suspended sediment concentration with ship-based measurements of large tidal-channel hydrodynamics and sediment transport. To capture the greatest possible range of variability, cross-channel transects designed to encompass a 12.4 h tidal cycle were performed in both dry and wet seasons during spring and neap tides.
Regional suspended sediment concentration begins to increase in August, coincident with a decrease in local salinity, indicating the arrival of the sediment-laden, freshwater plume of the combined Ganges–Brahmaputra–Meghna rivers. We observe profound seasonality in sediment transport, despite comparatively modest seasonal variability in the magnitude of water discharge. These observations emphasize the importance of seasonal sediment delivery from the main-stem rivers to this remote tidal region. On tidal timescales, spring tides transport an order of magnitude more sediment than neap tides in both the wet and dry seasons. In aggregate, sediment transport is flood oriented, likely as a result of tidal pumping. Finally, we note that rates of sediment and water discharge in the tidal channels are of the same scale as the annually averaged values for the Ganges and Brahmaputra rivers. These observations provide context for examining the relative importance of fluvial and tidal processes in what has been defined as a quintessentially tidally influenced delta in the classification scheme of Galloway (1975). These data also inform critical questions regarding the timing and magnitude of sediment delivery to the region, which are especially important in predicting and preparing for responses of the natural system to ongoing environmental change.
... The formation of the Ganges Brahmaputra Delta, or Bengal Delta, is characterised by a huge sediment load, tectonic subsidence and a near shore canyon system that resulted in widespread sediment distribution across the delta (Goodbred Jr & Kuehl, 2000;Datta et al., 2008). Two major carriers of sediment are the monsoon river discharges and the daily tidal prisms carrying sediment 120 km inland through tidal channels (Barua et al., 1994;Goodbred Jr & Kuehl, 2000;Rogers et al., 2013). The Ganges and Brahmaputra rivers jointly produce a total sediment flux of 1,060 MT per year, making it the third-largest sediment flux in river systems worldwide, after the Amazon and Huanghe rivers (Milliman et al., 1995). ...
Many urbanised deltas face development challenges due to growing economies, populations and climate change. Changes in land–water strategies are often required, as ‘business-as-usual’ solutions are no longer sufficient. The aim of this paper was to study tidal river management (TRM) as a strategic innovation, and trace how it is appreciated by people and used in master plans to address congested rivers and waterlogging in Bangladesh. In this context, a strategic innovation can be categorised as having four features: (i) it is a fundamental reconceptualisation of business as usual strategies; (ii) it is rule breaking and reshapes markets; (iii) it offers value improvement for livelihoods; (iv) it is sustainable. The case study analysis was built from 17 interviews, a focus group discussion and numerous documents. The case analysis revealed that tidal river management is very different (local, natural, complex) from mainstream engineering strategies for tidal rivers and polder systems, and is strongly supported by local people for its potential livelihood improvement. The paper concludes that tidal river management has strategic potential, though is hardly recognised in master plans. To advance practice, reconceptualisations are needed that focus on the diverse benefits of TRM, such as restored tidal rivers, flora and fauna. Further research could elaborate livelihood models that thrive on these benefits, and evaluate their costs and benefits accordingly.
... The overall dominant transport direction of sediments for most part of the year in the shelf of marine Bangladesh is southwestward (Barua et al. 1994) and the highest rate of accumulation is near the head of the canyon Swatch of No Ground (off the Sundarbans; Kuehl et al. 1989). However, locally at Hiron Point the maximum flood velocity was higher than the maximum ebb velocity -indicating a landward residual transport of sediments (Barua 1991). ...
This paper deals with the distribution and the physical and biological environment of the mangroves of Bangladesh. It also discusses the importance and management of mangroves of Bangladesh. The mangroves of Bangladesh are significant globally because of their size, geological settings, biological diversity and natural resource and livelihood support potentiality. Among these mangroves, the Sundarbans on the south western coastal Bangladesh covers an area of 5700 km 2 on the southern fringe of the Bengal Delta and represents one of the largest single tract mangrove forest of the world. This area represents physiographically an immature delta, geologically most recent and tectonically most active region of the world. Because of the favorable physical environment, biologically these mangrove ecosystems are one of the richest in the world. It represents about 50% of all major and minor mangroves in old-world (Indo-Pacific) and 41% of all mangroves in the world. The presence of Bengal Tiger among the terrestrial fauna qualifies the Sundarbans as one of the famous in the world. The importance of the Sundarbans among the mangroves in Bangladesh lies in the fact that it earns 41% of the total revenue, and produces 45% of the total timber and fuelwood in Bangladesh. These forests also provide habitat of 45% mammals, 42% birds, 46% reptiles, and 36% amphibians of the country. However, recently top-dying disease broke out affecting 17% of the sundri trees. The protective role of the mangroves from coastal natural hazards are well recognized in Bangladesh. Approximately 600,000 people maintain their livelihood directly or indirectly from this forest. The management of mangroves in Bangladesh has a long history of 125 years. However, the mangroves are still degrading, and the reasons has been identified as the very large population living along the coastline, poverty, inadequacy of knowledge and lack of responsiveness of the Forest Department to social issues.
... The geology and structural setting of the Bengal basin can be seen elsewhere (Morgan and McIntire, 1959;Sengupta, 1966;Imam and Show, 1985;Reimann, 1993). Discrete reports on the statistical parameters of grain size distribution of riverine sediments of the Bengal basin can be seen in Morgan and McIntire (1959), Coleman (1969), Chaudhri (1987), Subramanian and Jha (1988), Jahan et al., (1990), Alam et al., (1990), Kranck et al., (1993), Barua (1994), Chakrapani et al., (1995) and Datta and Subramanian (1997). However, no study has yet been made that has considered the grain size distribution of the total Bengal basin with respect to other major river sediments of the subcontinent. ...
The annual sediment discharge of the Ganges-Brahmaputra-Meghna (G-B-M) river system-estimated in the Bengal basin-is one of the highest globally, and more than 60% of this sediment load is delivered in the delta itself. The sediments typically consist of fine to very fine sand, silt and clay. The sediments are poorly sort ed and positively skewed suggesting dominance of the finer grained portion relative to the mean size. The distribution is mostly leptokurtic. A relatively high-energy hydraulic environment affects their deposition which takes place mostly under a graded (for bed sediments) and uniform suspension (for suspended sediments) condition. The riverbank and flood plain sediments show close similarity in size with the riverine sediments of the Bengal drainage basin, and could be a major source of river sediments. Sediments transported by the Himalayan rivers are relatively fine grained compared with that of the sediments transported by the Peninsular rivers to the Bay of Bengal.
... During low-flow winter months, tides propagate 70 km upstream through secondary and tertiary channels of the lower main stem Meghna River and sediment is kept in suspension by spring-neap tidal fluctuations (Barua, 1990). Based on measurements of sediment discharge below the Ganges and Brahmaputra confluence and suspended sediment concentrations estimated within the active river mouth estuary, distributary channels of the main stem river are likely carrying up to an order of magnitude higher suspended sediment during the peak monsoon freshet compared with the dry season (Barua et al., 1994, Islam et al., 1999. Wet season river processes influence sediment transport in the central fluvial-tidal delta and should be reflected in sediment routing and flux across the delta plain, though polders control sediment dynamics here just as they do in tidaldominated areas. ...
