Tidal Influence Research Articles

Tidal Currents and Their Reshaping Impact on Estuarine Morphology

The Selagan Jaya River Estuary frequently undergoes shape changes, prompting this study to unravel the influence of currents and tides on its morphological evolution. Field measurements encompassing current and tidal data, supplemented by secondary tidal data from TMD and estuary morphology data from USGS, were employed. Findings reveal a mixed semidiurnal tide with a Formzhal value of 0.491 for the Selagan Jaya Estuary. The lowest ebb elevation stands at 0.0117 m, contrasting with the highest flood elevation of 0.6046 m. Flood current velocities range from 0.193 to 1 m/s, while ebb currents span 0.127 to 0.307 m/s. Validation between field and TMD tidal data attains a 98.44% accuracy. Importantly, currents and tides distinctly shape the Selagan Jaya River Estuary; during flood tides, sediment-laden currents ingress, while ebb tides with slower currents hinder sediment egress, underscoring their profound morphological impact.
 Highlight:
 
 Estuarine Morphological Dynamics: Investigates the dynamic nature of the Selagan Jaya River Estuary's shape changes and its underlying processes.
 Tidal and Current Influence: Unravels the intricate interplay between currents and tides, revealing their significant role in shaping the estuarine morphology.
 Formzhal Analysis: Introduces the Formzhal value as a key indicator of the estuary's tidal characteristics and their implications for morphological evolution.
 
 Keyword: Estuarine Morphodynamics, Currents, Tidal Dynamics, Morphological Evolution, Sediment Transport

Macaronichnus ‘co-occurrence’ in offshore transition settings: Discussing the role of tidal versus fluid muds influence

Macaronichnus is a key trace fossil in palaeoceanographic, palaeoclimatic, and petroleum exploration research. Small ichnosubspecies such as Macaronichnus segregatis segregatis, M. s. lineiformins, M. s. maeandriformis and M. s. spiriformis, typically occur in wave-dominated foreshore sands where large M. s. degiberti was never found. The latter shows a wide environmental distribution, occurring in sandy deposits of tidal channels, tidal bar sand sandridges, tidal-flat sand sheets, shorefaces, bioturbated sandy shelf, shelf storm-sheets, shelf sand ridges, and upper slopes. Small M. segregatis and large M. s. degiberti have not been observed to date due the ecological segregation of the tracemakers. An abundant record of large M. s. degiberti in a Tortonian (Late Miocene) mixed carbonate-siliciclastic unit from the Betic Cordillera (southern Spain) has been studied. Occurrence of M. s. degiberti is the result of the interaction of tidal and waves, storm influenced environment determining high-energy conditions and associated palaeoenvironmental parameters as shifting substrates, organic matter availability, and oxygenated pore and bottom-waters. Locally, associated to M. s. degiberti appear small, sinuous traces infilled by light material that were originally assigned to M. s. maeandriformis, and very rare M. s. spiriformis. However, the absence of the typical rim of Macaronichnus avoid a conclusive assignment. The coexistence of both small traces (?M. s. maeandriformis) and large M. s. degiberti is identified in the deposits underlying mudstone layers, revealing the importance of mud deposition during tidal slack water intervals or linked to fluid mud events favouring the co-occurrence of the trace makers of both Macaronichnus ichnosubespecies. This fact would have significant palaeobiological and palaeoecological implications, and could be the first record of both ichnosubspecies in the same intervals.

Evaluating the estuarine tidal discharge through water stage component analysis and in-situ measurement — Case study Mandovi–Zuari estuary of Goa, India

Determining tidal reach and long-term monitoring of tidal discharge are vital in an estuarine system due to their potential use in water resource management and inland navigation. The upstream river discharge contributes to the nonstationary and nonlinear features of estuarine discharge. The tidal reach of an estuary can be identified based on the tidal influence on its water stage components. In the present study, a novel approach based on the Hilbert–Huang transform (HHT) and complete ensemble empirical mode decomposition with adaptive noise (CEEMDN), is used to decompose water stages into several Intrinsic mode functions (IMF). Those IMFs with different amplitudes and periodicities represent those factors that influence the water stages in an estuary. The efficiency of the present method is analyzed with the measured water level data from the estuary of Mandovi–Zuari in Goa, India. Principal ocean lunar semidiurnal constituent M2 is found to be the major tidal constituent, which is consistent in the coastal region. Subsequently, the high-frequency discharge is computed and validated with direct discharge measured by Rio Grande Acoustic Doppler Current Profiler (ADCP). Post-monsoonal discharge is computed using pre-monsoonal calibration coefficients, and a strong correlation is found between post-monsoonal measured and computed discharge. The proposed study will be helpful to estimate reliable estuarine tidal discharge with minimum operational maintenance.

