Sediment Supply Research Articles

Impact of Reduced Fluvial Sediment Supply on Saltwater Intrusion in the Yangtze Estuary

AbstractA decline of the fluvial sediment supply leads to coastal erosion and land loss. However, the fluvial sediment load may influence not only coastal morphodynamics but also estuarine hydrodynamics and associated saltwater intrusion. Previous studies revealed that suspended sediments influence estuarine hydrodynamics through various flow–sediment interactions. In this contribution, we systematically investigate how changes in fluvial sediment load and other climate‐change‐induced environmental change influence estuarine hydrodynamics and sediment dynamics. For this purpose, we utilize a well‐calibrated fully coupled model in which hydrodynamics, saltwater intrusion, and sediment transport interact with each other, to explore saltwater intrusion in the Yangtze Estuary in response to a decline in the sediment load, modified discharge, and sea‐level rise. Model results suggest that a 70% decline in the suspended sediment load weakens the impact of sediments on salinity‐induced stratification and thereby reducing saltwater intrusion. Sea‐level rise or discharge peak reduction increases saltwater intrusion. However, a fully coupled model accounting for sediment effects predicts a much larger increase in saltwater intrusion compared to noncoupled models. Whether this effect is important depends on estuarine sediment concentrations and therefore the potential role of sediments should be carefully investigated before applying a noncoupled model. This work highlights not only the relevance of a suspended sediment decline but also the use of fully coupled models for predicting saltwater intrusion in turbid estuaries and has broad implications for freshwater resource management in turbid estuarine systems influenced by human interventions and climate change.

The long-term dynamics of invasive signal crayfish forcing of fluvial sediment supply via riverbank burrowing

Animals are important drivers of sediment dynamics. Invasive signal crayfish (Pacifastacus leniusculus) have been shown to supply sediment to rivers by burrowing into riverbanks. Burrowing directly transfers excavated sediment into the river and also has an additional indirect affect by promoting riverbank failure. While previous research has isolated burrow densities at a point in time, rates of burrow construction and of burrow loss due to erosion are unknown, which introduces uncertainty into estimates of how much sediment burrows contribute to rivers. Here we report results from a 5-year study that evaluated the temporal dynamics of crayfish populations, rates of burrow loss due to erosion, and the mass of sediment directly supplied to rivers by burrow excavation. At ten reaches across five lowland streams in England, we estimated the mass of sediment displaced by 1861 new and previously constructed burrows. Both crayfish and burrow densities were variable over time, suggesting that burrows contribute temporally variable amounts of fine sediment to riverine systems. 42 % of observed burrows were constructed within the previous 365 days, and individual burrows lasted on average 461 days. Applying this to comparable historical data, an average of 2.0 t km−1 a−1 of sediment was excavated to construct burrows in the study reaches, which is eight times more than estimated in previous studies. Whilst total burrow densities in each year were not consistently correlated with contemporary crayfish densities, the mass of sediment excavated over the prior year was strongly correlated with contemporary crayfish densities. Current fine sediment management practices are largely aimed at controlling fine sediment delivery, predominately from agricultural activities, but biotic burrowing into riverbanks may represent an important and overlooked source of fine sediment supply. Incorporation of biotic processes in sediment dynamics would improve the accuracy of fluvial sediment budgets and enhance the knowledge base underpinning effective fine-sediment management practices.

STUDY OF PILLAR DIMENSION VARIATIONS MODELLING ON RIVER GEOMORPHOLOGY

The behavior of river flow patterns on the condition of the pillar dimensions can affect the geomorphology conditions of the river, namely the unbalanced local scouring between sediment transport and sediment supply so that it will affect the strength of the pillar. This study aims to study the effect of pillar dimensions on scour depth and channel base changes in the pillar area. The method of observation was carried out by simulations using Nays2DH with the same parameters for each test of each dimension with discharge (Q) = 0.5 m3/s, sediment diameter (D50) = 0.35 mm, channel width (B) = 5 m, channel length (L) = 25 m, slope (i) = 0.005 and drainage time (t) = 7200 s. The simulation results show the basic scour pattern of the channel which has the smallest value, the diamond type pillar. Degradation - Basic channel agitation on diamond type pillars, showing a pattern that tends to be evenly distributed compared to semi rounded, rectangle, or octagonal types. Period (t) 7200 s downstream of the pillar, semi rounded type scours change <0.3 m, rectangle type scours changes <0.4 m, octagonal type scours changes <0.4 m, and diamond type scours changes <0.2 m. So the dimensions of the diamond type are more effective in reducing the depth of scour.