The Ganges-Brahmaputra-Meghna (Bengal) Delta in Bangladesh has been described as a delta in peril of catastrophic coastal flooding because sediment deposition on delta plain surfaces is insufficient to offset rates of subsidence and sea level rise. Widespread armoring of the delta by coastal embankments meant to protect crops from flooding has limited natural floodplain deposition, and in the tidally dominated delta, dikes lead to rapid compaction and lowered land surface levels. This renders the deltaic floodplains susceptible to flooding by sea level rise and storm surges capable of breaching poorly maintained embankments. However, natural physical processes are spatially variable across the delta front and therefore the impact of dikes on sediment dispersal and morphology should reflect these variations. We present the first ever reported sedimentation rates from the densely populated and human-controlled floodplains of the central lower Bengal Delta. We combine direct sedimentation measurements and short-lived radionuclides to show that transport processes and lateral sedimentation are highly variable across the delta. Overall aggradation rates average 2.3 ± 9 cm y–1, which is more than double the estimated average rate of local sea level rise; 83% of sampled sites contained sediment tagged with detectable 7 Be, indicating flood-pulse sourced sediments are widely delivered to the delta plain, including embanked areas. A numerical model is then used to demonstrate lateral accretion patterns arising from 50 years of sedimentation delivered through smaller order channels. Dominant modes of transport are reflected in the sediment routing and aggradation across the lower delta plain, though embankments are major controls on sediment dynamics throughout the delta. This challenges the assumption that the Bengal Delta is doomed to drown; rather it signifies that effective preparation for climate change requires consideration of how infrastructure and spatially variable physical dynamics influence sediment dispersal on seasonal and decadal time scales.
... The present study shows that the eastern coastal zone is comparatively less dynamic than the central and the western coastal zones. The reason could be due to the flat and unbroken coast (Huq et al., 1999), and the northerly transportation of sediments along this coastal zone (Barua et al., 1994). Although the rates of changes were very low in comparison with the other zones, the rates of erosion were higher than the rate of accretion in the zone for all of the periods except from 1985 to 1995. ...
This paper draws upon the application of GIS and remote sensing techniques to investigate the dynamic nature and management aspects of land in the coastal areas of Bangladesh. The geomorphological characteristic of the coastal areas is highly dynamic where land erosion and accretion with different rates remain a constant phenomenon. This study focuses on three coastal zones: western, central and eastern that comprise the entire coastal area of the country. At its core, this study uses the past 30 year Landsat satellite images. This research reveals that the rate of accretion in the study area is slightly higher than the rate of erosion. Overall land dynamics indicate a net gain of 237 km² (7.9 km² annual average) of land in the area for the whole period from 1985 to 2015. The results also demonstrate that the rates of both erosion and accretion are higher in the central zone compared to the western and the eastern zones of the coastal area. This is the first time that the entire coastal areas of Bangladesh have been considered for assessment. This study also recommends that coastal managers, planners and policymakers to consider the identified dynamic trends of coastal land before opting for any specific measure. Constant monitoring using the GIS and remote sensing techniques would be a viable management for this purpose. This study has identified some causes of land dynamics, particularly for the three coastal zones, that might be helpful for policymakers in identifying the nature of interventions needs to be taken for specific coastal zones.
... While on the coastal shelf, the river sediment plume is dispersed westward by prevailing currents. This sediment plume remains in suspension and available for transport onshore through a series of tidal creeks and onto the subaerial delta plain (Kuehl et al., 1989;Barua et al., 1994;Rogers et al., 2013). ...
The Sundarbans is one of the largest coastal wetland sites in the world and covers an area of approximately one million hectares of the western delta of the Ganges and Brahmaputra (G-B) rivers (located across Bangladesh and India). Since the late Holocene, the western delta has not been directly fluvially sourced, due to the Ganges shift to the east (present-day Bangladesh). The depositional facies (Thin Mud 2 Facies) of the late-Holocene abandoned western region (The Sundarbans) is derived from dominant estuary-tidal dynamics, however the provenance of the associated TMF sedimentation in this far western zone (Indian Sundarbans per se) is as yet equivocal. In this study, sediment cores from the Indian Sundarbans (Saptamukhi-Thakuran estuary) were closely examined for grain-size distributions (GSDs), mineralogy through X-ray diffraction (XRD), and geochemistry with X-ray fluorescence (XRF). The TMF in the West Bengal Sundarbans has been determined to show intensively weathered, terrestrial sediment, derived principally from the Ganges Alluvial Plain (GAP). There is a predominance of quartz, mica and clay minerals, with quartz interpreted as a product of low-relief tropical weathering sourced via the G-B Rivers draining the Himalayas. Lithofacies interpreted through GSD analysis of the TMF is indicative of a muddy tidal flat environment with aggradation and a general fining-up trend between the adjacent estuaries. The sediment provenance indicates a continuing G-B sediment source, which moves westward along the Bay of Bengal, from the active delta front and is then reworked over the far-western abandoned delta by tidal–estuarine forcing.
... Thus, in the Mekong system, sediment is not only stored on the topset during the highdischarge season, but also generally retained on the topset (albeit advected along isobath) during the energetic, windy monsoon season (Fig. 15). This persistent retention of sediment near shore contrasts with systems like the Ganges-Brahmaputra, where sediment is transported beyond 20-m water depth during the high-flow season, possibly owing to seasonal increases in bed stresses, storms, and gravity flows (Segall and Kuehl, 1992;Barua et al., 1994;Wilson and Goodbred, 2015). For moderate-sized systems like the Gulf of Papua and Adriatic, delivery to the foreset is also common during energetic monsoons and Fig. 9) and x-radiograph negatives from the upper 30-90 cm are shown for cores from the upper foreset, lower foreset, and bottomset. ...
Compound deltas, composed of a subaerial delta plain and subaqueous clinoform, are common termini of large rivers. The transition between clinoform topset and foreset, or subaqueous rollover point, is located at 25–40-m water depth for many large tide-dominated deltas; this depth is controlled by removal of sediment from the topset by waves, currents, and gravity flows. However, the Mekong Delta, which has been classified as a mixed-energy system, has a relatively shallow subaqueous rollover at 4–6-m depth. This study evaluates dynamical measurements and seabed cores collected in Sep 2014 and Mar 2015 to understand processes of sediment transfer across the subaqueous delta, and evaluate possible linkages to geometry. During the southwest rainy monsoon (Sep 2014), high river discharge, landward return flow under the river plume, and regional circulation patterns facilitated limited sediment flux to the topset and foreset, and promoted alongshore flux to the northeast. Net observed sediment fluxes in Sep 2014 were landward, however, consistent with hypotheses about seasonal storage on the topset. During the northeast rainy monsoon, low river discharge and wind-driven currents facilitated intense landward and southwestward fluxes of sediment. In both seasons, bed shear velocities frequently exceeded the 0.01–0.02 m/s threshold of motion for sand, even in the absence of strong wave energy. Most sediment transport occurred at water depths <14 m, as expected from observed cross-shelf gradients of sedimentation. Sediment accumulation rates were greatest on the upper and lower foreset beds (>4 cm/yr at <10 m depth, and 3–8 cm/yr at ~10–20 m depth) and lowest on the bottomset beds. Physically laminated sediments transitioned into mottled sediments between the upper foreset and bottomset regions. Application of a simple wave-stress model to the Mekong and several other clinoforms illustrates that shallow systems are not necessarily energy-limited, and thus rollover depths cannot be predicted solely by bed-stress distributions. In systems like the subaqueous Mekong Delta, direction of transport may have a key impact on morphology.