Paleoenvironmental evolution of a Carboniferous marine succession during active volcanism in southern Mexico and its implications in the western Pangea margin configuration

Carboniferous units in southern Mexico consist of mixed carbonate-volcaniclastic facies representing shallow marine environments, from the middle ramp to shallow subtidal environments in the inner ramp. The Tournaisian-Visean Santiago Formation is characterized by the dominance of mixed clastic and bioclastic sediments in a mid-ramp setting without a supply of volcanic material within the basin. Instead, the newly reported locality of the Mississippian-Pennsylvanian Ixtaltepec Formation is a mixed succession of volcaniclastic and carbonated sediments deposited under a tidal influence in the middle to the inner ramp. The age is determined to be late Visean and early Serpukhovian (∼333-325 Ma) based on the presence of Actinoconchus lamellosus, Syringothyris sp., and Punctospirifer sp. and corroborated by geochronological data. This study provides new evidence of Carboniferous volcanism in the sediment-sources area in southern Mexico. Volcanic activity in the area is linked to the subduction of the oceanic crust of the Rheic Ocean, which resulted in the formation of magmatic arcs on the Gondwanan realm and the assembly of Pangea. During the Early Mississippian to early Permian, extensive areas of the northwest of South America and southwest of North America were covered by epicontinental seas. Sedimentation in the area was mainly characterized by shallow clastic-carbonate-dominated environments and deep marine settings, where basins within the peri-Gondwana Terranes received volcanic detritus from the North Gondwana and western Pangea arcs.

Spatial and temporal variations of geochemical processes and toxicity of water, sediments, and suspended solids in Sibuti River Estuary, NW Borneo

A comprehensive geochemical study was conducted in the Sibuti River estuary by considering water, suspended solids (SS), and sediment samples from 36 stations during southwest monsoon (SWM) and northeast monsoon (NEM). In this study, the distribution of in situ parameters, major ions, nutrients, trace metals, and isotopes (δD, δ18O) were analyzed in water samples, whereas sediments and SS were studied for trace metals. The distribution revealed that suspended solids were the major carrier of Cd, Zn, and Mn, whereas sediments worked as a major source of Co, Cr, Ba, Se, Cu, and Pb. Na-Cl water type and ion exchange dominated the lower part of the estuary during both seasons. However, the mixed mechanism of Ca–Cl, Ca–Mg–Cl, and higher weathering indicated reverse ion exchange in the intermediate and upper parts of the estuary. Isotopic signatures of δD and δ18O in estuarine water indicate that the precipitation over the Limbang area dominates during SWM, whereas higher evaporation was confirmed during NEM. The factor analysis revealed that seawater influence in the estuary majority controlled the water chemistry irrespective of seasons. Major ions were mainly regulated by the tidal influence during the low flow time of the river (SWM), whereas the mixing mechanism of weathering and seawater controlled the concentrations during NEM. Nutrients such as NO3, SO42−, NH3, and NH4+ mainly originated from the agricultural fields and nitrification along with ammonification were responsible for the recycling of such nutrients. Trace metals except Cd were found to be geogenic in nature and originating mainly from the oxidation of pyrites present in the sandstone and mudstones of the Sibuti Formation. Redox condition was catalyzed by microorganisms near the river mouth, whereas Al-oxyhydroxides and Fe-oxyhydroxides complexes in the intermediate and upper part under oxygenated conditions controlled the absorption of metals. Overall, the estuary was found to be absorptive in nature due to ideal pH conditions and was confirmed by the saturation index (SI) of minerals.

