Sediment Erosion Rates Research Articles

Design and Performance study of Francis turbine for high head applications.

Abstract Francis Turbines are widely employed globally for their adaptability and operational versatility across a broad range of water flow rates and pressure heads. This study explores the design and optimization of a Francis turbine configured for exceptionally high heads, specifically targeting a design head of 700 m. Traditionally, Pelton turbines are favoured for such high heads, but this study investigates the viability of Francis turbines in this range. Through comprehensive testing, including the development of hill chart diagrams to evaluate performance under various conditions, the study demonstrates promising efficiency levels. Utilizing a simplified approach, the design process involves empirical relations and the Khoj program, resulting in turbine designs representing three different speed numbers: 0.32, 0.42, and 0.52. Efficiency and sediment erosion rates were computed for each and compared between the three designs. The comparison indicates that while the turbine with a speed number of 0.32 exhibits lower efficiency, it offers superior erosion resistivity. Conversely, turbines with speed numbers 0.42 or 0.52 show better efficiency, especially in handling fluctuations in operating conditions. The findings suggest a balance between efficiency and erosion resistivity, recommending the turbine designed for a speed number of 0.32 for superior erosion resistance, and the turbine with a speed number of 0.42 for greater efficiency and operational range. This study broadens the horizon for Francis turbines in high-head applications, offering insights into their potential and viability in extreme conditions.

Research on Sediment Erosion and Anti-Wear Coating Materials for Water-Intake Components of Hydraulic Turbines in Sandy Rivers

The operational efficiency, stability, and lifespan of hydroelectric power plants operating on sediment-laden rivers are affected by sediment erosion. A numerical simulation of the sand–water flow in the water-intake components of a turbine at a specific power station was conducted using the Euler–Lagrange method. Additionally, sediment erosion tests were carried out on the water-intake components coated with epoxy mortar material. The results indicate that sediment erosion on the stay vane surface mainly occurs on the front face, with the most severe erosion at the head, while sediment erosion on the stay ring surface primarily occurs near the stay vane head. The extent of erosion is mainly influenced by the distribution characteristics of sediment particles. The wear of epoxy mortar coating material is minimally affected by the spraying thickness. Adding 30% hardener to the epoxy mortar material can significantly improve the erosion resistance of the stay vane surface by about 30%. The erosion rate on the frontside of the stay vane is approximately 2.6 times that of the backside. Based on the sediment erosion tests and numerical simulation results of the sand–water flow, an estimation formula for the sediment erosion rate of the epoxy mortar erosion-resistant coating was established. This formula can be used to predict the anti-sediment erosion performance of epoxy mortar materials applied to the water-intake components of this turbine and similar river turbines.

Decadal-scale assessment of sediment denudation rates in the Zhoukou River Basin, Taiwan: insights from improved DEMs of differencing based on spectral analysis

This study assesses sediment erosion rates in the Zhoukou River Basin in southern Taiwan over the past three decades, focusing on the impact of extreme rainfall events. While various established methods determine erosion rates over different temporal scales, we employ an independent approach for decadal-scale erosion rate calculation by utilizing open-source global and regional digital elevation models (DEMs) data for two distinct periods. We introduce a new method that applies Fourier analysis to vertically register the DEMs, significantly minimizing their vertical bias. Through spectral analysis, we identify long wavelength topography crucial for correcting vertical offsets. Erosion rates, computed through DEMs of Difference (DoD), exhibit a similar trend of significant reduction in sediment export rates from 1990–2010 to 2011–2020 due to decreased extreme rainfall events, aligning with erosion rate estimates derived from mean suspended load data at gauge stations. Over the entire period from 1990 to 2020, the calculated denudation rate was 14.19 mm/yr, whereas in the recent decade (2011–2020), it decreased to 10.46 mm/yr. Our study suggests that the improved DoD method can effectively estimate sediment transport rates by leveraging underutilized DEMs captured at distinct points in time, especially when the erosional signal dominates data noise.