Equilibrium configurations of sandy-muddy transitional beaches on South China coasts: Role of waves in formation of sand-mud transition boundary

Sandy-muddy transitional beaches (SMT-Beaches) consisting of the upper sandy beach and lower mudflat can be found on coasts with weak wave conditions and sufficient sediment supply. Currently, such SMT-Beaches receive limited attention, and their morphological characteristics as well as the mechanisms of the formation of sand-mud transition (SMT) boundary are still not clear. Based on field surveys on SMT-Beaches at South China Coasts, traditional equilibrium functions are assessed on SMT-Beaches, and an equilibrium profile function for SMT-Beaches is parameterized with only inputs of the offshore distance of the SMT boundary, tidal ranges and offshore wave heights. The present function exhibits good performance on both mesotidal and macrotidal SMT-Beaches. It improves the accuracy of profile curvature of the upper beachface compared with the traditional power function and reduces uncertainties in determining coefficients. Sediment characteristics differ between both sides of the SMT boundary. Shoreward of the SMT boundary, the probability distribution of grain size is generally unimodal with a peak located at sand grains and turns to be bimodal or trimodal with a significant increase at clay-to-silt grains seaward of the SMT boundary. Based on the exceeded Shields parameter, the critical position for the deposition of mud generally corresponds well with the SMT boundary, indicating that wave-related oscillatory motions play a dominant role in the formation of the SMT boundary.

Suspended sediment change along the Zhejiang-Fujian coast revealed using MODIS 250-m imagery

Suspended sediment concentration (SSC) is one of the important parameters in the ecological environment evaluation of marine water bodies, as it provides an important basis for research on the ecological environment of nearshore sea areas, shoreline beach erosion and deposition, water quality of coastal zones and nearshore sedimentation zones. In recent years, the amount of sediment carried by the Yangtze has declined dramatically due to the Three-Gorges Dam (TGD), which might change the sediment supply to the sea and thus affect sediment transport to the coasts of Zhejiang and Fujian. This study aims to both develop specific algorithms and reveal the interannual and seasonal dynamics of the suspended sediment belt from the Yangtze Estuary to the Zhejiang-Fujian provincial shoal using MODIS 250 m data from 2000 to 2020. MODIS time-series analysis shows that the suspended sediments were originally from the Yangtze Estuary and formed a suspended sediment belt in the winter season. Moreover, driven by monsoon winds and coastal current dynamics, the sediment belt has obvious seasonality. Furthermore, we also suggest that the coastal sediment area has a tendency to shrink in winter, and the lifetime of the sediment belt also shortens, showing early disconnection in spring and delayed recovery in autumn, in response to the decline in sediment discharge from the Yangtze River basin to the sea. Impressively, the impact of the TGD in 2003 led to a drop in the sediment concentration in the coastal area. In addition, further analysis confirms that a sudden decrease in suspended sediment concentration was observed at Zhoushan (S2), Wenling (S4), Minjiang River Mouth (S7), Pingtan Island (S8), and many other stations along the coast in 2003, and the closer to the Yangtze Estuary, the more serious this phenomenon becomes, on the basis of monthly SSC analysis. These findings can assist us in better understanding the new pattern of the sediment belt from the Yangtze Estuary to the Zhejiang-Fujian coastal area. The findings of this study have theoretical and practical implications for the study of the hydrological sediment field in offshore areas, the protection of the marine ecological environment, changes in the shoreline and mudflats, and the maintenance of waterways and ports in estuaries.

