Tidal Delta Plain Research Articles

Monitoring of drainage system and waterlogging area in the human-induced Ganges-Brahmaputra tidal delta plain of Bangladesh using MNDWI index

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.

Conceptual sedimentation model as the basis of geological correlation in a case study of AT6-8 reservoirs of Beregovoye field in West Siberia

For the Beregovoye field (West Siberia, Yamalo-Nenets Autonomous Okrug), a new subsurface geologic model for AT6-8 group of reservoirs in the Lower Cretaceous (Hauterivian-Aptian) interval within Ereyamskaya formation was built following the reprocessing and reinterpretation of 2D and 3D seismic data in 2020, drilling of new wells (new wells, which were drilled after 2016, account for about 10 % of the total well stock that penetrated the AT6-8 reservoirs), updating the structural framework, additional core studies, and revising the well test results. Well log correlation in the AT6-8 interval was based on the reservoirs formation in alluvial and tidal delta plain environments. Multidirectional source areas formed disconnected zones in the net reservoir interval, which controlled the oil and gas content of these reservoirs. Taking into account the results of the geochemical survey, a stage-by-stage approach to hydrocarbon accumulations (light oil and gas condensate) formation was proposed, which led to vertically and laterally varied phase compositions of the discoveries.

Sedimentation and Coastal Area Management in the human-modified Ganges-Brahmaputra tidal delta plain of Bangladesh

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.

Morphodynamic evolution of paraglacial spit complexes on a tide-influenced Arctic fjord delta (Dicksonfjorden, Svalbard)

Recent global warming triggered pronounced geomorphic changes such as coastal retreat and delta progradation along the coastlines of the Arctic regions. Coastal morphodynamics and associated sediment transport at the Arctic fjord head remain relatively unexplored due to the logistically limited accessibility to the field area, especially at short-term temporal scales. A repeat survey using an unmanned aerial vehicle (UAV)-assisted photogrammetry was conducted to quantify the annual morphodynamics of gravel spit complexes developed on the tidal delta plain of the deglaciated Dicksonfjorden, Svalbard of Arctic. Results show that the spit morphodynamics varies in time and space with an overall downfjord increase in the growth and migration rate of the spits. The youngest spits elongated 22 m yr−1 and migrated landward 4.3 m yr−1 between 2015 and 2019, marking the most pronounced spit morphodynamics documented to date in the Svalbard fjord systems. The spit morphodynamics is driven primarily by longshore drift and to a lesser degree by overwash processes. Gravels constituting the spits originate from the unconsolidated debris-flow deposits of old alluvial fans, which locally retreat 0.5 m yr−1. The growth of the spit complexes is also fed by snow meltwater discharge on the alluvial fans, accounting for a downfjord imbrication of angular gravel layers that are intercalated with interlaminated sands and muds on the landward sides of the spits. The breached spits at the most upfjord location have remained stationary during the study period and presumably since the 1930s. Rapid delta progradation combined with an isostatic rebound after the Little Ice Age (LIA) has decreased spit morphodynamics on the tidal delta plain upfjord in Dicksonfjorden with infrequent and insignificant wave influence. Sparse distribution of the isolated spits signifies the intermittent spit development, which is constrained by the proximity to the protruded alluvial fans. The spit complexes in Dicksonfjorden highlight that climate change accelerates coastal geomorphic changes at the fjord head by enhancing wave intensity and regulating episodic sediment delivery that led to the downfjord shift in the locus of wave shoaling.

Flood risk of embanked areas and potential use of dredge spoils as mitigation measures in the southwest region of the Ganges–Brahmaputra–Meghna Delta, Bangladesh

