Tidal Amplification Research Articles

Observing the influences of climatic and environmental variability over soil salinity changes in the Noakhali Coastal Regions of Bangladesh using geospatial and statistical techniques

Climate change is an intensive threat to the coastal areas of Bangladesh. Low-lying coastal areas suffer from salinity intrusion, especially in the geomorphologically active central coastal areas of Bangladesh (CAB). The intrusion of soil salinity has been particularly evident from studies on the CAB, which exhibits the effects of climate change and salinity changes along the coast. On a local scale, studies like these are essential, but current, when dealing with climate-driven changes considering the field of study, it is rarely observed and recorded in the literature. Therefore, this present study focuses on observing the influence of climate and environmental variability over soil salinity changes trend in the Noakhali coastal regions (NCR) of Bangladesh using geospatial and statistical techniques. To achieve this goal, the present study analyzed the historical climatic data of Bangladesh Meteorological Department (BMD) from 1987 to 2018, which showed that there were irregularities with deviation from normal distribution of temperature and rainfall data in the region. The study also shows maximum temperature raise of about 3°C and 1500 mm annual rainfall in the area. In addition, amplification of tides have been observed in the east and decreases in the west coast. On the other hand, the study suggested a significant reduction of about 22 dS/m in soil salinity data analysis from 1996 to 2019, which was a maximum of 2.27 dS/m in 2019 compared to 25 dS/m in 1996. However, in-depth observations of temperature and rainfall data revealed strong seasonal control over soil salinity levels. Abundance of rainfall is negatively correlated where tidal level is positively correlated with soil salinity level of the study area. There is a positive correlation between the soil salinity level and a minimum level of temperature change. The present study systematically focuses on the change in soil salinity of the NCR on climate and environmental changes, which provides information on historical influence. Finally, the study confirms that rainfall and tides are the main and temperature as the secondary soil salinity controlling factors of the CAB. The results of this study might be used scientifically to attain the sustainable agricultural practices, coastal development and regional planning of the country.

Seasonal Tidal Dynamics in the Qiantang Estuary: The Importance of Morphological Evolution

Despite the increasing number of studies on the river-tide interactions in estuaries, less attention has been paid to the role of seasonal morphological changes on tidal regime. This study analyzes the seasonal interplay of river and tide in the Qiantang Estuary, China, particularly focusing on the influences of the active morphological evolution induced by the seasonal variation of river discharge. The study is based on the high and low water levels at three representative stations along the estuary and daily river discharge through 2015, an intermediate flow year in which a typical river flood occurred, as well as the bathymetric data measured in April, July and November, 2015. The results show strong seasonal variations of the water level in addition to the spring-neap variation. These variations are obviously due to the interaction between river discharge and tide but can only be fully explained by including the effect of morphological changes. Two types of the influences of the variation of the river discharge on the tidal dynamics in the estuary can be distinguished: one is immediately induced by the high flow and the other continues for a much longer period because of the bed erosion and the following bed recovery. Tidal range in the upper reach can be doubled after the flood because of bed erosion and then decrease under normal discharge periods due to sediment accumulation. Over a relatively short term such as a month or a spring-neap tidal cycle, there exist good relationships between the tidal range, tidal amplification in the upper reach and the tidal range at the mouth, and between the hydraulic head over the upper and lower reaches. Such relationships are unclear if all data over the whole year are considered together, mainly because of the active morphological evolution.