The different erosion fate of the headland-embayed beaches on the muddy and sandy coasts of China

<p>China’s beaches exhibit different geomorphic characteristics depending on location. Due to increasing contemporary climate change, induced storm activities and human activities, beaches along the Chinese coast have been exposed to the risk of erosion. This article examines the different shoreline evolution processes from 1973 to 2021 as well as the erosion vulnerability of 9 headland-embayed beaches (of which 5 beaches, each at Baishawan, Dasha, Dongdan, Nanshajiao, and Mushao are on the muddy coast in Southern China and 4 beaches, namely, Bathing Beach 1, 2, 3 and Shilaoren Beach are on the sandy coast in Northern China) based on the inherent geomorphic characteristics and nearshore hydroclimatic factors of the beaches. In the analysis, there were 3 stages. During the first stage, erosion dominated both the muddy and sandy coasts as a result of intense storm conditions. During the second stage, the beaches had earlier recovered as a function of natural processes, however, storm activities later eroded the beaches. During the third stage, most of the beaches accreted as a result of coastal engineering interventions and beach nourishment project. The shoreline analysis results indicate that beaches on the muddy and sandy coasts have been eroding in the long term. During the first erosion stage, erosion is more severe on the muddy coast than on the sandy coast in the short term. On the sandy coast, the beaches recorded severe erosion from 1973 to 1998. Of the 9 beaches, the most eroded location was at Dasha on the muddy coast (LRR: –5.315 m/y; EPR: –5.671 m/y; NSM: –141.94 m) between 1974 and 1998. In summary, beaches on muddy coasts are more vulnerable to erosion than those on sandy coasts. On the muddy coast, there has been a shortage in the supply of sediment from the Yangtze River-derived sediment to the coast. The primary source of sand material for the studied beaches on the muddy coast has been the regular storm condition that changes the sand-mud transition line on the coast. For the sandy beaches, the primary factor responsible for the vulnerability and beach modification includes a shortage in the natural supply of beach material and storm activities, however, recent beach nourishment and coastal protection procedures are gradually stabilizing the beaches. Ultimately, the outcome of this research is suitable for beach management procedures on the Chinese coast.</p>

Modern Sedimentation Patterns in Lake Issyk Kul (Kyrgyzstan), Derived From Surface Sediment and Inlet Stream Samples

AbstractThis study provides a descriptive characterization of the modern sedimentary processes in Lake Issyk Kul, Kyrgyzstan, important for the selection of a suitable coring or deep‐drilling site, interpretation of future core data and applicability of proxies. The quasi‐equidistant sampling grid of 66 sediment surface samples covers the entirety of the lake basin and is complemented by 10 samples from the major inflows. The methodological approach includes geochemical, granulometric, lipid biomarker, diatom, and statistical analyses. The quantitative and qualitative changes in sediment composition yield information on its generic origin and prevailing transport and depositional environments. The composition of the surface sediments in Issyk Kul is highly heterogenous. Nearshore deposition is mainly controlled by wave action and by fluvial sediment supply with highest quantities of detrital input coming from the high‐energetic, eastern tributaries. Sediments in the deep central basin are mainly produced in situ and dominated by authigenic calcite. Biogenic accumulation is overall low, except for the western extremity of the lake, where the nearshore, shallow‐water, and low‐energetic environment favors aquatic productivity and subsequent preservation of organic material and diatoms. Redeposition of sediments is a dominant process along the slopes across the southern and western basin floor, where run‐out distances of mass movement deposits are up to 5 km. Directional sediment transport by lake currents appears to be less important, except for the transport of very fine‐grained organic matter. Biomarker‐inferred temperature reconstructions suggest lake surface temperatures of ∼15°C in the western littoral zone and in Tyup Bay and a decrease to ∼13°C basinward.

Late Quaternary river evolution and its response to climate changes in the upper Mekong River of the Qinghai–Tibetan Plateau