Riparian trees in mercury contaminated riverbanks: An important resource for sustainable remediation management

Mining operations generate sediment erosion rates above those of natural landscapes, causing persistent contamination of floodplains. Riparian vegetation in mine-impacted river catchments plays a key role in the storage/remobilization of metal contaminants. Mercury (Hg) pollution from mining is a global environmental challenge. This study provides an integrative assessment of Hg storage in riparian trees and soils along the Paglia River (Italy) which drains the abandoned Monte Amiata Hg mining district, the 3rd former Hg producer worldwide, to characterize their role as potential secondary Hg source to the atmosphere in case of wildfire or upon anthropic utilization as biomass. In riparian trees and nearby soils Hg ranged between 0.7 and 59.9 μg/kg and 2.2 and 52.8 mg/kg respectively. In trees Hg concentrations were below 100 μg/kg, a recommended Hg limit for the quality of solid biofuels. Commercially, Hg contents in trees have little impact on the value of the locally harvested biomass and pose no risk to human health, although higher values (195–738 μg/kg) were occasionally found. In case of wildfire, up to 1.4*10−3 kg Hg/ha could be released from trees and 27 kg Hg/ha from soil in the area, resulting in an environmentally significant Hg pollution source. Data constrained the contribution of riparian trees to the biogeochemical cycling of Hg highlighting their role in management and restoration plans of river catchments affected by not-remediable Hg contamination. In polluted river catchments worldwide riparian trees represent potential sustainable resources for the mitigation of dispersion of Hg in the ecosystem, considering i) their Hg storage capacity, ii) their potential to be used for local energy production (e.g. wood-chips) through the cultivation and harvesting of biomasses and, iii) their role in limiting soil erosion from riparian polluted riverbanks, probably representing the best pragmatic choice to minimize the transport of toxic elements to the sea.

Study on erosion characteristics of turbine in sediment-laden river

Abstract Sediment erosion of hydro turbines is prevalent and serious in the mountain river region. The effect of operating head and particle diameter on sediment erosion was investigated in this paper. A Eulerian-Lagrangian approach was applied to numerically simulate the solid-liquid flow in a Francis turbine under the minimum, design and maximum heads. Five typical diameters (0.1, 0.3, 0.5, 0.7, 0.9 mm) were firstly determined based on field measurements at a hydropower station. After tracking the trajectories of sediment particles, McLaury model was selected to predict the erosion rate. The results show that sediment erosion regions of runner blades are similar under different operating heads, while the sediment erosion rates amplify significantly with the increase of operating heads. The erosion rate of blade suction side is significantly higher than that of pressure side because of the effect of inter-blade vortices between runner blades. Sediment diameters also play an important role in accelerating erosion rate, which indicate that setting up sedimentation tanks to settle large-size sediment particles is a powerful method to alleviate sediment erosion. This study can provide a reference for erosion estimation and operational maintenance of hydro turbines in mountain river region.

Benthos as a key driver of morphological change in coastal regions

Abstract. Benthos has long been recognized as an important factor influencing local sediment stability, deposition, and erosion rates. However, its role in long-term (annual to decadal scale) and large-scale coastal morphological change remains largely speculative. This study aims to derive a quantitative understanding of the importance of benthos in the morphological development of a tidal embayment (Jade Bay) as representative of tidal coastal regions. To achieve this, we first applied a machine-learning-aided species abundance model to derive a complete map of benthos (functional groups, abundance, and biomass) in the study area, based on abundance and biomass measurements. The derived data were used to parameterize the benthos effect on sediment stability, erosion rates and deposition rates, erosion and hydrodynamics in a 3-dimensional hydro-eco-morphodynamic model, which was then applied to Jade Bay to hindcast the morphological and sediment change for 2000–2009. Simulation results indicate significantly improved performance with the benthos effect included. Simulations including benthos show consistency with measurements regarding morphological and sediment changes, while abiotic drivers (tides, storm surges) alone result in a reversed pattern in terms of erosion and deposition contrary to measurement. Based on comparisons among scenarios with various combinations of abiotic and biotic factors, we further investigated the level of complexity of the hydro-eco-morphodynamic models that is needed to capture long-term and large-scale coastal morphological development. The accuracy in the parameterization data was crucial for increasing model complexity. When the parameterization uncertainties were high, the increased model complexity decreased the model performance.

