Sediment Grain Size Research Articles

Microplastics in Sediment of Kuakata Beach, Bangladesh: Occurrence, Spatial Distribution, and Risk Assessment

Kuakata beach, known as Daughter of Sea in Bangladesh, has drawn a growing number of tourists from all over the world, leading to the higher use of single plastic products. This study was a first attempt to describe the occurrence, spatial distribution, and ecological risk of microplastics (MPs) in Kuakata beach sediments. A total of 24 surface sediment samples were collected from the intertidal zone of the beach, and MPs were extracted using the density separation method and a stereomicroscope. Fourier transform infrared (FTIR) spectroscopy was used for qualitative and quantitative identification. The results revealed that the average MPs in the beach sediment were 232 ± 52 items kg–1 dry weight, which was much higher than many other sandy beaches throughout the world. Analyses of variance showed a significant (p < 0.01) difference among the mean abundance of MPs in sampling points. Fibers were dominated in every sampling point with an average of 123 ± 27 item kg–1. Most of the MPs observed were colored (60%), and the rest were transparent (40%). It was found that the size range of 1–5 mm MPs constituted over half (55%) of total MPs covering an average value of 127 ± 34 items kg–1. Three polymer types were identified in the sediment samples through FTIR analysis which followed the decreasing order of polyethylene terephthalate > polyethylene > polypropylene. Correlation analysis showed a positive relationship between the abundance of MPs and the finer grain size of sediment (p = 0.055; r = 0.7), indicating grain size-controlled the density of MPs. The pollution load index was assessed to estimate the ecological risk and found that the beach sediment of Kuakata belonged to the risk category I of the pollution index. This investigation provided preliminary information on MPs pollution in the marine ecosystem that the policymakers can use to take appropriate management approaches.

Late Pleistocene–Holocene Palaeoenvironmental Evolution of the Makgadikgadi Basin, Central Kalahari, Botswana: New Evidence From Shallow Sediments and Ostracod Fauna

The Makgadikgadi Basin in Botswana hosts a system of salt lakes, which developed from the Upper Pleistocene onward due to the gradual shrinking of the giant Lake Palaeo-Makgadikgadi. Stratigraphic and palaeoclimatic studies of this area are difficult due to the influence of several factors, such as a complex history of regional tectonic activities, as well as climatic changes coupled with dryland diagenetic processes. This lake, in the central Kalahari, is the key to understanding the climatic variability in the southern part of Africa in the Quaternary and has played an important role in the evolution of numerous taxa, including our own. In this study, detailed sedimentological analyses (grain size and major elements distribution) of shallow sediments from the Makgadikgadi Pans were combined with the first comprehensive study of the encountered ostracod fauna to establish trends in the environmental changes in the area from the late Quaternary. Ostracod fossil assemblages from the cores of the Makgadikgadi Pans are dominated by theLimnocytheressp., an opportunistic taxa commonly colonizing the littoral areas of shallow evaporative, ephemeral lakes, together with the subordinate occurrences ofSarcypridopsis ochracea,Sclerocypriscf.bicornis, Candonopsisspp., andIlyocyprisspp. The sediments from the pans show fluctuations in the Cl/K and Ca/Cl ratios, often in phase with the relative abundance ofLimnocytheresuggesting a cyclicity induced by changes of salinity and alkalinity in the water. This multi-proxy study of the cores collected from the pans suggests a Late Pleistocene shallow, playa lake environment with strongly alkaline water, interrupted by a prolonged drought with sustained aeolian conditions between ∼16 and 2 ka BP. Increasing diversity of ostracod fauna in the top 20–30 cm of the cores indicates that a temporary shift toward higher humidity occurred around 2–1.5 ka BP and lasted through the Medieval Warm Period. This humid period was followed by an overall desiccation trend that started with the Little Ice Age and continues until the present day.