Simulation of suitable habitats for typical vegetation in the Yellow River Estuary based on complex hydrodynamic processes

Estuarine ecosystems are influenced by the combination of freshwater runoff and ocean tides, and the natural environment is intricate and complex, with special natural environmental conditions and biological habitat characteristics. The Yellow River is characterized by high sediment-bearing water flow, and the estuary forms a unique clear and turbid confluence. The Water and Sand Regulation (WSRS) from the Xiaolangdi Reservoir to the estuary and nearshore has had significant impacts on the environmental conditions of water, sediment and nutrients, further affecting the growth and development of vegetation in the nearshore zone. In recent years, more studies have focused on the effects of climate change and ocean tides on vegetation in the Yellow River Estuary; however, few studies have been conducted on the effects of the WSRS on klysmoeionion. In this study, we investigated the influence of water flow, sediment and nutrients on klysmoeionion based on long-term field monitoring and indoor control experiments; constructed a three-dimensional hydrodynamic water quality model to simulate the hydrodynamic characteristics, sediment transport patterns and nutrient salt distribution in the Yellow River Estuary under the combined influence of tides and riverine water and sediment; and further combined the growth competition model of typical vegetation (including the native species Zostera japonica and invasive species Spartina alterniflora) to construct a suitable habitat simulation model for typical vegetation communities in the Yellow River Estuary. Habitat and biomass changes in Z. japonica and S. alterniflora in the Yellow River Estuary were simulated under the influence of the WSRS and climate change. The results showed that the distribution of suitable habitats for Z. japonica and S. alterniflora in the Yellow River Estuary and offshore was characterized by a zonal distribution along both sides of the estuary, with an increasing trend of suitable habitat area from the estuary to both sides of the estuary; due to environmental conditions and inter- and intra-species competition, the biomass of Z. japonica and S. alterniflora was mainly distributed in the estuary and near the northern and southern shores, with the southern shore have the most distribution. In contrast, S. alterniflora was mainly distributed in the northern shore away from the estuary and in Laizhou Bay. During the WSRS period, the suitable habitat area of Z. japonica decreased by 25%, and the biomass decreased by 50%, while the suitable habitat and biomass of S. alterniflora were not significantly affected. The growth of Z. japonica was severely affected by the dual effects of global warming and the WSRS, while the biomass of S. alterniflora showed an increasing trend. The WSRS can further consider the growth pattern of vegetation to protect the resources of Z. japonica and to curb the expansion of the invasive S. alterniflora.

Influences of Tides, Winds, and Bathymetry on Surface Transport Through a Complex Nearshore System: Measurements from GPS Drifters and an Uncrewed Aerial Vehicle

Bathymetric features such as dredged channels, constricted-inlet basins, and nearshore islands have known influences on coastal transport, which are typically studied on an individual basis. However, their interactive effects on flow when found in the same area are poorly characterized. A multi-season study was conducted to characterize surface transport in and around Wood Island Harbor, in the southwestern Gulf of Maine, where these features all exist in close proximity. Patterns of surface transport were assessed with repeated series of Uncrewed Aerial Vehicle flights and deployments of GPS drifters across tidal conditions and winds typical to the area. Transport followed a major axis through the harbor, as a result of the combined effects of the linked inlet-channel system. Despite the complex topography, the individual dynamics of transport are maintained. Tidal variations explained a majority of displacement (R2 = 0.752) along the channel axis, while across-channel wind velocity explained the majority of orthogonal displacement (R2 = 0.646). Flow in the area was friction-dominated, and both inter-island passages and the deep channel served to direct flow. The offshore transport of tidally driven flows from a constricted-inlet basin was enhanced by deep channels, with flow primarily following bathymetric contours. Our observations suggest that surrounding topography lessened the role of wave energy in governing transport. The co-occurrence of bathymetric features around a constricted-inlet basin can disrupt the typical influences of physical mechanisms on surface transport, leading to a greater importance of local conditions and the larger-scale circulation of the area.

Numerical study on the influence of internal tides on tropical-cyclone-induced sea surface temperature cooling in the South China Sea

Numerical study on the influence of internal tides on tropical-cyclone-induced sea surface temperature cooling in the South China Sea

Short-Period Variation of the Activity of Atmospheric Turbulence in the MLT Region over Langfang

In this paper, we investigate the activity of atmospheric turbulence in the MLT region and the relationship between the activity of atmospheric turbulence and atmospheric wave activity. We use data from the Langfang MF radar (39.4∘N, 116.7∘E) from July 2019 to June 2020 and NRLMSIS 2.0 to calculate the parameters of atmospheric wave activity and atmospheric turbulence energy dissipation rate (ε). Atmospheric ε is modulated by different periods at different altitudes, and while there are 12 h and 24 h periods at all altitudes, the main period is different at different altitudes. A comparison of the ε with atmospheric tide activity shows that tides have an effect on ε, and the influence of tides on ε may be different at different altitudes. The pattern of variation in ε is similar to that of the atmospheric activity of the gravity wave, with both ε and the atmospheric activity of the gravity wave showing significant semi-annual variation.

