Reduced Sediment Supply Research Articles

A multi‐site and hypothesis‐driven approach to identify controls on the bedload transport regime of an anthropised gravel‐bed river

AbstractUnderstanding the effects of human disturbance on the bedload transport regime of anthropised rivers is a topic of growing importance, as such information is of interest for adequate river diagnosis, correct implementation of restoration measures and appropriate design of post‐action monitoring programs. However, such assessments are complex, especially in sites where multiple factors simultaneously influence the bedload transport regime, so that it is difficult to establish simple causal relationships between human disturbances and changes in the sediment transport regime, notably on bedload. To overcome this, there is a need for rigorous hypothesis‐driven approaches to assess the isolated effects of each driver. With this in mind, we have characterised the dynamics of bedload transport in the Upper Garonne (Central Pyrenees, Spain‐France), a river impacted by sediment retention, flow diversion and mining that influence its morphological conditions and transport regime. We assessed the effects of (1) surface grain size distribution, (2) river morphology, (3) sediment supply and (4) flow diversion on the bedload transport regime. Four sites with different degrees of river anthropisation were selected. After defining hypotheses on the most likely bedload transport conditions for each site, we proposed a set of discriminating criteria to test these hypotheses, based on temporal within‐site and spatial between‐site comparisons of coarse particle tracking measurements over four years. The results of this research showed that the hydrosedimentary regime of the Garonne is controlled by a complex combination of drivers such as valley physiography, which exerts a first‐order control on differences in reach‐scale bedforms and bedload dynamics; and human disturbances which contribute to a reduction in sediment supply through changes in land cover and hydropower dams, or to changes in hydrology (i.e., flow competence) due to water diversion and abstraction.

An overview of Upper Pleistocene coastal deposits on Mallorca island

This review paper delves into the geological history of Mallorca, focusing on the Middle to Late Pleistocene period and its impact on the landscape and climate of the island. The Quaternary, particularly, the Last Interglacial, represents a relatively short timeframe on the global chronological scale, yet it is notable for its climatic instability and complex alternation of glacial and interglacial phases, leading to fluctuations in sea levels. During the last interglacial, millennial-scale fluctuations, known as Dansgaard–Oeschger and Heinrich events, occurred. Aeolianites are thought to have formed during periods of sparse vegetation cover and abundant sediment availability from the sea bed or platform (Heinrich events), while colluvial deposits and paleosols are believed to represent periods of reduced sediment supply and enhanced pedogenesis (Dansgaard–Oeschger events). These different deposits reflect climatic shifts and sea-level changes, offering insights into the environmental history of the island. Fossil evidence, including Strombus bubonius (currently known as Thetystrombus lauts) or “Senegalese hosts,” reveals past warm climatic fluctuations, particularly during the Last Interglacial. Myotragus balearicus exemplifies evolutionary adaptation and isolation on the island, offering a unique perspective on its paleontological legacy. The integration of stratigraphic characteristics and literature research on Optically Stimulated Luminescence (OSL) dating techniques provides a comprehensive temporal framework, spanning from Marine Isotope Stage 6 (MIS 6) to Marine Isotope Stage 2 (MIS 2), enabling the precise dating of geological events and a stratigraphic correlation.

Assessment of soil erosion in the Upper Citarum watershed for sustainability of the Saguling reservoir: unmixing model approach.

The Citarum watershed and the Saguling reservoir are vital natural resources in Indonesia, affecting the livelihood of West Java and the DKI Jakarta population. This study aimed to assess the soil erosion in the Upper Citarum watershed and identify its source. The study used the fallout radionuclide technique, geochemical tracers, and an unmixing model to measure soil erosion and the contribution of suspended sediment sources due to erosion. Soil bulk transects and surface soil were sampled using a coring tool on the Ciwidey and Cisangkuy sub-watersheds. Riverbank and suspended sediment samples were collected from tributaries and rivers. With 137Cs, 40% of the samples had values below the minimum detectable activity, and vice versa for 210Pbex, all samples are detectable. For mitigation, bare land needs to be recovered due to its erosion (25.6 t ha-1year-1) exceeding the tolerance erosion value (17 t ha-1year-1). Statistically, Mg and Na were the most appropriate composite tracers for suspended sediment contribution. The unmixing model predicted the sediment contributors from bare land (58%), the riverbank (32.7%), and plantation land (9.3%). Proper land conservation could reduce sediment supply by almost 14.7% and extend the reservoir's life. This is the first study to report the feasibility of the unmixing model in Indonesia.