Tidal and seasonal controls on the stratigraphic architecture of blind tidal channel deposits in the fluvial-tidal transition of the macrotidal Sittaung River estuary, Myanmar

The morphodynamics and sedimentology of tide-dominated estuaries are controlled by the interaction of tidal currents and river processes. Decoupling tidal and river processes is challenging and rarely documented for the distal part of the estuary, where river influences are cryptic due to the large tidal prism. Dendritic blind tidal channels drain the inshore tidal flats and saltmarshes in the distal part of the fluvial-tidal transition zone (FTZ) of the macrotidal Sittaung River estuary, Myanmar. The present study details the sedimentology and stratigraphic architecture of point bars of the blind tidal channels to evaluate the relative importance of tidal and seasonal river processes in the distal part of the FTZ. The point bars consist of silty tidal rhythmites that display hierarchical tidal cycles manifested in the layer thicknesses. The rhythmites formed during the dry season are slightly muddier and more thinly laminated with accentuated neap-spring tidal cycles, whereas those formed during the wet season exhibit anomalously thick layers with indistinct tidal cycles and abundant subcritical climbing ripples. The wet-season rhythmites are commonly associated with bank-normal, high-relief truncation surfaces presumably formed by heavy rainfall at low tides during river floods. Chevron-like upbuilding of the tidal rhythmites overlies the surfaces in the upper part of tidal point bars, attesting to the rapid removal of accommodation space by sedimentation during the river floods with sustained high sediment loads. A time series of digital elevation models exhibited a rapid infill of the blind tidal channels, suggesting high sediment supply onto the tidal flats throughout the year. Despite being located in the seaward end of the FTZ of the estuary, where tidal amplification occurs, the blind tidal channels evolve in response to seasonal river floods as well as tidal currents, accounting for the dynamic geomorphic changes of the inshore tidal flats and saltmarshes in the Sittaung River estuary.

IDENTIFYING THE EFFECT OF TIDES ON GROUNDWATER LEVEL FLUCTUATIONS ON GILI KETAPANG ISLAND, INDONESIA

This research aims to identify the effect of tides on groundwater level fluctuation in Gili Ketapang Island by using a combination of field monitoring and hydrodynamic modeling. Groundwater data were collected from 5 July to 17 August 2018 from two wells monitoring, while the hydrodynamic model was adopted to identify sea-level conditions. The result explains the sea level around the island is similar among extremely strong correlations between the points. The hydrodynamic model proves a standing wave due to tidal amplification in Madura Strait waters. The effect of tides on the groundwater level characterized by decreasing in amplitudes and time lags as increasing the distance from the coast.

Tidal Oscillation and Resonance in Semi-Closed Estuaries—Empirical Analyses from the Elbe Estuary, North Sea

Many tidal influenced estuaries and coastal basins feature tidal amplification because of, e.g., convergence and reflection. Increasing amplification rates were observed in the Elbe estuary, with consequences for construction measures, nautical manoeuvring, flood protection, riverbed morphology and ecosystems. Although many studies were conducted investigating the tidal wave transformation in estuaries, studies based on spatially well-distributed empirical data covering periods over more than a year are rare. To fill this gap, a self-developed adapted harmonic analysis method of least squares was applied to hydrographs from 25 gauges, distributed over the tidal influenced estuary from the river mouth to the tidal border which is given by the weir 160 km upstream of the river mouth. The investigation period for the harmonic analyses covers a whole nodal cycle of 18.613 a beginning in the year 2000. The tidal constituents’ oscillatory behaviour including the appearance of compound tides, generated by nonlinear shallow water processes, and the formation of reflection induced partially standing waves are determined. The tidal constituents show shared frequency-group specific partial clapotis, but also have significant differences in amplification within those groups. The latter fact contributes to the detected inverse proportionality of tidal range amplification inside the estuary to incoming tidal wave height. As reflection can cause resonance in tidal influenced rivers, tests are developed to analyse whether criteria for resonance are met. To determine the system’s specific resonance frequency, a new method was introduced with the three-parameter Lorentzian curve-fitting. As the detected resonance frequency is not close to tidal frequencies, full-established resonance of the tidal wave and of the tidal constituents is not observed in the Elbe estuary. Migrating nodes of the partially standing tidal wave hint at increasing latent resonance.

