Neap Tidal Cycle Research Articles

Response of Sediment Dynamics to Tropical Cyclones under Various Scenarios in the Jiangsu Coast

The Jiangsu Coast (JC), China, is an area susceptible to the impact of tropical cyclones (TCs). However, due to the lack of available on-site observation data, nearshore sedimentary dynamic processes under the impact of TCs have not been fully explored. This study developed a 3D wave–current–sediment numerical model for the JC based on the Finite Volume Community Ocean Model (FVCOM) to investigate sediment dynamic responses to TCs under various scenarios, including different tracks, intensities of TCs and tidal conditions. The validation results demonstrated the model’s satisfactory performance. According to the simulation results, typhoons can significantly impact the hydrodynamics and sediment dynamics. During Typhoon Lekima in 2019, strong southeasterly winds substantially increased the current velocity, bottom stress, wave height, and suspended sediment concentration (SSC). Three typical landfall-type typhoons, with prevailing southeasterly winds, brought significant sediment flux from southeast to northwest along the coast, while the typhoon that moved northward in the Yellow Sea induced a relatively small sediment flux from north to south. Typhoons could also induce stripe-like erosion and deposition, which is closely related to seafloor topography, resulting in seabed thickness variations of up to ±0.3 m. Additionally, strengthening typhoon wind fields can lead to increased sediment flux and seabed morphological changes. Typhoon Winnie, particularly at spring tide, had a greater impact on sediment dynamics compared to other landfall typhoons. Numerical simulations showed that the typhoon-induced net sediment flux within the spring tidal cycle could increase by 80% to 100% compared to the neap tidal cycle, indicating the significant influence of tidal conditions on sediment transport during TC events.

Tidal, density and wind-driven subtidal circulation in a hypersaline coastal lagoon

Subtidal circulation patterns were studied along an elongated hypersaline system. Month-long data were collected using three acoustic Doppler current profilers. These profilers were moored at depths of around 15 m in three locations: 10 km inland from the mouth of the lagoon, within the central region, and near the head of the lagoon. The variability of temporal and spatial subtidal circulation was explored using Empirical Orthogonal Functions analysis, which explained 93% of the variability through the first three modes. The first mode was influenced by tidal advection, resulting in a tidally-driven residual circulation for elongated channels, with inflows occurring over the shoals and outflows in the deeper channel sections. During the neap-tide cycle, inverse density gradients became relevant, coinciding with the peaks of mode 2 amplitude that accounted for 15% of the total variability. The gravitational circulation exhibited its highest intensity near the head, characterized by a vertically sheared flow with inflow occurring near the surface and outflow underneath. Predominant onshore wind stresses had a significant impact on the subtidal circulation and displayed a strong correlation with the mode 3 amplitude, explaining 11% of the total variability. Wind-driven circulation exhibited a vertically sheared profile, characterized by downwind near-surface flow and upwind flow underneath in the lagoon mouth and head, and a homogeneous downwind profile in the central part of the lagoon. It is concluded that tidal stresses governed the subtidal circulation, followed by density gradients, and wind stresses modified spatial circulation patterns induced by tides and density gradients. This research provides new observational evidence on tidal residuals in long channels and contributes to the understanding of the interactions between the main subtidal circulation drivers in hypersaline lagoons.

Assessment of tidal current potential in the Pará River Estuary (Amazon Region – Brazil)

World energy consumption will grow by almost 50 % between 2018 and 2050, resulting in a 79 % increase in electricity generation over this period. In general, renewable energy has been an important source of electricity generation for several years due to its ability to produce unlimited energy. Among the renewable energies are the oceanic ones, which depend on the resources of the tidal waters and the oceans. Tides are a promising energy source, given their periodicity and predictability, using techniques of harmonic analysis or ocean modeling. The Amazon is a region with multiple sparsely distributed populations, with difficult access to energy. Renewable sources are an alternative to bring energy and development to these communities. This paper focuses on the assessment of the power of tidal currents in the Pará River Estuary (PRE, Brazilian Amazon) as an alternative source of electricity generation in the region. This estuary is characterized by particularities such as not having its own source, it has an extension of 300 km and a depth between 0 and 70 m. The methodology was based on the two-dimensional hydrodynamic model (2DH) of SisBahia. The results show the occurrence of five areas with potential for tidal energy exploitation. Power densities for these areas are in the range of 0.4–0.7 MWh/m2 in a spring and neap tidal cycle. This article demonstrates the relevance of choosing parameterized quantities that do not depend on specific equipment, as well as the importance of an adequate characterization of the hydrodynamic patterns in the PRE, necessary to optimize the use of hydrokinetic energy.

