River Sediment Discharge Research Articles

Large‐scale sedimentary shift induced by a mega‐dam in deltaic flats

ABSTRACTDeltas are crucial for land building and ecological services due to their ability to store mineral sediment, carbon and potential pollutants. A decline in suspended sediment discharge in large rivers caused by the construction of mega‐dams might imperil deltaic flats and wetlands. However, there has not been clear evidence of a sedimentary shift in the downstream tidal flats that feed coastal wetlands and the intertidal zone with sediments. Here, integrated intertidal/subaqueous sediment samples, multi‐year bathymetries, fluvial and deltaic hydrological and sediment transport data in the Nanhui tidal flats and Nanhui Shoal in the Changjiang (Yangtze) Delta, one of the largest mega‐deltas in the world, were analysed to discern how sedimentary environments changed in response to the operations of the Three Gorges Dam. Results reveal that the coarser sediment fractions of surficial sediments in the subaqueous Nanhui Shoal increased between 2004 to 2021, and the overall grain size coarsened from 18.5 to 27.3 μm. Moreover, intertidal sediments in cores coarsened by 25% after the 1990s. During that period, the northern part of the Nanhui Shoal suffered large‐scale erosion, while the southern part accreted in recent decades. Reduced suspended sediment discharge of the Changjiang River combined with local resuspension of fine‐grained sediments are responsible for tidal flat erosion. This study found that the spatial pattern of grain‐size parameters has shifted from crossing the bathymetric isobaths to being parallel to them. Higher tide level and tidal range induced by sea‐level rise, an upstream increase in bed shear stress and larger waves likely further exacerbated erosion and sediment coarsening in deltaic flats. As a result, this sediment‐starved estuary coupled with sea‐level rise and artificial reclamations have enhanced the vulnerability of tidal flats in Changjiang Delta, this research is informative to the sedimentary shift of worldwide mega‐deltas.

Data-driven approaches for estimation of sediment discharge in rivers

Data-driven approaches for estimation of sediment discharge in rivers

Evolution of tidal flats in the Yellow River Qingshuigou sub-delta: spatiotemporal analysis and mechanistic changes (1996-2021)

The Yellow River Delta has undergone significant changes, developing a temperate chalk-sand silty coast with expansive and level tidal flats. However, the area is currently facing the threat of erosion due to coastal area construction, decreased river sediment discharge into the sea, and intensified marine hydrodynamic forces. Remote sensing technology has important applications in spatial and temporal monitoring of tidal flats. This study employs tidal data to establish the threshold range for the extraction of tidal flats in the Yellow River Qingshuigou sub-delta, using the water frequency method based on the image element proposed by previous authors. The study successfully identifies and analyzes the range of tidal flats, and further delves into their evolutionary process and underlying mechanisms. The study concludes that the area of tidal flats has shown a decreasing trend and was divided into a “rapid decline phase” and an “overall stabilization phase” with 2006 as the boundary. The spatial and temporal evolution of tidal flats is primarily influenced by three main factors: river sediment discharges, marine hydrodynamic forces, and anthropogenic reclamation activities. Specifically, the river sediment discharges and marine hydrodynamic forces impact the extent of tidal flats by modifying the boundary conditions, primarily at the sea boundary. Reclamation activities have a direct and rapid effect on the extent of tidal flats, primarily at the land boundary. In this study, we determined the contribution of three factors to the spatial and temporal evolution of tidal flats in different periods. During the two phases of the tidal flats’ spatial and temporal evolution in the Yellow River Qingshuigou sub-delta, the driving mechanisms were identified as “reclamation - marine hydrodynamic forces - river sediment discharges” and “marine hydrodynamic forces - river sediment discharges - reclamation”.

Global change scenarios in coastal river deltas and their sustainable development implications