Identification of the sources and influencing factors of the spatial variation of heavy metals in surface sediments along the northern Jiangsu coast

Determining the sources and factors influencing the spatial variation of heavy metals in the surface sediments of tidal flats is important for controlling and reducing heavy metal pollution. We used principal component analysis and partial least square regression to analyse the distribution characteristics, sources, and factors influencing the spatial variation of nine heavy metals (As, Co, Cr, Cu, Mn, Ni, Pb, V, and Zn) in the surface sediments of tidal flats along four sites located on the northern Jiangsu coast. Our results indicate that the northern three sampling areas had a high heavy metal content, whereas the southern sampling area had a low heavy metal content. The heavy metals Mn and Pb are related to both natural and anthropogenic sources, whereas V and Cr are primarily derivatives of human activities, and As, Co, Cu, Ni, and Zn are primarily a result of natural processes. The spatial variation of Mn and Pb was primarily affected by the total inorganic carbon and clay content (positive affect) as well as the sand and silt content (negative affect). The spatial variation of V and Cr was primarily related to silt, clay, and total phosphorus content (positive influence) as well as sand (negative influence). In addition, the spatial variation of As, Co, Cu, Ni, and Zn was primarily influenced by the total organic carbon and clay content (positive effect) as well as sand and pH (negative effect). The grain size of sediments plays an important role in the spatial variation of heavy metals in the surface sediments of tidal flats, with fine-grained clay materials being related to the enrichment of heavy metals. The results of our study are important for the control and reduction of heavy metal pollution in coastal sediments, protecting both the ecological environment and human health.

Factors affecting average grain size changes in rivers of a catchment area (Ardak catchment area, northeast Iran)

Investigation of sediment control factors in river channels of a catchment can play a very important role in the sustainable management of that catchment. For this purpose, Ardak catchment was selected, on which a reservoir dam was constructed for drinking and agricultural purposes large city of Mashhad in northeastern Iran. Then 57 samples were collected from the surface to the depth of 30 cm along the four rivers: A (14), B (23), C (13), and D (7). The samples were subsequently weighed and sieved for particle-size distribution. Cumulative curves were plotted followed by the determination of statistical parameters. Thirty-four representative fresh samples were selected for study using macroscopic features. Lithofacies were identified in the field using parametersand their mutual association following the classification scheme of Walker and Cant and Miall. Data on the textural parameters of different rivers showed that rivers A, D, B, and C produce coarse sediments. In terms of average grain size, river A has a greater role in the production of gravel sediments and river B has a greater role in the production of sand and mud sediments. According to the studies, the effect of formation of this catchment (Mozduran 3, Tirgan, and Mozduran 1) on the production of gravel sediments and other formation (Chaman Bid, Sarcheshmeh, and Shurijeh) which include more shale layers is more in the production of sand and mud sediments. Examination of sedimentary facies of alluvial terraces shows that river B is more effective in producing gravelly sediments and river A is more effective in producing sandy and mud sediments. Finally, a comparison of average grain size in sediments formed in the riverbeds of this catchment area and a comparison of this parameter in past sedimentary facies can show the effect of tectonic activity as the most important factor controlling textural parameters in this basin over time.

River flooding response to ENSO-related monsoon precipitation: Evidence from late Holocene core sediments in the Jianghan Plain

In the Jianghan Plain, known as the province of thousands of lakes, monsoon precipitation-induced floods carrying large amounts of sediments can shape the depositional landscape, particularly after frequent river flooding. Therefore, the sediments document abundant information on climate change and its interaction with the fluviolacustrine system and paleoflooding. To date, late Holocene paleoflood records are limited, as continuous paleoflood archives are scarce. Here, using grain size end-members and sensitive components in combination with element records from the JH001 sediment core in the Jianghan Plain, nine continuous paleoflood regimes were identified during 3400–3000, 2750–2550, 2050–1820, 1800–1650, 1550–1500, 1350–1100, 1050–1000, 800–750, and 620–600 cal yr BP, respectively. Exploration of the climatic driving factors on flood regimes indicated a good agreement among paleoflood regimes and El Niño-Southern Oscillation (ENSO)-related hydrological records in the Middle Yangtze Valley (MYV) and reconstructed ENSO records. This suggests that ENSO is the dominant influence on flood occurrence in the Jianghan Plain. Strong ENSO activity exerted a southward shift in the subtropical high in the western Pacific and produced a long-lasting rainbelt in the mid-lower Yangtze Valley, which induced the occurrence of floods in the Jianghan Plain. The evolution pattern of sedimentary environment demonstrated that the floods led to river overflow and overbank expansion, even causing the rivers to divert and bifurcate. The overflow channels carried and deposited silt sediments in the floodplain, which shaped the specific alluvial-lacustrine landscape of the Jianghan Plain. Our results indicate that the grain size of core sediments is sensitive to the occurrence of paleofloods, and that ENSO-related paleofloods significantly influenced the fluviolacustrine sedimentary evolution. These findings are highly useful for regional flood planning and environmental protection in the eastern Jianghan Plain in the context of global warming.