Connecting Sediment Retention to Distributary‐Channel Hydrodynamics and Sediment Dynamics in a Tide‐dominated Delta: The Ayeyarwady Delta, Myanmar

AbstractThe largest deltas on Earth are tide‐dominated with multiple distributaries, which can experience different forcings. Yet, most observational studies of sediment dynamics in these systems focus on a single distributary. Comparison of the tidal‐to‐estuarine reaches of three representative distributaries of the Ayeyarwady Delta, Myanmar highlights the variability in sediment retention and export within a megadelta. The Ayeyarwady River has no mainstem dams, providing an opportunity to examine a relatively natural, monsoon‐dominated system. Observations were collected during high flow (September 2017) and low flow (March 2018 and 2019). The eastern Yangon distributary empties into the sheltered Gulf of Mottoma while the central Bogale and western Pathein distributaries are exposed to seasonal waves. During high flow, suspended‐sediment concentration (SSC) was consistent across all three distributaries (0.2–0.4 g/L). The upper delta retained 20%–60% of the mainstem sediment load, while the lower distributaries were tidal rivers exporting sediment. During low flow, the lower distributaries were partially mixed estuaries with little net discharge. However, SSC increased to >1 g/L in the Yangon distributary, remained constant in the Bogale distributary, and decreased in the Pathein distributary compared to high flow. This difference was driven by offshore sediment supply; fine sediment is retained near the mouth of the Yangon and advected away from the Pathein distributary. Consequently, nearshore sediment residence time in monsoon‐influenced megadeltas is controlled by coastal orientation, the timing of discharge with respect to ocean conditions, and tidal amplification. Changes to fluvial sediment delivery can cause non‐uniform coastal impacts due to these differing distributary processes.

IMPACT OF SEA LEVEL RISE ON HYDRODYNAMICS OF ESTUARIES WITH RESTRICTED ENTRANCES

Worldwide, hundreds of millions of people who live on or near estuarine environments are vulnerable to sea level rise (SLR). Using clustering techniques and moving beyond static models and case studies, this study used a large ensemble of idealised estuary models of varying scale, geometry, level of entrance constriction, and SLR scenarios. It was found that tidal forcing, degree of entrance restriction, and estuarine length can primarily control the tidal dynamics of prismatic estuaries under SLR. Further, restricting an entrance can be presented as a potential solution to offset SLR induced tidal amplification if the associated impacts on entrance stability, navigation, and flooding are considered.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/vOptOAbqN3U

Intensifying saline water intrusion and drought in the Mekong Delta: From physical evidence to policy outlooks

This paper assesses the recently intensified saline water intrusion (SI) and drought in the Vietnamese Mekong Delta (VMD). While the existing literature predominantly points the cause of drought to the hydropower dams in the upstream of the Mekong Basin, we contribute new physical evidence of the intensification of saline water intrusion (through backwater effect) in the VMD caused by three anthropogenic drivers: riverbed incision (due to both riverbed mining and dam construction), sea level rise and land subsidence. Thereupon, we highlight that it is critical to not underestimate the impacts from the localized factors, especially the riverbed-mining which can incise the channel by up to 15 cm/year and amplify the salinity intrusion. Our analysis is based on the extensive sets of hourly-to-daily hydrological time series from 11 gauge stations across the VMD. First, several signs of significantly increased tidal amplification (up to 66%) were revealed through the spectral analysis of the hourly water level data. This trend was further validated through the changes in slopes of the rating curves at the tidal zones, implying the relationships between the shift of the backwater effects on the rivers in VMD and the lowered water levels caused by the riverbed incision. Finally, we introduce a novel approach using the annual incision rates of the riverbed to compare four SI driving factors in terms of their relative contributions to the balance between fresh and saline water in the VMD.