Formation of a scour funnel upstream of an orifice affected by a bottom-mounted cylinder

A scour funnel before a bottom orifice on a dam is able to help intaking water with low sediment concentration and thus protect the water turbine. Appropriate placement of pier-like structures in front of a bottom orifice may increase the size of a scour funnel. This work experimentally investigates the formation and development of a scour funnel upstream of a bottom orifice affected by a bottom-mounted cylinder. It is found that the cylinder blocks the flow so as to reduce the flow rate by as much as 8.27%. Nevertheless, the sizes of equilibrium scour funnel generally increase due to the use of a cylinder; for instance, the equilibrium scour depth may increase by as much as 36.1%. The enlargement of scour funnel is closely related to the motion of large-scale turbulent vortices caused by the cylinder. We found that the turbulent kinetic energy around the cylinder is significantly increased due to the presence of a cylinder with a maximum relative increase in 192.5%. The placement of a cylinder leads to a drastical increment of sediment erosion rate. For cases with a cylinder, it may take only 1.1% of the time (for the first time) to reach the same level of equilibrium scour depth in a same case without a cylinder. Therefore, by appropriately placing a cylinder before a bottom orifice, it is possible to achieve a larger-sized scour funnel in a very shorter time with smaller abandoned water. A formula is established to calculate the maximum equilibrium scour depth.

Assessing the Erosional Properties of Muddy Sediments: Asan Bay Case Study

The study's outcomes reveal that both the critical shear stress needed to initiate erosion and the sediment erosion rate are influenced by various parameters, including the bulk density of sediment and bed shear stress. This comprehensive analysis underscores the vital role that these parameters play in determining sediment erosion characteristics. The presented research contributes to the understanding of erosional behaviors of muddy sediments and their implications. The findings emphasize the importance of comprehending such properties, especially in designing effective erosion control structures and managing water bodies prone to erosion.

Importance of Bed Liquefaction‐Induced Erosion During the Winter Wind Storm in the Yellow River Delta, China

AbstractData obtained during a 5‐month field observation of the Yellow River Delta show that the storm wave‐related oscillatory flows are mainly in an intermittently turbulent regime (400 < ReΔ < 1,200). Wave‐induced seabed liquefaction causes changes to the bed erodibility and increases the sediment entrainment rate, resulting in the formation of fluid mud layer (FML) during winter wind events. The strong NE winter winds lead to most of the wave‐induced liquefaction events in the study area. It was found the sediment erosion rate is underestimated when we solely focus on the wave‐induced bottom shear stress. To parameterize wave‐induced excess pore pressure buildup and sediment liquefaction, a series of experiments were conducted in a large wave flume and a new liquefaction‐erosion method is proposed to calculate the sediment erosion flux. Analysis of model results shows that the FML thickness is about 2–12 cm at the storm‐arrival stage during the entire observation period. The thickness of FML significantly increases with the increase of wave height and decrease of water depth. Once the FML is formed, strong currents can remove fluid mud 10 hr after the storm‐arrival stage, while the background currents on calm days can hardly cause suspension. The erosion rate in liquefaction zones can increase 5–10 times compared with the erosion rate in nonliquefaction conditions. The whole Bohai Sea is highly turbid after intense NE winter wind, and the liquefaction zones during winter winds would be the major sediment source. Enhanced sediment erosion can cause the subsequent degradation of the subaqueous Yellow River Delta.

Assessing gully erosion and rehabilitation using multi temporal LiDAR DEMs: Case study from the Great Barrier Reef catchments, Australia

Millions of dollars are being spent on gully rehabilitation to help reduce excess fine sediment delivery to the Great Barrier Reef (GBR). There is an urgent need for (i) prioritisation of active gullies for rehabilitation and (ii) the development of methodologies to inform the effectiveness of remediation. In this study we analyse DEMs of Difference derived from 0.5 m resolution 2–3 year interval multi-temporal LiDAR data collected pre and post rehabilitation at three variable gully morphologies in the Burdekin catchment. Our analysis indicates that the highest annual average fine sediment erosion rates for the untreated control gullies occur at the linear gully (53.38 t ha−1 y−1) followed by linear-alluvial gully (34.24 t ha−1 y−1) and least at the alluvial gully (14.41 t ha−1 y−1). The proportional loss or export of fine sediment from the gullies in their un-treated condition ranges from ∼68 to 90% of what is eroded, and when the gullies are treated the proportion of fine sediment that is retained in the gully proportional to what is eroded increases to ∼60% at all sites. Without pre-treatment baseline erosion rates, and additional post treatment LiDAR captures, it is difficult to quantify the treatment effectiveness. Our results offer insights in the erosion mechanisms within different geomorphic gully morphologies and rehabilitation effects in these erosional landforms. This study provides crucial knowledge of gully dynamics that can be coupled with other lines of evidence for better prioritisation of rehabilitation in the GBR catchments.