Tidal Dynamics And Physicochemical Parameters In A Tropical Estuary: Qua-Iboe River Estuary, Southeast Coast Of Nigeria

Estuaries are important ecosystems that support biodiversity and influence the economy of the regions they occupy. Major physicochemical variables of Qua-Iboe River estuary in the Niger Delta region of Nigeria were assessed for ecosystem quality. The influence of tidal current on the spatial distribution of physicochemical parameters in the estuary was examined. Physicochemical parameters, including transparency, temperature, salinity, dissolved oxygen, density and pH were measured in-situ during the dry season. Tidal current velocities were monitored over spring and neap tidal cycles using the Langragian method and results indicated ebb dominance. The relationships between physicochemical parameters were statistically analyzed using Pearson’s correlation matrix. Based on Pearson’s correlation, density showed significant correlations (p < 0.05) with temperature, pH and salinity. Surface and bottom water temperatures significantly correlated (p < 0.05) with density, salinity, dissolved oxygen and pH. Spatial distribution maps of physicochemical parameters were plotted using ArcGIS Pro. Similarities and dissimilarities of surface and bottom water parameters were interpreted using cluster analysis. Results revealed two groups reflecting distinct physicochemical characteristics in the study area. Group one, representing upper and central portions of the estuary is defined by waters which are limnetic to oligohaline, slightly acidic to neutral with low dissolved oxygen (<4mg/l) conditions. The estuary showed limnetic conditions at the upper reaches due to the strong fluvial input by the Qua-Iboe River. Group two, representing the lower estuary, is characterized by mesohaline to normal marine waters; neutral to slightly alkaline pH, and low dissolved oxygen (< 4mg/l) conditions. This study is relevant for future monitoring and assessment of the estuary and similar tidal ecosystems.
 

Hydrodynamic Modelling: Estuary Dynamic Implication to Morphological Changes

Estuaries are transition zones between the sea and inland rivers, where oceanic tidal waves and fluvial flows control hydrodynamic processes. However, these natural changes and implications are not thoroughly understood, particularly in tropical estuaries. The interactions between tidal and riverine flows in the Kuala Pahang Estuary were examined using numerical modelling techniques applying TELEMAC2D. Our results model demonstrate that prolonged heavy rainfall significantly impacted water levels in the estuary system during the neap tide cycle. Moreover, the results show that the tidal range and current speed significantly influenced estuary morphological changes. The flood-ebb current can affect sediment transport near the river bank, leading to erosion. Meanwhile, the ebb-flood current significantly affected the bed sediment and suspension in the estuary mouth area while deposition happens during the slack water, either flood or ebb. Our results have a major impact on estuary morphology and sediment transport in the estuary water column, which will lead to a reduction in estuary water quality and the local economy, particularly fisheries.

Longitudinal and lateral circulation and tidal impacts in salt-plug estuaries

This study presents a comparison of forcings between density gradient and tides in idealized funnel-shaped salt-plug estuaries. Three-dimensional numerical model results also examine longitudinal and lateral circulations. In salt-plug estuaries, a positive longitudinal estuarine circulation is observed landward of a salinity maximum zone or salt plug. Seaward of the salt plug, the estuary shows an inverse circulation cell. The longitudinal flows show a fortnightly variability inside the salt plug. Also, the salt plug is saltier in spring tides than in neap tidal cycles mainly owing to higher landward salt transport by tidal advection during the spring tides. The lateral circulation and divergence dv/dy have the influence of Coriolis acceleration. In the absence of Earth’s rotation, the tidally averaged lateral circulations have nearly the same directions within the positive and inverse cells. Directions of lateral flow reverse in the salt-plug area. Inside this area, the lateral circulation also shows clear neap–spring variability, including downwelling cells during spring tides and upwelling in neap tides. The salinity maximum zone exhibits a vertically homogeneous condition particularly in meso-tidal salt-plug estuaries. Finally, this study introduces the threshold of “log(tidal Froude number)=3” (tidal forcing 3 orders of magnitude ≥ density gradients) for salt-plug estuaries as the condition under which the tidal forcing can overcome the density gradient, and consequently salinity inside the salt plug zone is reinforced by tides with a landward movement. This robust salinity maximum zone is also identified by a high Ekman number (log(Ekman number)>0.25).

