Increase In Sediment Discharge Research Articles

Holocene floodplain aggradation in the central Grampian Highlands, Scotland

ABSTRACT Radiocarbon ages for samples of organic material within and overlying the highest Holocene floodplain and fan terraces in Glen Feshie and Glen Tromie imply sediment aggradation after ∼4.3 cal ka and probably incision after ∼3.7 cal ka, and in the Edendon Valley aggradation after ∼2.8 cal ka, with incision after ∼2.7 cal ka. The timing of sediment aggradation at all three sites postdates the onset of pine forest decline (∼4.8 cal ka) at nearby high-level sites, and coincides with wet periods characterised by high water tables. This coincidence in timing suggests that forest decline may have caused upstream extension of the tributary network, headwater incision and flashier flood responses, and that increased rainfall enhanced sediment discharge from headwater tributaries, with consequent sediment accumulation downstream on low-gradient fans and floodplains. More speculatively, exhaustion of readily entrained sediment from headwater areas may have stimulated subsequent floodplain and fan incision. Our results show that the highest Holocene terrace (the Main Holocene Terrace) is a diachronous feature, even in valleys emanating from the same upland source area, and support the conclusions of simulation models that predict marked increases in sediment discharge when deforestation is succeeded by an episode of increased rainfall.

Carbon isotope chemostratigraphy, geochemistry, and biostratigraphy of the Paleocene–Eocene Thermal Maximum, deepwater Wilcox Group, Gulf of Mexico (USA)

Abstract. The Paleocene–Eocene Thermal Maximum (PETM) represents the most pronounced hyperthermal of the Cenozoic era and is hypothesized to have resulted in an intensification of the paleohydrologic cycle, including enhanced seasonality and increased sediment discharge to the coastal ocean. Although the PETM has been widely documented, there are few records from deposits that form the distal, deepwater components of large sediment-routing systems. This study presents new constraints on the stratigraphic placement of the PETM in the deepwater Gulf of Mexico basin through analysis of geochemical, carbon isotopic, and biostratigraphic data within a ∼124 m cored interval of the Wilcox Group. Biostratigraphic and carbon isotopic data indicate that the PETM extends over ∼13 m based on acmes in the dinoflagellate Apectodinium homomorphum and calcareous nannoplankton Rhomboaster cuspis as well as a ∼-2 ‰ shift in bulk organic δ13C values. A decrease in bioturbation and benthic foraminifera suggests that a reduction in oxygen of Gulf of Mexico bottom waters and/or an increase in sedimentation rates were coincident with the onset of the PETM. A ∼2 m lag in the depositional record separates the onset of the PETM negative carbon isotope excursion (CIE) and deposition of a 5.7 m thick interval of organic-lean claystone and marlstone that reflects a shut-off of the supply of sand, silt, and terrestrial palynomorphs to the basin. We interpret deposits of the PETM in the deepwater Gulf of Mexico to reflect the combined effects of increased erosional denudation and rising sea level that resulted in sequestration of sand and silt near the coastline but that allowed delivery of terrigenous mud to the deep sea. The similarity of oceanographic changes observed in the Gulf of Mexico and Atlantic Ocean during the PETM supports the inference that these water masses were connected during latest Paleocene–earliest Eocene times. Although deposition of typical Wilcox Group facies resumed during and after the PETM recovery, an increased influx of terrestrial detritus (i.e., pollen, spores, terrestrial organic debris) relative to marine dinoflagellates is suggestive of long-lasting effects of the PETM. This study illustrates the profound and prolonged effects of climatic warming on even the most distal reaches of large (≥1×106 km2) sediment-routing systems.

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.