Comparative analysis of discharge and sediment flux from two contiguous glacierized basins of Central Himalaya, India

In the present study, suspended sediment load (SSL), sediment yield and erosion rates in Pindari Glacier basin (PGB) and Kafni Glacier basin (KGB) have been estimated using daily discharge and suspended sediment concentration (SSC) data for three ablation seasons (2017-2019). For this, one meteorological observatory and two gauging sites have been established at Dwali (confluence point), and water samples have been collected twice in a day for high flow period (July to September) and daily for lean period (May, June and October). An area-velocity method and stage-discharge relationship has been established to convert water level into discharge (m3 s-1). For estimating SSC (mg/l), collected water samples have been filtered, dried, analysed and confirmed with an automatic suspended solid indicator. Further, SSL, sediment yield and erosion rates have been computed using SSC data. The results reveal that mean annual discharge in PGB (35.06 m3s-1) has been found approximately 1.7 times higher than KGB (20.47 m3s-1). The average SSC and SSL in PGB have been observed about 396.07mg/l and 1928.34 tonnes, and in KGB, it is about 359.67mg/l and 1040.26 tonnes, respectively. The SSC and SSL have followed the pattern of discharge. A significant correlation of SSC and SSL has been found with discharge in both the glacierized basins (p < 0.01). Interestingly, average annual sediment yield in PGB (3196.53 t/km2/yr) and KGB (3087.23 t/km2/yr) have been found almost identical. Likewise, the erosion rates in PGB and KGB have been witnessed about 1.18 and 1.14mm/yr, respectively. Sediment yield and erosion rates in PGB and KGB have been found in correspondence with other basins of Central Himalaya. These findings will be beneficial for engineers and water resource managers in the management of water resources and hydropower projects in high-altitude areas and in the planning and designing of water structures (dams, reservoirs etc.) in downstream areas.

Impacts of hydrodynamic conditions and microscale surface roughness on the critical shear stress to develop and thickness of early-stage Pseudomonas putida biofilms.

Biofilms can increase pathogenic contamination of drinking water, cause biofilm-related diseases, alter the sediment erosion rate, and degrade contaminants in wastewater. Compared with mature biofilms, biofilms in the early-stage have been shown to be more susceptible to antimicrobials and easier to remove. Mechanistic understanding of physical factors controlling early-stage biofilm growth is critical to predict and control biofilm development, yet such understanding is currently incomplete. Here, we reveal the impacts of hydrodynamic conditions and microscale surface roughness on the development of early-stage Pseudomonas putida biofilm through a combination of microfluidic experiments, numerical simulations, and fluid mechanics theories. We demonstrate that early-stage biofilm growth is suppressed under high flow conditions and that the local velocity for early-stage P. putida biofilms (growth time < 14 h) to develop is about 50 μm/s, which is similar to P. putida's swimming speed. We further illustrate that microscale surface roughness promotes the growth of early-stage biofilms by increasing the area of the low-flow region. Furthermore, we show that the critical average shear stress, above which early-stage biofilms cease to form, is 0.9 Pa for rough surfaces, three times as large as the value for flat or smooth surfaces (0.3 Pa). The important control of flow conditions and microscale surface roughness on early-stage biofilm development, characterized in this study, will facilitate future predictions and managements of early-stage P. putida biofilm development on the surfaces of drinking water pipelines, bioreactors, and sediments in aquatic environments.