Deltas play a critical role in the ambition to achieve global sustainable development given their relatively large shares in population and productive croplands, as well as their precarious low-lying position between upstream river basin development and rising seas. The large pressures on these systems risk undermining the persistence of delta societies, economies, and ecosystems. We analyse possible future development in 49 deltas around the globe under the Shared Socio-economic and Representative Concentration Pathways until 2100. Population density, urban fraction, and total and irrigated cropland fraction are three to twelve times greater in these deltas, on average, than in the rest of the world. Maximum river water discharges are projected to increase by 11–33 % and river sediment discharges are projected to decrease 26–37 % on average, depending on the scenario. Regional sea-level rise reaches almost 1.0 m by 2100 for certain deltas in the worst-case scenario, increasing to almost 2.0 m of relative rise considering land subsidence. Extreme sea levels could be much higher still—reaching over 4.0 m by 2100 for six of the 49 deltas analysed. Socio-economic conditions to support adaptation are the weakest among deltas with the greatest pressures, compounding the challenge of sustainable development. Asian and African deltas stand out as having heightened socio-economic challenges—huge population and land use pressures in most Asian deltas and the Nile delta; low capacity for adaptation in most African deltas and the Irrawaddy delta. Although, deltas in other parts of the world are not immune from these and other pressures, either. Because of unique pressures and processes operating in deltas, as in other “hotspots” such as small islands, mountains, and semi-arid areas, we recommend greater consideration and conceptualisation of environmental processes in global sustainable development agendas and in the Integrated Assessment Models used to guide global policy.

Study on the mechanisms of interannual variation in suspended sediment concentration in the Bohai Sea based on GOCI

The launch of the Geostationary Ocean Color Imager (GOCI) satellite has made it possible to observe suspended sediment concentration (SSC) in the Bohai Sea (BHS) over a long time and a large area. We utilized remote sensing data from GOCI from 2011 to 2021 to study the spatiotemporal variation characteristics of SSC in the BHS and identified the contribution of each oceanic element to the changes in SSC. Our findings indicate that SSC is high in winter and low in summer, and related to the southeastward winds in winter (>7 m/s) and northwestward winds in summer (<4 m/s), with a correlation coefficient of 0.634. The average annual increase in SSC is 0.07 g/m3 (an increase of 1.2% per year). In terms of spatial distribution, SSC is highest in Bohai Bay and Laizhou Bay, followed by the Liaodong Bay, and is greater than that in the Central Bohai Sea, which is related to multiple river estuaries in the three bays. Additionally, the Bohai and Liaodong Bay in the BHS are covered by sea ice during winter, which affects the response of waves and currents to wind and changes the ocean current field, thereby affecting the resuspension and transport of sediment. Our EOF analysis revealed that wind-induced changes are the dominant factors in the variations of SSC, while changes in river runoff, sediment discharge, and sea ice cover are also important factors influencing SSC changes.

A Large Sediment Accretion Wave Along a Northern California Littoral Cell

AbstractThe northern California littoral cell of the Klamath River, which is a mixed rocky and sandy system with significant shoreline curvature, was investigated by examining ∼40 yr of satellite‐derived shoreline positions and historical records. We find that an accretion wave of sediment was initiated near the Klamath River mouth in the late 1980s and translated downcoast over the subsequent decades. The wave passed rapidly (∼2,500 m/yr) through a rocky coastal reach with more oblique wave directions and slowly through a sandy reach (∼200 m/yr) where wave crests approach at more normal angles. Within the sandy reach, the accretion wave extended over 200 m offshore, was ∼10 km long, incorporated 20 ± 6 million m3 of sediment, and averaged 1.3 ± 0.4 million m3/yr of longshore sediment transport over a 20‐yr interval. Diffusion of the accretion wave was observed, but the diffusivity coefficient (εobs ∼0.01 m2/s) was lower than values predicted by theory, which we attribute to net sediment transport convergence in the study area caused by the curvature of the shoreline. Examining historical records, we find that increased sediment discharge in the Klamath River occurred during the 20th century from industrial‐scale logging and climatic extremes. Thus, we hypothesize that increased river sediment discharge introduced new sediment to the littoral cell that initiated the observed accretion wave. These hypotheses can be tested with stratigraphic and mineralogic investigations of the broad study area beach that has formed during the past 150 years.

Spatial and temporal variability of suspended sediment fronts over the Yangtze Bank in the Yellow and East China Seas