This volume provides a state-of-the-art summary of biogeochemical dynamics at major river-coastal interfaces for advanced students and researchers. River systems play an important role (via the carbon cycle) in the natural self-regulation of Earth's surface conditions by serving as a major sink for anthropogenic CO2. Approximately 90 percent of global carbon burial occurs in ocean margins, with the majority of this thought to be buried in large delta-front estuaries (LDEs). This book provides information on how humans have altered carbon cycling, sediment dynamics, CO2 budgets, wetland dynamics, and nutrients and trace element cycling at the land-margin interface. Many of the globally important LDEs are discussed across a range of latitudes, elevation and climate in the drainage basin, coastal oceanographic setting, and nature and degree of human alteration. It is this breadth of examination that provides the reader with a comprehensive understanding of the overarching controls on major river biogeochemistry.
This volume provides a state-of-the-art summary of biogeochemical dynamics at major river-coastal interfaces for advanced students and researchers. River systems play an important role (via the carbon cycle) in the natural self-regulation of Earth's surface conditions by serving as a major sink for anthropogenic CO2. Approximately 90 percent of global carbon burial occurs in ocean margins, with the majority of this thought to be buried in large delta-front estuaries (LDEs). This book provides information on how humans have altered carbon cycling, sediment dynamics, CO2 budgets, wetland dynamics, and nutrients and trace element cycling at the land-margin interface. Many of the globally important LDEs are discussed across a range of latitudes, elevation and climate in the drainage basin, coastal oceanographic setting, and nature and degree of human alteration. It is this breadth of examination that provides the reader with a comprehensive understanding of the overarching controls on major river biogeochemistry.
This volume provides a state-of-the-art summary of biogeochemical dynamics at major river-coastal interfaces for advanced students and researchers. River systems play an important role (via the carbon cycle) in the natural self-regulation of Earth's surface conditions by serving as a major sink for anthropogenic CO2. Approximately 90 percent of global carbon burial occurs in ocean margins, with the majority of this thought to be buried in large delta-front estuaries (LDEs). This book provides information on how humans have altered carbon cycling, sediment dynamics, CO2 budgets, wetland dynamics, and nutrients and trace element cycling at the land-margin interface. Many of the globally important LDEs are discussed across a range of latitudes, elevation and climate in the drainage basin, coastal oceanographic setting, and nature and degree of human alteration. It is this breadth of examination that provides the reader with a comprehensive understanding of the overarching controls on major river biogeochemistry.
This volume provides a state-of-the-art summary of biogeochemical dynamics at major river-coastal interfaces for advanced students and researchers. River systems play an important role (via the carbon cycle) in the natural self-regulation of Earth's surface conditions by serving as a major sink for anthropogenic CO2. Approximately 90 percent of global carbon burial occurs in ocean margins, with the majority of this thought to be buried in large delta-front estuaries (LDEs). This book provides information on how humans have altered carbon cycling, sediment dynamics, CO2 budgets, wetland dynamics, and nutrients and trace element cycling at the land-margin interface. Many of the globally important LDEs are discussed across a range of latitudes, elevation and climate in the drainage basin, coastal oceanographic setting, and nature and degree of human alteration. It is this breadth of examination that provides the reader with a comprehensive understanding of the overarching controls on major river biogeochemistry.
This volume provides a state-of-the-art summary of biogeochemical dynamics at major river-coastal interfaces for advanced students and researchers. River systems play an important role (via the carbon cycle) in the natural self-regulation of Earth's surface conditions by serving as a major sink for anthropogenic CO2. Approximately 90 percent of global carbon burial occurs in ocean margins, with the majority of this thought to be buried in large delta-front estuaries (LDEs). This book provides information on how humans have altered carbon cycling, sediment dynamics, CO2 budgets, wetland dynamics, and nutrients and trace element cycling at the land-margin interface. Many of the globally important LDEs are discussed across a range of latitudes, elevation and climate in the drainage basin, coastal oceanographic setting, and nature and degree of human alteration. It is this breadth of examination that provides the reader with a comprehensive understanding of the overarching controls on major river biogeochemistry.
Given the dire state of the planet where scarce resources are being exhausted and “waste” is carelessly disposed of, we can find a hero within the concept of the circular economy—a model which aims to achieve a closed loop scenario by recycling the wastes back into the useful economic and ecological flows. Bangladesh, despite her growing population, lacks a dependable institutional waste management system, so the government should apply the concept of circular economy to modify and reconstruct it into a sustainable model. Organic food waste is the major constituent of household waste, containing nutrients and chemicals with too great of an influence on the chemical cycle to simply be disposed of. The current primary waste disposal method followed in Bangladesh is undesirable landfill dumping and such an easily recyclable component like food waste must not follow that path and should be re-circulated back into the loop instead. Currently, there are no comprehensive studies on food waste in Bangladesh to be used for policy guidance. Hence, proper quantification with composition of food wastes is essential to determine the reprocessing potential and provide policy guidance to restructure the existing inefficient waste management system into a sustainable one within the purview of circular economy concept. This chapter attempts to quantify the food waste generation of Bangladesh and explores the various options of available modern technology and methods to recover and reuse food waste. Circularity is the perfect instrument to promote decoupling and achieve sustainability, so its prospect and significance in food waste recycling shall be investigated.KeywordFood waste generationFood waste recyclingCircular economyBangladesh
The chapter analyses the short-run and long-run effects of international trade on the environment. For this purpose, the bounds testing method to cointegration is applied to a small island country setting of Mauritius and over the period 1980–2018. The result shows that that trade has adversely impacted on the environment. In addition to that, higher economic growth is as well observed to generate higher CO2 emission. Furthermore, the CUSUM and CUSUM square confirm the stability of the model for Mauritius.KeywordsTradeCO2 emissionsARDLMauritius
The Ganges, Brahmaputra and Meghna rivers are the three main rivers contributing to the formation the GBM (Ganges–Brahmaputra–Meghna) delta system. The delta system undergoes several environmental challenges both natural and manmade. A significant portion of population from different nations is directly and indirectly dependent on these three rivers for their sustenance. As transboundary rivers, they suffer a lack of transnational cooperation in terms of river protection and sustainable management. This review article analyses the environmental challenges in the GBM delta system to better understand the critical issues related to sustainable delta management. It also focuses on the transboundary issues and their solutions through cooperation, knowledge sharing and other joint activities between nations. The Ganges, Brahmaputra and Meghna rivers suffer from different forms of pollution. Significant sources include domestic, industrial and agricultural affluents along with direct defecation, bathing, washing, solid waste dumping and throwing of ritualistic burnt dead bodies into river water. The level of pollution creates havoc in aquatic ecosystem and leaves the water unusable for humans as well. The significant growth of population in these regions has modified the land cover of the GBM delta system. The rise in agricultural land, dams and other hydraulic structures has modified the erosion and sedimentation dynamics of the Ganges, Brahmaputra and Meghna rivers. As a result, the cases of riverbank erosion, riverbed siltation and river course shifting are also rising. The rivers also struggle with the various types of natural disasters like cyclones, storm surges, floods, droughts, salinity intrusions, coastal erosions and tidal