Hidden delta degradation due to fluvial sediment decline and intensified marine storms.

Deltas are threatened by erosion due to climate change and reduced sediment supply, but their response to these changes remains poorly quantified. We investigate the abandoned Yellow River delta that has transitioned from rapid growth to ongoing deterioration due to a river avulsion removing the sediment supply. Integrating bathymetric data, process observations, and sediment transport modeling, we find that while the subaerial delta was stabilized by engineering measures, the subaqueous delta continued to erode due to intensified storms, losing 39% of its mass deposited before the avulsion. Long-term observations show that winter storms initiate scouring of the subaqueous delta, contributing up to 70% of seabed erosion. We then analyze 108 global deltas to assess subaqueous delta erosion risks and identify 17 deltas facing similar situations of sediment decline and storm intensification during the past 40 years. Our findings suggest that subaqueous delta erosion must be integrated into delta sustainability evaluations.

Analysis of bed changes in the segment of the Tien river flowing through Tan Chau

The segment of the Tien River flowing through Tan Chau town is a meandering section with complex flow patterns, influenced by the flow dynamics and the lack of upstream sediment supply. This has resulted in the formation of deep erosional channels that affect the bank stability. This study focuses on evaluating the bed change of the Tien River within Tan Chau town through the analysis of measurement data and some scenarios from numerical modeling. The findings reveal that the Tan Chau - Hong Ngu curved section experiences severe channel erosion, particularly concentrated in the topographic channel and inclined towards the concave bank of Tan Chau (the inner bank). An analysis of causative factors indicates that reduced sediment supply due to dam construction and sand mining activities have led to changes in the riverbed, deeper erosion, and bank erosion near the sand mining area. The results quantifies the sediment deficit in the area at 0.86%, while the impact of sand mining in this area is 0.26%. The findings from this research provide a database to support local planning for bank protection projects and disaster mitigation measures due to bank erosion.

Tidal dissipation morphodynamic feedback triggers loss of microtidal marshes

Abstract Coastal marsh loss is commonly attributed to changes in external forcings, such as an increase in sea-level rise rate or a reduction in sediment supply. Here we show that extensive marsh loss can be caused by internal mechanisms alone, and specifically by autogenic tidal choking. This occurs when the marsh fills in, increasing tidal dissipation by bed friction and eventually decreasing the tidal range in its landward section. The reduced tidal range decreases sediment import on the marsh platform and increases ponding, both of which lead to interior marsh loss even with modest sea-level rise rates. This process is predicted to occur in dissipative microtidal marshes, which are experiencing some of the fastest rates of marsh loss worldwide. Considering this mechanism is essential to understanding the relationship between marsh loss, sea-level rise, and sediment supply and to eventually predicting future marsh evolution.

Analysis of the driving factors of the change of erosion-deposition in the Minjiang Estuary, Southeast China

Understanding the evolution and driving factors of sedimentation and erosion at the mouths of small and medium-sized mountain streams during various periods is essential for regional spatial utilization, development, and sustainable economic growth. This is particularly important when considering the combined impact of climate change and human activities. This paper presents an analysis of the changes in sedimentation and erosion of mouth isobaths and underwater deltas over different periods using nautical chart data (1950-2019) and analyzes the factors driving changes in sedimentation and erosion during different periods from 1950 to 2020 based on the runoff-sediment discharge of the Minjiang River (MR) and extreme climate factors such as typhoons, especially the driving factors that caused a sudden change in the sedimentation and erosion process between 1998 and 2005. The results indicate that runoff-sediment characteristics are crucial in driving sedimentation and erosion changes. In the past 70 years, the underwater delta of the MRE has mainly experienced four stages: deposition (1950-1992) -erosion (1993-1998) -deposition (1998-2011) -erosion (2011-2019). Taking the impoundment operation of the Shuikou Reservoir in 1993 as the node, the sediment load of the Minjiang River into the sea began to decrease sharply, and then the estuary quickly showed a state of erosion. The change of sediment flux into the sea is the main driving factor for the evolution of erosion and deposition in the Minjiang River Estuary (MRE). The critical value of the erosion-deposition transition is about 570.3 × 104 t/yr. After the estuary entered a state of erosion in 1993-1998, significant siltation suddenly occurred in 1998-2005. The reason for the siltation in this period may be related to the frequent transit typhoons and flood events in 1998-2005. Therefore, the change of river sediment supply into the sea is the main driving factor controlling the erosion and deposition evolution of the Minjiang River estuary on a long time scale of more than 10 years, which reflects the influence of large-scale human activities on the river and estuary area in the past half century. Extreme climate events are the incentive to adjust the estuary landform in the short term. Extreme climate events will not fundamentally change the process of estuarine geomorphological evolution under the background of sediment supply reduction.