AbstractIn the 1960s, earthen embankments, locally known as polders, were first constructed in the Ganges–Brahmaputra–Meghna (GBM) Delta with the intention of protecting agricultural lands from salinity; however, unintended consequences of the barriers have negatively impacted the region. Once‐active distributary channels have morphed into terminal, headless tidal channels as the sediment‐laden water that was once dispersed across the tidal delta plain is now restricted by embankments. Elevation loss within embankments and channel siltation adjacent to embankments are deleterious consequences of hard engineering and pose significant flood risk when effective sea level rise is considered. The purpose of this study is to assess the current and future flood risks of the embanked areas in the southwest region of the GBM Delta and to expand on the potential for local‐based solutions (e.g. mud plinths and embankments). We propose a novel source of material for these structures—repurposed dredge spoils.Project sites are along terminal tidal channels, proximal to coastal towns in the southwest region of the delta. We calculate here that as the effective sea level rise will continue to increase in the years to come, the flood risk will correspondingly increase (>67%). Geospatial analyses reveal that several prominent tidal channels have reduced in width over the last 30+ years, and recent dredging operations removed ~1.7 × 106 m3 of sediment infill. Using conservative values, we estimate that the dredge spoils could be used to elevate ~20 000 houses on mud plinths or raise 18 km of existing embankments. On a delta‐wide scale, if all infilled tidal channels were dredged to 10% of their original widths, ~547 000 houses could be raised above the flood levels and ~500 km of embankments could be elevated. With a mean household population of 4.3 people, repurposing dredge spoils can positively impact ~2.4 million people within the vulnerable southwest region of the GBM Delta.

Preliminary Evidence of Transport‐Limited Chemical Weathering and Element Immobility in the Ganges Tidal Delta Plain of Bangladesh

AbstractPrevious studies have documented a weathering‐limited regime in the upper reaches of the Ganges River Basin. Chemical weathering and element mobility at six sites in the lower reaches of the Ganges‐Brahmaputra tidal floodplain of southwest Bangladesh were investigated by comparing compositions of rice paddy soils, precursor tidal channel sediments, surface waters, and extract solutions, which represent the soluble fraction of solids. Little spatial variation in water and solid compositions is observed in each season, indicating similar processes are acting to transport elements across this region. Roughly one to several decades after deposition, rice paddy soils are not significantly different in mineralogy or composition from precursor tidal channel sediments, and both are similar to the composition of average upper continental crust. Soil and sediments contain mostly stable, cation depleted minerals, and coexisting waters are saturated in those minerals. There is no detectable change in composition of tidal channel water between upstream and downstream sites. Together, these observations indicate the dominance of weathered material and weak chemical weathering in the tidal floodplain, consistent with a transport‐limited regime. Multiple lines of evidence indicate a lack of exchange equilibrium between surface waters and coexisting solids, which may be a common feature in tidal river deltas where transport‐limited regimes likely dominate.

Surface elevation and sedimentation dynamics in the Ganges-Brahmaputra tidal delta plain, Bangladesh: Evidence for mangrove adaptation to human-induced tidal amplification

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.

Salinization and arsenic contamination of surface water in southwest Bangladesh

To identify the causes of salinization and arsenic contamination of surface water on an embanked island (i.e., polder) in the tidal delta plain of SW Bangladesh we collected and analyzed water samples in the dry (May) and wet (October) seasons in 2012–2013. Samples were collected from rice paddies (wet season), saltwater ponds used for brine shrimp aquaculture (dry season), freshwater ponds and tidal channels (both wet and dry season), and rainwater collectors. Continuous measurements of salinity from March 2012 to February 2013 show that tidal channel water increases from ~0.15 ppt in the wet season up to ~20 ppt in the dry season. On the polder, surface water exceeds the World Health Organization drinking water guideline of 10 μg As/L in 78% of shrimp ponds and 27% of rice paddies, raising concerns that produced shrimp and rice could have unsafe levels of As. Drinking water sources also often have unsafe As levels, with 83% of tubewell and 43% of freshwater pond samples having >10 μg As/L. Water compositions and field observations are consistent with shrimp pond water being sourced from tidal channels during the dry season, rather than the locally saline groundwater from tubewells. Irrigation water for rice paddies is also obtained from the tidal channels, but during the wet season when surface waters are fresh. Salts become concentrated in irrigation water through evaporation, with average salinity increasing from 0.43 ppt in the tidal channel source to 0.91 ppt in the rice paddies. Our observations suggest that the practice of seasonally alternating rice and shrimp farming in a field has a negligible effect on rice paddy water salinity. Also, shrimp ponds do not significantly affect the salinity of adjacent surface water bodies or subjacent groundwater because impermeable shallow surface deposits of silt and clay mostly isolate surface water bodies from each other and from the shallow groundwater aquifer. Bivariate plots of conservative element concentrations show that all surface water types lie on mixing lines between dry season tidal channel water and rainwater, i.e., all are related by varying degrees of salinization. High As concentrations in dry season tidal channel water and shrimp ponds likely result from groundwater exfiltration and upstream irrigation in the dry season. Arsenic is transferred from tidal channels to rice paddies through irrigation. Including groundwater samples from the same area (Ayers et al. in Geochem Trans 17:1–22, 2016), principal components analysis and correlation analysis reveal that salinization explains most variation in surface water compositions, whereas progressive reduction of buried surface water by dissolved organic carbon is responsible for the nonconservative behavior of S, Fe, and As and changes in Eh and alkalinity of groundwater.