Tidal sedimentary dynamics of the Early Pleistocene Messina Strait (Calabria, southern Italy) based on its modern analogue

The Messina Strait excursion opens a series of geological field trips associated with the 10th International Congress of Tidal Sedimentology (Tidalites), Matera, Italy, 5-7 October 2021. This guide aims at documenting a number of selected outcrops located along the eastern margin of the modern Messina Strait, in order to illustrate the sedimentary dynamics of the Early Pleistocene tide-dominated Messina Strait. Since the Pliocene, this extensional basin separated Sicily from Calabria, forming a wide non-tidal seaway. Successively, this basin turned into a ca. 10-15 km-wide and 40 km-long, tide-dominated strait during the Early Pleistocene, prior to its definitive closure following a Middle Pleistocene phase of tectonic uplift. As for today in its modern analogue, the ancient strait acted as a major conduit for marine water exchanges between the Ionian and the Tyrrhenian seas. Semi-diurnal, reverse bidirectional tidal currents flowed in phase opposition parallel to the strait margins, being subject to tidal amplification due to bathymetric restriction across the shallower strait-centre zone. This oceanographic setting partitioned the strait into specific environments. Nowadays, their sedimentary record is exposed in a series of outcrops across the western (Sicily) and eastern (Calabria) margins of the modern strait.A series of stops along a south-to-north transect covers a total distance of ca. 20 km. Outcrops of the first day show coarse-grained deposits lying adjacent to a block-faulted central horst and transgressively overlain by cross-stratified, mixed bioclastic-siliciclastic arenites. These strata record bypass and residual sedimentation in the strait-centre zone of the ancient strait. The second day, large- and medium-scale cross-stratification exhibiting a variety of tidal sedimentary indicators are observed, interpreted as the ancient northern tidal dune field. The third day focuses on one major section representing the north-eastern flank of the ancient strait, where subaqueous canyon-fill strata, mass-wasting deposits and tide-influenced delta front-facies are exposed.

Tidally trapped pulsations in binary stars

ABSTRACT A new class of pulsating binary stars was recently discovered, whose pulsation amplitudes are strongly modulated with orbital phase. Stars in close binaries are tidally distorted, so we examine how a star’s tidally induced asphericity affects its oscillation mode frequencies and eigenfunctions. We explain the pulsation amplitude modulation via tidal mode coupling such that the pulsations are effectively confined to certain regions of the star, e.g. the tidal pole or the tidal equator. In addition to a rigorous mathematical formalism to compute this coupling, we provide a more intuitive semi-analytic description of the process. We discuss three resulting effects: (1) Tidal alignment, i.e. the alignment of oscillation modes about the tidal axis rather than the rotation axis; (2) Tidal trapping, e.g. the confinement of oscillations near the tidal poles or the tidal equator; (3) Tidal amplification, i.e. increased flux perturbations near the tidal poles where acoustic modes can propagate closer to the surface of the star. Together, these phenomena can account for the pulsation amplitude and phase modulation of the recently discovered class of ‘tidally tilted pulsators.’ We compare our theory to the three tidally tilted pulsators HD 74423, CO Cam, and TIC 63328020, finding that tidally trapped modes that are axisymmetric about the tidal axis can largely explain the first two, while a non-axisymmetric tidally aligned mode is present in the latter. Finally, we discuss implications and limitations of the theory, and we make predictions for the many new tidally tilted pulsators likely to be discovered in the near future.

Use of airborne Lidar to investigate mangrove accommodation space in macrotidal estuaries of northern Australia