Environmental Assessment Impact of Acid Mine Drainage from Kizel Coal Basin on the Kosva Bay of the Kama Reservoir (Perm Krai, Russia)

The Kosva Bay is permanently affected by acid mine drainage (AMD) from Kizel Coal Basin in the Perm Krai of Russia. This discharge is released in the middle part of the Kosva River from the abandoned mines. This study investigates the current trace element (TE) concentrations for Zn, Cu, Pb, Ni, Cr, Cd, As, and Hg and the mineral composition, major oxides, grain size of sediments, and acute toxicity using two test organisms within the site of AMD downstream from the Kosva River and up to the Kosva Bay of Kama Reservoir. The objectives of this study were to analyze the quality of sediment and level pollution of Kosva Bay using pollution and ecotoxicological indices. The environmental indices, namely the contamination factor (CF), the geoaccumulation index (Igeo), and the potential ecological risk factor (Eri), indicate contamination by Cr and Pb in sediments at the site of AMD, with the highest values for Cr, Cu, and As in the Kosva Bay sediments downstream of abandoned coal mines. The results of Igeo and CF average values in bay of sediments showed different degrees of contamination, from moderate contamination to considerable contamination, respectively. According to the potential ecological risk index (RI) values, the Kosva Bay sediments exhibited low to moderate risk, and As and Cd have the highest contribution rate. According to LAWA and the Polish geochemical classification of sediments, sediments of the bay correspond to the highest levels (IV–III classes) for Cr, Ni, and Hg. Based on the SQGC, Hg, Cd, Cr, and Ni are the most probable for resulting in adverse effects on aquatic organisms in this study. The results of this study indicate that complex pollution and ecotoxicological indices must be supported by ecotoxicologal tests. High precipitation totals, low evaporation rates, and flow regulation stream by the Shirokovsky Reservoir located upstream from abandoned coal mines provide significant fluctuations in streamflow, which is probably the most important factor controlling the distribution and mobility of TE in the studied sediments.

STATISTICAL PARAMETERS ANALYSIS OF SEDIMENT GRAIN SIZE FROM RAYA RIVER BENGKAYANG REGENCY, WEST BORNEO

The study of the statistical analysis parameters of grain size in the waters of Sungai Raya was carried out in order to understand the characteristic of the sediment in the river and coastal areas. The grain size by sieving method, and analyze the statistical parameters including the average grain size, sorting, skewness, and kurtosis. Sediment samples were acquired from six stations with each station consisting of three sampling points representing river west bank, river midstream, river east bank, and the coastal area consisting of five stations. The results of the sediment analyses showed that the average grain size in the river ranges from 1.23 to 2.77 mm, and on the beach area is between 1,52 - 2,59 mm, classified as fine sand, medium sand, very fine sand, and coarse sand with predominant fine sand in all stations. The results of the statistical parameter analysis of the grain size of the bottom sediment in the waters of Sungai Raya exhibit the average diameter value ranging from 1.23 to 2.77 mm, classified as fine sand to medium sand. This value indicates that the type of sediment in this location is dominated by silt with grain sizes ranging from < 0.05 - 0.002 mm. The sorting value ranges from 0.14 to 1.59 mm, categorized into moderately sorted, moderately well-sorted, very well sorted, and poorly sorted classes. The average value of skewness is 2.17 with the classification of the bed load being very fine and kurtosis value ranges from 0.72 to 1.67 indicates the classification in these waters is platycuric, mesokurtic, leptokurtic, and very leptokurtic. These finding concluded that in the study area there is a variation in the angularity of the sediment grains due to the different hydrodynamic processes