Widespread infilling of tidal channels and navigable waterways in the human-modified tidal deltaplain of southwest Bangladesh

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 (<2% change). We attribute most of the channel infilling to the local reduction of tidal prism in poldered areas and the associated decline in current velocities. The infilled channels account for ~90 km2 of new land in the last 40–50 years, the rate of which, ~2 km2/yr, offsets the 4 km2/yr that is eroded at the coast, and is equivalent to ~20% of the new land produced naturally at the Ganges-Brahmaputra tidal rivermouth. Most of this new land, called ‘khas’ in Bengali, has been reclaimed for agriculture or aquaculture, contributing to the local economy. However, benefits are tempered by the loss of navigable waterways for commerce, transportation, and fishing, as well as the forced rerouting of tidal waters and sediments necessary to sustain this low-lying landscape against rising sea level. A more sustainable delta will require detailed knowledge of the consequences of these hydrodynamic changes to support more scientifically-grounded management of water, sediment, and tidal energy distribution.

Sources of salinity and arsenic in groundwater in southwest Bangladesh.

BackgroundHigh salinity and arsenic (As) concentrations in groundwater are widespread problems in the tidal deltaplain of southwest Bangladesh. To identify the sources of dissolved salts and As, groundwater samples from the regional shallow Holocene aquifer were collected from tubewells during the dry (May) and wet (October) seasons in 2012–2013. Thirteen drill cores were logged and 27 radiocarbon ages measured on wood fragments to characterize subsurface stratigraphy.ResultsDrill cuttings, exposures in pits and regional studies reveal a >5 m thick surface mud cap overlying a ~30 m thick upper unit of interbedded mud and fine sand layers, and a coarser lower unit up to 60 m thick dominated by clean sands, all with significant horizontal variation in bed continuity and thickness. This thick lower unit accreted at rates of ~2 cm/year through the early Holocene, with local subsidence or compaction rates of 1–3 mm/year. Most tubewells are screened at depths of 15–52 m in sediments deposited 8000–9000 YBP. Compositions of groundwater samples from tubewells show high spatial variability, suggesting limited mixing and low and spatially variable recharge rates and flow velocities. Groundwaters are Na–Cl type and predominantly sulfate-reducing, with specific conductivity (SpC) from 3 to 29 mS/cm, high dissolved organic carbon (DOC) 11–57 mg/L and As 2–258 ug/L, and low sulfur (S) 2–33 mg/L.ConclusionsGroundwater compositions can be explained by burial of tidal channel water and subsequent reaction with dissolved organic matter, resulting in anoxia, hydrous ferric oxide (HFO) reduction, As mobilization, and sulfate (SO4) reduction and removal in the shallow aquifer. Introduction of labile organic carbon in the wet season as rice paddy fertilizer may also cause HFO reduction and As mobilization. Variable modern recharge occurred in areas where the clay cap pinches out or is breached by tidal channels, which would explain previously measured 14C groundwater ages being less than depositional ages. Of samples collected from the shallow aquifer, Bangladesh Government guidelines are exceeded in 46 % for As and 100 % for salinity.

Source, transport, and evolution of saline groundwater in a shallow Holocene aquifer on the tidal deltaplain of southwest Bangladesh

AbstractDeltaic groundwater resources are often vulnerable to degradation from seawater intrusion or through interaction with saline paleowaters. The Ganges‐Brahmaputra‐Meghna River delta, in Bangladesh and West Bengal, India, is a particularly vulnerable area with an estimated 20 million coastal inhabitants directly affected by saline drinking water. The shallow groundwater of the coastal regions is primarily brackish with pockets of fresher water. A small‐scale hydrologic investigation of groundwater salinity beneath an embanked tidal channel island was undertaken to explore possible hydrogeological explanations of the distribution of water salinities in the shallow aquifer. This study employs a combination of 3H and 14C dating, electromagnetic subsurface mapping, and a 2‐D solute transport model. The authors conclude that the shallow groundwater salinity can best be explained by the slow infiltration of meteoric water into paleo‐brackish estuarine water that was deposited during the early‐mid Holocene.