Analisis Pengaruh Pembangunan Detached Breakwater terhadap Perlindungan Garis Pantai di Pulau Bengkalis Menggunakan Penginderaan Jauh

The coastline is the boundary between land and sea water whose position is always changing dynamically. Changes in coastlines are caused by disturbances to sediment transport along onshore, reduced sediment supply, disturbances of buildings, and low condition of the cliffs therefore they can’t cover the waves (Ministry of Public Works and Housing, 2015). The purpose of this study is to determine the response of the coastline after the construction of detached breakwater structure on the north coast of Bengkalis Island. Sentinel-2 imagery data used in this study and then analyzed using the Digital Shorelines Analysis System (DSAS) tool. The results of the analysis show that there are have variations of the shoreline response to the structures that have been built. In the period 2016 – 2022 the coastline hase changes, both accretion and abrasion. The maximum change in the forward coastline is 189.09 meters long with an accretion rate of 30.5 meters/year. While the maximum change in the backward coastline is 144.56 meters long with an abrasion rate of 23.32 meters/year. Overall, the construction of detached breakwaters has a positive impact on the coastline, thus increasing the land area on the north coast of Bengkalis Island.
 

Depositional model of clastic and carbonate rocks in a continental rift basin: a case study of the Paleogene Shahejie Formation in the Cangdong Depression, Bohai Bay Basin

The study of a clastic and carbonate rock depositional model in a continental rift basin is helpful to better understand the sedimentary processes active in continental rift basins. However, the spatiotemporal evolution characteristics and controlling factors of clastic and carbonate rocks in continental rift basins are still unclear. Therefore, the sedimentary characteristics of the clastic and carbonate rocks in the Paleogene Eocene Shahejie Formation (Es) are analysed via seismic, well log, core, thin section, and geochemistry data. Then, the impacts of tectonic movement, sediment supply and hydrologic conditions on sedimentary characteristics are discussed, and a depositional model is finally established. Five 3rd-order sequences, named SQ1–SQ5 from bottom to top, are identified in the Es. Fan delta, braided delta, meandering river delta, and lake sedimentary systems are identified in the Es; clastic rock sedimentary systems are identified in SQ1–SQ4, while concurrent clastic and carbonate rock sedimentary systems are identified in SQ5. During the SQ1–SQ4 period, the palaeogeomorphology formed by differential faulting activity or fault interactions and the inherited palaeogeomorphology resulted in different distributions of the sedimentary systems. The changes in accommodation space and sediment supply caused by tectonic movement led the delta to prograde or disappear. During the SQ5 period, the clastic rock sedimentary systems underwent a transition into clastic rock and carbonate rock sedimentary systems as a result of rift weakening, broad crustal thermal subsidence, sediment supply reduction and high-salinity lake water. This study provides a typical case for the evolution characteristics of the clastic rock and carbonate rock systems in continental rift basins, identifies changes in the tectonic, sediment supply, and hydrologic characteristics during their mutual transformations, and provides an important reference for hydrocarbon reservoir prediction.