Multiple continental-scale large rivers originate from the Qinghai–Tibetan Plateau (QTP), known as the “Asia Water Tower”, because of glacial melting. However, the timing of fluvial processes in the headwaters of these large rivers and their correlation with global climate changes remain uncertain. Among these rivers, the Mekong River, the third-longest in Asia, flows through the southeastern QTP. In this study, we investigated fluvial terraces, buried paleochannels, hillslope-derived colluvium, and remnants of dammed lake in the Nangqian Basin, which is in the headwaters of the Mekong. We reconstructed the fluvial aggradation and incision history employing optically stimulated luminescence (OSL) dating. Our results suggest that orbital-scale climate changes have significantly influenced fluvial processes in the Nangqian Basin since the Late Pleistocene. During marine isotope stage (MIS) 3 (57–29 ka), a relatively warm period within the last glacial epoch, the Nangqian Basin experienced aggradation of fluvial sediment. Subsequently, during the later part of the Last Glacial Maximum (LGM) (21–17 ka), episodic landslides and debris flows caused valley infilling, resulting in river blockage and the formation of a dammed lake. At around 16 ka, the Mekong in the Nangqian Basin shifted from aggradation to fluvial incision. Since the late Holocene, the basin returned to aggradation, forming a new terrace at around 1.1 ka. This may be due to the combination of decreased stream power and increased sediment supply, potentially linked to intensified human activities and its associated vegetation degradation. Overall, our study highlights the close links between glaciation and fluvial processes in the region. Specifically, we propose that the observed aggradation pattern during the last glacial period on the plateau primarily resulted from substantial detrital input from glacier activities. The transition from aggradation to incision during the last deglacial period is likely influenced by increased discharge from glacier meltwater. The presence of colluvium interbedded within the late LGM fluvial sediment sequences highlights the significant impact of climate changes on fluvial processes in the headwater areas of the QTP, especially those events stemming from hillslope activity, such as landslides and debris flows.

Long-distance recovery of nonuniform suspended load in the Middle Yangtze River owing to upstream damming

The amount of suspended sediment entering the Middle Yangtze River (MYR) was greatly reduced owing to the operation of the Three Gorges Dam (TGD) and other upstream dams, which has caused the process of sediment transport in the MYR at a severe non-equilibrium state. A detailed investigation was conducted into the transport of nonuniform suspended load in the MYR using about 30-year observed data: (i) firstly, the spatiotemporal variations in fractional sediment fluxes were investigated during the pre- and post-TGD periods. The total sediment flux decreased after the TGD operation, and the magnitude for the coarse fraction (d > 0.125 mm) was smaller than those for the fine (d < 0.031 mm) and medium (d = 0.031–0.125 mm) fractions; moreover, the sediment flux for each fraction continuously recovered along the 955-km-long reach, with the recovery distance for the coarse fraction being much shorter than the other two fractions; it was mainly attributed to the fact that the riverbed could supply abundant coarse sediment to the flow, and this fraction experienced a considerably higher recovery degree than the other two fractions; (ii) secondly, the sediment budgets for the fine, medium and coarse fractions were calculated to assess the contributions of different sediment sources to the recovery of graded sediments in the MYR. About 50 %, 29 % and 14 % of the increased fractional sediment amounts for fine, medium and coarse fractions were originated from the tributary confluence, while the remaining sediment was supplied from the riverbed during the process of channel erosion. Therefore, the recovery of suspended load was largely attributed to the supply of sediment from the riverbed, especially for the coarse fraction; (iii) finally, the impacts of the altered flow-sediment regime and the bed-material composition on sediment recovery efficiency were analysed. The sediment recovery efficiency greatly increased after the TGD operation, which was caused by the reduced sediment concentration; and the sediment recovery efficiency for the coarse fraction was much larger than those for the fine and medium sediment fractions. It is attributed to the fact that the channel can provide abundant coarse sediment to the flows in the MYR, while the sediment recovery efficiencies of fine and medium fractions were restricted by the low degree of sediment supply from the riverbed.

Consequences of limited sediment supply for long-term evolution of offshore tidal sand waves, a 3D model perspective

Field data show that offshore tidal sand waves in areas where sediment supply is limited have different characteristics (shape and dimensions) compared with their counterparts in areas with sufficient sediment supply. So far, only the initial formation of tidal sand waves on a sediment-starved shelf has been studied with a 2DV model that ignores variations along the crests. In this study, a 3D non-linear morphodynamic model is used to investigate the effects of sediment availability on the long-term evolution of offshore tidal sand waves. Overall, the simulated sand waves have characteristics that resemble those of observed sand waves. The mature sand waves that develop in the case of limited sediment supply (i.e., thickness of erodible sediment layer is smaller than the height of sand waves) are more three-dimensional, i.e., having isolated and more irregular crestlines compared with those in the case of sufficient supply. With decreasing sediment supply, sand waves have larger spacings between successive crests, smaller heights and they migrate faster. These differences in the characteristics of the sand waves start to occur once the hard bed underneath the erodible sediment layer is exposed.