Characteristics of the Sediment Transport Process in Vegetation Hillslopes under Different Flow Rates

Vegetation filter strips (VFSs) have always been an important measure to control agricultural soil erosion, especially in mountainous and hilly areas with more sloping farmland. To investigate the mechanism of the sediment-trapping process by VFSs, a series of tests were conducted with four gradients of flow rate, 7.5–45 L min−1 m−1, and two different sediment concentrations of 40 and 120 g L−1. The whole process of overland flow was monitored, and sediment and particle size samples from the inflow and outflow were collected and measured. The results showed that the changes in sediment concentration did not significantly affect the corresponding coefficients in the power function relationship between overland flow rate and velocity. Using the Reynolds number alone cannot effectively indicate the flow pattern of overland flow on vegetation hillslopes. The peak particle size and linear function were effective in describing the relationship between sediment particle composition and delivery rate during the sediment-trapping process by VFSs. During the sediment-trapping process, the sediment-trapping capacity of VFSs continued to decrease. The increase in sediment discharge was accompanied by a higher proportion of coarse particles. Under the same flow rate conditions, when the sediment concentration was higher, the coarse particles and their proportion also increased faster. Therefore, using only a certain particle size threshold to distinguish suspended and transported sediment may lead to inaccurate estimation of the sediment-trapping performance of VFSs. This study deepened the understanding of the mechanism of water–sediment processes on vegetation hillslopes and promoted the widespread and efficient application of VFSs management technology.

Modeling the spatially distributed nature of subglacial sediment transport and erosion

Abstract. Glaciers expel sediment as they melt, in addition to ice and water. As a result, changing glacier dynamics and melt produce changes to glacier erosion and sediment discharge, which can impact the landscape surrounding retreating glaciers, as well as communities and ecosystems downstream. Currently, numerical models that transport subglacial sediment on sub-hourly to decadal scales are one-dimensional, usually along a glacier's flow line. Such models have proven useful in describing the formation of glacial landforms, the impact of sediment transport on glacier dynamics, and the interactions among climate, glacier dynamics, and erosion. However, these models omit the two-dimensional spatial distribution of sediment and its impact on sediment connectivity – the movement of sediment between its detachment in source areas and its deposition in sinks. Here, we present a numerical model that fulfills a need for predictive frameworks that describe subglacial sediment discharge in two spatial dimensions (x and y) over time. SUGSET_2D evolves a two-dimensional subglacial till layer in response to bedrock erosion and changing sediment transport conditions. Numerical experiments performed using an idealized alpine glacier illustrate the heterogeneity in sediment transport and bedrock erosion below the glacier. An increase in sediment discharge follows increased glacier melt, as has been documented in field observations and other numerical experiments. We also apply the model to a real alpine glacier, Griesgletscher in the Swiss Alps, where we compare outputs with annual measurements of sediment discharge. SUGSET_2D accurately reproduces the general quantities of sediment discharge and the year-to-year sediment discharge pattern measured at the glacier terminus. The model's ability to match the measured data depends on the tunable sediment grain size parameter, which controls subglacial sediment transport capacity. Smaller grain sizes allow sediment transport to occur in regions of the bed with reduced water flow and channel size, effectively increasing sediment connectivity into the main channels. The model provides the essential components of modeling subglacial sediment discharge on seasonal to decadal timescales and reveals the importance of including spatial heterogeneities in water discharge and sediment transport in both the x and y dimensions in evaluating sediment discharge.

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.

Воздействие отработанных месторождений россыпной платины на речную систему р. Вывенки (Камчатский край)

Geological exploration and mining operations have been conducted in the Vyvenka River basin on the Seinav-Galmoenan platinum deposit since 1994. The majority of the explored and prospective reserves are located in the valleys of spawning salmon tributaries, raising concerns about the environmental impact of mining activities. Despite the cessation of platinum mining in the 2010s, there has been a continuing impact on the river system in recent years, with extensive sediment plumes forming along the length of the rivers. This research aims to assess the dynamics of disturbed lands and determine the scale of sediment export into the river network from the territory of the ore mining sites of Vetvisty, Penisty, Ledyanoy, Yuzhny, and Levtyrinivayam. Field and satellite data were combined to analyze the suspended sediment discharge regime and the main sediment sources, i.e., the riverbank deformations along the mining sites. As of 2021, the total area of disturbed lands in the Vyvenka basin due to open-cast platinum mining is estimated to be 19.2 km2 corresponding to 0.15 % of the area of the Vyvenka River basin. Up to 75 % of the disturbed area consists of bare lands, making them prone to surface erosion. The largest amounts of sediment influx from the mining-affected river basins were observed in 2018 during the unevenly high freshet. The increased sediment discharge was due to the overflow of sedimentation ponds in certain areas. The study results indicate a prolonged anthropogenic impact on river ecosystems, manifested in the change of hydrological regime, activation of channel deformations, and the increased sediment influx to the river network from ore mining sites.