Synoptic observations of sediment transport and exchange mechanisms in the turbid Ems Estuary: the EDoM campaign

Abstract. An extensive field campaign, the Ems-Dollard Measurements (EDoM), was executed in the Ems Estuary, bordering the Netherlands and Germany, aimed at better understanding the mechanisms that drive the exchange of water and sediments between a relatively exposed outer estuary and a hyper-turbid tidal river. More specifically, the reasons for the large up-estuary sediment accumulation rates and the role of the tidal river on the turbidity in the outer estuary were insufficiently understood. The campaign was designed to unravel the hydrodynamic and sedimentary exchange mechanisms, comprising two hydrographic surveys during contrasting environmental conditions using eight concurrently operating ships and 10 moorings measuring for at least one spring–neap tidal cycle. All survey locations were equipped with sensors measuring flow velocity, salinity, and turbidity (and with stationary ship surveys taking water samples), while some of the survey ships also measured turbulence and sediment settling properties. These observations have provided important new insights into horizontal sediment fluxes and density-driven exchange flows, both laterally and longitudinally. An integral analysis of these observations suggests that large-scale residual transport is surprisingly similar during periods of high and low discharge, with higher river discharge resulting in both higher seaward-directed fluxes near the surface and landward-directed fluxes near the bed. Sediment exchange seems to be strongly influenced by a previously undocumented lateral circulation cell driving residual transport. Vertical density-driven flows in the outer estuary are influenced by variations in river discharge, with a near-bed landward flow being most pronounced in the days following a period with elevated river discharge. The study site is more turbid during winter conditions, when the estuarine turbidity maximum (ETM) is pushed seaward by river flow, resulting in a more pronounced impact of suspended sediments on hydrodynamics. All data collected during the EDoM campaign, but also standard monitoring data (waves, water levels, discharge, turbidity, and salinity) collected by Dutch and German authorities are made publicly available at 4TU Centre for Research Data (https://doi.org/10.4121/c.6056564.v3; van Maren et al., 2022).

Saltwater Intrusion‐Induced Flow Reversal in the Changjiang Estuary

AbstractSaltwater intrusion is a common feature in the Changjiang Estuary affected by river discharges and tidal flows. It leads to a two‐layer flow structure during the flood‐to‐ebb tidal transient period: the seaward tidal flow in the upper water column and onshore intruded salt flow in the lower column, even though the lower column water usually experiences an ebb flow eventually. Our recent measurements with a tripod deployed in a tidal channel of the North Branch in the East China Sea challenged this feature. We detected that the two‐layer flow structure disappeared in the ebb tide period during the neap cycle due to intense saltwater intrusion. A constant onshore flood‐like flow predominated the entire water column. The physical mechanism for the flow reversal was examined using the Finite Volume Community Ocean Model (FVCOM). The FVCOM was robust to capture the observed flow reversal in the tidal channel during the neap tidal cycle. The momentum balance analysis results suggest that the flow reversal occurred when the saltwater intrusion‐induced onshore baroclinic pressure gradient force and baroclinic tidal rectification overwhelmed the seaward barotropic pressure gradient force. A parameter‐driven criterion was derived theoretically to determine the potential occurrence of a stable ebb flow reversal in the tidal channel.