Petrology of Bengal Fan turbidites (IODP Expeditions 353 and 354): provenance versus diagenetic control

ABSTRACT High-resolution petrographic and heavy-mineral analyses of Bengal Fan turbidites from six cores drilled during IODP Expeditions 353 and 354 elucidate factors controlling their intersample compositional variability as a key to understanding sedimentary processes and erosional evolution of the Himalayan belt since the Miocene. Bengal Fan turbidites are feldspatho-quartzose to litho-feldspatho-quartzose with plagioclase &amp;gt; K-feldspar; slow-settling micas increase in abundance in very fine sand and coarse silt. The feldspar/quartz ratio and higher-rank metamorphic rock fragments notably increase from uppermost Miocene to Pleistocene deposits, which is ascribed to the onset of rapid exhumation of the Eastern Himalayan syntaxis since ∼ 5 Ma. The same trends are documented in Nicobar Fan turbidites, confirming that they belong to the same sedimentary system. Both Bengal and Nicobar fans record a pulse in mass accumulation rate at Tortonian times, when supply of sedimentary and very-low-grade metasedimentary detritus reflected accelerated exhumation of the Lesser Himalaya. In contrast to foreland-basin sediments, where ferromagnesian minerals have been completely dissolved in strata as young as Pliocene–Pleistocene, in both Bengal–Nicobar and Indus fans amphibole invariably represents about half of the moderately rich to rich transparent-heavy-mineral suite, demonstrating that amphibolite-facies Greater Himalaya metamorphic rocks were widely exposed in the Himalayan range well before the late Miocene and possibly since the late Oligocene, as indicated by a few sillimanite and kyanite grains in Bengal Fan sediments as old as 23 Ma and 28 Ma, respectively. Diagenetic dissolution strongly affected olivine and pyroxene in strata older than the middle and early Pleistocene, respectively, whereas amphibole decreases markedly through progressively older Miocene strata. Ferromagnesian minerals and sillimanite are almost completely dissolved in lower Miocene strata, where durable zircon, tourmaline, rutile, and apatite make up half of the strongly depleted heavy-mineral assemblage. Quaternary turbidites from the six studied cores have virtually the same compositional signatures, testifying to efficient homogenization by turbidite transport and reworking across the fan. Turbidites in western cores closer to peninsular India (U1444A and U1454B) are not different from those in eastern cores, indicating very minor supply from the subcontinent. Forward-mixing calculations based on integrated petrographic and heavy-mineral data indicate that sand supply from the Brahmaputra River to Quaternary turbidites was four times larger than supply from the Ganga River, indicating up to six times higher sediment yields and erosion rates in the Brahmaputra than in the Ganga catchment, largely reflecting superfast erosion of the Eastern Himalayan syntaxis.

Capturing of organic carbon and nitrogen in eelgrass sediments of southern Scandinavia

AbstractThe ability of seagrass meadows to filter nutrients and capture and store CO2 and nutrients in the form of organic carbon (OC) and nitrogen (N) in their sediments may help to mitigate local eutrophication as well as climate change via meadow restoration and protection. This study assesses OC and N sediment stocks (top 50 cm) and sequestration rates within Danish eelgrass meadows. At four locations, eelgrass‐vegetated and nearby unvegetated plots were studied in protected and exposed areas. The average OC and N sediment 50 cm stocks were 2.6 ± 0.3 kg OC m−2 and 0.23 ± 0.01 kg N m−2, including vegetated and unvegetated plots. In general, OC and N stocks did not differ significantly between eelgrass meadows and unvegetated sediments. Lack of accumulation of excess 210Pb suggested sediment erosion or low rates of sediment accumulation at most sites. OC accumulation rates ranged from 6 to 134 g m−2 yr−1 and N from 0.7 to 14 g m−2 yr−1. Generalized additive models showed that ≥ 80% of the variation in sediment OC and N stocks was explained by sediment grain size, organic matter source, and hydrodynamic exposure. Long cores, dated with 210Pb, showed declining OC and N densities toward present time, suggesting long‐term declines in eelgrass OC and N pools. Estimates of potential nation‐wide OC and N accumulation in eelgrass sediments show that they could annually capture up to 0.7% ± 0.5% of CO2 emissions and 6.9% ± 5.2% of the total terrestrial N load.