Kalimantan hydroclimate variability since the last glacial period

Changes in convective activity and hydroclimate over Northeastern Kalimantan are key features to understand glacial to interglacial climate evolution in the center of the West Pacific Warm Pool during the Late Pleistocene to Holocene. We use high-resolution X-ray fluorescence (XRF) scanner-derived elemental ratios in sediment Core SO217-18522 (1º 24.106’ N, 119º 4.701’E, 975 m water depth) recovered from the northern Makassar Strait to reconstruct changes in precipitation-related weathering and erosion over Northeastern Kalimantan over the last 50 kyr. Enhanced seasonality of rainfall and an extended dry season during Heinrich Stadials (HS4 to HS1) and the Younger Dryas (YD) suggest weakening of the tropical convection associated with a southward shift of the tropical rain belt and the annual mean position of the Intertropical Tropical Convection Zone. Increasing sediment discharge and intensification of convective activity occurred during the early to mid-Holocene during an interval of high Northern Hemisphere insolation, elevated atmospheric pCO2 levels and global warming. Our reconstructions in comparison with regional terrestrial and marine records highlight the high spatial variability of Kalimantan hydroclimate on millennial to glacial-interglacial timescales.

The supply-generated sequence: A unified sequence-stratigraphic model for closed lacustrine sedimentary basins with evidence from the Green River Formation, Uinta Basin, Utah, U.S.A.

Abstract Closed lakes and oceans are stratigraphically distinct systems. However, closed-lake stratigraphy is often interpreted using conventional sequence stratigraphic concepts which were generated for marine settings. As a consequence, lacustrine stratigraphy has long been vexing and applied on an ad-hoc basis. To remedy this, we present a novel, unified sequence stratigraphic model for hydrologically closed (endorheic) basins: the Supply-Generated Sequence (SGS) Model. This model was generated to interpret our outcrop-based correlation—the largest to date at ∼ 30 km—across the Sunnyside Interval member of the middle Green River Formation in Nine Mile Canyon near Price, Utah, USA. The SGS model is based on the fundamental sedimentological and hydrodynamic differences between closed lakes and marine settings wherein the relationship between water discharge and sediment discharge is highly correlated. The SGS model divides packages of genetic lacustrine strata by bounding correlative surfaces, conformable or unconformable, separating facies and surfaces associated with low clastic supply (e.g., carbonates, mudstones, or exposure surfaces) from facies characteristic of relatively higher amounts of clastic supply (subaerial channelized sandstones, subaqueous siltstones, and pedogenic mudstones). We use the SGS model to correlate regional sequences at a higher resolution than previous interpretations and find the greatest amount of clastic deposition occurs during periods of lake-level rise, indicating that the SGSs are characteristically transgressive. Additionally, this model removes the implicit and explicit base-level assumptions of previous sequence stratigraphic models while being agnostic to the source of increased sediment discharge and therefore generalizable to other closed lacustrine settings. We use the high-resolution supply-generated sequences (meters thick) to argue for a climatic origin of the cyclic Sunnyside interval deposits based on sequence durations (40–50 kyr), and aligning sequences with recognized early Eocene transitory hyperthermal event timing and their associated climatic shifts across the region, increasing riverine discharge of sediment and water.

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.