Suspended sediment fronts (SSFs) are an important sub-mesoscale phenomenon prevalent in estuaries, coasts, and continental shelf seas, affecting ocean hydrodynamics, fisheries, and ecology. Using archived (2011–2021) Geostationary Ocean Color Imager (GOCI) data, the spatial distributions and long-term variations in SSFs over the Yangtze Bank were quantitatively investigated by calculating frontal probability (FP). The findings indicate that the majority of SSFs occurred in regions inside the 50-m isobath. Their distributions were considerably limited by depth gradients. The FP values were divided into three main levels according to their statistical characteristics. The boundaries of moderate-FP zones (FP = 3%) were aligned with the locations of tidal mixing fronts. Empirical orthogonal function (EOF) analysis revealed that frontal activity exhibited strong seasonal cycles, together with large interannual variations. Its spatio-temporal pattern was associated with multiple factors. Among them, topographic effects combined with tidal mixing dominated frontogenesis. The seasonal development of SSFs was mainly controlled by stratification and wind waves associated with monsoons and was partially affected by wind-driven regional circulations. The direct impact of the Yangtze River sediment discharge was restricted to waters inside the 30-m isobath. The El Niño-Southern Oscillation affected interannual frontal dynamics to a large extent. After El Niño years, due to weakening winter winds and warmer sea surfaces, frontal activity was suppressed by the intensification of stratification and weakening of vertical mixing. In contrast, frontal activity increased after La Niña years. These findings will broaden the understanding of SSF variability and its ecological effects over the Yangtze Bank and in other systems.

Effects of cascading reservoirs on streamflow and sediment load with machine learning reconstructed time series in the upper Yellow River basin

Large reservoirs are widely built along rivers for flood control, hydropower generation, water supply for industry and agricultural production, and recreation. Many large reservoirs/dams built to provide multiple functions for local people are strongly influenced by the downstream flow and sediment discharge in the upstream of the Yellow River, China. In this study, we assessed the changes in the river flow and sediment discharge in responses to the construction of cascade reservoirs/dams by using continuously hydrological time series observed at six stations during 1956–2018. The temporal change in the streamflow was insignificant, but the sediment load decreased dramatically due to the trapping effects of the cascade reservoirs. The change points were detected in 1969 and 1986, which were consistent with the timing of the construction of gigantic reservoirs. We then applied the abrupt changing points to divide the whole time series into a reference period (1956–1968) and two changing periods (1969–1986 and 1987–2018). The linear regression and three machine learning models were employed to reconstruct the natural streamflow and sediment load at the downstream station, before quantifying their responses to cascade reservoirs. The annual mean streamflow and sediment load decreased by 21% and 84%, respectively, after constructing the Longyangxia and Liujiaxia reservoirs. Cascade reservoirs contributed 85.39% and 94.91% to the decreases in the streamflow and sediment discharge, respectively. Hydrological regimes have been greatly altered by the 13 cascade reservoirs, with seasonal redistribution of the streamflow and flood peak reduction because their high trapping efficiencies exceed 95%. These findings indicate that large and medium sized reservoirs have drastically altered the streamflow and sediment load, thereby intensely influencing the fluvial processes in the upstream of the Yellow River.

Erosion of four Brazilian coastal deltas: how dam construction is changing the natural pattern of coastal sedimentary systems.

Reservoir-building on rivers is still "the Brazilian energy culture", such that many other dams are being constructed and planned, with many other rivers potentially suffering the same impact in their sediment flux. In this study, we evaluated the impacts of reservoir-building on the erosion of the São Francisco, Jequitinhonha, Doce, and Paraíba do Sul river deltas. A time series of Landsat images from 1973 to 2020 was analyzed, with three key highlighted moments (1973, 1997 and 2020) that summarize the erosion processes on these deltas. In addition to the images, continuous river water discharge, sediment discharge and basin rainfall data were analyzed between 1940 and 2020, providing river parameters over a period exceeding that of the satellite data. The findings suggest that coastal erosion has progressed in the four deltas, with higher estimated losses in the São Francisco and Paraíba do Sul over the 47 years of available satellite images. However, despite minimal overall estimated gains, the Jequitinhonha and Doce experienced high erosion at their river mouths, as in the other two rivers, compensated by accretion in distal coastal areas. These results can be explained by reductions in river flux and consequent sediment transport capacity due to reservoirs.

Investigation of Correlation Between Suspended Sediment Discharge and Activity Concentration of Natural Radionuclides in Langat River Basin

Investigation of the correlation between natural radionuclide activity concentrations (RC) in river sediment and suspended sediment discharge (SD) is carried out for Langat river basin over a period of 4 years. Three river stations with suspended sediment discharge rate derived from validated sediment rating equation are paired with three radionuclide observation stations by geographical proximity. Long-term correlation of the quarterly RC observation with maximum SD is generally poor. However, for specific time window, the data trend correlation ranges from moderate to good, indicative of the influence of SD on the transport process of NORM within the river basin. Examination of the seasonal variation shows that Station 2 exhibits good correlation of RC and SD in the months of March and August/September, corresponding to the pre-storm seasons, with R2 value as high as 0.9799. Despite the limitation of the undated RC observation and the use of hindcasted SD data, the present study shows the increase of radionuclide activity concentration of Ra-226, Ra-228 and K-40 in river sediment can be associated with the suspended sediment discharge rate and hence, the river streamflow especially during the pre-storm season.