Dredging of the Shenzhen-Zhongshan Link in the Pearl River Estuary, China: Short-Term Siltation Mechanism for a Trial Trench

Han, Z.; Li, H.; Xie, H.; Yang, H.; Ouyang, Q., and Gong, S., 2022. Dredging of the Shenzhen-Zhongshan Link in the Pearl River Estuary, China: Short-term siltation mechanism for a trial trench. Journal of Coastal Research, 38(5), 988–998. Coconut Creek (Florida), ISSN 0749-0208. Understanding the short-term, particularly daily, siltation mechanism of a dredged trench is critical for immersed tunnel construction, as well as for offshore engineering and coastal management. This study explored the short-term siltation mechanism of the trial trench dredged for the Shenzhen-Zhongshan Link in the Pearl River Estuary, China. Monthly silting characteristics over a period of ∼11 months indicate that the silting intensity during the flood season was much larger than during the dry season. Local suspended sediment concentrations (SSCs) also varied on a flood–dry seasonal cycle, which were impacted by the suspended load from the Pearl River. Daily silting characteristics indicate that the silting intensity during spring tides were higher than during neap and moderate tides and that the local SSCs and flow velocities also varied on a spring–neap tidal cycle, which was dominated by periodic variations in the astronomical tide. The flow velocity deceased substantially from the surface to the bottom of the trench, and flow circulation occurred at the bottom of the trench, which aided the silting process. Seasonal changes in the sediment load from the Pearl River were the dominant cause of the changes in suspended sediment around the trench, which produced seasonal siltation changes in the trench. Daily changes in trench siltation were directly caused by the amount of sediment that entered the trench, which was affected by changes in the flow velocity and bottom SSC around the trench during the neap–spring tidal cycle. Although the high-siltation intensity during the flood season was unhelpful, the low-siltation intensity during neap and moderate tides was helpful for the construction of immersed tunnel during the flood season.

Simulating the role of tides and sediment characteristics on tidal flat sorting and bedding dynamics

AbstractUnderstanding sediment sorting and bedding dynamics has high value to unravelling the mechanisms underlying geomorphological, geological, ecological and environmental imprints of tidal wetlands and hence to predicting their future changes. Using the Nanhui tidal flat on the Changjiang (Yangtze) Delta, China, as a reference site, this study establishes a schematized morphodynamic model coupling flow, sediment dynamics and bed level change to explore the processes that govern sediment sorting and bedding phenomena. Model results indicate an overall agreement with field data in terms of tidal current velocities, suspended sediment concentrations (SSCs), deposition thicknesses and sedimentary structures. Depending on the variation of tidal current strength, sand‐dominated layers (SDLs) and mud‐dominated layers (MDLs) tend to form during spring and neap tides, respectively. Thinner tidal couplets are developed during daily scale flood–ebb variations. A larger tidal level variation during a spring–neap tidal cycle, associated with a stronger tidal current variation, favours the formation of SDLs and tidal couplets. A larger boundary sediment supply generally promotes the formation of tidal bedding, though the bedding detail is partially dependent on the SSC composition of different sediment types. Sediment properties, including for example grain size and settling velocity, are also found to influence sediment sorting and bedding characteristics. In particular, finer and coarser sediment respond differently to spring and neap tides. During neap tides, relatively small flow velocities favour the deposition of finer sediment, with limited coarser sediment being transported to the upper tidal flat because of the larger settling velocity. During spring tides, larger flow velocities transport more coarser sediment to the upper tidal flat, accounting for distinct lamination formation. Model results are qualitatively consistent with field observations, but the role of waves, biological processes and alongshore currents needs to be included in further studies to establish a more complete understanding.

The Intra-Tidal Characteristics of Tidal Front and Their Spring–Neap Tidal and Seasonal Variations in Bungo Channel, Japan

The intra-tidal variations of a tidal front in Bungo Channel, Japan and their dependence on the spring–neap tidal cycle and month were analyzed utilizing high-resolution (~2 km) hourly sea surface temperature (SST) data obtained from a Himawari-8 geostationary satellite from April 2016 to August 2020. A gradient-based front detection method was utilized to define the position and intensity of the front. Similar to previous ship-based studies, SST data were utilized to identify tidal fronts between a well-mixed strait and its surrounding stratified area. The hourly SST data confirmed the theoretical intra-tidal movement of the tidal front, which is mainly controlled by tidal current advection. Notably, the intensity of the front increases during the ebb current phase, which carries the front toward the stratified area, but decreases during the flood current phase that drives the front in the opposite direction. Due to a strong dependence on tidal currents, the intra-tidal variations appear in a fortnight cycle, and the fortnightly variations of the front are dependent on the month in which the background stratification and residual current changes occur. Additionally, tidal current convergence and divergence are posited to cause tidal front intensification and weakening.