Changes in sediment erosion rate in the Azuma River basin after the Hokkaido eastern Iburi earthquake in 2018

The Hokkaido Eastern Iburi earthquake in 2018 induced the largest landslide over the Azuma River basin in Hokkaido, the north island of Japan. This study estimated the sediment erosion rate over the Azuma River basin using the universal soil loss equation (USLE) for the pre- and post-earthquake conditions. The results showed that the potential sediment erosion rate from the entire basin increased by 8–29 times more than the pre-earthquake condition. The primary cause of the increase in sediment erosion rate was deforestation due to the landslide, which generated 13% of the bare land area in the entire basin. In addition, the influence of interannual variability of rainfall on sediment erosion rate was calculated from observed rainfall data. As a result, the minimum annual sediment erosion rate after the earthquake exceeded the maximum before the earthquake, indicating that the post-earthquake sediment erosion rate is substantially higher than the pre-earthquake rate. These findings suggest that in order to assess the sediment erosion risk over the Azuma River basin, it is necessary to predict and monitor the vegetation recovery, considering the interannual variability of rainfall.

Contrasting strategies to cope with storm‐induced erosion events: a flume study comparing a native vs. introduced bivalve

AbstractStorm‐induced erosion events may alter the diversity of tidal flat communities by selecting species that can better tolerate such disturbances. Introduced and invasive species are highly adaptable to a wide range of abiotic characteristics, and this adaptability may make them better able to withstand erosion events. With a novel flume method, we compared the ability of two bivalve species to resist storm‐induced erosion: Cerastoderma edule, a native species to the Scheldt estuary in the Netherlands, and Ruditapes philippinarum, an introduced species that is successful in the Netherlands and worldwide. We used three sediment erosion rates to simulate storms of increasing severity. At the 10.6 and 15.9 cm h−1 sediment erosion rates, all R. philippinarum were surfaced, whereas only half C. edule were surfaced. However, after being brought to the sediment surface, C. edule were more readily transported by currents and waves than R. philippinarum due to differences in their shell shape. We concluded that the two bivalve species had different strategies to avoid mortality by severe storm erosion: C. edule avoided being surfaced and R. philippinarum avoided being transported. In this case, it appears that extreme storms favor the specific adaptations of a native species over the broad adaptability of a non‐indigenous one. Indeed, C. edule may be more likely to survive moderately extreme storms than R. philippinarum, though the most extreme storms would be equally devastating to both species.

Optimization of Francis Turbine Runner for Variable Speed Operations with minimization of sediment erosion

Sediment erosion is one of the most notorious phenomenon damaging the turbines, in the Himalayan region. Francis turbine, which is one of the most used type of hydro turbine in the world as well as in Nepal, is affected quite severely, more so when operated in off design conditions. This study has taken a reference design of a Francis turbine from a hydropower in Nepal to optimize it for operation in variable speeds. Minimization of sediment erosion and maximization of efficiency are taken as the objective functions of the optimization, for which blade angles at trailing edge and blade angle distribution are taken as the design variables. The design space of the runner are constrained such that the optimized design could replace the existing runner in the turbine. Latin Hypercube Sampling technique is used to populate the design space such that the design variables are divided randomly to create required number of designs in the design space. Computational Fluid Dynamic analysis are performed on simplified numerical models of the samples to predict their performance and Sediment Erosion Rate Density (SERD), under various operating conditions. The results of output parameters, obtained from CFD, along with the design variables, are used to develop an approximation model relating the objective functions with the design parameters. NSGA-II optimization technique is used to search for the optimum design. The paper presents the comparison of the sediment erosion and efficiency of the reference runner and optimized runners under various operating conditions. It also presents an outline of the process used to optimize the runner for variable speed operation with minimum sediment erosion.

Spatial and Temporal Variability of Bed Exchange Characteristics of Fine Sediments From the Weser Estuary

Sedimentation of fine-grained sediments in estuaries is a natural physical phenomenon influenced by biogeochemical processes. In the estuarine turbidity maximum (ETM), enhanced net deposition of sediments is observed even in areas with higher hydrodynamic exposure, such as the navigational channel. Maintenance dredging is a common method to maintain the navigational channel, which requires large financial effort and has potential negative impacts on the environment. Research at the Institute for River and Coastal Engineering addresses the challenge of understanding the processes leading to net sedimentation and accumulation in estuarine navigational channels in reach of the ETM. In this contribution, investigations of bed exchange properties of estuarine cohesive sediments conducted in field and laboratory studies are presented. The results provide rarely available and estuary-specific parameters characterizing sediment transport, mainly related to erosion processes. By performing field campaigns within the ETM of the Weser estuary, cores of freshly deposited sediments have been sampled from two sites (Blexer Bogen and Nordenham) along the center of the navigational channel. Sediment characteristics (grain size distribution, water content, loss on ignition, density profiles) have been derived, and the erodibility of the deposits is investigated both quasi in situ and in the laboratory using an erosion microcosm system. Erodibility experiments are run in a closed system so sediment concentration above the lutocline increases during the experiment. This is a unique feature of this study, and it is expected to produce more natural characteristics of net erosion. By proving the reproducibility of the natural structure of the deposited sediments (stratification and density profiles) in the laboratory, systematic studies for analyzing the sensitivity of determined parameters (shear stresses and erosion rates) to varying environmental conditions (settling conditions and density) could be performed. Temporal development of suspended sediment concentration and erosion rates is the main result of the erodibility experiments, from which we derive bandwidths for erosion parameters, like floc erosion rate, critical shear for floc erosion, and critical shear for mass erosion.