Landscape ecological risk assessment and driving mechanism of coastal estuarine tidal flats—A case study of the liaohe estuary wetlands

Estuarine tidal flats are located in the land-sea crisscross-sensitive zone. The degraded and shrinking state of major tidal flat landscapes weakens their ecological role in maintaining the safety of coastal wetlands in the world. However, little attention has been paid to tidal flat landscape evolution and ecological risks due to the tidal flats being located in the periphery of estuarine wetlands. This study was conducted in the tidal flats of the Liaohe estuary wetlands. The GIS spatial analysis and a transition matrix were combined to explore the tidal flat landscape evolution characteristics. The ecological risk faced by the tidal flats was evaluated by a landscape ecological risk model constructed by landscape indexes. The key driving factors and threshold values that affect the ecological risks were clarified using geographical detectors and regression analysis methods. The results showed that the tidal flats mainly evolved into a natural wetland before 2000 and rapidly changed into an artificial landscape between 2000 and 2010; the proportion of artificial landscapes then began to decline after 2010. Over the past 30 years, the landscape ecological risk in this area has generally been at a medium risk level. The ecological risk index showed a decreasing trend with the increases in landscape diversity. The low-risk artificial plaques have gradually replaced high-risk and medium-risk plaques of exposed tidal flats and natural wetlands. The study showed that human development activities were the key factors affecting the landscape ecological risks of the tidal flats. River sediment discharge was the key natural factor in maintaining the landscape ecological risk at a stable level. Therefore, the human development activities should be controlled within an appropriate limit, and the material supply of the upstream basin should be guaranteed to maintain the stability of the tidal flat landscape form in the protection and utilization of estuarine tidal flats in the future.

Monitoring 23-year of shoreline changes of the Zengwun Estuary in Southern Taiwan using time-series Landsat data and edge detection techniques

More than 80% of sandy beaches in Taiwan have been experiencing a severe recession, although the sediment discharge of rivers in Taiwan is significantly higher than the world average producing almost 2% of global fluvial sediment discharge. This contradiction is primarily due to the widespread constructions of reservoirs and intensive anthropogenic activities in coastal regions. In addition, coasts are particularly vulnerable to hazards due to climate change, such as sea-level rise, as they are located at the transition zone of terrestrial and marine environments. Along with the fact that Taiwan is an island and is one of the most climate-vulnerable regions globally, coastal management and sustainability are nationally critical topics, especially considering the ongoing reformation and legislation of Taiwan's coastal conservation laws.As stated in the Sustainable Development Goals (SDGs) goal 14, accurate and continuous shoreline positions information is essential for coastal conservation. However, by reviewing previous global studies and projects commissioned by the Taiwanese government aiming at monitoring shoreline changes, they usually exhibit several limitations, such as limited band selections or conservative band ratio-derived water indices, relying on either manual digitization or simple thresholding methods, focusing on either artificial or smoothly shaped coasts, and using images acquired at considerably different tidal height levels.Therefore, in the present study, a subpixel shoreline extraction approach based on a sustainable cross-generation dataset and a robust edge detection algorithm is proposed. This approach is exemplified by the Zengwun River Estuary located in southwestern Taiwan—Taiwan's most critical coastal preservation region. By quantitatively analyzing the resultant time-series shoreline positions from 1999 to 2021, several hotspots of shoreline recession have been identified: an extreme erosional rate up to −69.4 m year−1 is revealed in the northern sand bank; while the offshore sand bar demonstrates an overall landward retreat rate of −35.4 ± 1.24 m year−1.