Extensive mangroves in the Northern Territory of Australia reflect the large tidal range along that coast. Airborne Lidar enables interpretation of ground elevation along transects through mangroves and across coastal and estuarine plains, supplementing past levelling surveys of elevation range of the principal mangrove taxa. Mangrove distribution in the upper intertidal varies geographically in response to tidal variations. In the well-mapped Darwin and Bynoe Harbours, Sonneratia alba grows at elevations below mean sea level, whereas Rhizophora occupies a prominent zone that is inundated by neap tides. Ceriops is generally found at higher elevations, often showing a canopy height gradation. A number of mangroves, including Avicennia, occur at the upper bound of the mangrove forest adjacent to the hinterland, which occurs at ~2.8 m above Australian Height Datum (AHD) on the open coast and at ~3.6 m AHD, reflecting tidal amplification, in the embayments. Along the less known macrotidal estuaries of the Alligator Rivers, accommodation space and species distribution varies with distance from the open coast. On the coastal plain that has formed along the southern margin of van Diemen Gulf, Lidar reveals that landwardmost mangroves also occur up to an elevation of ~2.8 m AHD. Amplification of tides in the cuspate-meandering middle sections of the South and East Alligator Rivers enables mangroves to occur up to 3.5–3.6 m AHD, in contrast to the less infilled West Alligator River. The Lidar reveals substrate topography and vegetation structure. The Rhizophora zone is particularly distinct and its steeper gradient implies rapid sediment accretion; Rhizophora has played a disproportionately large role in the sequestration of blue carbon. Low-lying parts of these estuarine plains, particularly former channels, are already experiencing tidal creek extension and saline intrusion, and topography of the landscape behind the mangroves will mean many areas will be vulnerable to future salinization in response to rising sea level.

Estuarine tidal response to sea level rise: The significance of entrance restriction

Estuarine environments, as dynamic low-lying transition zones between rivers and the open sea, are vulnerable to sea level rise (SLR). To evaluate the potential impacts of SLR on estuarine responses, it is necessary to examine the altered tidal dynamics, including changes in tidal amplification, dampening, reflection (resonance), and deformation. Moving beyond commonly used static approaches, this study uses a large ensemble of idealised estuarine hydrodynamic models to analyse changes in tidal range, tidal prism, phase lag, tidal current velocity, and tidal asymmetry of restricted estuaries of varying size, entrance configuration and tidal forcing as well as three SLR scenarios. For the first time in estuarine SLR studies, data analysis and clustering approaches were employed to determine the key variables governing estuarine hydrodynamics under SLR. The results indicate that the hydrodynamics of restricted estuaries examined in this study are primarily governed by tidal forcing at the entrance and the estuarine length. In addition, SLR increases the average water depth and alters the nodal point location in a seaward direction, thereby significantly affecting tidal wave propagation patterns and reducing the degree of flood domination. In estuaries with restricted entrances, tidal range diminishes drastically in the restricted zone by 20–60%, and the maximum tidal current velocity is higher in the restricted part but lower in the upstream part compared to unrestricted estuaries. For estuary types examined, inundation extent under SLR will likely be greater than the extent expected through simply adjusting existing water levels upwards. The findings also underline that an engineered entrance restriction could regulate tidal wave propagation within an estuary to offset SLR induced tidal range amplification. However, this may pose additional management challenges.

Tidal amplification and along‐strike process variability in a mixed‐energy paralic system prograding onto a low accommodation shelf, Edgeøya, Svalbard

AbstractThe study describes the depositional development and sediment partitioning in a prograding paralic Triassic succession. The deposits are associated with the advance of large prism‐scale clinoforms across a shallower platform area. Approaching the platform, the limited accommodation and associated relative higher rates of deposition generated straighter clinoforms with lower foreset angles. The vertical restriction across the platform is interpreted to have amplified the tidal signature. Sediment was redistributed from the coast into increasingly sandy delta‐front deposits, compared to offshore equivalents. The deposits comprise extensive compound dune fields of amalgamated and increasingly clean sandbodies up‐section. Rapid deposition of significant amounts of sand led to differential subsidence and growth‐faulting in the delta front, with downthrown fault blocks further amplifying the tidal energy through funnelling. A mixed‐energy environment created along‐strike variability along the delta front with sedimentation governing process‐regime. Areas of lower sedimentation were reworked by wave and storm‐action, whereas high sedimentation rates preserved fluvially dominated mouth bars. A major transgression, however, favoured tidally dominated deposits also in these areas, attributed to increasing rugosity of the coastline. Formation of an extensive subaqueous platform between the coast and delta front dampened incoming wave energy, and tidally dominated deposits dominate the near‐shore successions. Meanwhile formation of wave‐built sand‐bars atop the platform attest to continued wave influence. The strong tidal regime led to the development of a heterolithic near‐shore tidally dominated channel system, and sandier fluvial channels up‐river. The highly meandering tidal channels incising the subaqueous platform form kilometre wide successions of inclined heterolithic stratification. The fluvially dominated channels which govern deposition on the delta plain are narrower and slightly less deep, straighter, generally symmetric and filled with cleaner sands. This study provides important insight into tidal amplification and sand redistribution during shallowing on a wide shelf, along with along‐strike process‐regime variability resulting from variations in sediment influx.