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.

Impact of sediment size, channel velocity and plant density on sediment deposition inside Phragmites australis canopies

In this study, laboratory experiments were performed to investigate the impacts of sediment size, channel velocity and plant density on sediment deposition inside an emergent canopy constructed with Phragmites australis (P. australis) that included real plant morphology. Three sediment sizes (ds = 10, 22 and 48 μm), three plant densities (n = 0.006, 0.018 and 0.027 cm−2) and various mean channel velocities (U0 = 6 to 17 cm/s) were considered. Different mean channel velocities corresponded to the presence or absence of sediment resuspension in the bare channel, which led to distinct deposition patterns. Inside canopies, sediment deposition was dominated by the near-bed turbulent kinematic energy (near-bed TKE). At very low mean channel velocities (i.e., no resuspension in the bare channel), the pure deposition inside the canopy was the same as that in the bare channel. As the mean channel velocity increased but resuspension did not occur in the bare channel, the local vegetation-induced turbulence led to an increase in near-bed TKE within the canopy leading edge region, further causing the formation of a diminished deposition region. When the mean channel velocity was sufficiently large to produce resuspension in the bare channel, diminished and enhanced deposition regions were observed inside the canopy compared to the bare channel. A prediction method considering the impacts of sediment size, channel velocity, plant density and plant morphology was proposed to predict the length of the in-canopy diminished deposition region. The predictions and measurements were consistent. Different sediment sizes, channel velocities and plant densities led to two distinct sediment supply conditions inside the canopies. When sediment supply was not limited, finer sediment and/or sparser canopies led to longer diminished deposition regions, but the pure deposition inside the canopies remained constant. When sediment supply was limited, a different deposition pattern was observed. The pure deposition decreased along the canopy length, but the resuspension in the bare channel and canopy leading edge region might compensate for the limited sediment supply inside the canopy. A threshold representing the ratio of the advection time throughout the canopy to the particle settling time, κ, was defined to evaluate whether the sediment supply inside the canopy was limited. The sediment supply was not limited at κ < 0.4 but was limited at κ > 1.

Depositional architecture and sequence stratigraphic framework of the fluvio‐lacustrine Ash Shumaysi Formation, Jeddah‐Makkah Region, Saudi Arabia: Implications for climatic and tectonic changes in a local‐scale sub‐basin

AbstractThis study aims to interpret and document the depositional architecture styles and sequence stratigraphic framework of the Ash Shumaysi Formation in the Jeddah‐Makkah region, the west‐central part of Saudi Arabia, and presents an example of rarely discussed, local‐scale sub‐basins (half grabens). It also shows the relationships between synchronous sedimentary processes and pre, syn and post‐rift conditions. The described lithofacies and their facies associations indicate the presence of seven architectural depositional styles: proximal‐distal braided fluvial, meandering fluvial (point bar), crevasse splay, floodplain, estuarine and lacustrine. A proposed depositional model for the Ash Shumaysi Formation is drawn. The Ash Shumaysi Formation forms a second order depositional sequence, which is organised into two third order depositional sequences (sequences I and II) bounded by three sequence boundaries. Each third‐order sequence encloses the low accommodation systems tract and high accommodation systems tract. The low accommodation systems tract represents the coarse‐grained, braided‐distal fluvial facies developed during low accommodation space associated with high sediment supply (high discharge). The high accommodation systems tract encloses the fine‐grained deposits of point bar, estuarine and lacustrine facies that reflect the creation of significant accommodation space and low sediment supply (low discharge). Vertical and lateral variations of the inferred depositional architectural styles, sequences and systems tracts reflect that tectonic forces and climate are the main controlling factors during deposition of the Ash Shumaysi Formation, although base‐level changes in response to sea‐level changes cannot be ruled out.

