Tidal Phase Research Articles

Establishing a long-term Tidal Water Elevation for Thane City, India

Future extreme tide levels and flood risk along the coastal region will be impacted strongly by climate change. Flooding in urban areas is a big challenge affecting coastal cities worldwide due to heavy rainfall coupled with high tides. The key factor in the design and planning of coastal town drainage systems is the selection of appropriate tide levels at drainage outfall location. The present study aims to define the long-term tidal water level for Thane city, India and this level will perform effectively under current and future tidal water conditions. The present study uses the observed tidal data from 2009 to 2018 about Thane Salt Bundar tide gauge station and Kasheli Bridge tide gauge station. Initially, the tidal levels are defined for various tidal phases using the fundamental statistical analysis method, the trend of tide level analysed by the linear regression model and then Gumbel's extreme value type I (EV-I) distribution method used to define the long-term tide levels for different return periods. The obtained results indicate that the tidal level may rise by 300 mm at Thane salt bundar and 270 mm at Kasheli bridge. The long-term tidal level at Thane salt Bundar may increase 3.3m and 2.3 m at Kasheli bridge. Established long-term tidal levels give good results under climate change conditions and can be safer for avoiding flood risk and improving the drainage outfall design for Thane city.

Influence of river discharge on circulation and tidal process in the Java Sea, Indonesia

This study investigates the effect of river discharge in transport and tidal processes in the Java Sea using the Coastal and Regional Ocean Community (CROCO) hydrodynamic model. The model has 20 vertical layers and a horizontal resolution of 1/18 degrees. The oceanic and atmospheric forcing of this model is taken from the global Copernicus Marine Environment Monitoring Service (CMEMS) model and the fifth generation ECMWF atmospheric reanalysis (ERA5) hourly data. Daily Global Flood Awareness System (GloFAS) data has been successfully implemented as river flow data for this study. Two scenarios have been applied, namely, with and without river discharge. This study shows that the two scenarios and the satellite observational data agree in terms of water level with Root Mean Square Difference RMSD) about 4 cm, Sea Surface Temperature with RMSD about 0.29 °C, and Sea Surface Salinity with RMSD about 0.39 psu. The model was also validated using seven tide gauges and produced a good agreement. River discharge increase eastward transport in the eastern part of the Java Sea up to 0.1 Sv (1 Sv= 106 m3s−1). Both scenarios produce similar tidal amplitude and phase and agree well with previous studies and other tidal data sources.

Analysis of interaction between morphology and flow structure in a meandering macro-tidal estuary using 3-D hydrodynamic modeling

This study employs a 3-D numerical model to examine the behavior of primary and secondary flows on a macro-tidal estuarine bend with seasonally evolving morphology. Three scenarios were developed based on actual bathymetric surveys at the meander bend in the upper region of Japan's Chikugo macro-tidal estuary namely, S1: one month post-flood (channel at maximum capacity), S2: two months post-flood (onset of mud bar formation) and S3: nine months post-flood (most stable channel with low capacity). Secondary circulation begins before the center of the curve at −1/3.5 of the radius of curvature (θ), and fully develops between the center until +θ/3.5 for both flood and ebb phases. Primary flow mainly dominates the morphological evolution in the estuary, as secondary flow, which is responsible for the lateral transport of the sediments accounts for 10–15% of the primary flow. The residual currents for one tidal cycle are generally inwards, resulting in net lateral sediment transport near the bottom. Primary current decelerates due to the increase in cross-sectional area at the curve exit while secondary currents exhibit bottom inward flows at the curve center (S1). Since flood currents are larger than ebb due to tidal asymmetry, suspended sediments are transported landward, resulting in the mud bar formation inside the curve upstream (S2). Shear stress (τ) exceeds critical value of 1.8 N/m2 especially at the inner bank during flood tide, but falls below this value during ebb. Since having τ greater than 1.8 N/m2 causes large scouring, it is presumed that erosion, resuspension and transportation occur more actively during flood tide than ebb. The channel becomes more stable as its flow capacity decreases and together, the water flow is mainly directed towards the outer bank during the entire dry season due to further sedimentation at the inner bank (S3). The process is repeated until the occurrence of the next flood, hence giving insight on the seasonal variation in deposition and erosion in the estuarine curve. The overall flow structure and the trend of seasonal flow volume variation in the channel are similar along the streamline from entry to exit for both tidal phases. This study successfully establishes the roles of primary and secondary flows in the evolution of morphology in an actual macro-tidal estuarine setting with strong tidal asymmetry.