Flood risk of natural and embanked landscapes on the Ganges–Brahmaputra tidal delta plain

Controlled embankment breaches could reduce flood risk for the Ganges–Brahmaputra tidal delta plain as sea level rises.

Construction and Maintenance of the Ganges-Brahmaputra-Meghna Delta: Linking Process, Morphology, and Stratigraphy

We present a review of the processes, morphology, and stratigraphy of the Ganges-Brahmaputra-Meghna delta (GBMD), including insights gained from detailed elevation data. The review shows that the GBMD is best characterized as a composite system, with different regions having morphologic and stratigraphic attributes of an upland fluvial fan delta; a lowland, backwater-reach delta; a downdrift tidal delta plain; and an offshore subaqueous-delta clinoform. These distinct areas of upland and lowland fluvial reaches and tidal dominance vary in time and space, and we distinguish late-Holocene phases of delta construction, maintenance, and decline similar to delta-lobe cycling in other systems. The overall stability of the GBMD landform, relative to many deltas, reflects the efficient, widespread dispersal of sediment by the large monsoon discharge and high-energy tides that affect this region. However, we do identify portions of the delta that are in decline and losing elevation relative to sea level owing to insufficient sediment delivery. These areas, some of which are well inland of the coast, represent those most at risk to the continued effect of sea-level rise.

Monsoon sedimentation on the ‘abandoned’ tide-influenced Ganges–Brahmaputra delta plain

Annual sediment delivery by the Ganges and Brahmaputra rivers to the Bengal margin has kept pace with sea level rise since the mid Holocene, sustaining subaerial growth of the delta. However, the Sundarbans region of the tidal delta is disconnected from major distributary sources of sediment and is often thought to be sediment starved, eroding, and susceptible to the meter of sea level rise predicted for the 21st century. Despite these assumptions, direct sedimentation measurements on the tidal delta plain reveal widespread mean annualized accretion rates of ∼1.1 cm yr−1, although heterogeneous depositional patterns indicate that topography and internal creek networks influence local sediment distribution. Short-lived radioisotope inventories (7Be: t1/2 = 53.3 days) measured on the freshly accumulated sediments indicate that about ½ of the mass deposited on the lower delta was sourced directly from the seasonal flood pulse of the river; the remaining ½ is derived from older (≥1 yr) reworked sediments. Net sedimentation on this part of the delta traps ∼10% of annual Ganges–Brahmaputra sediment load, with accretion rates roughly equivalent to the mean regional rate of relative sea-level rise (RSLR) of ∼1.0 cm yr−1. If these sedimentation rates are representative of longer-term trends and subsidence rates remain stable over the next century, the lower delta plain may continue to maintain its elevation and stability despite documented mangrove retreat around its seaward edges.

Distribution, isotopic variation and origin of sulfur in coals in the Wuda coalfield, Inner Mongolia, China

This paper describes coal petrographic characteristics, sulfur abundance, distribution and isotopic signature in coals in the Wuda coalfield, Inner Mongolia, northern China. Petrographic studies suggest that depositional environment influences petrographic composition. The No. 9 and No. 10 coal seams, which are thought to have formed on a tidal delta plain, have high collodetrinite contents (up to 66.1%) indicating enhanced gelification and bacteria activity during coal accumulation, and also have the highest sulfur content (3.46% and 3.42%). Both organic and pyritic sulfur isotope values (−12.3‰ to 5.8‰ and −18.7‰ to 1.1‰, respectively) are variable and generally tend to be more negative in high-sulfur coals than those in low-sulfur coals in the Wuda coalfield. The negative sulfur isotope values indicate that a large portion of sulfur in the high-sulfur coals has a bacterial origin. Sulfur isotopic compositions and variations within the section were used to propose a model to explain the origin of sulfur in these coals. The presence of pyritized rod-like bacteria, cyanophyte's gelatinous sheaths and degraded algae organic matter suggests that bacteria, and perhaps algae, may play an important role in the formation of these high-sulfur coals.