The influence of hydrology and sediment grain-size on the spatial distribution of macroinvertebrate communities in two submerged dunes from the Danube Delta (Romania)

The influence of hydrology and sediment grain-size on the spatial distribution of macroinvertebrate communities in two submerged dunes from the Danube Delta (Romania)

Coarsening of sediments from the Huanghe (Yellow River) delta-coast and its environmental implications

We studied temporal-spatial variations in the grain size of sediments from the Huanghe (Yellow River) delta-coast. Four sediment push-cores and more than 200 surface sediment samples were collected. Key factors that control the sediment variability were revealed, as well as their environmental significance. Analytical results indicated that sediments of the Huanghe delta-coast have coarsened over the past decades, and that this coarsening has intensified in recent years. The average mean grain size (Mz) of two cores in the offshore area from the abandoned delta lobe increased by approximately 18%–32% in the 2010s compared to the 1990s. Surface sediments in the nearshore area were coarsened by about 25% on average from 28.6 μm in 1992 to 35.7 μm in 2000. The sediment coarsening of the abandoned delta lobe began in the 1990s due to the extensive erosion caused by the absence of riverine-sediment supply. Coarse-grained sediments that were resuspended and transported offshore also made the sediment of the abandoned delta lobe coarser. In contrast, sediments of two cores in the offshore area of the active delta lobe were coarsened by more than 30% in the 2010s compared to the 1990s. The average Mz of surface sediments in the nearshore area increased by more than 22% from 23.5 μm in 1992 to 28.7 μm in 2000. The sediment coarsening of the active delta lobe began in the 1990s and became more significant in the 2000s. This trend was due to the rapid decrease of the sediment load from the Huanghe, as well as the coarsening of river-delivered sediments (from 17.7 μm to 22.9 μm) caused by the Water-Sediment Regulation Scheme that has been in place since 2002. Increasing coastal hydrodynamics and the loss of riverine sediment also account for the coarsening of sediments of the delta lobe. Our data clearly indicate that the critical bed-shear stress of the Huanghe delta-coast increased significantly. As a result, it was more difficult to initiate motion of the seabed sediments, leading to the decline of resuspension and erosion. Moreover, since the content of sedimentary organic carbon is negatively correlated with sediment grain size, the coarsening sediment of the study area will have inevitably degraded the carbon-related biogeochemical processes of the Huanghe delta-coast. Our findings have significant implications for the evolution of large river deltas, which are now threatened by sediment starvation and erosion.

Study on reservoir physical properties of Nadanhada terrane

The Nadanhada terrane is basically a large tectonic sedimentary hybrid, which is an accretionary complex belt formed by the subduction of the Pacific plate to the Eurasian plate. Through field geological survey, paleomagnetic analysis, paleontological analysis and microscopic observation of core slices, its structural and petrological characteristics were studied. The results show that the accretionary complex is mainly composed of 7 parts, including undeformed sedimentary rocks, shear deformed sedimentary rocks, shear deformed volcanic rocks, turbidites, deformed siliceous rocks, limestone blocks and basic ultrabasic rocks. Shear deformation structures of deformed sedimentary rocks and volcanic rocks are developed. The sediment grain size of the Lower Cretaceous reservoirs in each basin in the northern region is relatively coarse, the sorting is moderately deviated, and the rounding is poor, mainly secondary to angular. The grain size of the lower Cretaceous sediments of each basin in the southern region is relatively fine, with coarse, medium and fine sandstones, but mainly medium fine sandstones, with medium sorting preference and sub angular to sub round roundness.

Barents Sea coastline dynamics in the Holocene in the Kola region: grain-size and LOI analyses of lake sediments

Abstract. The research of Barents Sea coastline dynamics is based on isolated basins method: bottom sediments were sampled in the chain of lakes located in the area of Musta-Tunturi ridge by the Barents Sea coastline and on the isthmus between Sredniy Peninsula and mainland part of the region, the expedition took place in July 2021. This report presents common results of grain-size and LOI (losses on ignition) analyses of 4 lakes bottom sediments. Deepest uncovered sediments are associated with marine genesis and characterized by the biggest average particle diameter and the lowest percentage of organic matter. Layers previously supposed as transgression sediments also have dissimilar grain-size structure from other layers of the core and low percentage of organic matter.