Resolving Estuarine Nitrogen Use by Phytoplankton Communities Using a Whole Ecosystem Tracer Approach

The use of nutrients by diverse phytoplankton communities in estuarine systems, and their response to changes in physical and biogeochemical processes in these natural systems, is a significant ongoing area of research. We used a whole ecosystem 15NO3− tracer experiment to determine the uptake of different nitrogen (N) forms in phytoplankton functional groups over a mid- to neap tidal cycle in a salt marsh creek in Plum Island Estuary, Massachusetts, USA. We quantified the biomass and δ15N for three groups corresponding to micro- (20–200 μm; microP), nano- (3–20 μm; nanoP), and picophytoplankton (< 3 μm; picoP). All three size classes showed distinct use of recycled N sources throughout the 11-day sampling period and minimal direct assimilation of the 15NO3− tracer. MicroP consistently used high amounts of creek-derived 15NH4+, even with a shift at neap tide from diatom- to dinoflagellate-dominated communities (including members of the harmful genus Alexandrium). NanoP use of recycled 15NH4+ increased over the mid-neap tidal cycle, while picoP use decreased. Both biomass and NH4+ use (as highest δ15N values) of all size groups were maximized during neap tide. This study demonstrates partitioning of recycled N use among size-based phytoplankton groups in the estuary, with distinct effects of tidal cycle on the nutrient uptake of each group, and with important implications for the roles of diverse phytoplankton communities in estuarine nutrient cycling.

Assessing the Influence of Changes in Shoreface Morphology since the 19th Century on Nearshore Hydrodynamics and Shoreline Evolution in Northern France: A Modeling Approach

Latapy A.; Hequette A.; Nicolle A., and Pouvreau N., 2020. Assessing the influence of changes in shoreface morphology since the 19th century on nearshore hydrodynamics and shoreline evolution in northern France: A modeling approach In: Malvarez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 542–547. Coconut Creek (Florida), ISSN 0749-0208.The shoreface off the coast of northern France is characterized by the presence of numerous tidal sand banks, forming linear shore-parallel or slightly oblique massive sand bodies. Digitization of historic hydrographic field sheets from the French Hydrographic Service (Shom) archives enabled to produce Digital Elevation Models (DEMs) of the seabed morphology at different dates since the early 19th century. Our analyses of bathymetry changes based on differential DEMs showed significant morphological variations across the shoreface near Calais during the 19th and 20th centuries, which are largely due to changes in nearshore sand bank morphology and position. Sand bank mobility can be partially explained by tidal current asymmetry, inducing an elongation and an alongshore migration of this sand body in the direction of the net residual tidal current. Observed changes in shoreface and nearshore morphology attempt to be related with possible variations in hydrodynamics, this paper presenting the results obtained on tidal currents. TELEMAC-2D model was used to simulate 2D velocity field and water depth during a spring and neap tidal cycle over bathymetry grids based on historical bathymetric soundings. Depending on the bathymetric setting, modeling of tidal residual currents revealed notable increases or decreases in residual current velocity during distinct time periods, particularly in nearshore areas, which may lead to seabed erosion where current strengthening is observed through time or conversely to sediment accumulation in areas of decreasing current velocity. These results are consistent with the shoreline changes that occurred during the 20th century, with a correspondence between nearshore areas of decreasing (or increasing) residual current velocity and adjacent shorelines that were affected by seaward progradation (or erosion), suggesting strong links between changes in seabed morphology over the shoreface and shoreline evolution.