Fluid mud dynamics in a tide-dominated estuary: A case study from the Yangtze River

Fluid mud plays an important role in the maintenance of the estuarine turbidity maximum (ETM) and makes a significant contribution to estuarine sediment transport. A seabed tripod system and a shipboard vertical profile were deployed to explore the spatial-temporal variation and mechanism of fluid mud in the ETM of the Yangtze Estuary. The near-bed suspended sediment concentration (SSC) measured by OBS and ABS exceeded 50 kg/m3, confirming the existence of fluid mud. The fluid mud layer varied frequently in thickness during spring and intermediate tides, with a maximum of up to 0.6 m, but maintained stability during neap tides. The thickness rose rapidly during the larger ebb tide in semidiurnal tidal cycles of intermediate tides when the gradient Richardson number (Ri) increased to exceed the critical value of 0.25, and the velocity at 0.2 mab was in the range of 0.5–0.8 m/s. The fluid mud layer transported more than twice as much sediment as the low-concentration upper layer did. Waves and intense currents at the flood phase of tide enhanced vertical mixing within the water column, with Ri being reduced to lower than 0.25 and turbulent kinetic energy (TKE) exceeding 0.01 m2/s2, leading to the breakdown of the fluid mud layer. Sediment stratification damps turbulence close to the seabed and leads to gradual solidification of the bottom fluid mud. Approximately 0.2 m of fluid mud deposit was formed on the seabed within 148 h of the observation period.

The Determination of the Rotational State and Interior Structure of Venus with VERITAS

Abstract Understanding the processes that led Venus to its current state and will drive its future evolution is a major objective of the next generation of orbiters. In this work we analyze the retrieval of the spin vector, the tidal response, and the moment of inertia of Venus with VERITAS, a NASA Discovery-class mission. By simulating a systematic joint analysis of Doppler tracking data and tie points provided by the onboard synthetic aperture radar, we show that VERITAS will provide accuracies (3σ) in the estimates of the tidal Love number k 2 to 4.6 × 10−4, its tidal phase lag to 0.°05, and the moment of inertia factor to 9.8 × 10−4 (0.3% of the expected value). Applying these results to recent models of the Venus interior, we show that VERITAS will provide much-improved constraints on the interior structure of the planet.

Winter observations alter the seasonal perspectives of the nutrient transport pathways into the lower St. Lawrence Estuary

Abstract. The St. Lawrence Estuary connects the Great Lakes with the Atlantic Ocean. The accepted view, based on summer conditions, is that the estuary's surface layer receives its nutrient supply from vertical mixing processes. This mixing is caused by the estuarine circulation and tides interacting with the topography at the head of the Laurentian Channel. During winter when ice forms, historical process-based studies have been limited in scope. Winter monitoring has been typically confined to vertical profiles of salinity and temperature as well as near-surface water samples collected from a helicopter for nutrient analysis. In 2018, however, the Canadian Coast Guard approved a science team to sample in tandem with its ice-breaking and ship escorting operations. This opportunistic sampling provided the first winter turbulence observations, which covered the largest spatial extent ever measured during any season within the St. Lawrence Estuary and the Gulf of St. Lawrence. The nitrate enrichment from tidal mixing resulted in an upward nitrate flux of about 30 nmol m−2 s−1, comparable to summer values obtained at the same tidal phase. Further downstream, deep nutrient-rich water from the gulf was mixed into the subsurface nutrient-poor layer at a rate more than an order of magnitude smaller than at the head. These fluxes were compared to the nutrient load of the upstream St. Lawrence River. Contrary to previous assumptions, fluvial nitrate inputs are the most significant source of nitrate in the estuary. Nitrate loads from vertical mixing processes would only exceed those from fluvial sources at the end of summer when fluvial inputs reach their annual minimum.

Mobile bedform dynamics approaching a bedload parting site: Pentland Firth, northeast UK