Sources of Coarse Sediment Grain-Size Variability Along the Upper Sandy River, Oregon Revealed by Uav Remote Sensing

Sources of Coarse Sediment Grain-Size Variability Along the Upper Sandy River, Oregon Revealed by Uav Remote Sensing

Effects of bottom trawling on trace metal contamination of sediments along the submarine canyons of the Gulf of Palermo (southwestern Mediterranean)

Submarine canyons are preferential pathways for transport of particulate matter and contaminants from the shelf to the deep sea. The Gulf of Palermo continental margin has a very narrow shelf (about 2–3 km wide on average) and is incised by several submarine canyons that favour shelf-slope sediment transfer. A sediment core collected on the outer shelf and six sediment cores taken at different depths along the Oreto, Eleuterio and Anerella submarine canyons were analysed to study the transfer and historical record of trace metal contamination in the Gulf of Palermo continental margin. Trace metals, major elements, organic carbon and sediment grain size were analysed in these cores, which were dated with 210Pb to assess their historical compositional evolution since the late 19th century. Hg, Pb, Cu, Zn and Cd content increased until the 1970s and 1980s, associated with the increase in urbanization and industrial activities in the Palermo area, and Hg was the contaminant that reached the highest enrichments. However, the increasing trend of these metals contamination was reversed in the 1970s and 1980s, coinciding with drastic changes in the terrigenous content and grain size of sediments in the canyon axes. These changes occurred when bottom trawling fleets expanded to deeper fishing grounds equipped with powerful trawlers around the Gulf of Palermo canyon heads and flanks and along the Oreto canyon axis. Bottom trawlers have resuspended large amounts of sediment, which have been transferred into the canyons since the 1970s and 1980s and have thus increased sediment accumulation rates. This resuspended sediment has been mixing with the sediment transferred and accumulated along the canyons, diluting and reducing its trace metal contamination levels since the expansion of the bottom trawling fleets.

Distribution of organophosphate esters influenced by human activities and fluvial-tidal interactions in the Dong Nai River System, Vietnam

Limited information is known about organophosphate esters (OPEs) in sediments of the Dong Nai River System (DNRS) in Vietnam and the influences of complex hydro-sedimentary dynamics on their fate. In this study, 48 surface sediment samples were collected from the Dong Nai-Soai Rap River and its tributary Vam Co River for the determination of 11 target OPEs, together with grain size and total organic carbon (TOC). The total concentrations of OPEs were in the range of 39.4 ng/g dw–373 ng/g dw (mean: 128 ng/g dw), and tris(1-chloro-2-propyl) phosphate (TCPP) was the predominant one with an average contribution of 81%, followed by tri-n-butyl phosphate (TNBP), tris(2-ethylhexyl) phosphate (TEHP). The composition profiles of OPEs at different locations of the DNRS showed no significant differences (p > 0.05). In addition, the distribution of OPEs had been influenced by both human activities and the fluvial-tidal interactions. The highly frequent and various human activities in Ho Chi Minh City (HCMC) leaded to the highest total concentration of OPEs in the midstream site. Based on our dataset, TOC content and grain size of sediments had significant correlation with certain OPEs (p < 0.05), and sediments with higher TOC content and finer grain size in the DNRS were more likely to be deposited in the downstream reach, contributing to the estuary of the DNRS was identified as another hotspot with the second highest concentration of OPEs. Furthermore, the distribution of OPEs in the transects had distinct characteristics, which reflected the joint influence of the human activities and fluvial-tidal interaction as well. However, the mechanism of their influence needed further investigation.