Saltmarsh sustainability throughout the Holocene in Boston Harbor: A new sea-level curve for the lower Gulf of Maine and implications of recent anthropogenic alteration

Saltmarsh evolution is closely linked to sea-level rise (SLR) and sediment supply. In a regime of accelerating SLR, saltmarsh survival depends on the ability of the marsh platform to grow vertically through organic and inorganic accumulation, and laterally via transgression over adjacent uplands. In formerly glaciated settings, the formation and maintenance of a saltmarsh are complicated by steep uplands, low sediment availability, isostatic rebound, changes to tidal amplitudes, and anthropogenic alteration. While much work has examined the development of large saltmarshes in New England since the mid-Holocene and Anthropocene, little is known about the smaller isolated saltmarshes that form in pockets along the glaciated coastline. We use radiocarbon dates from newly uncovered index points (basal peat) within Boston Harbor to date one of the oldest saltmarshes in the region (∼4.2 ka), and calculate the polynomial relative sea-level curve (RSLR) for the lower Gulf of Maine taking into account tidal amplification throughout the Holocene. This study informs the historical persistence of isolated saltmarshes under varying rates of sea-level rise (magnitude as much as 2 mm yr−1). Additionally, we refine the timing and rates of the mid-Holocene sea-level deceleration in this region: between 2.8 and 3.3 ka, slowing from 2.1 mm yr−1 to 0.5 mm yr−1. RSLR determined from the past century of tide gauge data is 2.86 mm yr−1, five times greater than historic rates over the past 2000 years. Recent vertical accretion rates (determined from marker horizons and 210Pb data) indicate that portions of Boston Harbor’s saltmarshes are able to keep pace with current RSLR. However, historical diking and ditching practices appear to have resulted in elevation loss and conversion from high saltmarsh platforms to low saltmarsh platforms, largely influencing the amount of inorganic sediment needed to help maintain marsh surfaces. These results highlight that knowledge of historical anthropogenic modifications and changes in hydroperiod are critical when interpreting vertical accretion results. More importantly, anthropogenic modification within sediment starved saltmarshes may be the cause for cannibalization of marsh shorelines, whereby inorganic sediment eroded from marsh edges is recycled and delivered to the surface, facilitating vertical accretion that offsets RSLR. Under modern accelerated sea-level rise and decreased sediment supply, this cannibalization process may now be the only pathway for saltmarsh survival.

Tidal circulation in an Early Permian epicontinental sea: Evidence of an amphidromic system

In epicontinental seas, as well as in oceans, amphidromic systems control tidal movement, acting on coastal dynamics and, therefore, influencing the depositional record. Inland seas were common in Pangea during the Permian. However, estimating the conditions that governed the tide in these seas is highly complex. Although Permian marine deposits are well documented on a regional scale, the tidal behavior in the epicontinental sea has not been analyzed yet. In this work, we present a theoretical perspective on the behavior of tides in the epicontinental sea of the Paraná Basin during the Early Permian (Guatá Sea), covering an area of ~1200,000 km2. Mathematical models were applied to test the existence of amphidromic points in the basin (determined from the Rossby radius of deformation) as well as to verify the possibility of resonance. Since paleodepth is unknown and the basin tectonism is still under discussion, an estimated bathymetry from 20 to 500 m was used to cover even some unlikely scenarios. We also tested, using mathematical models, the amplification of the tide inside two paleovalleys. The obtained results were compared to Hudson Bay, considered here to be a modern analog of the Guatá Sea. According to the paleogeography, paleolatitude (Southern Hemisphere), and depositional records, the Guatá Sea had clockwise-rotation amphidromic systems. However, resonant effects may also have affected circulation, especially at sea depth below 100 m. In the simulated scenarios, the tide amplification in both valleys was variable but concentrated between micro to mesotidal amplitudes. This study presents the first contribution to the understanding of the tidal behavior of the Early Permian epicontinental sea of the Paraná Basin in the basinal context of the Gondwanan paleocontinent. Recognition of amphidromic circulation in an inland sea is important because it may help to explain the general behavior of tidal deposits recorded in the basin, which were built and reworked under the influence of this system.