Entrainment of bed sediment composed of very fine material

AbstractIn an estuary bed composed of very fine sediment material, tidal currents cause active sediment transportation, bar morphology changes, and severe bank erosion. However, it is difficult to evaluate the erosion rate of a sediment bed composed of particles measuring five to several tens of micrometres using methodologies found in the available literature that estimate entrainment in the presence of fine sediment suspension. This paper proposes a new method to evaluate the erosion rate of a sediment bed composed of very fine material by introducing the concept of entrainment developed for density stratified flows. A series of flume experiments were conducted to investigate the erosion rates of bed material with water–sediment mixtures of different concentrations, sediment particle sizes, and overall Richardson numbers. The results of the flume experiments are compared with past theories and experimental results conducted for density stratified flows. The data show that the erosion rate of a water–sediment mixture is proportional to the inverse of the overall Richardson number, as proposed in previous studies on density stratified flows. In cases with a strong cohesion, the erosion rate can be evaluated using a modified Richardson number, introduced to evaluate the influence of viscosity on entrainment rates. The applicability of the proposed method is also tested by comparing the results of the present study with those of previous studies in terms of equilibrium suspended sediment concentration and size distributions of suspended sediment. The present results show that the proposed method can be used to evaluate not only the erosion rate of suspended sediment but also the characteristics of equilibrium transport processes.

The Segmented Zambezi Sedimentary System from Source to Sink: 2. Geochemistry, Clay Minerals, and Detrital Geochronology

Elemental geochemistry, Nd isotopes, clay minerals, and U-Pb zircon ages integrated by petrographic and heavy-mineral data offer a multiproxy panorama of mud and sand composition across the Zambezi sediment-routing system. Detrital zircon geochronology highlights the four major episodes of crustal growth in southern Africa: Irumide ages predominate over Pan-African, Eburnean, and Neoarchean ages. Smectite, dominant in mud generated from Karoo basalts or in the equatorial/winter-dry climate of the Mozambican lowlands, prevails over illite and kaolinite. Elemental geochemistry reflects quartz addition by recycling (Uppermost Zambezi), supply from Karoo basalts (Upper Zambezi), and first-cycle provenance from Precambrian basements (Lower Zambezi). Mildly negative for sediments derived from mafic granulites, gabbros, and basalts, εNd values are most negative for sand derived from cratonic gneisses. Intrasample variability among cohesive mud, very coarse silt, and sand is principally caused by the concentration of Nd-rich monazite in the fine tail of the size distribution. The settling-equivalence effect also explains deviations from the theoretical relationship between εNd and TNd,DM model ages, suggesting that monazite carries a more negative εNd signal than less dense and less durable heavy minerals. Elemental geochemistry and Nd isotopes reveal that the Mazowe-Luenha river system contributes most of the sediment reaching the Zambezi delta today, with minor supply from the Shire River. Sediment yields and erosion rates are much lower on the low-relief Kalahari Plateau than in rugged Precambrian terranes. On the plateau, mineralogical and geochemical indices testify to extensive breakdown of feldspars and garnet unjustified by the present dry climate. Detrital kaolinite is recycled by incision of Cretaceous–Cenozoic paleosols even in the wetter lower catchment, where inefficient hydrolysis is testified to by abundant fresh feldspars and undepleted Ca and Na. Mud geochemistry and surficial corrosion of ferromagnesian minerals indicate that, at present, weathering increases only slightly downstream the Zambezi River.