A Conversion for Average Grain Size Based on Gamma Ray Well Log: Application in the Second Member of Xujiahe Formation of Anyue Area, Sichuan Basin

The grain sizes of clastic rock sediments serve as important depositional indicators that are significant in sedimentology and petroleum geology studies. Generally, gamma ray, spontaneous-potential and resistivity well logs are utilized to qualitatively characterize variations in sediment grain size and determine the lithology in clastic reservoirs. However, grain size analysis of modern sedimentary samples collected from active rivers and deltas indicates that the percentage of fine depositional component has a logarithmic relationship with the average grain size in delta and river systems. Using the linear relationship to process the lithology interpretation, siltstones or mudstone is likely to be interpreted as sandstone. Therefore, a logarithmic conversion formula was built up between the gamma ray logs and measured grain size for the second member of the Xujiahe Formation of Anyue Area in the Sichuan Basin. Using the formula, the average grain size and lithology of the exploration wells were determined for the interest intervals. Furthermore, the calculated grain size gives a better understanding of the controlling factors of hydrocarbon-bearing reservoirs in the study area.

Spatial variability of the physicochemical characteristics of surface sediments in the “Three Rivers and One Dune” Region in the Kuye River Basin, Mu Us Sandy Land, China

Eolian-fluvial interactions are a common geomorphic phenomenon and Earth surface process in arid regions. We selected the “Three Rivers and One Dune” region, in the Kuye River Basin, in the northeastern Mu Us Sandy Land in Northern China, for a study of eolian-fluvial interactions, based on a geomorphological survey and analysis of the grain size and chemical element contents of surface sediments. The results showed that the interactions of wind and water flow enhanced have caused the mixing of the surface sediments, which is reflected by the spatial heterogeneity of their physicochemical properties, which are different from those of the local parent rock and local sandy environments. In detail, the variations of the chemical weathering-related indicators, including the CIA, CIW, CPA, Na/K and Rb/Sr, along four transects all indicated that the degree of chemical weathering was generally low in the region (CIA < 50), implying that the materials are unweathered or in an incipient chemical weathering state. Combined with the grain size results, in the upper reaches (transect KYH-1) the mean grain size was coarser, the degree of sorting was lower, and there was a lower degree of chemical weathering. Transect KYH-4, located in the most southeasterly part of the study area, has the lowest mean grain size and is relatively well sorted; hence, it has undergone a greater degree of chemical weathering. Transects KYH-2, KYH-3 and KYH-4 span the “Three Rivers”, and their sediments are relatively well mixed and heavily influenced by locally-derived materials. The differences in physicochemical properties between the transects can be attributed to the diversity of slope directions and of the angles between the wind direction and the river flow. Overall, our results, combined with regional comparisons, indicate that the sand sources of the dunes in the study area are mainly locally derived.

Distribution of modern dinocysts in surface sediments of southern Brittany (NW France) in relation to environmental parameters: Implications for paleoreconstructions

Dinoflagellate cyst assemblages from 15 modern surface sediment samples of the Bay of Quiberon (Southern Brittany shelf) have been examined to assess their potential as marine bio-indicators for paleoenvironmental reconstructions in a shallow coastal environment. Some discrepancies are noted in the distribution of dinocyst taxa in the study area, and particularly regarding dinocyst concentration and diversity (26 different taxa identified in total) as well as heterotrophic taxa percentages. We suggest that the proportion of heterotrophic taxa is, in an embayment of 15 m deep in average, mainly attributed to bottom water oxygenation and sediment granulometry, both acting on species-selective degradation after dinocyst deposition. More precisely, higher heterotrophic abundances are found under lower oxic conditions and in fine grain-size sediment samples, leading to caution about their use as productivity indicators in coastal environments when these parameters are not fully addressed. The comparison of the Bay of Quiberon data with surface sediment samples and top cores from previously published data makes it possible to establish a transect of the modern dinocyst distribution from inshore to offshore areas in the northern Bay of Biscay, allowing to identify different ecological groups according to the hydrological and bathymetric contexts: i) an estuarine assemblage strongly dominated by Lingulodinium machaerophorum, ii) a proximal coastal assemblage dominated by L. machaerophorum and, to a lesser extent, Spiniferites bentorii, iii) a neritic assemblage dominated by L. machaerophorum, Spiniferites ramosus and cysts of Pentapharsodinium dalei, and iv) an oceanic group dominated by Spiniferites mirabilis and Operculodinium centrocarpum.