The Pentland Firth between mainland Scotland and Orkney is an area of the UK continental shelf that experiences extreme tidal flows (>3 m/s). This study presents a time series of bathymetric data coupled with a tidal flow model to examine hydrodynamics and bedform response at the eastern approaches to the Pentland Firth. These observations were additionally augmented by sediment grain type and ADCP data to validate the numerical flow model. Tidal flows of the Pentland Firth result from a tidal phase difference between the east and west approaches to the channel. A resulting barotropic pressure gradient leads to flow accelerations that locally exceed 5 m/s. The extreme tidal setting of the Pentland Firth's eastern approach was found to promote distinct bedforms that are spatially varied in geomorphology with distance from the Pentland Firth. Sediment analysis also showed a decreasing grain size trend also with distance from the Pentland Firth. The modelled residual tidal current shows strong agreement with the sediment transport pathways, supported by the bedform migration direction. The energetic tidal flows of the Pentland Firth's eastern approach interacts with the highly irregular coastline, generating residual tidal counter currents leeward of flow obstructions. These counter currents (i.e. residual tidal current vortices) were reflected in bedform migrations and thence the sediment transport pathways. As the Pentland Firth is considered as a bedload parting site, the residual tidal current vortices are expected to influence the rate of erosion at the bedload parting site, by recirculating sediment back upstream as a counter current inshore of the main flow. The modification of sediment transport pathways by residual tidal current vortices may affect the development of nearshore and offshore engineering, and should be considered in any initial site assessment.

Partially penetrating lake-aquifer interaction in a laboratory-scale tidal setting

Freshwater lakes close to the coast are important ecotones with strong functional dependency on ocean forces. Tides are likely to create dynamic lake-groundwater interactions, which have not been previously assessed for partially penetrating lakes. This study investigates tidal lake-groundwater interaction in a laboratory-scale aquifer cross-section with and without the existence of a partially penetrating lake. Experimental observations were found to be well matched to the results of numerical modeling in terms of the amplitudes and phases of pressure fluctuations at 19 locations. The results show that the lake dampened tide-induced pressure head fluctuations, indicating that the enhanced storage capacity was more influential in modifying tidal propagation relative to the increased transmissiveness imparted by the lake. The lake also caused stronger depth-dependency in groundwater tidal fluctuations, as determined from vertical profiles of tidal amplitude and phase. Vertical profiles also reveal that the tidal amplitude and phase responded differently to the addition of the lake, in a manner that reflects previous studies of tidal propagation in layered aquifers. Tides created significant aquifer-lake fluxes, leading to strong rates of lake flushing, particularly through the seaward lake boundary. Including a clogging layer at the lake bottom (within numerical models) tended to reduce the lake’s effects on tidal propagation, although clogging layer effects were complex. This study highlights the important role of tides on partially penetrating lake-groundwater exchange dynamics and the influence of partially penetrating lakes on tidal propagation in coastal settings, albeit at the laboratory scale.

Diving deeper into the underlying white shark behaviors at Guadalupe Island, Mexico.

Fine‐scale movement patterns are driven by both biotic (hunting, physiological needs) and abiotic (environmental conditions) factors. The energy balance governs all movement‐related strategic decisions.Marine environments can be better understood by considering the vertical component. From 24 acoustic trackings of 10 white sharks in Guadalupe Island, this study linked, for the first time, horizontal and vertical movement data and inferred six different behavioral states along with movement states, through the use of hidden Markov models, which allowed to draw a comprehensive picture of white shark behavior.Traveling was the most frequent state of behavior for white sharks, carried out mainly at night and twilight. In contrast, area‐restricted searching was the least used, occurring primarily in daylight hours.Time of day, distance to shore, total shark length, and, to a lesser extent, tide phase affected behavioral states. Chumming activity reversed, in the short term and in a nonpermanent way, the behavioral pattern to a general diel vertical pattern.

Morphology of estuarine bedforms, Weser Estuary, Germany

AbstractLarge bedforms (dunes) are present in many shallow‐water environments. Knowledge of their dimensions and dynamics is required for river and coastal management. In tidal rivers and estuaries, the interaction among hydrodynamics (which results from the action of river and tidal flows), sediment transport and bedform shape and size is complex. In the present study, the distribution and morphology of bedforms in the Weser Estuary, Germany, were investigated. Bedforms were identified in bathymetric data of monthly multibeam echosounder surveys along the navigation channel during the years 2009 to 2013. Their size and shape were characterized. Bedforms were present along most of the channel, except at the position of the estuarine turbidity maximum and where dredging is carried out. Average bedform length varied between 20 and 60 m and bedform height between 0.3 and 1.6 m. Bedform asymmetry varied spatially and temporally along the estuary. In times of high river discharge, bedforms were generally more ebb‐asymmetric than in times of low discharge. The bedforms were predominantly two‐dimensional low‐angle dunes with their steepest slope situated near the bedform crest. A significant proportion of bedforms possessed a steep face (portion of the lee side steeper than 15°). This implies that they are likely to create flow separation and a turbulent wake, with a strong potential to induce high bedform roughness. However, important variations in steep face area density were noted, both spatially along the estuary and temporally as a function of the tidal phase (ebb or flood) and the seasonal variation in river discharge. The results have wide implications in terms of understanding and modelling hydrodynamics and sediment transport in estuaries.