Experiments on Gravel‐Sand Transitions: Examination of Washload Deposition

AbstractAn abrupt transition in bed grain size occurs in river systems. Over a short downstream distance, often only a few channel widths, the bed surface fines from gravel (∼10 mm) to sand (∼1 mm). This is the gravel‐sand transition (GST), and it is the only abrupt downstream reduction in grain size within fluvial systems. There are several theories for the origin of the GST, including size‐selective deposition of bimodal grain size distributions and the rapid onset of washload deposition due to changes in particle suspension properties at shear velocities of ∼0.1 m/s. Here, we present a laboratory experiment examining changes in fluid and sediment dynamics across a GST. We developed a stable gravel bed reach that was just below the threshold of motion, then fed sand. We observed sand carried as washload in the gravel reach fall out of suspension, forming a sand bed and a stable GST. Shear velocity was 0.09–0.10 m/s upstream of the GST and <0.10 m/s downstream, consistent with the washload deposition hypothesis. We were then able to perturb the position of the GST by systematically varying discharge and/or sand supply, shifting it downstream with an increase in discharge or a reduction in sediment supply. A decrease in discharge or increase in sand supply caused upstream migration. Our observations support an abrupt change in washload transport conditions across a narrow range of shear velocities, consistent with the washload deposition theory and measurements taken across GSTs in natural river systems.

Impacts of Riverine Floods on Morphodynamics in the Yellow River Delta

The geomorphological stability and ecological environment of megadeltas worldwide are of vital importance for their sustainable development. Deltaic hydro-morphodynamics is extremely sensitive to high riverine flow due to reduced sediment supply. However, the morphological evolution and response of deltas under high riverine flow have remained inadequately quantified. As one of the typical megadeltas, the Yellow River Delta (YRD), is becoming increasingly sensitive to environmental changes and intensified human interventions. In this study, a numerical model and field data were used to investigate the hydrodynamic changes and morphodynamic evolution induced by extreme river discharge in the YRD. The numerical experiments with different runoff scenarios reveal that high-energy riverine floods can cause significant hydrodynamic changes in bed shear stresses, water levels, and flow velocities, particularly in the abandoned river mouth. Moreover, it enhances the ebb-dominated tidal asymmetry, which considerably intensifies fluvial sediment resuspension and transport processes. The results also show high-energy riverine floods in the flood seasons trigger severe erosion in the Yellow River submerged delta, with a net erosion volume reaching −0.07 × 108 m3/year. The hydrodynamic increment in the abandoned river mouth is more significant, and therefore, severe erosion occurs, with the maximum erosion thickness reaching 7 m. These findings highlight the role of high riverine floods on the hydro-sediment dynamics of large river deltas under a sediment starvation condition.

Biocompacting livestock accelerate drowning of tidal salt marshes with sea level rise

A global concern for coastal ecosystems is the predicted rise in sea-level for which salt marshes must keep pace by increasing in surface elevation sufficiently. Variables that control this elevation change need to be identified to predict the adaptability of marshes to future sea-level rise. Many European marshes are grazed by livestock and these heavy grazers can biocompact the soil, a process often underestimated in studies assessing the long-term survival of marshes. We measured elevation changes for thirteen years in the field in grazed and non-grazed marshes. With a statistical model the most important factors controlling rates of surface elevation change were identified and provided the input for a mathematical model to study future elevation change of grazed and non-grazed salt marshes up to 2100 under three Sea Level Rise and sediment supply scenarios. We found that trampling by grazing cattle significantly reduced the annual rates of elevation gain from 11.9 mm yr-1 in the non-grazed marsh to 3.6 mm yr-1 in the grazed marsh. Next to biocompaction by livestock, precipitation deficit and extreme drought resulted in extra compaction. Our model results showed that cattle presence had a negative impact on the future adaptability of salt marshes to grow vertically for rising sea levels. Biocompaction reduced the total elevation change by 42% if the current linear SLR does not accelerate. For an accelerating and high SLR to 109 cm +NAP in 2100, biocompaction reduced elevation changes by 12% and the grazed marsh can no longer outcompete the rise in sea level from around 2050 onwards, compared to the non-grazed marsh. The grazed marsh will slowly drown but this will not lead to a significant change in vegetation composition yet. For an extreme SLR to 195 cm +NAP in 2100 the elevation changes in both the grazed and non-grazed marshes cannot keep pace with the rise in sea level and the marsh vegetation is expected to show regression to plants typical for a low marsh. A reduction in sediment supply will aggravate the effects of SLR and may result in highly increasing inundation frequencies and subsequent disappearance of the marsh vegetation.