Evolution patterns and spatial sources of water and sediment discharge over the last 70 years in the Yellow River, China: A case study in the Ningxia Reach

The hydrology and sediment processes in large rivers play important roles in maintaining aquatic and coastal ecosystems and advancing civilization and production in human systems. Therefore, quantitatively analyzing the spatiotemporal variability and dynamics of water and sediment discharge in large rivers is essential for improving watershed management and sustainable development in the areas surrounding rivers, especially the Yellow River, which is one of the most sediment-laden rivers in the world. In this study, we analyzed the evolution patterns and spatial sources of water and sediment discharge in the Yellow River from 1951 to 2020 and determined the impacts of different factors on water and sediment discharge variations. The results showed that the annual water and sediment discharge significantly decreased (p < 0.05) over the past 70 years, with an abrupt change occurring in 1986. The first dominant periodicity of water discharge was approximately 29 years, while the first dominant periodicity of sediment discharge was approximately 28 years. In terms of the water and sediment discharge sources, the dominant factor affecting variations in water discharge was water diversion from 1951 to 2020, while the dominant factor affecting variations in sediment discharge was sediment aggradation from 1951 to 1985 and changed to tributary inflow sediment from 1986 to 2020. In addition, the water and sediment discharge changes were also affected by anthropogenic activities, such as water and sediment diversions, dams and reservoirs, and water and soil conservation measures. In particular, the water and sediment interception capabilities of the established soil and water conservation measures gradually became saturated over time. Specifically, the maximum water and sediment interception capabilities of the current soil and water conservation measures were 12.2 billion m3 and 1.9 Gt, respectively. Overall, the results of the present study can help tailor water and sediment regulation countermeasures in the future.

Remote Sensing of Sediment Discharge in Rivers Using Sentinel-2 Images and Machine-Learning Algorithms

The spatio-temporal dynamism of sediment discharge (Qs) in rivers is influenced by various natural and anthropogenic factors. Unfortunately, most rivers are only monitored at a limited number of stations or not gauged at all. Therefore, this study aims to provide a remote-sensing-based alternative for Qs monitoring. The at-a-station hydraulic geometry (AHG) power–law method was compared to the at-many-stations hydraulic geometry (AMHG) method; in addition, a novel AHG machine-learning (ML) method was introduced to estimate water discharge at three gauging stations in the Tisza (Szeged and Algyő) and Maros (Makó) Rivers in Hungary. The surface reflectance of Sentinel-2 images was correlated to in situ suspended sediment concentration (SSC) by support vector machine (SVM), random forest (RF), artificial neural network (ANN), and combined algorithms. The best performing water discharge and SSC models were employed to estimate the Qs. Our novel AHG ML method gave the best estimations of water discharge (Szeged: R2 = 0.87; Algyő: R2 = 0.75; Makó: R2 = 0.61). Furthermore, the RF (R2 = 0.9) and combined models (R2 = 0.82) showed the best SSC estimations for the Maros and Tisza Rivers. The highest Qs were detected during floods; however, there is usually a clockwise hysteresis between the SSC and water discharge, especially in the Tisza River.

Fire (plus) flood (equals) beach: coastal response to an exceptional river sediment discharge event

Wildfire and post-fire rainfall have resounding effects on hillslope processes and sediment yields of mountainous landscapes. Yet, it remains unclear how fire–flood sequences influence downstream coastal littoral systems. It is timely to examine terrestrial–coastal connections because climate change is increasing the frequency, size, and intensity of wildfires, altering precipitation rates, and accelerating sea-level rise; and these factors can be understood as contrasting accretionary and erosive agents for coastal systems. Here we provide new satellite-derived shoreline measurements of Big Sur, California and show how river sediment discharge significantly influenced shoreline positions during the past several decades. A 2016 wildfire followed by record precipitation increased sediment discharge in the Big Sur River and resulted in almost half of the total river sediment load of the past 50 years (~ 2.2 of ~ 4.8 Mt). Roughly 30% of this river sediment was inferred to be littoral-grade sand and was incorporated into the littoral cell, causing the widest beaches in the 37-year satellite record and spreading downcoast over timescales of years. Hence, the impact of fire–flood events on coastal sediment budgets may be substantial, and these impacts may increase with time considering projected intensification of wildfires and extreme rain events under global warming.