Wildfire probability models calibrated using past human and lightning ignition patterns can inform mitigation of post-fire hydrologic hazards

Most wildfires are started by humans, however, geographic variation of potential ignition sources is not often explicitly accounted for in wildfire simulation modelling or risk assessments. In this study, we investigated how patterns of human and lightning ignitions can influence modelled fire simulations and demonstrate how these data can be used to assess post-fire flooding and sediment transport. We used historical ignition data (1992–2015) to characterize ignition patterns for thirteen mountain ranges in southern Arizona, United States, and developed FlamMap burn probability (BP) models for three scenarios: human ignition, lightning ignition, and random ignition. We then developed a watershed-scale case study assessing the impacts of ignition scenarios on post-fire hydrology using the KINEROS2 model that simulates runoff and erosion. BP models illustrated considerable differences in landscape fire risk between the three ignition scenarios. Results from the watershed model indicate the greatest impacts from the post-fire human ignition scenario, with a 10-fold increase in sediment discharge and four-fold increase in peak flow compared to pre-fire conditions. Our results show that consideration of ignition source and location is important for assessing fire risk, and our modelling approach provides a planning mechanism to identify locations most at risk to fire-induced flood hazards, where prevention and mitigation activities can be focused.

Discharge Determines Avulsion Regime in Model Experiments With Vegetated and Unvegetated Deltas

AbstractThe dynamics and morphological evolution of deltas and their channel networks involve interactions between many factors, including water and sediment discharge and cohesion from fine sediment and vegetation. These interactions are likely to affect how strongly vegetation influences deltas. Altering water or sediment discharge may affect channel mobility, magnitude of deposition and erosion events, and may result in the delta being less suitable for vegetation colonization. Using the numerical model DeltaRCM Vegetation, we explore how water and sediment discharge affects delta evolution and under which conditions vegetation exerts a stabilizing effect on the channel network. We propose new insights into delta evolution under different discharge conditions. First, we observe a regime shift in avulsion dynamics, driven by delta‐scale water surface slopes, with increasing water discharge: from a few active channels undergoing occasional complete, global avulsions (with low discharge) to many active channels experiencing frequent partial, local avulsions (with high discharge). Second, with vegetation, increased sediment discharge results in more frequent switching of the dominant channels but also prevents vegetation from establishing in non‐dominant channels, resulting in more frequent channel reoccupation and therefore greater stability in channel network planform. These insights have important implications for understanding the distribution of water, sediment, and nutrients on deltas in the face of future changes in climate and human modifications of fluxes of sediment and water to the coast, especially for restored or engineered deltas with controlled water or sediment discharges.

Multi‐decadal changes in the relationships between rainfall characteristics and debris‐flow occurrences in response to gully evolution after the 1990–1995 Mount Unzen eruptions

AbstractExplosive volcanic eruptions can cause long‐term landscape change, leading to increased sediment discharge that continues after the cessation of the eruptions. During the period 1990–1995, eruptions of Mount Unzen, Japan, generated large amounts of pyroclastic material, resulting in 57 debris‐flow events during 1991–2018. To investigate changes in the relationships between rainfall characteristics and debris‐flow occurrence, we conducted the following: geometric analysis of two gullies (i.e., debris‐flow initiation zones) using LiDAR (light detection and ranging)‐generated 1 m DEMs (digital elevation models); rainfall analysis, based on the relationship between rainfall duration and mean intensity (i.e., considering the intensity–duration, or ID, threshold); and debris‐flow monitoring during 2016–2018. Since 1991, rainfall runoff has caused erosion of the supplied pyroclastic material, generating a channel network consisting of incised gullies. With sufficient rainfall, debris flows formed, accompanied by further gully erosion; this resulted in both vertical and lateral adjustments of the cross‐sectional geometry. In the two decades since the eruptions ceased, readily mobilized pyroclastic material has become scarce as the gullies have adjusted to local hydrographic conditions. At the same time, the infiltration capacity of the volcanic flank has increased, reducing the capacity for overland flow. As a result, since 2000, rainfall events with intensities above the ID threshold have occurred; however, the lack of sediment supplied by the gullies appears to have hindered the occurrence and development of debris flows. This suggests that debris flows in volcanically perturbed landscapes may occur at lower rainfall thresholds as long as the corresponding upland channels are evolving as a result of intense overland flow. However, as such channels evolve towards equilibrium geometries, the frequency of debris flows decreases in response to the reduction in sediment availability.