Magnitude and timing of transient incision resulting from large‐scale drainage capture, Sutlej River, Northwest Himalaya

AbstractFew studies have constrained the magnitudes and timescales associated with large‐scale drainage captures (areas &gt;103 km2), even though these constraints are crucial to reconstruct sediment budgets, assess the potential for drainage reorganization to be preserved in the rock record, and determine the extent to which environmental signals (i.e., structures, composition and fossil assemblages within sedimentary rocks that are influenced by sediment supply and transport) are representative of conditions during deposition. In this work, we characterize the Pleistocene capture of the Zhada Basin, an ~23 000 km2 extensional basin in southern Tibet, by the Sutlej River, a prominent tributary to the Indus River. We quantify the magnitudes and timescales of capture‐driven erosion using knickpoint celerity modelling, paleotopographic reconstructions, 10Be‐derived denudation rates, and topographic analyses of drainage divides. We find that capture has removed 2010 ± 400 km3 of sediment from the Zhada Basin, increasing sediment supply to the Sutlej network by 17%–29% since 735 ± 269 ka. This work represents a crucial step towards reconstructing the Pleistocene sediment budget of the Indus sedimentary system and identifying potential impacts from sediment redistribution. We also identify several plausible tectonic or autogenic mechanisms that may have facilitated capture of the Zhada Basin, including: (1) preferential erosion of weak lithologies along active faults, (2) headward erosion in response to prior capture of the Spiti River and (3) headward erosion generated by breaching of a structural culmination downstream (the Kullu‐Rampur Window). This provides a framework to assess the mechanistic links between arc‐parallel extension, large‐scale drainage capture, landscape evolution and orogenic wedge deformation.

Effects of Patch Density and Incoming Sediment on Flow Characteristics and Bed Morphology

This study focuses on the effects of vegetation patch density, bed condition, and incoming sediment on flow structure and bed morphology within and around a patch. The variation in upstream adjustment velocity is not well defined for low-density vegetation patches but decreases with increasing patch density in high-density patches. The length of the upstream adjustment region is greater for high-density vegetation. Incoming sediment causes a reduction in both the steady wake velocity and the length of the steady wake at a low density. The length of the recovery region increases with density when vegetation is sparse, but remains constant in a dense patch. Additionally, the length of the recovery region decreases due to incoming sediment. Turbulent kinetic energy is not affected by the bedform and incoming sediment when reaching its first peak. However, the second maximum of the turbulent kinetic energy increases when the bed is movable. The evolution of bed morphology is closely related to the flow structure and the growth of the von Karman vortex street. Both the rising length and the adjusted length decrease with increasing patch density, while the incoming sediment causes an increase in the adjusted length. Behind the patch wake, the first minimum elevation, maximum elevation, and second minimum elevation decrease as the patch density increases. These values, in turn, increase with the sediment supply upstream of the flume.

A tale of two Arabian Cenozoic ooids: Factors controlling the differential growth of ooids across the Arabian Plate

Ooids are a critical yet enigmatic carbonate grains in the geological record. Previous studies have focused their efforts on studying the mode of ooid formation throughout the Phanerozoic via experimental, numerical simulations, and field studies. To date, however, there is little discussion around factors governing the texture, mineralogy, structure, size, and geometry of Cenozoic ooids, particularly in Saudi Arabia. The present article aims to provide insights on key factors controlling the differential growth of Cenozoic ooids in Saudi Arabia by comparing two contrasting carbonate environments and tectonic settings. Here, we studied the Holocene (0.4–2 kya) ooids developed in the tidally dominated carbonate ramp of the Arabian Gulf under a passive margin setting and the Miocene (15–21 mya) ooids from the Red Sea that were developed in a tectonically active, wave-dominated fault-bounded carbonate platform. The Holocene ooids were characterized by aragonitic, heavily micritized and small (max:550 μm) ooids with bioclast-dominated nuclei. In contrast, the Miocene ooids were replaced by dolomite, larger in size (max:820 μm) and significantly less micritized with variable nuclei compositions, such as rock fragments and bioclasts. The variation in the intensity of bioerosion and micritization suggest the Miocene ooids experienced lesser resting stage time than the Holocene ooids developed under the tide regime due to active wave actions. In addition, the Miocene ooids were developed in a fault-bounded margin allowing a higher carbonate saturation state to concentrate in that zone while the Holocene ooids were developed under a widespread distribution of carbonate saturation state in a vast carbonate ramp setting. These findings indicate that original platform geometry and depositional system (tide-vs wave-dominated) are substantial in dictating the duration of ooid resting stage and distribution of carbonate saturation state across the platforms. All these combined, allows the Miocene ooids to grow larger and reaching its equilibrium size faster. This study highlights the importance of coupling platform architecture and other key environmental parameters (wave actions, sediment supply, and tectonic activities) in understanding the formation of Cenozoic ooids.