Efficient measurement of large-scale decadal shoreline change with increased accuracy in tide-dominated coastal environments with Google Earth Engine

Most of the worlds’ population relies on the processes and ecosystems in the coastal zone. Understanding the long-term change of coastlines is critical for the effective management of these complex, and heavily utilised regions. There has been a recent increase of studies focused on large-scale shoreline change mapping. However, most current methods are optimized for extracting shorelines of wave-dominated sandy beaches, which are only 30% of the global coasts, resulting in uncertainty for other environments such as tidal flats and bedrock. Here, we propose a new shoreline change mapping workflow, using the Landsat archive and Google Earth Engine, which increases compute efficiency and is suitable for retrieving shoreline changes for various coastal landforms at high tide instead of mean sea level. By validating against regional and continental datasets in Australia, we found the approach here produced high mapping accuracy and showed particularly better performance at tide-dominated coasts, where tidal flats and intertidal bars and ridges are present, when compared to past approaches. This is an important step forward since tide-dominated and tide-modified coasts are widely distributed at tropical low latitudes. We also explored the global application of the proposed method and derived hotspots of shoreline erosion and accretion that agreed with multiple regional studies across the world. Most of these hotspots were related to river sediment discharge and human intervention on the coast, as expected. Although it requires further validation, the global application of our method demonstrates the significance of this approach in identifying potential threats to coastal zones, especially in complex tide-dominated environments, which can facilitate effective coastal management.

Dominant updriftward sediment transport on the updrift-side of a modern deflected delta, Ishikari coast, Hokkaido, Japan

The Ishikari coast of Hokkaido, Japan is a wave-dominated semi-enclosed coast, the sediments of which are primarily sourced from a single river (the Ishikari River), forming a northeast deflected delta. Asymmetric and deflected deltas are generally considered to be formed under net longshore sediment transport downdriftward; however, through sedimentological and grain-size trend analysis (GSTA) of seafloor sediments, along with some other observational data, we recognised dominant updriftward sediment transport on the updrift-side of the deflected delta system. GSTA showed that the most important trend in the coast is coarser, better-sorted, and more positively skewed (CB+). CB+ trends around the river mouth converge from the lower shoreface zone to the river mouth spit on the updrift side. The trends reflect the combined effects of shore-normal wave action and river sediment discharge. Wave-driven surface currents crossing over the delta lobe converge at the southwestern side of the river mouth. In the deeper zone (lower shoreface), southwest (updriftward) oriented CB+ trends prevail. These trends reflect successive re-deposition of delta front sands, with progressive mud winnowing, driven by the strong south-westerly bottom currents observed in the bay during winter storms. Although GSTA did not show any apparent trend field in the nearshore zone, gradual fining of sand fractions to the southwest, along with the chronological transition of the coastline after the construction of a large port (trap of drifted sands on the downdriftside of the port and regression of coastline on the updriftside of the port), suggest a dominant longshore sediment transport updriftward on the updrift-side of the delta. Because the growth of deflected deltas depends on dominant sediment accretion on the updrift-side, these results can be rationalised in the case of internally sourced deflected deltas. Uncertainties remain regarding the net alongshore sediment transport of the entire Ishikari coast; however, the deflected planform is controlled by deflected mouth bars and river mouth jets due to the waves approaching the river mouth at slightly oblique angle in winter, and is likely independent on the direction of dominant sediment transport.

Decreasing land growth and unique seasonal area fluctuations of two newborn Mississippi subdeltas

A total of 491 Landsat satellite images from 1973 to 2017 were used to study the evolution of two newborn subdeltas of the Mississippi-Atchafalaya system, the Wax Lake Delta (WLD) and the Atchafalaya Delta (AD). The exposed delta areas derived from the satellite images using an unsupervised classification method were applied to evaluate the long-term land growth and seasonal area variations in the WLD and AD, as well as their main influencing factors. The results show that the land growth rate of the WLD decreased relative to the mean sea level from 1.67 km2/year to 0.35 km2/year from 1983–1999 to 1999–2017, and the growth rate of the AD decreased from 1.51 km2/year to 0.80 km2/year from 1973–1999 to 1999–2017. The land building of the two subdeltas during the 1980s to 1999 was more efficient than that from 1999 to 2017. The significant decreases in the land growth rate and land building efficiency are attributed both to the increased impact from relative sea level rise and to the decrease in river sediment discharge, and the extension of the distributary channels, as well. The satellite-derived exposed areas of the WLD and AD show unique seasonal loops affected by the growth of vegetation during the warmer weather from April to June and the decay of vegetation mainly due to the cold fronts in December, which is different from the monotonic changes in the areas of other deltas, mainly driven by the rise and fall of water levels. A new method based on the correction between the exposed areas and water levels from December to April was used to estimate the vertical growth rates of the WLD and AD. After deducting the relative sea level rise rate, the net sediment deposition rates of the WLD and AD were estimated to be 20.8 and 24.2 mm/year, respectively. Approximately 72% and 62% of the sediments deposited in the WLD and AD were used to offset the effects of relative sea level rise when building deltaic lands from 1999 to 2017. This study not only improves our understanding of the growth of newborn subdeltas but also aids in assessing whether river course shifts and sediment diversions can restore deltas or deltaic wetlands that are at risk of drowning.