Tidal impacts on the discharge division and freshwater transport in the Berau Delta, East Kalimantan, Indonesia

Flow division at bifurcations is important in controlling material carried from the terrestrial to the coastal zone in tidally influenced deltas. The present study aims to identify the impacts of tides on flow division and freshwater transport in the Berau Delta, East Kalimantan. The Princeton Ocean Model (POM) is applied to simulate the hydrodynamics with forcing from river discharges and tides. Tides at open seas and observed river discharge at upstream locations were used to set model boundaries. Model validation was accomplished by comparing measurements of tidal elevation and salinity time series with model results. The model results reproduce the observed in temporal variations of tidal elevation and salinity. Model results highlight that the tidal amplitude has large influence on discharge division at the tidal junction. Tides enhance equal subtidal flow distribution in the river junctions, with the influence of tide is 15% on subtidal flow distribution in the bifurcations. Freshwater discharge at the Berau Delta flows mainly to the north and middle outlet with 90% of the total discharge. Based on an analysis model results, the decomposition of freshwater transport reveals that advection and tidal pumping are major factor, which are fluctuate in fortnight cycle.

WATER LEVEL PROFILE IN PARIT BERKAT DRAINAGE CHANNEL OF PUNGGUR BESAR

Floods are common natural catastrophes in Indonesia, particularly during the rainy season. The Parit Berkat water catchment region is one of the locations that frequently experience flooding. The channel's capacity must be increased to accommodate the current water discharge, resulting in this condition. Several reasons are responsible for the diminished capacity of the canal. For instance, inhabitants' activities along canal banks might contribute to the siltation of channels. This research aimed to evaluate the influence of tides and rainfall on the flow of the Parit Berkat Canal. This research provided the use of both primary and secondary data. The leading information consists of measurements of the cross-section and length of the channel and the elevation of the tidal water level over 15 days. Secondary data consists of the maximum daily rainfall data received from Balai Wilayah Sungai Kalimantan I (BWSK I) and the location map for the research. The flow profile is simulated using HEC-RAS. The findings demonstrate that when maximum rainfall and low tides coincide, 13 stations are safe in the 2-year return period of rain, eight stations are safe in the 5-year return period, and six stations are safe in the 10-year return period.

Lunar nodal tidal cycle influences on the input of coarse sediments during the last glaciation in the deep South China Sea

Cross-slope transport of coarse sediments is thought to be dominated by gravity-flow or hyperpycnal-flow processes triggered by hydrodynamic events. However, whether long-term non-event hydrological processes can affect longitudinal transport is still unclear. Here, high-resolution (1 mm) X-ray fluorescence core scanning combined with coarse-fraction observation has been conducted on a laminated sequence (dated as last glacial 14.3–20.8 cal ka BP) of Core MD05-2892 in the South China Sea. The laminae show interbedded millimeter-scale dark and light layers. Dark layers are distinguished by coarse-grained (sand-containing clayey silt) sediments rich in Ca, Zr, and Si, whereas light layers are characterized by fine-grained (silty clay) deposits rich in K, Ti, and Fe. Accordingly, Zr/K and Si/K ratios are used to indicate lamina occurrences, and the ratios are well correlated to grayness. Spectral analyses of the ratios and grayness all present a persistent ∼18.6-yr cycle, which is interpreted as the lunar nodal tidal cycle. This signal in laminae could be related to remobilization of coarse sediments via mean high-water-line variations driven by the lunar nodal tide during the last glacial sea-level lowstand. Coarse sediments were more likely transported from the shelf break via suspended-load-dominated hyperpycnal flow to the slope to form dark layers during periods of strengthened tidal activity, while fine sediments were dispersed via hemipelagic transport to form light layers during periods of weakened tidal activity. Our study highlights for the first time that the lunar nodal tide could periodically trigger the transport of coarse sediments from the shelf to the deep sea.