Runoff and sediment characteristics of a typical watershed after continuous soil erosion control in the red soil region of Southern China

To investigate the runoff and sediment characteristics of the watershed after continuous soil erosion control in the red soil region of southern China, hydrological data were collected from 2013 to 2020 in the Zhuxi watershed, which is located in the southeast of the red soil region. The results showed that the monthly suspended sediment concentration and annual sediment yield have significantly decreased since 2017. At the event scale, 180 floods were classified into four flood patterns (A, B, C and D) by the K-medoids clustering method based on the flood duration, runoff depth and peak discharge. Each flood pattern resulted in differing levels of the event sediment yield (D > C > B > A), and Pattern C contributed the most to runoff and sediment yield. The study period was divided into two stages according to the annual sediment yield. Compared with 2013–2016, the total runoff and sediment yield of the 3 major flood patterns (A, B and C) in 2017–2020 decreased by 17.88%∼25.12% and 78.92%∼90.34%, respectively. These flood patterns decreased not only in frequency but also in suspended sediment concentration, which was caused by the changes in the runoff-sediment relationships of flood events. Six types of suspended sediment concentration and discharge (SSC-Q) relations appeared in the flood events, indicating that the sediment sources and sediment transport processes were complex and dynamic. In 2017–2020, the single-valued relation became the dominant type of SSC-Q relations for all flood patterns, which suggests that the sediment supply was inhibited. The results provide a more comprehensive understanding of the runoff and sediment processes of a watershed after continuous soil erosion control, which is beneficial for revealing the development of soil erosion in the red soil region of southern China.

El Niño–Southern Oscillation (ENSO)-driven hypersedimentation in the Poechos Reservoir, northern Peru

Abstract. Although extreme El Niño events (EENEs) have always impacted hydrological anomalies and sediment transport in South America, their intensification by global warming and their association with changes in human activities and land cover after humid periods may lead to the acceleration of sediment transfers in river systems and dam reservoirs. This situation may threaten soil and water resources in arid and semiarid regions highly dependent on water originating from large dams. In this study, we investigated the sediment sequence accumulated in the Poechos Reservoir (northern Peru) and provided a retrospective reconstruction of the interactions of El Niño–Southern Oscillation (ENSO), agricultural practices and vegetation cover changes with sediment dynamics (1978–2019). To this end, a sediment core was dated and characterized by physical and chemical analyses (e.g., scanner tomography, X-ray fluorescence, particle size analysis) to estimate the evolution of sedimentation rates and changes in sediment sources during the last 5 decades. Sediment tracing results indicated the occurrence of changes in sediment sources associated with positive and negative phases of the Eastern Pacific index with a greater contribution of the lowland dry-forest area in comparison to that of the Andean area to sediment during the El Niño events (mean contribution of 76 %; up to 90 % during the coastal El Niño events (CENEs) of 2016–2017). This source contribution was mostly controlled by the stationary rainfall occurring during the EENEs in the lowland dry-forest area characterized by a low vegetation cover. Overall, after an extreme phase of ENSO, like after the EENE 1982–1983, the normal discharges and persistent sediment supplies from the middle- and upper-catchment parts led to river aggradation and the storage of substantial amounts of sediment in alluvial plains. In the absence of a significant EENE between 1983 and 1997, the large volume of sediment stored in the alluvial plains was exported by the EENE 1997–1998 resulting in an increase in sedimentation rate of 140 % after 1997 with a significant aggradation of the deltaic zone of the reservoir. In addition to the impact of extreme climate events on sediment dynamics, the development of agriculture along the riverine system after an extreme phase of ENSO increased the availability of sediments in the main channel of the rivers, easily transported by the next EENE. This study suggests that intensification of human activities associated with a higher frequency of extreme rainfall events amplified the quantity of sediment transported by the river system, which will significantly decrease the lifespan of the reservoir, which is essential to meeting the freshwater demands of the farmers and the populations living in this arid and semiarid region.