Shoreline change on a tropical island beach, Seven Mile Beach, Grand Cayman: The influence of beachrock and shore protection structures

Contemporary and near-future shoreline change is widely regarded as an issue on small tropical islands. While it is widely anticipated that sea-level rise will precipitate shoreline recession on tropical islands, studies to date record both accretion and recession at historical timescales. This study of Seven Mile Beach, Grand Cayman presents a case study of historical shoreline change in which the local geomorphic setting is shown to be an important influence on shoreline behaviour. Consistent with its leeside setting, historic shoreline analysis (1958–2019) reveals erosion on the margins and accretion in the central part of the headland-embayment beach where no beachrock is present. The beach comprises five discrete, but interlinked subcells delineated by low headlands of exposed beachrock. These headlands have emerged through shoreline recession post-1971 but once exposed have become loci of persistent erosion, suggesting a positive feedback between beachrock and waves. A Category 5 Hurricane generated waves directly opposed to long-term modes and throughout the beach, long-term patterns of shoreline change were temporarily reversed, however, the historic pattern of shoreline change was restored within 2 years. The contemporary patterns of erosion and cell development suggest a reduction in sediment supply leading to cannibalization of relict beachridges on the margins of the embayment and emergence of formerly buried beachrock. The effects of coastal structures and erosion abatement measures were assessed and recommendations for coastal management, including development setback lines are presented.

Late Pleistocene to Holocene sedimentary history in the Pearl River Delta revealed by OSL and radiocarbon dating

A reliable chronology is essential to understand deltaic sedimentation processes in response to sea-level fluctuations and climatic changes. In the Pearl River Delta (PRD), high-resolution chronostratigraphy is still limited which hinders the detailed interpretation of sedimentary history. In this study, optically stimulated luminescence (OSL) dating and radiocarbon (14C) dating were applied to establish a chronological framework of core P5-4 (30 m depth) in the southern PRD. Fifteen OSL and four 14C samples produced ages ranging from c. 8 ka to 0.5 ka, and two older OSL samples produced minimum ages of 75 and 62 ka. 14C ages are generally older (up to c. 1 ka) than the corresponding OSL ages and the discrepancy increases with depth. The ages and sedimentary data of sixteen published cores have been compiled to correlate the regional stratigraphy and reconstruct the late Pleistocene to Holocene deltaic process. It is concluded that: (1) The bottom subaerially-exposed deposits of core P5-4 were dated to marine isotope stage (MIS) 5, which is consistent with similar OSL-based records in the PRD. The laterally correlation shows that a depositional hiatus during c. 70–8 ka prevailed in the PRD and may attribute to strong erosion due to weathering and fluvial scouring. (2) Due to the relatively high altitude of bedrock, the post-glacial marine deposition of the PRD started at c. 8 ka, postdating other megadeltas by at least 2–4 ka. During 8–0.5 ka, a three-stage sedimentary process that is common in the PRD occurred in P5-4, i.e., early-Holocene (8–6 ka) rapid accumulation rate (5.44 m/ka) associated with rapid sea level rise, followed by mid-Holocene (6–3 ka) decelerated sedimentation rate (1.17 m/ka) in response to reduced sediment supply and strong tidal forces and late-Holocene (3–0.5 ka) accelerated sedimentation rate (2.28 m/ka) due to enhanced human activity.