bores. There is also a lack of understanding and cooperation between countries when it comes to sustainable delta management. To top it off, other issues like extreme poverty, lack of education, dependency on nature-based agricultural practices have also clouded the long-term development of the delta system and its people. The above-mentioned challenges require holistic and integrated action plans among respective nations. This can be achieved through effective policy dialogues, transnational protection and conservation plans, information sharing, co-learning, joint research programs, transstate accountability and technological advancements which can be ensured by each of the participating nations. The Ganges–Brahmaputra–Meghna (GBM) delta system extends for more than hundreds of kilometres along the Bay of Bengal coastline. The Ganges and Brahmaputra River drains roughly around 75% of the Himalayan mountain range. These two rivers carry around 1.1 GT/yr of sediment and dump on the Bengal Basin. This amount is equal to around 6–8% of the total sediment input dumped on the oceans globally. These rivers are extensively affected by different types of pollutions, erratic flood intensities and altering tidal characteristics. They also suffer from river stage fluctuations in the downstream riparian regions due to the construction and operation of hydraulic structures in the upstream regions. This study provides a critical analysis of the present issues and challenges regarding the active rivers of the GBM delta system and recommends a holistic and sustainable management plan for the protection and conservation of the rivers by analysing previously published research works and secondary datasets. The Ganges, Brahmaputra and Meghna river basins experience various point and non-point pollutions. The downstream regions face a higher rate of pollution than the upstream ones. As the rivers approach the delta mouth, land use patterns and land covers change due to increased rate of urbanization, industrialization, agricultural practices and climate change effects. Their flow is also being controlled by several upstream hydraulic structures like dams, barrages and reservoirs. Consequently, the river basins face many natural and anthropogenic disasters. The combined effect of these issues elevates the vulnerability of the downstream delta mouth population and destabilizes their socio-economic conditions. These crises can be solved through transnational cooperation, regional capacity building and sharing of information between upstream and downstream riparian countries. Inclusive and flexible strategies with appropriate policy dialogue may lead to amendments of current agreements which may eventually create a sustainable platform to overcome the environmental challenges of the Ganges–Brahmaputra–Meghna delta system.KeywordsGBM deltaGanges RiverBrahmaputra RiverMeghna RiverPollutionLand useDisasterClimate changeTransboundary riverCooperationManagement
The Ganges-Brahmaputra-Meghna (GBM) Delta is one of the largest deltas in the world, covering more than 100,000 km². The GBM River system transports over one billion tons of sediment annually and delivers ∼750 million tons to the Bay of Bengal, ∼25% of which is advected by tides into the fluvially abandoned western delta plain. The Sundarbans National Forest (SNF) is located within the GBM Delta and, covering more than 10,000 km², is the world's largest continuous mangrove stand. The present rate of sediment delivery allows the SNF platform elevation to keep pace with regional sea level rise, but India's National River Linking Project (NRLP) could decrease the suspended sediment loads of Ganges and Brahmaputra Rivers by 39–75% and 9–25% respectively, and thus may change future sedimentation. In this study, we examine whether sediment stored in the tidal channels could serve as an additional source for platform nourishment, if resuspended and delivered to the mangrove platform throughout the year. Over two field seasons 70 shallow (<50 cm) sediment cores were collected from the channel banks, imaged using x-radiography to observe fine scale changes in depositional characteristics on tidal channel banks, and subsampled for subsequent textural analysis. In more than half of the cores examined there is no change in the style of the laminations, suggesting that the processes controlling deposition remain constant throughout the year, despite a change in the external sediment supply. Cores where the depth of laminations decreases from wet to dry season demonstrate that sediment is being eroded from the banks and advected elsewhere, with the mangrove platform a likely sink. Over 70% of cores showed surface (0–2 cm) grain sizes coarsening from the wet to the dry season, which could be the product of the winnowing of fine sediments as overall sediment supply decreases and material with lower bulk density is preferentially taken away. Total organic carbon (TOC) results indicate constant reworking of sediment on the banks throughout the year. These results demonstrate that sediment in this system is persistantly reworked, and that at least a portion of the sediment delivered to the platform during the dry season can be sourced from the banks.
Bangladesh, one of the countries most vulnerable to climate change, is threatened by sea level rise (SLR) and land subsidence. The tidal river management (TRM) practised in coastal regions of Bangladesh has the potential to raise the land by sedimentation, to counteract SLR and subsidence. TRM is an indigenous method in which dikes are breached to readmit sediment-rich water into a polder, which results in sediment being deposited in depressions in the polder called ‘beels’, while simultaneously preventing silting-up of the tidal rivers. However, after several years of continuous sedimentation from TRM, deposition has been uneven and less than expected.
To study the effectiveness of TRM, this research analyses different scenarios to identify which operation schemes most effectively trap sediment to raise the land surface. The scenarios developed considered the number of inlets, flow regulation through the beel using open inlets or gated inlets with different operation schemes, and seasonality. The study area was Pakhimara Beel in southwest Bangladesh, where TRM is ongoing. To simulate the scenarios, a two-dimensional (2D) morphodynamic model with variable cell size was set up, calibrated and used. The simulation results were analysed for total sediment deposition, uniformity of spatial distribution of sediment deposition and trapping efficiency.
Sediment deposition shows clear seasonal variability, with greatest deposition in the pre-monsoon period, less during monsoon and least in the dry season. Greatest deposition combined with high spatial uniformity was found for TRM that uses two inlets located at opposite sides connected to different watercourses. Regulated flow using successively opened gates resulted in highest sediment deposition in all seasons, about double that of the existing situation without gate. However, given the complexity and cost of gate operation, TRM with two inlets located at opposite sides of the beel without flow regulation may be considered more feasible, and still effective despite 20-30% less sediment deposition. To also increase acceptability by local affected stakeholders we propose to restrict this improved TRM to the monsoon period, to allow crops to be grown on the land in the dry and pre-monsoon periods, and ensure salinity is minimized. Such well-planned TRM has the potential to also counteract sea level rise in sinking deltas elsewhere in the world by enhancing sedimentation.
Abstract: The Suspended Sediment Concentrations (SSC) have been mapped along the entire coast of Bay of Bengal (BOB) during 2002-2003 (Pre and Post SW monsoon at low and high tide levels), using high-resolution ocean colour data acquired by Oceansat-1 onboard the Indian Remote Sensing Satellite (IRS-P4-OCM). The SSC distribution in the coastal waters of BOB are highly controlled by diverse forces that include littoral
currents, tidal currents, fluvial currents, geostrophic current, coastal circulation and wind stress. The regional SSC distribution pattern along the coast shows seasonal gradient trend from south to north which following the corresponding monsoon and inter monsoon circulations. However, there are also localized variations in the regional SSC distribution occur at selected river mouths, bays and tidal inlets. These variations are mainly
controlled by tidal energy, resuspension, shallow bathymetry, coastal and fluvial currents. The regions from south to north have shown to gradually increase in SSC due to the correlation with the micro tidal regime in the south and macro tidal ranges in the northern part of the BOB. The coastal sediment dynamics also show a strong influence on the open ocean. A clear understanding of the pathways of source material and their final
destination (probably a sink) as well has been acquired from this study.
Key words: Suspended Sediment Concentration, Bay of Bengal, Ocean Colour, Coastal and Tidal currents.