Sediment budget and morphological change in the Red River Delta under increasing human interferences

The Red River sediment dispersal system has been increasingly altered by human activities, but the resultant morphodynamic change and sediment budget in the Red River Delta (RRD) has been little studied in a holistic perspective. In this study, time-series analysis of river water and sediment discharges from 1960 to 2010 was carried out to trace human interferences on sediment routing processes, and historical charts from 1930 to 2008 were digitized to reconstruct the Digital Elevation Models for investigating spatiotemporal variations in the RRD morphology and calculating their depositional and erosional fluxes. The results show that sediment reduction by 61.3% in the Red River is mainly induced by constructing the Hoa Binh Dam at the lower reach of the Da river, and deforestation and other land-use change have recently increased significantly the sediment loads in the Thao and Lo rivers. Intertidal and subtidal morphological changes in the RRD are highly site-specific in response to natural and anthropogenic drivers. Rapid intertidal accretion majorly occurs near the distributary mouths because of abundant sediment supply and artificially promoted siltation by land reclamation projects, but severe coastal erosion may occur secularly after the distributary outlet shifting or temporarily by storm events. The subaqueous RRD is generally featured by the contour-parallel zonation of erosion and accretion determined by strong longshore currents associated with monsoon winds. Net accretion was observed in the tide-dominated northern subaqueous RRD and has recently changed into a slight erosion due to reduced sediment input in the last two decades, while net erosion was monitored in the southern subaqueous RRD predominated by wave and longshore currents and has recently entered into a slight accretion phase in response to the distributary outlet shifting and secular morphodynamic adjustment. The study RRD region received on average 41.1 Mt./yr (Mt = 106 t) of suspended sediments, and 14.2 Mt./yr and 28.6 Mt./yr were estimated to accumulate on the intertidal flats and in the northern subaqueous RRD, respectively. However, erosion in the southern subaqueous RRD could contribute 42.9 Mt./yr of sediment to the system. Therefore, 41.2 Mt./yr of suspended sediment was indicated to escape from the study RRD region and transport further southward by longshore current to form the distal muddy clinoform along the inner Vietnam shelf. This sediment budget estimation should bear a significant uncertainty, but provides important information for multi-decadal sediment dispersal in a mega-delta system and coastal management.

Sediment budget of the Yellow River delta during 1959–2012, estimated from morphological changes and accumulation rates

The Yellow River (Huanghe) is a major sediment source of sediment to the Bohai Sea and Yellow Sea, resulting in the formation of a large and dynamic delta complex. However, the river sediment discharge reaching the delta has decreased dramatically since the middle 1960s. This study examines the sediment accumulation and morphological changes of the delta complex during 1959–2012 using a repeated bathymetry dataset, high-resolution seismic profiles, and sediment cores obtained in 2012 and 2013. Sediment accumulation rates were determined using the downcore concentrations of 137Cs and 210Pb. The average accumulation rates calculated for the delta complex range from 6.9–24 cm y−1 on the delta front slope, 1–2 cm y−1 at the toe of the slope, and < 1 cm y−1 in the northeastern shelf area. Morphological changes and sediment accumulation rates show that 1) the Yellow River delta has entered a destruction phase after 1996, 2) the amount of erosion of the whole delta reaches 210 MT y−1 of sediment, which is at least required to maintain the whole delta morphology, 3) three-quarters (487 MT y−1) of the Yellow River-derived sediment was deposited in the subaerial and subaqueous delta during 1959–2012, and 4) Whereas, one-quarter (144 MT y−1) was transported to the Bohai Sea, and as much as one fifth (117 MT y−1) was transported further offshore to the Yellow Sea, passing through the Bohai Sea during this period. The source of these sediments is a combination of the Yellow River sediment discharge and the erosion of the previous Yellow River delta lobes. Despite the sharp decline in the Yellow River sediment discharge over the past five decades, the offshore dispersal of the river discharge and eroded sediment to the Yellow Sea was more than 100 MT y−1 on average. Results of this study show that the erosion of the abandoned delta lobes can sustain the supply of sediment to remote offshore depocenters even after decades of declining river sediment discharge.