Seawater intrusion in fractured coastal aquifers influenced by tides: Laboratory and numerical investigations

Fractured aquifers are particularly vulnerable to seawater intrusion (SWI), but the mechanism of SWI in such aquifers remains largely unknown. This study aims to investigate the influence of tides on SWI in fractured aquifers through laboratory experiments and numerical models. Six fractured aquifers and one homogeneous aquifer were employed to conduct laboratory and numerical experiments to preliminary analyze the comprehensive effects of tides and fractures on salinity distributions, flow fields and submarine groundwater discharge (SGD). Sensitivity analyses were performed to further explore the impact of each fracture characteristic (i.e., horizontal position, vertical position, length and orientation) on SWI properties (i.e., saltwater wedge, mixing, saline volume) and concentration distribution under non-tidal and tidal conditions. The results indicate that tides can expand the variation range of most SWI properties in fractured aquifers and alter the influence patterns of fracture characteristics. Fractures tend to converge around freshwater, and their effect on SWI properties depends on their relative position to the saltwater. Toe length (TL) reached its maximum when the fractures had just separated from the saltwater wedge and were fully submerged in freshwater. Longer fractures promoted SWI and relieved the tides’ restriction on TL. Moreover, horizontal fractures can significantly amplify high-concentration (50%-90%) saltwater distribution, SGD and upper saline plume (USP) with the help of tides, while vertical fractures only had an impact on the mixing zone. These results have important implications for improving our understanding of comprehensive impact of tides and fractures on SWI in coastal subsurface environments.

Field survey and analysis of water flux and salinity gradients considering the effects of sea ice coverage and rubber dam: a case study of the Liao River Estuary, China

Predicting net river fluxes is important to promote good water quality, maritime transport, and water exchange in estuaries. However, few studies have observed and evaluated net water fluxes to estuaries under complex conditions. This study used advanced survey techniques to obtain high-frequency monitoring data of cross-sectional current velocity, water level, and salinity in the Liao River Estuary (LRE) from 2017 to 2020. The net water flux into the sea was computed based on field data and the impacts of the rubber dam and sea ice cover on water flux and salinity processes were analyzed in the study region. In the Liao River Station (LRS), the fluctuations of water level and discharge were not obvious in winter due to the sea ice cover. There were significant seasonal and inter-annual changes in water fluxes due to variability in river discharge and tidal oscillations. The results also showed that the net water flux into the sea from the LRS was positive in wet season, and greater during ebb tides than flood tides. The net water fluxes in the normal and dry seasons were mostly negative due to the influence of tides, indicating that the annual runoff from the Liao River fluctuated greatly throughout the year. The water flux in the LRS was more suitable for representing water flux into the sea than the Liujianfang Hydrometric Station (LHS) in the LRE. The impacts of the rubber dam and Panshan Sluice on water fluxes to the sea were both significant. Lower salinity in the study area coincided mostly with height water fluxes to the sea and periods when the rubber dam was raised. This study results provide us new insights to measure the water flux into sea under the condition of ice cover in the tidal reach of estuary and the method can be used for water flux observation for other estuaries.

Influence of Seasons and Tides on the Distribution of Enteric Protozoa on the Shores of the Atlantic Ocean in Kribi (South Region of Cameroon): Health Risks Related to Bathing

Beaches are major sites of microbiological pollution. Assessment of the abundance of resistant forms of enteric protozoa on these recreational waters is important for the prevention and management of health risks. Based on sedimentation and flotation methods, this study found that Kribi beach waters concentrate considerable amounts of enteric protozoa, which are potentially pathogenic. They include Coccidia (Cryptosporidium sp. and Cyclospora cayetanensis), Amoebae (Endolimax nana, Entamoeba histolytica and Entamoeba coli) and Flagellates (Giardia intestinalis). In general, seasonal changes and tidal cycles have significantly impacted the spread of these parasites along Kribi beaches. Thus, at all sites surveyed (Mpalla, Ngoyè and Mboamanga), maximum protozoan abundances were recorded at low tide and during the rainy seasons. It should also be noted that at each sampling site, significant correlations were recorded between certain protozoa and certain physico-chemical variables (p < 0.05). At Mboamanga, for example, Cryptosporidium sp. and Endolimax nana were positively correlated during the Short Rainy Season with temperature (r = 0.601, p = 0.044 and r = 0.632, p = 0.042). At Mpalla, a positive and significant correlation was observed during the Short Rainy Season between Entamoeba coli and pH (r = 0.605, p = 0.033). The high concentration of resistant forms of these enteric protozoa at Kribi beaches is a real public health threat for bathers. Therefore, in this tourist town, it is urgent to put in place an effective plan for the collection and sustainable treatment of solid and liquid waste, which are the main sources of contamination.