The landscape of southwest Bangladesh, a region constructed primarily by fluvial processes associated with the Ganges and Brahmaputra Rivers, is now maintained almost exclusively by tidal processes as the fluvial system has migrated to the east through the Holocene. In natural areas such as the Sundarbans National Forest, year-round spring-tide inundation delivers sufficient sediment for vertical accretion to keep pace with relative sea-level rise. However, recent human modification of the landscape in the form of embankment construction has terminated this pathway of sediment delivery for much of the region, resulting in a startling elevation imbalance, with inhabited areas often sitting >1m below mean high water. Restoring this landscape, or preventing land loss in the natural system, requires an understanding of how rates of water and sediment flux vary across time scales ranging from hours to months. In this study, we combine time-series observations of water level, salinity, and suspended sediment concentration, with ship-based measurements of large tidal channel hydrodynamics and sediment transport. To capture the greatest possible range of variability, cross-channel transects designed to encompass a 12.4-h tidal cycle were performed in both dry and wet seasons, during spring and neap tides.
Regional suspended sediment concentration begins to increase in August, coincident with a decrease in local salinity, indicating the arrival of the sediment-laden, freshwater plume of the combined Ganges-Brahmaputra-Meghna rivers. We observe profound seasonality in sediment transport, despite somewhat modest seasonal variability in the magnitude of water discharge, indicating the importance of this seasonal sediment delivery. On tidal time-scales, spring tides transport an order of magnitude more sediment than neap tides in both the wet and dry seasons. In aggregate, sediment transport is flood-oriented, likely a result of tidal pumping. Finally, we note that rates of sediment and water discharge in the tidal channels are of the same scale as the annually averaged values for the Ganges or Brahmaputra rivers. These observations provide context for examining the relative importance of fluvial and tidal processes in what has been defined as the quintessential tidal delta in the classification scheme of Galloway. These data also inform critical questions regarding the timing and magnitude of sediment delivery to the region, which are especially important in predicting, and preparing for, future change under changing environmental conditions.
The physical sustainability of deltaic environments is very much dependent on the volume of water and sediment coming from upstream and the way these fluxes recirculate within the delta system. Based on several past studies, the combined mean annual sediment load of the Ganges-Brahmaputra-Meghna (GBM) systems has previously been estimated to vary from 1.0 to 2.4 BT/year which can be separated into components flowing from the Ganges (260 to 680 MT/year) and Brahmaputra (390 to 1160 MT/year). Due to very limited data and small contribution of the Meghna system (6-12 MT/year) to the total sediment flux of the GBM system, the data of the Meghna is not considered in the analysis assuming the sediment flux from GB system as the sediment flux of GBM. However, in this paper our analysis of sediment concentration data (1960-2008) collected by Bangladesh Water Development Board shows that the sediment flux is much lower: 150 to 590 MT/year for the Ganges versus 135 to 615 MT/year for the Brahmaputra, with an average total flux around 500 MT/year. Moreover, the new analysis provides a clear indication that the combined sediment flux delivered through these two major river systems is following a declining trend. In most of the planning documents in Bangladesh, the total sediment flux is assumed as a constant value of around 1 billion tons, while the present study indicates that the true value may be around 50% lower than this (with an average decreasing trend of around 10 MT/year).
Since the 1960s, ~5000 km2 of tidal deltaplain in southwest Bangladesh has been embanked and converted to densely inhabited, agricultural islands (i.e., polders). This landscape is juxtaposed to the adjacent Sundarbans, a pristine mangrove forest, both well connected by a dense network of tidal channels that effectively convey water and sediment throughout the region. The extensive embanking in poldered areas, however, has greatly reduced the tidal prism (i.e., volume of water) transported through local channels. We reveal that >600 km of these major waterways have infilled in recent decades, converting to land through enhanced sedimentation and the direct blocking of waterways by embankments and sluice gates. Nearly all of the observed closures (~98%) have occurred along the embanked polder systems, with no comparable changes occurring in channels of the Sundarbans (
The N Inlet, S Carolina, salt marsh estuary is dominated by tidal and coastal far-field forcing. In general, net currents flow in opposite directions on different sides of the same estuarine cross-section. The net longitudinal salt transport is largely a balance between ebb-directed advective salt flux and flood- directed dispersive tidal flux due to triple-correlation between tide, velocity, and salinity. The salt flux is in general not in steady state from one tidal cycle to the next and there is a pronounced transverse circulation contribution to the mean advective salt flux. -from Author
The processing algorithms used for relating the apparent color of the ocean observed with the Coastal-Zone Color Scanner on Nimbus-7 to the concentration of phytoplankton pigments (principally the pigment responsible for photosynthesis, chlorophyll a) are developed and discussed in detail. These algorithms are applied to the shelf and slope waters of the Middle Atlantic Bight and also to Sargasso Sea waters. In all, four images are examined, and the resulting pigment concentrations are compared to continuous measurements made along ship tracks. The results suggest that over the 0.08–1.5-mg/m³ range the error in the retrieved pigment concentration is of the order of 30–40% for a variety of atmospheric turbidities. In three direct comparisons between ship-measured and satellite-retrieved values of the water-leaving radiance the atmospheric correction algorithm retrieved the water-leaving radiance with an average error of ~10%. This atmospheric correction algorithm does not require any surface measurements for its application.
The chapter describes the principal features of the vertical current structure and the turbulence properties observed in the various kinds of bottom boundary layer encountered in shelf seas. The approach is primarily observational, though a basic theoretical framework is introduced to enable the measurements to be fitted into a generalisable pattern. Spectra of turbulence and the 'bursting phenomenon' are also discussed, in as far as they aid the understanding of boundary-layer processes. -Author
In the Potomac River, satellite imagery shows a turbidity maximum developed in the lower estuary with concentrations estimated at 10-20 times normal. Significant quantities of sediment entered the Bay from the Potomac. The satellite data further indicate differences in grain size between the Potomac and the Upper Bay. The imagery shows the movement and high concentrations of suspended sediment in the upper estuary, with estimated concentrations of 3 times 1979 and 100 times normal. -from Author
Bengal Submarine Fan, with or without its eastern lobe, the Nicobar Fan, is the largest submarine fan known. Most of its sediment has been supplied by the Ganges and Brahmaputra Rivers, probably since the Early Eocene. The “Swatch-of-No-Ground” submarine canyon connects to only one active fan valley system at a time, without apparent bifurcation over its 2500- km length. The upper fan is comprised of a complex of huge channel- levee wedges of abandoned and buried older systems. A reduction of channel size and morphology occurs at the top of the middle fan, where meandering and sheet flow become more important.
The amount of suspended sediment transported by rivers to the seas each year is tabulated. The major rivers are ranked in order of tons of sediment transported per year and drainage area and water discharge data are included. The rivers are listed by, continents in subsequent tables with data on drainage area, annual sediment yields in tons, sediment production rates in tons per square mile per year, the years of sediment measurement, and the sources of data. This sample represents more than one-third of the land contributing water-borne sediment to the seas and, if representative, indicates an annual world sediment yield of 20 billion (20 × 109) tons. The data suggest that Africa, Europe, and Australia have very low sediment yields (
Grain size distributions commonly show discontinuities and straight line segments when plotted (using the phi transformation) on probability paper. The different components making up the distribution have commonly been interpreted as subpopulations of grains moved by different transport mechanisms. The paper considers this hypothesis for the restricted case of the two coarsest components observed in many samples of river bed materials. By comparing analyses of bed materials and suspended load, it is shown that the components may be interpreted as (C) traction, and (A) intermittent suspension subpopulations, moved by the dominant discharge of the river. From an analysis of theory, experiments and field data on turbulence in open channels, it is shown that a satisfactory though approximate hydraulic criterion for separating the two mechanisms of sediment transport is as follows: grains move by traction if their settling velocity is larger than the shear velocity of the stream, and by intermittent suspension if their settling velocity is smaller than the shear velocity. This criterion is confirmed by comparison with hydraulic and sediment data for the Middle Loup, Niobrara, Elkhorn, Missouri, Mississippi and Rio Grande Rivers. Consequently, it should be possible to determine the dominant shear velocity of ancient fluvial sands by determining the size "break" between their two coarsest components.