Sand spit dynamics in a large tidal‐range environment: Insight from multiple LiDAR, UAV and hydrodynamic measurements on multiple spit hook development, breaching, reconstruction, and shoreline changes

AbstractSand spits with distal hooks have been well documented from coasts with low to moderate tidal ranges, unlike high tidal‐range environments. Datasets from 15 LiDAR and 3 UAV surveys between 2009 and 2019 on the Agon spit in Normandy (France), a setting with one of the largest tidal ranges in the world (mean spring tidal range: 11 m), combined with in‐situ hydrodynamic records between 2013 and 2017, highlight a three‐stage pattern of spit hook evolution. Stage 1 (2009–2013) commenced with the onshore migration and attachment of a swash bar, followed by persistent spit accretion updrift of the bar and erosion downdrift because of the slow speed of bar migration in this large tidal‐range environment. In stage 2 (2013–2016), three overwash events and a 220 m‐wide breach culminating in the total destruction of the spit during winter 2015–2016 involved the landward mobilization of thousands of cubic metres of sand. These events occurred during short durations (a few hours) when spring high tides coincided with relatively energetic waves, underscoring the importance of storms in rapid spit morphological change. Strong spring tidal currents maintained the breach. Stage 3 (2016–2019) has involved new hook construction through welding of a swash bar and spit longshore extension, highlighting the resilience of the spit over the 10‐year period, and involving a positive sediment balance of 244 000 m3. The three stages bring out, by virtue of the temporal density of LiDAR and UAV data used, a high detail of spit evolution relative to earlier studies in this macrotidal setting. The large tidal range strongly modulates the role of waves and wave‐generated longshore currents, the main process drivers of spit evolution, by favouring long periods of inertia in the course of the spring–neap tidal cycle, but also brief episodes of significant morphological change when storm waves coincide with spring high tides. © 2020 John Wiley &amp; Sons, Ltd.

Effects of Spring–Neap Tidal Cycle on Spatial and Temporal Variability of Satellite Chlorophyll-A in a Macrotidal Embayment, Ariake Sea, Japan

We investigated the spatio-temporal variability of chlorophyll-a (Chl-a) and total suspended matter (TSM) associated with spring–neap tidal cycles in the Ariake Sea, Japan. Our study relied on significantly improved, regionally-tuned datasets derived from the ocean color sensor Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua over a 16-year period (2002–2017). The results revealed that spring–neap tidal variations in Chl-a and TSM within this macrotidal embayment (the Ariake Sea) are clearly different regionally and seasonally. Generally, the spring–neap tidal variability of Chl-a in the inner part of the Ariake Sea was controlled by TSM for seasons other than summer, whereas it was controlled by river discharge for summer. On the other hand, the contribution of TSM to the variability of Chl-a was not large for two areas in the middle of Ariake Sea where TSM was not abundant. This study demonstrates that ocean color satellite observations of Chl-a and TSM in the macrotidal embayment offer strong advantages for understanding the variations during the spring–neap tidal cycle.

Transport modelling of sediment re-suspended during submarine cable laying operation

A two-dimensional depth-averaged hydrodynamics and suspended sediment transport numerical model is presented, which is discretized with finite volume method based on a triangular mesh. It is applied to investigate the diffusion and transport characteristics of sediment re-suspended during a demonstration staged submarine cable laying across the Jintang Channel, Zhejiang Province, China. The model is validated primarily against the observed data during the spring, middle and neap tidal cycles in July 2015. The model results show well agreements between the predicted and observed processes of tidal level, velocity and suspended sediment concentration (SSC). The validated model is then applied to simulate the distribution of the tidal velocity and additional SSC during the cable laying. The results show a close relationship between the additional SSC distribution and tidal hydrodynamics.

Spatial and temporal analysis of extreme storm-tide and skew-surge events around the coastline of New Zealand

Abstract. Coastal flooding is a major global hazard, yet few studies have examined the spatial and temporal characteristics of extreme sea level and associated coastal flooding. Here we analyse sea-level records around the coast of New Zealand (NZ) to quantify extreme storm-tide and skew-surge frequency and magnitude. We identify the relative magnitude of sea-level components contributing to 85 extreme sea level and 135 extreme skew-surge events recorded in NZ since 1900. We then examine the spatial and temporal clustering of these extreme storm-tide and skew-surge events and identify typical storm tracks and weather types associated with the spatial clusters of extreme events. We find that most extreme storm tides were driven by moderate skew surges combined with high perigean spring tides. The spring–neap tidal cycle, coupled with a moderate surge climatology, prevents successive extreme storm-tide events from happening within 4–10 d of each other, and generally there are at least 10 d between extreme storm-tide events. This is similar to findings from the UK (Haigh et al., 2016), despite NZ having smaller tides. Extreme events more commonly impacted the east coast of the North Island of NZ during blocking weather types, and the South Island and west coast of the North Island during trough weather types. The seasonal distribution of both extreme storm-tide and skew-surge events closely follows the seasonal pattern of mean sea-level anomaly (MSLA) – MSLA was positive in 92 % of all extreme storm-tide events and in 88 % of all extreme skew-surge events. The strong influence of low-amplitude (−0.06 to 0.28 m) MSLA on the timing of extreme events shows that mean sea-level rise (SLR) of similarly small height will drive rapid increases in the frequency of presently rare extreme sea levels. These findings have important implications for flood management, emergency response and the insurance sector, because impacts and losses may be correlated in space and time.