Future Interactions Between Sea Level Rise, Tides, and Storm Surges in the World's Largest Urban Area

AbstractThe Pearl River Delta contains the world's largest urban area in both size and population. It is a low‐lying flood‐prone coastal environment exposed to sea level rise (SLR) and extreme water levels caused by typhoons. A Finite Volume Community Ocean Model implementation for the South China Sea and the Pearl River Delta is used to understand how future SLR, tides, and typhoon storm surges will interact and affect coastal inundation. The SLR signal and extreme surge levels provide the major contributions to flooding; however, amplification of tides could exceed 0.5 m for 2.1 m SLR and should be considered when planning future coastal defences. On the other hand, if typhoons like Hato or Mangkhut, the latest and strongest ones hitting the area, were to happen in the future, a surge level reduction up to 0.5 m could be expected in coastal areas.

Numerical simulation of mid-Holocene tidal regime and storm-tide inundation in the south Yangtze coastal plain, East China

Coastal flooding is reported as one of the major causes for the interruptions of Neolithic cultures inhabiting the south Yangtze coastal plain, China during the middle Holocene. The archaeologically sterile transgressive sedimentary layers could be the results of extreme events or relative sea-level rise induced by the enlargement of the local tidal range or global/regional sea-level rise. To clarify the mechanism of the transgressive deposits at typical Neolithic sites and the termination of Neolithic Liangzhu Culture in the south Yangtze coastal plain, this study reconstructed the paleo-topography at 7.6, 6.4 and 4.5 cal ka BP on the basis of a database including 107 sediment cores and 644 radiocarbon ages and simulated the paleo-tidal regimes and the inundation areas induced by typhoon events under the paleo-topography and sea levels using a hydrodynamic model of high-resolution Changjiang Estuary Finite-Volume Community Ocean Model (CE-FVCOM). The simulation results of tidal regimes showed that the tidal amplitude increased from 1.6 m to 2.2 m in the head of Hangzhou Bay induced by the paleo-topographic change during the period 7.6–4.5 cal ka BP. By contrast, it reduced from 1.6 m to 1.0–1.2 m in the Ningbo Plain, southeast of the bay, which was unable to explain the long-lasting marine inundation there and supported our previous assumption of an event of abrupt sea-level rise at 4.5–4.4 cal ka BP. Simulation of the storm-induced inundation further suggested that coastal flooding mainly occurred along the coastline of the Taihu Plain under the combined effect of tidal amplification and typhoon events at 4.5 cal ka BP, while the inundation almost covered half of the Taihu Plain if all factors including tidal amplification, extreme event and the abrupt sea-level rise were considered. We therefore suggest that the obvious increase in flood risk under the backdrop of sea-level rise was a major cause for the abandonment of the Liangzhu Culture in the south Yangtze coastal plain at around 4.4 cal ka BP.