Shelf sediment dispersal seaward of the Ganges-Brahmaputra river system is examined during January/February (low discharge) through analysis of sediment cores and surface grab samples for clay-size mineralogy. Illite is the dominant mineral in surficial sediments landward of the 50-m isobath, comprising >40% of the clay-size assemblage. Kaolinite concentrations of surficial sediments generally are greater than 20% throughout the area; however, very high abundances occur off Bhola Island (25 33%) and the Chittagong coast (34 39%), probably as a result of extensive shoreline erosion of cultivated areas enriched in kaolinite. Smectite and illite/smectite concentrations, reflecting input from distributaries of the Ganges, are enriched on the western inner shelf and along the western margin of the ``Swatch of No Ground'', a major submarine canyon west of the present river mouth. Although surficial chlorite abundances on the inner shelf in water depths
Laplace's tidal equations are solved for the M2 and S2 tides in the world oceans on the basis of a knowledge of the tidal potential alone. Tidal dissipation was taken to be limited to the coastline, where a fraction of the tidal energy incident on the coast was assumed to be absorbed. The coast was assumed to be either vertical or to have a sloping shelf, the latter model yielding results in better agreement with observations. The main purpose of this investigation was to determine the effects of tidal self-attraction and of tidal loading. A fast iterative method was developed by which these secondary effects could be evaluated. The resulting change is of the order of 10%, and somewhat improves the agreement between the theoretical and observed tides. Tidal dissipation from the M2 and S2 tides totals 3.1 × 1019 erg/s. The total retarding couple exerted by the M2 and S2 tides on the Earth comes out at 4.2 × 1023 dyn cm, yielding a deceleration of the Earth's rotation from these sources of 1080' '/cy2. The theoretical tidal values were compared with observations on the northeastern and western coasts of the Pacific, on the western coast of the Atlantic, on New Zealand, and on islands. The theoretical tidal phases are generally within one hour of the observed values; the amplitudes are in reasonable agreement with observations except in zones where there is amplification by the fiord effect. A solution obtained for a 'smoothed' coastline showed little change. Note on units. 1 erg = 10-7 J. 1 dyn = 10-5 N. In this paper the symbol cy is used for century.
A study was made of bathymetric charts of those coastlines characterized by large vertical tidal ranges (greater than 10 feet). In these areas tidal currents are strong (1-5 knots) and may significantly affect sedimentation. Two characteristic types of sand accumulation were found which appear to be formed by these tidal currents. Both exhibit wave-like profiles, and are of a scale significant to oil exploration. The first type is here called "tidal current ridges." These are a rhythmic series of ridges oriented parallel with a tidal current. They are 25-100 feet high, 5-40 miles long, and spaced 1-6 miles apart. Most are composed of sand, but some may be mud or silt. Their spacing is proportional to the depth of water and current velocity. This suggests that their origin is related to the similar problem of the hydraulic geometry of stream channels. Although best developed in the Bay of Korea and the Gulf of Cambay, these ridges appear to be present wherever tidal current velocities range between 1 and 5 knots and a supply of sediment is available. The second type is sand waves. These are large ripple marks oriented perpendicular to the current direction. Recent evidence by European oceanographers has indicated that, whereas in rivers these waves are fairly small-scale features, in the open ocean they commonly have heights greater than 25 feet. Cartwright and Stride have shown a wide distribution of sand waves of this size, particularly in the North Sea. Their relationship to tidal current ridges is not known, although they appear to occur in the same environment. Since tidal currents are now significant in shallow ocean areas, their effect should be visible in large percentage of the shallow-water deposits of the geologic past. In particular it is suggested that some of the lenticular sands of the Chester Series of Illinois, of the Cardium Formation of Canada, and of the Clinton sands of Ohio show tidal current effects. The rhythmic pattern of tidal current ridges and sand waves should be considered in the study of the distribution of these and other shoestring sands. End_of_Article - Last_Page 274------------
I. General Considerations .
1. Introduction —It may seem presumptuous in one who is neither a geologist nor has any pretension to geological knowledge to venture to address this Society on a subject so nearly akin to their special science. My excuse must be that, having resided for five years on the banks of one of the most active of the Bengal rivers, I have had opportunities which are not vouchsafed to every one of observing their phenomena, and have been a witness of the changes I am about to describe. I may also, perhaps, be allowed to state that, when I first became aware of the disturbance that was taking place around me, I set myself carefully to measure and observe what was passing; and , in 1835, made a sketch-survey of the lower Ganges and Brahmapootra, from Jaffiergunge to the sea. This was published by Mr. Tassin a few years afterwards, and is, so far as I know, the only survey that was made—certainly the only one published—between that made by Major Rennell and the survey now in progress, but which has not yet been given to the world. I may also mention, in extenuation, that I have waited for more than a quarter of a century in order that some one more worthy might undertake the task; but, as no one has come forward, I may perhaps be now excused for venturing upon it.
In order, however, to obviate the reproach of presumption, my intention is to confined myself
Reconnaissance of the Bengal provinces of East Pakistan and India indicates that structural activity, primarily faulting, has significantly influenced Quaternary geology. Two areas of Pleistocene terrace border the Bengal basin on the east and west and flank Tertiary and older hills of India. Two large inliers of Pleistocene sediments within the basin are surrounded by Recent flood-plain deposits of the Ganges and Brahmaputra rivers and their combined deltaic plain. Block faulting and echelon faulting have so disturbed the topography of the Pleistocene terrace that the reconnaissance was insufficient to permit determination of whether multiple terraces are present within the basin. Faulting and structural uplift have continued into the Recent epoch, necessitating a physiographic subdivision into an early and a late phase. Changes in the courses of the Ganges and Brahmaputra rivers through Bengal during the last few hundred years can be attributed to faulting and resultant tilting of fault blocks. These changes have caused the Ganges to abandon numerous western distributaries in favor of joining the Brahmaputra-Meghna system to the southeast. At present about 12,000 square miles of former Ganges deltaic plain in southwest Bengal has been abandoned. A series of surface echelon faults plus evidence of structural control of stream courses suggest the presence of a subsiding structural trough or major fault at depth. This active structural zone apparently has controlled both the Brahmaputra and Ganges rivers in their lower reaches. Subsurface information is lacking, but this subsiding trough may possibly be related to the arcuate chain of mountains in adjacent Burma.
We have constructed new tidal charts of the northern Indian Ocean by empirical means from existing data and from recent deep pressure gage measurements. A brief comparison is made to Dietrich's (1944) charts and to some existing numerical models of the region. The deviations from Dietrich's charts are significant but not drastic.