Impact assessment of reservoir desiltation measures for downstream riverbed migration in climate change: A case study in northern Taiwan

Typhoon Aere in 2004 induced severe sedimentation and loss of storage capacity of the Shihmen Reservoir in northern Taiwan. The resulting dramatic increase in the turbidity of the water seriously affected the water supply. To effectively maintain the stability of the water supply and maintain the reservoir’s storage capacity, the government of Taiwan began to plan and construct a series of improvement measures, such as a sediment flushing tunnel, the JhongJhuang Bank-Side Reservoir, and the Amuping Desilting Tunnel. However, previous studies only focused on the impact of the sediment flushing tunnel and the Amuping Desilting Tunnel on the downstream riverbed, and did not consider the possibility of increasing sediment discharge after the completion of the JhongJhuang Bank-Side Reservoir. In addition, climate change will cause the intensity of extreme rainfall to increase enormously in the future. That rainfall and extra sediment flushing will challenge the existing flood prevention facilities. Therefore, this study considered that the JhongJhuang Bank-Side Reservoir will increase sediment discharge of the Shihmen Reservoir, and used dynamical downscaling extreme typhoon data of climate change under the RCP 8.5 scenario to explore the flood prevention and riverbed migration of the main channels of the Dahan and Tamsui Rivers in the future. We used the rainfall–runoff model of Hydrologic Modeling System to simulate rainfall and runoff, and used the hydraulic and sediment transport model of CCHE1D to holistically simulate flood events and consequent river scouring and deposition behaviors. Our results showed that the projected peak discharge during the late 21st century (2075 to 2099) will be at least 50% higher than that during the baseline (1979 to 2003) period. In terms of flood prevention, the potential of overbank flooding will increase in the downstream area, and the trend of long-term change in the riverbed will be dominated by degradation (-0.489 ± 0.743 m) in the future. The improvement measures will have a limited impact on riverbed migration (0.011 ± 0.094 m) in the Dahan and Tamsui Rivers. After the operation of the JhongJhuang Bank-Side Reservoir, the Shihmen Reservoir is expected to increase the sediment discharge ratio by 70% during floods, and it will not cause excessive water turbidity that may affect downstream water supply.

Heavy metal contamination and provenance of sediments recovered at the Grijalva River delta, southern Gulf of Mexico

This study examines textural characteristics, carbonate, organic carbon and chemical composition of sediment samples recovered in the nearshore region (~3–19 m water depth) at the Grijalva River mouth, southern Gulf of Mexico. The aim of this study is to assess the heavy metal contamination and provenance variations in sediments between the dry and northers (rainy) seasons. The proportion of sand, carbonate, and organic carbon contents were abundant at sites located near to the Grijalva River mouth, principally in northers season. Silt content is dominant in the dry season, especially in the sites located away from the Grijalva River mouth. The Chemical Index (CIX) values obtained for the dry (~76–84) and northers (~75–87) seasons indicated a moderate to high intensity of weathering and a humid climate in the source area. The low K2O/Al2O3 ratio values (<0.5) in the dry and northers seasons indicated a higher proportion of aluminosilicates than detrital minerals, due to moderate-to-high intensity of weathering. The correlation among elemental concentrations revealed the association of Cr, Nb, Zr, Hf, Y, and rare earth elements (REE) with detrital minerals rather than aluminosilicates. The major element concentrations, Th/Sc, La/Sc, Co/Th, and Cr/Ni elemental ratios indicated the derivation of sediments from intermediate and mafic igneous rocks. A similarity in the North American Shale Composite (NASC) normalized rare earth element (REE) patterns and Eu anomaly between the dry and northers seasons suggested that the variations in sediment input played a less significant role and the provenance of sediments remained constant. The enrichment factor (EF) showed moderate to severe enrichment for Cr, Ni, and Sb (EF = ~2.63–7.57), highest in the northers season, especially at the Grijalva River mouth, although the index of geo-accumulation (Igeo) for these elements suggested as moderately polluted (Igeo = 0.34–2.0). The slightly elevated EF values for Cr, Ni, and Sb during northers season were probably due to an increase in sediment discharge into the sea during the northers season. We inferred that the enrichment of Cr, Ni, and Sb in sediments was due to both lithogenic and anthropogenic origins. The sediments were influenced by the intermediate and basic igneous rocks of the Chiapas Massif Complex, and the waste generated by the urban areas located within the Grijalva River drainage basin.