Simulated effects of tidal inundation and light reduction on Zostera muelleri flowering in seagrass nurseries

Zostera muelleri is an abundant seagrass species distributed through intertidal and shallow subtidal waters on the subtropical coasts of Australia. The vertical distribution of Zostera is likely defined by tidal influences, particularly desiccation and light reduction stresses. These stresses were expected to affect the flowering of Z. muelleri; however, it is difficult to quantify the effects of tidal inundation with field studies due to multiple confounding environmental factors affecting flowering (e.g., water temperature, herbivory, nutrients). A laboratory aquarium experiment compared the effects of two levels of tidal height (intertidal and subtidal) and light intensity (shaded and unshaded) on flowering timing, abundance, the ratio between flowering shoots and vegetative shoots, the morphology and duration of flower development. The earliest and greatest flowering intensity was recorded in the subtidal-unshaded group, with no flowers observed in the intertidal-shaded group. Notably, the peak flowering time was the same across shaded and unshaded treatments. Shading prolonged the timing of the first flowering and reduced the density of flowering shoots and spathes, while tidal inundation had a more significant effect on the density of flowering shoots and the density of spathes. Results showed that Z. muelleri could flower under low light conditions or tidal stress but not when exposed to both stresses simultaneously in a laboratory ‘nursery setting’. Therefore, applying subtidal-unshaded conditions appears to be beneficial for seagrass nurseries aimed at improved flower abundance despite the plants previously being collected from and adapted to intertidal meadows. Further studies that explore the suitable conditions for triggering and optimising the flowering will be beneficial in designing cost-effective seagrass nurseries.

Floating buoy fatigue analysis for barge-to-ship operations

Transshipment operations around the world are in high demand, notably so in the Amazon. Usually, this type of operation occurs with mooring buoys. In Brazil, transshipment operations are handled with temporary use contracts of a maximum of 4 years. An adaptation of a method for fatigue analysis of this type of structure is proposed. This method uses computational modeling and simulation via the FEM, focusing on the points of greatest cyclical loading of the padeye, to estimate the useful life of this structural component through S–N curves and internationally recognized regulations. Thereunto, the main tensions in the buoy are obtained from the application of calculated forces for traction on a mooring cable at the bitt of the buoy, whose magnitudes satisfy ranges validated by means of instrumentation on board. The limitations of the method are based on environmental conditions corresponding to a river region with no tidal influence, a fact that allows simplification regarding the influence of waves. The results indicate that regular loads acting on the padeye can ensure a useful life as if the component were a medium-term anchored system since the minimum fatigue life for an applied load of 30 tf is 7.66 years.

Riverbank erosion and char stability along the fluvial-to-tidal transition zone in the Lower Meghna River and Tentulia Channel in the Ganges-Brahmaputra-Meghna Delta, Bangladesh

The Ganges-Brahmaputra-Meghna (GBM) delta has been named one of the world's most vulnerable sinking deltas. It is also one of the most dynamic deltas, with the world's highest sediment discharge and third highest water discharge, with semi-diurnal mesoscale (2–4 m) tides. The high fluvial discharge and significant tidal velocities (1–2 m/s) create an expansive fluvial-to-tidal transition zone (FTTZ) along the entire Lower Meghna River (LMR). Within this FTTZ, tidally elongated channel bars, locally called ‘chars’, rapidly evolve due to the high sediment and water discharges. Chars along the LMR have a total population exceeding 5 million people; thus, evaluating the interactions of fluvial and tidal processes along the FTTZ is crucial to understanding vulnerability among char communities with respect to flooding and sea-level rise. Here, we utilize a multi-faceted approach to assess the geomorphology, sedimentology, and hydrology of three chars longitudinally spaced within the LMR and one of its distributaries – the Tentulia Channel. Decadal land change analyses revealed that the FTTZ along the upper LMR and Tentulia Channel has been gaining land at a rate of 4.7 km2/yr, with erosion being most prevalent upstream and most accretion occurring within 50 km of the coast. Almost all the stratigraphic profiles show fine and/or medium sands at depth (0.5–2 m), implying that the three chars are similarly susceptible to erosion by tidal currents and high river discharge. Thus, we attribute the increased stability of chars in the downstream direction to increased tidal influence rather than sedimentology. Land elevation and water level surveys reveal that elevation relative to sea level decreases as the distance to the mouth of the LMR decreases: from 4.05 m (130 km from the mouth), to 2.56 m (100 km from the mouth), and 1.85 m (70 km from the mouth). By incorporating local water level data with the elevation surveys, we conclude that fluvial, namely monsoonal, processes control the geomorphology of the chars near the confluence of the LMR and the head of Tentulia Channel. The shift between fluvial-dominated and tidal-dominated regions of the FTTZ occurs just downstream of Char 3 (∼70 km from the LMR mouth). Therefore, char communities near the LMR confluence are most vulnerable to riverbank erosion and flooding associated with the monsoon season, and chars proximal to the coast are most vulnerable to erosion and flooding related to storm surges and relative sea-level rise.