Development of a Tsunami-tide Interaction Model with Simulation of Tsunami Wave Propagation to Korea Coasts

Kim, K.O.; Jung, K.T., and Ha, T.M., 2021. Development of a tsunami-tide interaction model with simulation of tsunami wave propagation to Korea coasts. In: Lee, J.L.; Suh, K.-S.; Lee, B.; Shin, S., and Lee, J. (eds.), Crisis and Integrated Management for Coastal and Marine Safety. Journal of Coastal Research, Special Issue No. 114, pp. 91–95. Coconut Creek (Florida), ISSN 0749-0208. Simulation studies of historic and prognostic tsunamis have been performed with a focus on tsunami-tide interactions. A modeling system based on the 3D POM (Princeton Ocean Model) has been developed that is composed of a global tsunami model and a regional multinesting tide/tsunami/tsunami-tide interaction model covering the Northwest Pacific region. Propagations of two historic tsunamis, the 2010 Chilean and 2011 Tohoku tsunamis, were simulated to estimate the effects of tsunami-tide interactions by comparing the results from tsunami-only and tsunami simulations with tide simulations. Calculations show that tsunami waves are significantly transformed after traveling a long distance and subsequently affected by the tidal phase. As prognostic tsunami propagation, the Nankai Trough earthquake-induced tsunami wave in Japan was investigated. A series of calculations were performed with a systematic shift of the tsunami occurrence time. The effects of tsunami-tide interactions were examined in terms of local tidal phase changes, focusing on the changes in the first tsunami heights and occurrence times arriving at the selected tidal stations in Korea. The tsunami-tide interaction mostly results in a positive change in the tsunami first peak heights with a maximum change near low tide probably due to the interaction between the tsunami-induced current and tidal current. The water depth increase near high tide advances the arrival of the tsunami first peaks, whereas the water depth decrease near low tide delays the arrival of the tsunami first peaks.

Initial Findings Suggest Box Jellyfish Encounters along Shallow Philippine Coastlines are Predictable

Jellyfish stings account for numerous deaths in the Philippines. Despite this, there is little scientific information on the identity, distribution, seasonality, and ecology of the box jellyfish species involved. We present initial findings of three months (April–June 2021) of monitoring the abundance of and envenomations by box jellyfishes in eastern and western Luzon. Emergent patterns suggest encounters between box jellyfish and people are more likely to occur in beach areas within a few kilometers from rivers when tidal ranges are large, particularly around the flood phase of high tide. Encounters are also more likely on coasts where the monsoon winds blow onshore during the warmer months, bringing these animals closer to beach areas. They then stay longer in the shallows when the seas are calm. Beachgoers are, thus, advised to plan their destinations and activities carefully and wear protective clothing when venturing into shallow coastal areas.

Investigations Exploring the Use of an Unstructured-Grid, Finite-Volume Modelling Approach to Simulate Coastal Circulation in Remote Island Settings—Case Study Region, Vanuatu/New Caledonia

Understanding coastal circulation and how it may alter in the future is important in island settings, especially in the South West Pacific, where communities rely heavily upon marine resources, and where sea level rise (SLR) is higher than the global average. In this study we explore the use of an unstructured-mesh finite-volume modelling approach to assist in filling the knowledge gaps with respect to coastal circulation in remote island locations—selecting the Vanuatu and New Caledonia archipelagos as our example study site. Past limited observations and modelling studies are leveraged to construct and verify a regional/coastal ocean model based on the Finite-Volume Community Ocean Model (FVCOM). Following verification with respect to tidal behaviour, we investigate how changes in wind speed and direction, and SLR, alter coastal water levels and coastal currents. Results showed tidal residual circulation was typically associated with flow separation at headlands and islands. Trade winds had negligible effect on water levels at the coast, however, wind-residual circulation was sensitive to both wind speed and direction. Wind-residual currents were typically strongest close to coastlines. Wind residual circulation patterns were strongly influenced by Ekman flow, while island blocking, topographic steering and geostrophic currents also appear to influence current patterns. Tidal amplitudes and phases were unchanged due to SLR of up to 2 m, while maximum current speeds altered by as much as 20 cm/s within some coastal embayments. Non-linear relationships between SLR and maximum current speeds were seen at some coastal reef platform sites. Under higher sea levels, tidal residual currents altered by less than ±2 cm/s which is relatively significant given maximum tidal residual current speeds are typically below 10 cm/s. Our findings indicate that under higher sea levels, coastal processes governing sediment transport, pollutant dispersal and larval transport are likely to alter, which may have implications for coastal environments and ecosystems. Given winds influence coastal circulation and subsequent coastal processes, changes in trade winds due to climate change may act to further alter coastal processes. It is felt that the current modelling approach can be applied to other regions to help fill critical knowledge gaps.