Characteristics of sediment dynamics following large-scale sediment supply events in mountain watersheds in Japan

Large-scale sediment supply events (LSEs) cause not only a sudden increase in sediment discharge immediately after their occurrence, but also a long-term condition of elevated sediment yield. In Japan, many LSEs, triggered by heavy rainfall or earthquakes have created high risks for sediment-related disaster. However, the difference between sediment dynamics following a LSE triggered by rainfall and one triggered by an earthquake is unclear, as the sediment dynamics due to rainfall and those due to earthquakes have not been compared using consistent methodology. In this study, we estimated the volume of sediment supply VL (m3), time series of the volume of sediment yield VY (m3year−1) and the non-dimensional parameter NVY as estimated by dividing VL by the catchment area and annual maximum 3-day rainfall amount using the same methodology for 16 events in eight study basins in Japan. We used annual maximum 3-day rainfall amounts as a surrogate for discharge. Using these parameters, we estimated the period over which the effect of a LSE continues (PLS); sediment retention time (residence time, Tr1), and time for active sediment yield to recover to the background level (recovery time, Tr2a). In addition, we estimated the recovery time (Tr2b) based on when the maximum of NVY immediately after the LSE decreased to a certain extent. This calculation allowed us to estimate the recovery time even when background level was unknown. Both residence and recovery times ranged from <1 year to 19 years, with recovery time tending to be shorter than residence time. We found that Tr2b can be used as a substitute for Tr2a. No differences in PLS between rainfall-induced and earthquake-induced events were apparent, although VL differed markedly between event types. This discrepancy may be due to the difference in sediment connectivity between hillslopes and river channels. VY during the first and second year after a LSE associated with an earthquake is likely to be greater than that after a rainfall-associated event.

Increased Subglacial Sediment Discharge in a Warming Climate: Consideration of Ice Dynamics, Glacial Erosion, and Fluvial Sediment Transport

AbstractWe evaluate changes to subglacial sediment discharge during glacier retreat by considering ice dynamics, bedrock erosion, and sediment transport processes. Coupling these components together within a single framework, we simulate sediment discharge from synthetic alpine glaciers experiencing accelerated glacier melt for 100 years. We find that sediment discharge increases by about 8 times the steady glacier values by the end of the simulation. The enhanced sediment discharge persists through peak water discharge and despite annual bedrock erosion volume decreasing by approximately 30% from the initial value. The greater sediment discharge results from increased melt at the glaciers' higher region, where transport was limited prior to glacier retreat. These findings suggest that large increases in sediment discharge may occur as glaciers retreat. The magnitude of the sediment discharge increase primarily depends on the quantity of sediment stored subglacially.

River base level change in mouth channel evolution: The case of the Yellow River delta, China

The base level is one of the key factors controlling river evolution. In the Yellow River mouth, a base-level rise takes place not only because of a relative sea-level rise, but also as a consequence of the increasing of river channel length associated with the large progradation rate of the delta. A change in river length results in a change in the gradient (slope) of the mouth channel, which further leads to a riverbed aggradation/degradation or a water level change at the deltaic apex. This process is principally associated with the morphodynamic backwater effect. The consequent water level change can be considered as a base-level change of the Yellow River. In this work, a relation was constructed between the water level at the deltaic apex and the riverine water and sediment inputs, as well as with the gradient of the mouth channel in the progradation period of each deltaic lobe. Also, for disclosing the importance of the base-level change due to variations of mouth channel length, it was estimated and compared with the relative sea-level change in the past decades, and for predicting the water level at the deltaic apex, the relation between mouth channel length and cumulative riverine sediment input was investigated. It was found that in the period 1953–2016, the base-level changes associated with variations of the mouth channel length could be larger than 1 m, or even larger than 2 m in one year, with a conservatively estimated annual mean of 34.8 mm/a, which is much larger than the relative sea-level change of 7.0 mm/a in the Yellow River mouth. The mouth channel length is closely and linearly related to the cumulative sediment discharge of the Yellow River. The relation for calculating the water level at the deltaic apex was found to be capable of giving a reliable result with the riverine water and sediment inputs, the relative sea-level, and the mouth channel length estimated from cumulative riverine sediment discharge. This relation was also used to estimate the equilibrium gradient of the mouth channel for sediment transport. The results show that after the year 2000, the equilibrium gradient has remained smaller than the actual gradient of the mouth channel, and both gradients decreased continuously with the decrease of riverine sediment inputs, but their values were becoming closer to each other. It is highly possible that the water level at the deltaic apex will be raised by the increase in sediment discharge of the river in the future.