During 1965–1966 a field program was conducted within six tidal channels in Puget Sound, Washington, and the Strait of Juan de Fuca. The purpose of the program was to investigate the logarithmic nature of the velocity distribution within 1.5 m of the bed, and the frictional influence that different bed configurations have upon the adjacent fluid. Measurements were made of the velocity distribution, bed configuration, and sediment textural characteristics in each of the channels. Velocity data were collected continuously for periods of several to 20 h. The bed configuration in the study areas varied from rocks and gravel to small ripples and sand deformed into irregular roughness patterns.Generally the flow within a channel can be divided into two categories conforming to “hydrodynamically rough” and “hydrodynamically transitional” flow conditions. This division is based on a significant decrease in the dispersion of the drag coefficient as the Reynolds number increased. The boundary between transitional and rough flow seems to be qualitatively related to the geometry of the bed, with the more complex beds becoming fully rough at higher Reynolds number than more simple roughness patterns.By grouping all data, a mean and 95% confidence limit was assigned to the drag coefficient, which represents a more or less average value for the shallow marine environment regardless of bed configuration and flow conditions. The grouped data are also divided into two regions (transitional and rough) according to a roughness Reynolds number, with representative values of the drag coefficient assigned to each region.
The nature of shelf sedimentation seaward of the world's largest sediment dispersal system is examined by using sedimentological and geochronological techniques on a unique suite of sediment cores and grab samples. Sediments from the Ganges-Brahmaputra river system are currently accumulating on the shelf in water depths of less than about 80 m, forming a clinoform-like deposit similar to subaqueous deltas found off other major river systems. The highest sediment accumulation rates on the shelf occur near the head of the Swatch of No Ground, a major submarine canyon that indents the shelf west of the present river mouths. This observation, together with textural data, suggests that river sediments are transported seaward and westward and that the Swatch of No Ground is currently a major conduit for the transport of sediments from the Bengal shelf.
Heathershaw, A.D., 1981. Comparisons of measured and predicted sediment transport rates in tidal currents. Mar. Geol., 42: 75-104. This paper describes bedload transport measurements in a large-scale sedimentary system on the British continental shelf. Steady-flow sediment transport theories are applied to oscillatory tidal currents, and various equations are compared with measured transport rates from tracer dispersion experiments. The effects of waves on both measured and predicted rates are briefly considered and the predicted directions of sediment movement compared with those inferred from sedimentological data.
A method is presented which enables the computation of the suspended load as the depth-integration of the product of the local concentration and flow velocity. The method is based on the computation of the reference concentration from the bed-load transport. Measured concentration profiles have been used for calibration. New relationships are proposed to represent the sizegradation of the bed material and the damping of the turbulence by the sediment particles. A verification analysis using about 800 data shows that about 76% of the predicted values are within 0.5 and 2 times the measured values.
Publisher Summary This chapter discusses the bottom boundary layer of shelf seas; and describes the principal features of the vertical current structure and the turbulence properties observed in the various kinds of bottom boundary layer encountered in shelf seas. Spectra of turbulence and the bursting phenomenon are also discussed. The oscillatory–planetary boundary layer in a shallow sea may be depth-limited, it will frequently exhibit significant thermal stratification effects, it may additionally be modified by sand-waves or sandbanks, and for non-uniform sediment distributions it may also develop an internal boundary layer. The boundary layer beneath tidal and other currents on the continental shelf exhibit many similarities with boundary layers in the atmosphere and the laboratory. Measurements of turbulence profiles would help to improve empirical expressions for the scaling of the spectral distribution of turbulent energy. Although considerable effort has gone into the study of the bursting phenomenon in its own right, especially in the laboratory, it still remains for a practical and usable method to be proposed that will incorporate the processes involved into a theoretical framework for the bottom boundary layer.
The treatment in the book is based on courses of lectures given in the University of Liverpool and elsewhere over the years. The approach throughout is a combination of the observational and the theoretical, since it is in this way that scientific progress is made. A certain amount of mathematics must play its part but it is kept as elementary as possible, while still being sufficiently rigorous, and does not require a mathematical knowledge beyond that provided by a first year university course in the subject. (from paper)
During the past decade, the science of dynamic meteorology has continued its rapid advance. The scope of dynamic meteorology has broadened considerably. Much of the material is based on a two-term course for seniors majoring in atmospheric sciences. This book presents a cogent explanation of the fundamentals of meteorology and explains storm dynamics for weather-oriented meteorologists. It discusses climate dynamics and the implications posed for global change. The new edition has added a companion website with MATLAB exercises and updated treatments of several key topics.
New data and new estimates from old data show that rivers with large sediment loads (annual discharges greater than about 15 x 106 tons) contribute about 7 x 109 tons of suspended sediment to the ocean yearly. Extrapolating available data for all drainage basins, the total suspended sediment delivered by all rivers to the oceans is about 13.5 x 109 tons annually; bedload and flood discharges may account for an additional 1-2 x 109 tons. About 70% of this total is derived from southern Asia and the larger islands in the Pacific and Indian Oceans, where sediment yields are much greater than for other drainage basins.-Authors
This paper is a summary and progress report of a study of the northeastern Indian Ocean, covering the areas of the Bengal and Nicobar Fans, the western Wharton Basin, the continental margins surrounding the Bay of Bengal, the Andaman Sea, the Andaman-Nicobar Ridge, the Sunda Arc off Sumatra and Java, and the adjacent land areas (Fig. 1). Bathymetry and topography of the study area are shown in Fig. 2. Combined, it is one structural province, extending from the Assam Valley on the northeast and the Ganges flood plain on the northwest, southward over oceanic crust to the distal ends of the Bengal and Nicobar Fans. This Mesozoic-Cenozoic structural basin will henceforth be referred to as the Bengal Geosyncline.
The spatial distribution of suspended particulatematter (SPM) was estimated in Mayagüez Bay on the west coast of Puerto Rico by using traditional ship board measurements and remotely sensed data acquired over four days during January 1990. This effort was part of a joint project between NASA and the University of Puerto Rico to develop techniques to monitor the water quality of a Caribbean coastal zone. This paper presents the methods and algorithms developed to map and analyze short term changes in the source and spatial distribution of SPM in Mayagüez Bay by using remotely sensed data acquired by the Calibrated Airborne Multispectral Scanner (CAMS). A PC-based data acquisition system was developed to record continuous ship measurements of select in- water variables. Spectral reflectances derived from CAMS red and near-IR data were corrected for atmospheric effects and then used to generate maps of SPM. These maps displayed SPM plumes associated with the mouths of the bay's three river systems. Significant day-to-day differences in the spatial characteristics were observed, suggesting that changes in river discharge occurred. However, an analysis of estimated river discharge, sediment yield, local wind field, and thermal river plume indicates that observed sediment plumes result primarily from wind-driven resuspension events.
Topographical, flow volume and suspended sediment concentration (SSC) measurements are presented in the large area of a turbidity maximum in the estuary of the Ganges-Brahmaputra-Meghna river system in Bangladesh. The zone of turbidity maximum is indicated by the satellite imagery taken during the low river discharge period. Topographical and flow volume measurements, the aspect ratio and the flow ratio, indicate that the system is divided into flood- and ebb-dominated channels. Suspended sediment movement in the estuary appears to be associated with the horizontal stratification of flow and with the residual movement induced by the asymmetry of the tidal wave. The flow ratio at different sections indicates that net import of suspended sediment takes place through flood channels while the net export occurs through ebb channels. Despite this difference in the direction of net movement of sediments, measurements show that the SSC, both in the flood channel and in the ebb channel, is dependent on the tidal range: the spring tidal SSC is about twice that of the neap tide, and locations with higher tidal range show higher magnitudes of SSC. The response of sediments to the varying energy environment of tidal range and tidal current, however, is not instantaneous; the SSC shows a time lag in its variation with the varying energy condition. The amplification and asymmetry of the tidal wave in the flood channels, and the slack-water asymmetry measured in a flood-dominant location, indicate that a possible landward residual transport of suspended sediment due to this factor is more pronounced through flood channels.