Microplastics pollution in the rivers of a metropolitan city and its estimated dependency on surrounding developed land

The spatial distribution and abundance of suspected microplastics (SMPs) in the surface water of a metropolitan city, as represented by four Hong Kong rivers, was studied during the dry season. Shing Mun River (SM), Lam Tsuen River (LT), and Tuen Mun River (TM) are located in urbanized areas, and SM and TM are tidal rivers. The fourth river, Silver River (SR) is situated in a rural area. TM had a significantly higher SMP abundance (53.80 ± 20.67 n/L) than the other rivers. The SMP abundance increased from upstream to downstream in non-tidal rivers (LT and SR), but not in tidal rivers (TM and SM), probably due to the tidal influence and a more homogeneous urban development along the tidal rivers. Inter-site differences in the SMP abundance were strongly correlated with the built area ratio (defined as the percentage of surrounding developed land area), human activities, and the nature of the river. About half (48.72 %) of the SMPs were <250 μm. Fibers and fragments were most abundant (>98 %), with most of them being transparent (58.54 %), black (14.68 %), or blue (12.12 %). Polyethylene terephthalate (26.96 %) and polyethylene (20.70 %) were the most common polymers. However, the MP abundance could be overestimated due to the presence of natural fibers. By contrast, an underestimation of the MP abundance could result from a smaller volume of water samples collected, due to a low filtration efficiency caused by high organic content and particle concentrations in the water. A more effective solid waste management strategy and upgrading of the sewage treatment facilities for removing microplastics are recommended to ameliorate the microplastic pollution in local rivers.

Water-level changes and subsidence rates along the Saigon-Dong Nai River Estuary and the East Sea coastline of the Mekong Delta

Southeast Asian deltas are highly threatened areas for flooding as a response to the combined effects of natural compaction and subsidence exacerbated by human impacts, e.g. oil, gas and water extraction, retention of sediment discharge due to river damming and sand mining, land use changes, sea-level rise and storm-induced water-level setup. Tide-induced water-level fluctuations on different time- and spatial scales, seasonal variations of freshwater runoff and sea-level setup can amplify the impact of sea-level rise and of storm surges on the coastal environment and its inhabitants. Moreover, increasing populations accompanied by growing societal demands can lead to further pressure on delta areas. For the Vietnamese Mekong Delta (VMD) and the Saigon-Dong Nai River (SGDNR), rates of subsidence of several cm/yr have already been determined, but for the coastal area, which is most vulnerable to sea-level rise and storm surges, a lack of detailed spatial and temporal information of this parameter exists. To assess the influence of tide- and water-level fluctuations, records from 11 stations, from the tectonically stable position Vung Tau north of the SGDNR entry to Ganh Hao in the southern VMD, were analyzed. The results reveal a relative sea-level rise at Vung Tau reaching 2.2 ± 0.3 mm/yr for the period from 1987-2015, while along the VMD those rates show a spatially variation from 5.6 ± 0.3 mm/yr at Ben Trai to 13.5 ± 0.7 mm/yr at My Thanh. Considering Vung Tau as tectonically stable, these numbers indicate subsidence rates of the coastal sections along the VMD ranging from 3.4 ± 0.3 mm/yr to 11.3 ± 0.7 mm/yr. It is likely that the rates of subsidence along the coast of VMD have accelerated since 2005. They are not only up to 4 times higher than the mean sea-level rise but are variable along the whole SGDNR estuary and the VMD East Sea coastline. Additionally, the importance of daily, fortnightly and seasonal water level fluctuations due to tides and atmospheric influence, which are acting on top of the long-term relative sea-level rise, is shown. Especially the identified regional variations make these data valuable for setting regional priorities for protection strategies to mitigate riverine and marine flooding, especially when both coincide.