The interaction of retention areas and submarine ground water (SGD) in small bays: Implications for phytoplankton growth

Modified Davis drifters were deployed repeatedly over spring and neap tidal cycles in Kinvara Bay, a small semi-enclosed bay on the west coast of Ireland, in order to map circulation pathways and assess their implications for the transport and dispersal of nutrients and phytoplankton. The results showed that limited movement of water during neap tides created a retention area in the inner bay, trapping nutrients and any suspended particles. It is proposed that this retention, coupled with enhanced nutrient inputs via submarine ground water discharge driven by precipitation, leads to enhanced biological growth conditions for phytoplankton and is the likely cause of intense dinoflagellate blooms in summer that have historically occurred from time to time within the bay. This study has implications for all small bays with nutrient-rich freshwater inputs which may develop tidally driven retention areas.

Enhanced Exchange Flow During Spring Tide and Its Cause in the Sumjin River Estuary, Korea

Knowledge of exchange flow and its variations is important, because it determines the distribution of salt, nutrient, pollutant, and suspended sediment in estuaries. Although exchange flow has been studied extensively, their variations over the spring–neap tidal cycle remain unclear. Many studies have indicated that the exchange flow is weaker during spring tides than neap tides, but others have reported that it might be stronger during spring tides. Therefore, we investigated the spring–neap variation of an exchange flow and its cause based on intensive observational data and an analytical model applied to the Sumjin River estuary (SRE), Korea. The observations revealed that exchange flow increased during the spring tide but decreased during the neap tide. The horizontal salinity gradient increased about seven times more during spring tide than during neap tide in the middle of the estuary. The analytical model implied that the exchange flow in the SRE is mainly driven by the horizontal salinity gradient. The large horizontal salinity gradient and tidal current amplitude ratio in the SRE between the spring and neap tides suggests that the horizontal pressure gradient driven by the salinity gradient overwhelms vertical mixing in driving exchange flow in the SRE.

Variation in microplastics composition at small spatial and temporal scales in a tidal flat of the Yangtze Estuary, China

Microplastics are small, degrade slowly, and easily persist in the water column because they are close to neutrally buoyant. Understanding the distribution of microplastics is fundamental to evaluating the ecological risks that they cause and to identifying ways to control microplastics pollution. Most of the existing research on the distribution of microplastics in the coastal zone has focused on large spatial and temporal scales. To build on past work, we investigated variation in microplastics in a tidal flat of the Yangtze Estuary on small spatial (sediment depth, mudflat vs. vegetation zone) and temporal (fortnightly and semidiurnal) scales. Microplastics were more abundant in surface (0–2 cm) sediments during neap versus spring tide cycles, likely indicating increased deposition during periods with calm waters and increased suspension when water was more turbulent, but did not vary at greater depths in the sediment. Individual microplastics particles were also larger during neap versus spring tide periods. In contrast to the variation between spring and neap tide periods, we found no variation in the abundance of microplastics on the semidiurnal scale. Microplastics were also more abundant in the transect in the vegetation than at slightly lower elevations in the adjacent mudflat. Across all samples, the abundance of microplastics was negatively correlated with the strength of hydrological processes such as submergence time and flow velocity. Our results showed that sampling of microplastics in the intertidal environment needs to consider variation among spring and neap tide cycles, and also among different intertidal habitats that may differ only slightly in elevation. We encourage coupling sampling with direct measures of hydrological processes so that variation in microplastics abundance and size can be rigorously linked to hydrological processes.