Measurements of velocity profiles have been obtained within 2 m of the sea bed over a rippled sand in Start Bay, Devon, U.K. Simultaneous observation by underwater television has enabled the threshold of movement of the sand to be determined. The threshold criteria are considerably higher than laboratory measurements for a flat bed. As the sand moves the ripples change their shape. This causes a change in the bottom roughness length, the drag coefficient and the apparent threshold of sand movement during the tidal cycle. The bottom roughness length and the threshold of movement also increase with decreasing tidal amplitude. The calculated suspended sediment concentration at the height of bed roughness length was proportional to the normalized excess shear stress with a constant of proportionality of 0·78×10−4. The concentration at a height of 10cm was proportional to u*5.
Four quarterly surveys were made of the sand-wave field in 1968–1969. The data on trough to crest height of the sand waves have been expressed as significant wave heights. These show a maximum of 7 m, decreasing to 2 m in the north and towards the coast. Wavelengths are from 200 to 500 m. Variability in the position of the northern and eastern margins of the field is attributed to wave activity. The position of the western boundary to the sand-wave field is due to scarcity of sand and to the existence of a gravel bottom in parts of the deep-water channel. Symmetrical and asymmetrical sand waves, and sand waves with superimposed megaripples are the forms recognised. The maximum height of 7 m is in agreement with predictions using an equation given by Kennedy (1969). Together with northward decrease in height there is decrease in grain size and absence of megaripples on sand waves. The northern boundary to the sand-wave field is caused by decreasing asymmetry of the tidal ellipse. This does not occur in a narrow tongue extending along the coast of Texel where an extension of the sand-wave field is found. The northwards decrease in height is mainly caused by increase in suspension transport of sand. This agrees with a re-evaluation of Bagnold's (1966) suspension criterion which shows that large amounts of sand < 230 μ in diameter will be transported in suspension. Megaripples on sand waves are taken to be large scale analogues of ripples on dunes seen in flumes. The megaripples represent perturbation of bed-load transport rate and the sand waves perturbation of suspended load transport rate. Thus the northwards absence of megaripples on sand waves, decrease in grain size, decrease in height and increase in suspension transport, fit in a coherent dynamical explanation of the characteristics of the sand-wave field.
The Ganges and Brahmaputra rivers combined have formed one of the largest deltas in the world, comprising some 23,000 sq. miles. These rivers originate within the Himalayan Mountains and drain an enormous land area before entering East Pakistan. Individually, each of the rivers discharges in excess of 2.5 million cusecs of water during flood, and combined they carry nearly 6 million cusecs of water to the Bay of Bengal, nearly three times the amount borne by the Mississippi River. Having such a large drainage area, the rivers are also heavily charged with sediment, transporting approximately 13 million tons of suspended sediment per day during flood. The large discharge and heavy sediment load cause the rivers to be extremely unstable, and the channels are constantly migrating laterally. Within Recent times both rivers have occupied and abandoned numerous river courses. The Brahmaputra followed a route some 60 miles to the east of its present course only 200 years ago. The long-term patterns of river migration indicate that the Ganges has been migrating eastward, whereas the preferred migration of the Brahmaputra is westward. These movements are obviously controlled by major faults or fractures in the earth's crust.
Recent advances in microcomputer technology have resulted in systems that rival the speed, storage, and display capabilities of traditionally larger machines. Low-cost microcomputers can provide a powerful environment for image processing. A new software program which offers sophisticated image display and analysis on IBM-based systems is presented. Designed specifically for a microcomputer, this program provides a wide-range of functions normally found only on dedicated graphics systems, and therefore can provide most students, universities and research groups with an affordable computer platform for processing digital images. The processing of AVHRR images within this environment is presented as an example.
WHEN the flow of fluid over a plane bed of loose grains is increased
from zero, a condition is reached where a few grains move. This is known
as the threshold of grain movement. This report describes the
determination of the threshold under flows of water and oil for beds of
mineral grains of uniform size with diameters between 20 and 200
µm.
The present analysis of results generated by a multilayer, adiabatic, monthly mean wind-driven numerical model of the upper Indian Ocean indicates that its simulated northeastern currents are in general agreement with observations and interpretations. Attention is given to the large anticyclonic flow during Northern Hemisphere winter. The model's layer-thickness reveals the propagation of Kelvin waves along the coast, travelling the entire perimeter of the Andaman Sea and Bay of Bengal; this wave excites westward-propagating Rossby waves into the bay's interior. A time-series analysis of transport calculations yields peaks in the 20-30 day and 50-60 day ranges which are unlikely to have been directly forced by the applied stress.
Global Ocean tides, Part II. Naval Surface Weapon Cent
- E W Schwiderski
Schwiderski, E.W., 1979. Global Ocean tides, Part II. Naval Surface Weapon Cent., Dahlgren, VA.
Sedimentary and tectonic processes in the Bengal deep sea fan and geosyncline
- Curray
Curray, J.R. and Moore, D.J., 1974. Sedimentary and tectonic processes in the Bengal deep sea fan and geosyncline. In: G.A. Burk and C.L. Drake (Editors), Geology of Continental Margins. Springer, New York, pp. 617-627.
Application of AVHRR satellite data to the study of sediment and chlorophyll in turbid coastal waters
- Stumpf
Stumpf, R.P., 1987. Application of AVHRR satellite data to the study of sediment and chlorophyll in turbid coastal waters. NOAA Tech. Mem. NESDIS AISC, 7, 50 pp.
Monatskarten fur den Indischen Ozean The balance of suspended sediments in the Gironde and Thames estuaries Velocity profile over a rippled bed and the threshold of movement of sand
- Germany
- Hamburg
- K R Dyer
Deutsches Hydrographisches Institut, 1960. Monatskarten fur den Indischen Ozean. Report #2422, Federal Republic of Germany, Hamburg. Dyer, K.R., 1978. The balance of suspended sediments in the Gironde and Thames estuaries. In: B. Kjerfve (Editor), Estuarine Transport Processes. Univ. South Carolina Press, Columbia. Dyer, K.R., 1980. Velocity profile over a rippled bed and the threshold of movement of sand. Estuarine Coastal Mar. Sci., 10:181 199.
Bangladesh The World's Coastline
- R E Snead
Snead, R.E., 1985. Bangladesh. In: E.C.F. Bird and M.L. Schwartz (Editors), The World's Coastline. Van Nostrand Reinhold, New York.
Coastal Embankment Project, Engineering and Economic Evaluation, 1. Dacca, East Pakistan. Land Reclamation Project, 1982. Sediment Concentrations and Salinities Between Bhola Island and Chittagong Coast
- Leedshill-De
- Leuw
Leedshill-De Leuw Engineers, 1968. Coastal Embankment Project, Engineering and Economic Evaluation, 1. Dacca, East Pakistan. Land Reclamation Project, 1982. Sediment Concentrations and Salinities Between Bhola Island and Chittagong Coast. Land Reclamation Project, Dhaka, Bangladesh, Tech. Rep., 7.