Tidal amplification and salt intrusion in the Mekong Delta driven by anthropogenic sediment starvation

Natural resources of the Mekong River are essential to livelihood of tens of millions of people. Previous studies highlighted that upstream hydro-infrastructure developments impact flow regime, sediment and nutrient transport, bed and bank stability, fish productivity, biodiversity and biology of the basin. Here, we show that tidal amplification and saline water intrusion in the Mekong Delta develop with alarming paces. While offshore M2 tidal amplitude increases by 1.2–2 mm yr−1 due to sea level rise, tidal amplitude within the delta is increasing by 2 cm yr−1 and salinity in the channels is increasing by 0.2–0.5 PSU yr−1. We relate these changes to 2–3 m bed level incisions in response to sediment starvation, caused by reduced upstream sediment supply and downstream sand mining, which seems to be four times more than previous estimates. The observed trends cannot be explained by deeper channels due to relative sea level rise; while climate change poses grave natural hazards in the coming decades, anthropogenic forces drive short-term trends that already outstrip climate change effects. Considering the detrimental trends identified, it is imperative that the Mekong basin governments converge to effective transboundary management of the natural resources, before irreversible damage is made to the Mekong and its population.

Surface elevation and sedimentation dynamics in the Ganges-Brahmaputra tidal delta plain, Bangladesh: Evidence for mangrove adaptation to human-induced tidal amplification

In the Ganges-Brahmaputra (G-B) delta, periodic flooding of the land surface during the tidal cycle coupled with enormous sediment delivery during the monsoon promotes sediment accretion and surface elevation gain through time. However, over the past several decades, widespread embankment (“polder”) construction in the G-B tidal delta plain has led to numerous environmental disturbances, including channel siltation and tide range amplification. While previous research indicates that rates of sediment accretion are relatively high in the G-B tidal delta plain, it remains unclear if and how surface elevation is maintaining pace with relative sea-level rise (RSLR) in this region. In this study, we utilize an array of surface elevation tables, sediment traps, and groundwater piezometers to provide longitudinal trends of sedimentation and elevation dynamics with respect to local platform elevation and associated hydroperiod. Two hydro-geomorphic settings of the Sundarbans mangrove forest are compared: higher elevation stream-bank and lower elevation interior. Seasonal measurements over a time span of 5 years reveal that elevation gain is occurring in all settings, with the highest rates observed at elevated stream-bank zones. Elevation gain occurs primarily in response to sediment accretion, with possible minor contributions from pore-water storage and swelling of clay minerals during the monsoon season (i.e., belowground biomass and organic contribution is minimal). As a result, elevation loss and shallow subsidence in the G-B delta is unlikely to be caused by compaction of organic-rich soils, but rather appears to be controlled by seasonal lowering of the groundwater table and compaction of clay minerals. Rates of surface elevation gain in the Sundarbans greatly exceed rates of RSLR and more closely follow rates of RSLR augmented from tide range amplification, indicating that this landscape is adapting to human-induced environmental change. The proceedings of this study underscore the adaptability of the natural G-B tidal delta plain to local environmental disturbances, with the caveat that these defenses may be lost to future upstream reductions in sediment supply.

Sea-level-rise-induced threats depend on the size of tide-influenced estuaries worldwide

The effects of sea-level rise on the future morphological functioning of estuaries are largely unknown because tidal amplitudes will change due to combined deepening of the estuary mouth and shifting amphidromic points at sea. Fluvial sediment supply is also globally decreasing, which hampers infilling necessary to maintain elevation relative to sea level. Here we model 36 estuaries worldwide with varying sizes, shapes and hydrodynamic characteristics, and find that small shallow estuaries and large deep estuaries respond in opposite ways to sea-level rise. Large estuaries are threatened by sediment starvation and therefore loss of intertidal area, particularly if tidal amplitude decreases at the mouth. In contrast, small estuaries face enhanced flood risks and are more sensitive to tidal amplification on sea-level-rise-induced deepening. Estuary widening can partly mitigate adverse effects. In large estuaries, expanded intertidal areas increase tidal prism and available erodible sediment for adaptation, whereas it slightly reduces tidal amplification in small estuaries. Small shallow estuaries face enhanced flood risk under climate change because of sea-level-rise-induced tidal amplification. In contrast, large deep estuaries are threatened by sediment starvation and therefore a loss of intertidal area. Both cases can potentially be mitigated by estuary widening.