Assessing the impact of climate change on sediment discharge using a large ensemble rainfall dataset in Pekerebetsu River basin, Hokkaido

Increased rainfall associated with climate change can increase sediment discharge. The supply of fine sediment from slope failures inhibits bed armoring of mountain rivers and increases sediment discharge to the downstream reaches. Floods without slope failures lead to bed erosion and armoring and may ultimately decrease sediment discharge. Thus, it is important to consider sediment discharge from slope failure and bed erosion as factors affecting sediment production. Climate change affects not only the rainfall amount, but also the temporal rainfall pattern; consequently, the pattern affects the sediment production factors and the amount of sediment discharge. However, changes in sediment discharge due to climate change based on sediment production sources have not yet been clarified. In this study, we statistically analyzed 1200 results simulated using a physics-based sediment runoff model to assess the impact of changes in temporal rainfall patterns on sediment discharge and sediment production sources in the Pekerebetsu River Basin. In the simulations, we used the rainfall predicted in d4PDF (Database for policy decision-making for future climate change), a large ensemble climate simulation database at 5 km and 20 km resolutions. Our results showed that the climate-driven increase in sediment discharge was considerably larger than that of rainfall. An increase in short-term heavy rainfall increased the supply of fine sediments from slope failure. This resulted in the suppression of bed armoring and a large increase in sediment discharge. Thus, the increase in sediment discharge is not only caused by an increase in rainfall but also by changes in temporal rainfall patterns and sediment production factors. The sediment discharge calculated for the 20 km resolution climate projection was nearly one order of magnitude smaller than that for the 5 km resolution. This suggests that the 20 km resolution climate projections do not adequately represent orographic rainfall in the mountains and thus, do not adequately reproduce extreme sediment discharge events. An increased sediment supply causes bed aggradation and decreases the river conveyance capacity of the downstream channel. The model developed in this study will contribute to flood risk analysis and flood control planning for increased rainfall due to climate change.

Multi-annual and multi-decadal evolution of sediment accretion in a saltmarsh of the French Atlantic coast: Implications for carbon sequestration

Coastal marshes offer natural solutions for adapting to and mitigating the effects of climate change and sea level rise. However, the resilience of the marsh physical system and, with it, the ecosystem services that it provides, is largely site specific. This calls for the increase in the spatial cover of coastal marsh studies in order to assess the controlling factors of marsh evolution, and their long-term carbon storage capacities. Here, we study the spatio-temporal variations in sedimentation rates and organic carbon (OC) sequestration capacity of the macrotidal minerogenic saltmarshes in Aiguillon Bay, belonging to one of the largest French coastal marshes. Supported by aerial photographs and satellite image analysis, we first show that saltmarshes of the Aiguillon Bay have prograded at very high rates, up to 14 m yr−1 since 1950. Sediment accumulation rates (SAR) were estimated at both multi-annual to multi-decadal scales based on two approaches: (i) LiDAR-based digital elevation models from multiple acquisition dates (2010–2021); and (ii) depth profiles of 210Pb in excess and 137Cs in sediment cores collected along cross-shore transects in the saltmarshes. Long-term SAR range from 0.8 to 2.2 cm yr−1 and are among the highest reported worldwide for equivalent systems. The positive accretion balance (accretion rate minus local sea-level rise rate) provides important clues on marsh resilience suggesting that the Aiguillon Bay is currently able to adapt to rising sea level. Despite relatively low organic carbon content (1.3–6.0%), high SAR leads to high carbon sequestration rates (99–345 gC m−2 yr−1; or a mean value of 2.5 Mg C ha−1 yr−1). The isotopic signature of sediment OC reveals a significant and rapid decomposition of organic material in surface cores, while allochthonous sediment of marine origin dominates the signature of chemically-stable OC of marsh sediments. This implies that the carbon sequestration capacity of minerogenic saltmarshes, such as those of the Pertuis Charentais, also depends upon the wealth of adjacent coastal environments through high sediment supply and primary productivity.