Sedimentary characteristics and formation of riverine source bordering dunes in a humid region: an example from the lower reaches of Kiso River, central Japan

Riverine source bordering dunes are formed by the interaction of fluvial and aeolian processes. Even in humid regions such as central Japan, small-scale, elongated aeolian dunes parallel to river channels occur intermittently; a typical example is identified where sand bars had formed along the left bank of the lower reaches of Kiso River. This paper presents the sedimentary characteristics and luminescence chronology of the aeolian dunes along the Kiso River. The sand constituting dunes is well-sorted, medium-grained and the grain size shows little variation with depth. The sand would be supplied from the riverbed via the bars by the strong west-northwesterly to westerly winds promoted by the monsoon in the dry winter when the water level of the Kiso River drops significantly. Fading corrected infrared-stimulated luminescence (IR50) dating suggests that the dunes occurred and grew rapidly upward after about 600–700 yr ago. Increased sediment yield caused by human activities such as deforestation in the drainage basin, and the resulting increase in sediment supply to the lower reaches may have promoted the formation of dunes. However, with the construction of continuous levees, the reduced sediment supply related to dam construction, and the lowering of the riverbed, the connectivity between sand bars and dunes declined, and most of the dunes appear to have been stable for the past century. Fluvial sand supply influenced by human activities as well as strong winds during the dry season plays a crucial role in the formation of riverine source bordering dunes in humid regions.

Morphological evolution of paired sand spits at the Fudu river mouth: Wave effects and anthropogenic factors

Sand spits are fragile landforms that have rich values in economic, ecology, landscape and many other respects. They are quite diverse in morphology and their evolution mechanisms are influenced by various hydrodynamic and morphologic factors. Although morphological evolution of sand spits has been well documented, emanative paired sand spits stretching in opposite directions are relatively unexplored. In this study, a pair of emanative sand spits, which have experienced a dramatic evolution in the past decades, at the Fudu river mouth, on the eastern coast of the Bohai Sea, China, are investigated. The shoreline data detected from satellite images between 1984 and 2020 obtained through a free cloud-based platform Google Earth Engine were used to quantify the morphological evolution of the paired sand spits. The offshore wave data derived from a high temporal and spatial resolutions wave hindcast database along the Chinese coast were combined to explore their evolution mechanisms. Results show that the morphological evolution of the paired sand spits is dominated by the different breaking wave characteristics at flanks of the river mouth, which is formed by the geomorphic modulation of the bimodal offshore wave climate. The sand spit at south flank is typically formed by longshore sediment transport like many other cases, while the spit at north flank is fed by intertidal bars through onshore sediment transport and presents good resilience to storm waves and human disturbances. Although several degradation characteristics were caused by storm waves, it is indicated that sediment supply reduction and interception of sediment transport caused by intense human activities are the primary reasons for the degradation of the paired sand spits. The present investigation enriches the maintaining and degradation mechanisms of paired sand spits and suggests that sufficient attention should be paid to the resilience of geomorphic systems in coastal exploitation.

Radiolarian assemblage from radiolarites of the Krs Gradac section (SW Serbia): On the way to a better reconstruction of the Middle-Late Jurassic geodynamic history of the Inner Dinarides

The type section of the Gonje Formation (Krs Gradac, Sjenica area) is characterized by an abundance of radiolarian tests, which, however, are gene rally recrystallized and poorly preserved. This paper focuses on the age of radiolarite sequence below the series with intercalated turbidites and mass transport deposits. The Callovian/Oxfordian (?early Oxfordian) moderately preserved radiolarian assemblage was determined from the higher part of the radiolaritic sequence without mass transport deposits. The new biostratigraphic age is consistent with the previously published data of the authors and reveals that the radiolaritic sequence without mass transport deposits of the Gonje Formation was deposited between the Bathonian and Oxfordian. The biostratigraphic age of this part of the Gonje Formation evidence that the Ljubis Basin underwent in Late Oxfordian to earliest Tithonian times a phase of starvation with reduced sediment supply or production. The reasons for this starvation are discussed.

Landscape response to hydroclimate variability shown by the post-Bonneville Flood (ca. 18 ka) fluvial-geomorphic history of the middle Snake River, Idaho, USA