Precursory tidal triggering and b value variation before the 2011 Mw 5.1 and 5.0 Tengchong, China earthquakes

Recent studies have suggested that tidal triggering may preferentially occur when a region is critically stressed, and that tidal stresses provide a potential tool for forecasting earthquakes. Most tidally triggered precursors before large earthquakes are found in subduction zones. We examined tidal triggering of earthquakes near the intracontinental Gaoligong Shear Zone, which is also a geothermally active region. We observed statistically significant correlations between tidally induced stresses and earthquake times, which only appeared approximately two years before the 2011 Tengchong Mw 5.1 and 5.0 earthquakes. The Schuster spectrum shows two peaks at ∼0.5 and ∼1.0 days related to the tidal period. The tidal phase distribution in the pre-mainshock time period shows that most earthquakes occur during phases of positive tidal shear stress and dilatancy strain. Our findings provide a typical case of tidal triggering of earthquake precursors in a continental setting. The stress state can be inferred using the Gutenberg–Richter b value, which characterizes the relative proportion between small and large earthquakes and correlates negatively with differential stress. We investigated the temporal changes of the b value and found that it retains a long-term low value but has a prominent short-term decline prior to mainshocks. The coincidence of low Schuster probability ps with a low b value suggests that the crustal medium is near a critical state, making it more sensitive to external perturbations. The results of epidemic-type aftershock sequence (ETAS) modelling revealed that the seismic activity is affected by not only cascading earthquake triggering but also external forcing. Taking the collective results of the Schuster test, b value, ETAS modelling, and other evidence, we suggest that tidal triggering of earthquakes is likely when the fault is in a near-critical stress state, and geothermal fluid may play an important role in the triggering mechanism.

Foraging habitat selection by breeding Herring Gulls (Larus argentatus) from a declining coastal colony in the United Kingdom

Individual foraging specialisms among generalist species can have important consequences for demographic rates. We investigated the habitat preferences of breeding Herring Gulls (Larus argentatus) at South Walney, a declining coastal colony in the UK, comparing results to a previous assessment made three decades before, when the breeding population was higher, which indicated substantial use of terrestrial, intertidal and anthropogenic resources. We used a two-stage Resource Selection Function (RSF) analysis to assess the habitat selection of 21 Herring Gulls fitted with GPS tags over three breeding seasons (2014–16). By generating pseudo-absences within the minimum convex polygon surrounding individual level GPS fixes, we first used logistic regression models to relate presence/absence for each individual to habitat. Selection for mussel bed areas was also modelled specifically as a function of phase of tide, time of day, period within the breeding season, year as well as individual covariates of mass, sex and hatching success. Second, individual RSF coefficients were extracted from the logistic regression models and used as response variables in linear models.Herring Gulls significantly selected for mussel beds and other coastal habitats (75% of all fixes away from the colony occurring in these habitats). Selection for mussel bed sites was significantly greater during the first half of the breeding season, March–May, compared with June–August, while there was also significant positive selection for this habitat between −3 and −2 m tide height (ordnance datum).There has been an apparent reduction in the range of food resources utilised by Herring Gulls at South Walney in recent years with a greater propensity for intertidal specialists within the tracked sample and, in particular, little use was made of urban and landfill areas. Therefore, better understanding of the intertidal resources available and considered management will be beneficial to balance the needs of conservation and commercial and social interests of the nearby shellfisheries.