Measurement and Analysis of Acoustic Backscatter Value for Bottom Classification of waters Tidung Island

Sediment in a water has an important role for organisms, namely as a habitat, a place for foraging for food, and a place for spawning. These sediment can affect the composition of organisms in the water. The purpose of this study is to calculate the value of acoustic backscatter for the classification of the bottom of the water and to see the effect of sediment grain size on the backscatter value obtained from a single beam acoustic instrument. Data collection was carried out from 10 to 12 June 2021 in the water of Tidung Island, Seribu Islands, using the SIMRAD EK-15 single beam, single frequency 200 kHz instrument. Sediment sampling was carried out at 13 stations. The results showed that the waters of Tidung Island were dominated by muddy substrate which was classified based on the Surface Backscattering Strength (SS) value. Meanwhile, the grain size of the sediment affects the SVb value, where the large the grain size of the bottom sediment, the SVb value will be higher. The higher SVb value the SS value will be higher.&#x0D; Keywords: Bottom Classification, Acoustic Backscatter, Tidung Island

Hydrogeomorphic Recovery and Temporal Changes in Rainfall Thresholds for Debris Flows Following Wildfire

AbstractWildfire‐induced changes to soil and vegetation promote runoff‐generated debris flows in steep watersheds. Postfire debris flows are most commonly observed in steep watersheds during the first wet season following a wildfire, but it is unclear how long the elevated threat of debris flow persists and why debris‐flow potential changes in recovering burned areas. This work quantifies how rainfall intensity‐duration (ID) thresholds for debris‐flow initiation change with time since burning and provides a mechanistic explanation for these changes. We constrained a hydrologic model using field and remotely sensed measurements of soil‐infiltration capacity, vegetation cover, runoff, and debris‐flow activity. We applied this model to estimate rainfall ID thresholds for debris‐flow initiation within three burned areas in the southwestern United States over a postfire recovery period of three to four years. Modeling suggests ID thresholds are lowest immediately following the fire (below a one‐year recurrence interval [RI] storm) and increase with time, such that a 10‐ to 25‐year RI storm would be required to generate a debris flow after three years of recovery. Modeled changes in rainfall ID thresholds result from increases in soil infiltration capacity, canopy interception, hydraulic roughness, and median grain size of sediment entrained in an incipient debris flow. The relative importance of each of these factors varied among our three sites. Results improve our ability to assess temporal changes in postfire debris‐flow potential, highlight how site‐specific factors may alter the persistence of postfire debris‐flow hazards, and provide additional constraints on the timescale of recovery following wildfire.

Operating Effects of the Three Gorges Reservoir on the Riverbed Stability in the Wuhan Reach of the Yangtze River

Reservoir construction may modify the downstream flow and sediment transport, and correspondingly result in adjustments in morphodynamics of a river, especially riverbed instability. Based on hydrological datasets from 2003 to 2019 during the post-dam period using the topographic data of 57 fixed cross-sections in the Wuhan reach of the Yangtze River, we calculated the indexes representing the channel stability. Moreover, considering the effects of flow, sediment concentration, grain size of sediment, and water depth, we propose a method for calculating the equilibrium values of cumulative erosion and the lateral migrate intensity of thalweg in this paper, and the method combines with the delayed response model (DRM) to comprehensively analyze the variations in the longitudinal and lateral stabilities of the riverbed. The results revealed that the channel has been obviously eroded in the downstream reach, resulting in a 76% decrease in sediment discharge after the impoundment of the Three Gorges Reservoir (TGR). Specifically, in the past 17 years, the cumulative erosion in the Wuhan reach of the Yangtze River reached 1.72 m, while the bankfull depth increased by 1.87 m. The lateral migrate intensity of thalweg increased in response to Coriolis force, with an increase of 22.3%. Taken together, the results show that the proposed formula can effectively simulate the variation process of channel stability, and it also quantifies the extent of the influence weight of interannual flow and sediment regimes. The morphodynamics adjustments in the channel stability of our studied reach were closely related to the previous five-year flow and sediment regimes, implying that channel evolution may lag behind the changes in flow and sediment discharges.