Geospatial analysis and assessment of 226Ra, 235U, 232Th, 137Cs, and 40K at Anzali wetland, north of Iran

Achieving better monitoring and radiation risk assessment is among the main issues in environmental studies. In this regard, natural radioactivity measurements in sediments can provide useful information about the environmental transport mechanism and about the sources of radionuclides. Anzali wetland, as the study area of this research, is located on the southwestern shore of the Caspian Sea with rapidly changing ecosystems. Because of its strategic location, increasing pollution levels, decreasing water table, and increasing sediment discharge from rivers, Anzali wetland has a unique significance in terms of studying its radioactivity from sediment and soil samples. The average 226Ra, 235U, 232Th, 137Cs, and 40K values for 33 sediment and soil samples were 24.66, 3.72, 31.94, 11.66, and 506.38Bqkg-1, respectively. Variography analyses revealed a spatial structure with minimum/maximum variance equal to 3/8 from 135°/45° azimuth direction. In addition, the fractal geometry indicates values higher than 3.7, 24.5, 30, 25, and 475Bqkg-1 as anomalous values for 235U, 226Ra, 232Th, 137Cs, and 40K using Kriging estimated data with a search radius of 5000m, respectively. Compared with the average values published by UNSCEAR (2008) for earth's crust average and compared with the values for the southern areas of Iran (Oman Sea), the estimated average and anomalous data for a pollutant at Anzali wetland do not exceed the reference data. The higher values in the samples of this study could be related to specific environmental problems such as industrial wastewater from about 30 polluted factories transported by 10 major rivers. Agricultural wastes, such as herbicides, pesticides, and fertilizers from rice fields, and being a tourist attraction zone at sea beach are the other most possible pollution sources in the study area.

Regional and Annual Variability in Subglacial Sediment Transport by Water for Two Glaciers in the Swiss Alps

Because the majority of sediment originates subglacially, as opposed to proglacially, focus must be given to subglacial sediment discharge. The latter, however is poorly constrained. We present a subglacial sediment transport time-series from two glacierized catchments (Gornergletscher and Aletschgletscher) in the Swiss Alps, based on hourly suspended sediment transport data and bedload transport estimates. This dataset is used to identify interannual and regional variability and to quantify the relationship between sediment transport and water discharge. Analysis of the relationship suggests that the access of water to subglacial sediment exerts substantial control on the quantity of sediment discharged. Historical data from Gornergletscher since the 1970's shows that elevated amounts of sediment were discharged in the 1980's, following the onset of increasing glacier melt. However, by 2016 and 2017, the sediment discharge returns to quantities below those in the 1970's, suggesting that sediment discharge can stabilize to new hydrological regimes over decadal timescales. Erosion rates for the two catchments (0.28\,mm\,a$^{-1}$ to 0.49 \,mm\,a$^{-1}$) are smaller than in other glacierized catchments of the Swiss Alps ($\sim$\,1\,mm\,a$^{-1}$). In some cases they are even less than a third of those reported in earlier decades, highlighting substantial regional and interannual variability in catchment-scale erosion. Empirical models, calibrated with 2016--2017 data and relating water discharge to suspended sediment concentration, fail to capture the increase in sediment discharge over the 1980's. This suggests that processes secondary to runoff, such as changing access to subglacial sediment by meltwater, were responsible for the increase. Because subglacial sediment discharge depends on both water discharge and sediment availability, we argue that physically-based models are required to capture the evolution of subglacial sediment transport as glaciers retreat.