AbstractThe fluvial geomorphology and stratigraphy on the middle Snake River at Bancroft Springs, Idaho, provide evidence for numerous episodes of Snake River aggradation and incision since the Bonneville Flood at ca. 18 ka. A suite of seven terraces ranging from 20–1 m above modern bankfull elevation records multiple cut-and-fill cycles during the latest Pleistocene and Holocene in response to local base-level controls, variations in sediment supply, and hydroclimate change. Radiocarbon and luminescence dating show that the ages of fluvial aggradation generally coincide with increased sediment supply and likely wetter hydroclimate during onset of the Younger Dryas stadial (ca. 13.2 ka), deglaciation and termination of the Younger Dryas stadial (ca. 11.3 ka), Early Holocene cooling (ca. 8.8 ka), and Neoglacial (ca. 4.5, 2.9, 1.1 ka). Six intervening periods of incision and channel stability may also reflect either reduced sediment supply, drier hydroclimate, or both. The terrace chronology can be correlated to a variety of local and regional paleoclimate proxy records and corresponds well with periods of continental- and global-scale rapid climate change during the Holocene. The fluvial record demonstrates the geomorphic response and sensitivity of large river systems to changes in hydroclimate variability, which has important implications for inferring paleoenvironmental conditions in the region.

Geomorphologic evolution of the shallow-buried abandoned Yellow River delta during the last 2000 years

Different from the continuous development of many river deltas since the major Holocene transgression, the modern Yellow River delta (YRD) remained for nearly 1000 years (from 893 AD to 1855 AD) in an abandonment state, until after 1855 AD when it was buried after the Yellow River entered the sea via Lijin. In this study we used optically stimulated luminescence (OSL) dating and grain size and sedimentary facies analysis of four borehole cores, combined with an analysis of landform morphology, to reconstruct the history of the ancient abandoned YRD. The results indicate that after the delta was abandoned due to the reduced sediment supply by the Yellow River in 893 AD, a sandy coast developed under the influence of wave erosion, and offshore shell ridges were formed beyond the coastline. During the interval from the Medieval Warm Period (MWP) to the Little Ice Age (LIA), driven by climate change and storm surges, the shell ridges migrated laterally, widened and rose, while sediment accumulation in the landward-side interfluvial floodplains was relatively weak. This configuration subsequently constituted a coastal highland – plain depression system. After 1077 AD, this system, together with the abandoned delta, was eroded by the Daqing River estuary. After 1855 AD, the abandoned delta landform system was completely buried by Yellow River sediments, and the wave-controlled sandy coast was transformed to a tide-controlled silt-mud coast. We summarize these findings in the form of a geomorphic model of the evolution of the abandoned delta from the MWP to the LIA. Overall, our results highlight the geomorphic effects of the sediment-laden river delta and the response of the abandoned delta geomorphic system on the millennial timescale, and they provide a theoretical foundation for predicting the geomorphic evolution of a major river delta on different timescales, against the background of global change.

The future of coastal monitoring through satellite remote sensing

AbstractSatellite remote sensing is transforming coastal science from a “data-poor” field into a “data-rich” field. Sandy beaches are dynamic landscapes that change in response to long-term pressures, short-term pulses, and anthropogenic interventions. Until recently, the rate and breadth of beach change have outpaced our ability to monitor those changes, due to the spatiotemporal limitations of our observational capacity. Over the past several decades, only a handful of beaches worldwide have been regularly monitored with accurate yet expensive in situ surveys. The long-term coastal-change data of these few well-monitored beaches have led to in-depth understanding of many site-specific coastal processes. However, because the best-monitored beaches are not representative of all beaches, much remains unknown about the processes and fate of the other >99% of unmonitored beaches worldwide. The fleet of Earth-observing satellites has enabled multiscale monitoring of beaches, for the very first time, by providing imagery with global coverage and up to daily frequency. The long-standing and ever-expanding archive of satellite imagery will enable coastal scientists to investigate coastal change at sites vulnerable to future sea-level rise, that is, (almost) everywhere. In the past decade, our capability to observe coastal change from space has grown substantially with computing and algorithmic power. Yet, further advances are needed in automating monitoring using machine learning, deep learning, and computer vision to fully leverage this massive treasure trove of data. Extensive monitoring and investigation of the causes and effects of coastal change at the requisite spatiotemporal scales will provide coastal managers with additional, valuable information to evaluate problems and solutions, addressing the potential for widespread beach loss due to accelerated sea-level rise, development, and reduced sediment supply. Monitoring from Earth-observing satellites is currently the only means of providing seamless data with high spatiotemporal resolution at the global scale of the impending impacts of climate change on coastal systems.