Intelligent Buoy System (INBUS): Automatic Lifting Observation System for Macrotidal Coastal Waters

The west coast of South Korea is characterized by a wide macrotidal area with a maximum tidal range of ∼10 m. The sea surface elevation varies with the tidal phase, which leads to significant changes in the vertical structure of the physical, chemical, and biological properties of the water column, especially when the interaction by waves and/or the freshwater and sediment input from the river increases. Under such conditions, it is difficult to carry out continuous and consistent measurements of the vertical structures of the water qualities using a conventional observation system that is fixed to the seabed or sea surface because the thickness of the water column constantly changes. Based on the demand for long-term observations of the vertical structures of the water properties in macrotidal environments, the Intelligent Buoy System (INBUS) was developed by the Korea Institute of Ocean Science and Technology (KIOST). INBUS is a buoy equipped with an instrument frame, which is similar to other buoys that are fixed to the sea surface. During every measurement, INBUS detects the water depth and sends the frame down to the seabed to measure the water quality at every vertical level set up in the system. For example, if N levels are set in INBUS, the water depth of each measurement is divided by N layers and the instruments in the lifting frame measure the water properties of each of the N layers while they are descending to the bottom. Based on this procedure, the vertical structure of the water column is consistently measured in N layers regardless of changes of the water depth due to tides and waves. The lifting and measuring process is automatically controlled by INBUS once it is set up in the system. In addition, because INBUS allows bidirectional communication through code division multiple access (CDMA), the system can be controlled by stations on land. If the CDMA communication becomes inoperable owing to extreme wave conditions or the buoy is lost because of incidents such as a collision with a ship, the location of INBUS can be tracked by low-earth-orbit satellites.

Temperature Tides Across the Mid-Latitude Summer Turbopause Measured by a Sodium Lidar and MIGHTI/ICON.

Local full diurnal coverage of temperature variations across the turbopause (~90-115 km altitude) is achieved by combining the nocturnal observations of a Sodium (Na) Doppler lidar on the Utah State University (USU) campus (41.7°N, 248.2°E) and NASA Michelson interferometer for global high-resolution thermospheric imaging (MIGHTI)/Ionospheric connection explorer (ICON) daytime observations made in the same vicinity. In this study, utilizing this hybrid data set during summer 2020 between June 12th and July 15th, we retrieve the temperature signatures of diurnal and semidiurnal tides in this region. The tidal amplitudes of both components have similar vertical variation with increasing altitude: less than 5 K below ~98 km but increase considerably above, up to 19 K near 104 km. Both experience significant dissipation near turbopause altitudes, down to ~12 K up to 113 km for the diurnal tide and ~13 K for the semidiurnal tide near 110 km. In addition, while the semidiurnal tidal behavior is consistent with the theoretical predictions, the diurnal amplitude is considerably larger than what is expected in the turbopause region. The tidal phase profile shows a dominance of tidal components with a long vertical wavelength (longer than 40 km) for the semidiurnal tide. On the other hand, the diurnal tide demonstrates close to an evanescent wave behavior in the turbopause region, which is absent in the model results and Thermosphere ionosphere mesosphere energetics and dynamics (TIMED)/Sounding of the atmosphere using broadband radiometry (SABER) observations.

Contribution of sediments to stratification in a fluvial estuarine system during a low-discharge period

Changjiang River Estuary (CRE), as one of the largest fluvial estuarine systems worldwide, experiences great changes in river discharges, sediment loads and geomorphology, due to heavy anthropogenic activities. The contribution of sediments to stratification in CRE was examined by analyzing the field data of sea surface level, current, salinity, suspended sediment concentration (SSC) and sediment grain size. The data were simultaneously observed at 1-h intervals and six vertical layers at 10 stations during the low-river discharge period from January 11 to 19, 2016. During this winter period, the asymmetric tides at all the field stations had longer ebb durations. The tidal range in the north branch became the largest among all the stations. The tidal phase inside the north branch was more delayed than that in the south branch, due to the effect of the local bathymetry and coastlines. Currents and bed sediments impacted SSC through changing bottom friction and viscosity, and then impacted water stratification subsequently through modulating water density. Net sediment flux was mostly seaward during the entire spring-neap tidal cycle, because of the Eulerian velocity and tidal pumping effect. Turbidity-induced stratification accounts for more than 50% of the total stratification during spring tides.

Variation of salinity in the Sundarbans Estuarine System during the Equinoctial Spring tidal phase of March 2011

The Sundarbans Estuarine System (SES), comprising the southernmost part of the Indian portion of the Ganga-Brahmaputra delta bordering the Bay of Bengal, is India’s largest monsoonal, macro-tidal, delta-front estuarine system. The Sundarbans Estuarine Programme (SEP), covering six semi-diurnal tidal cycles during 18–21 March 2011 (the Equinoctial Spring Phase), was the first comprehensive observational programme in the SES. The 30 observation stations, spread over more than 3600 $$\hbox {km}^2$$ , covered the seven inner estuaries of the SES: the Saptamukhi, Thakuran, Matla, Bidya, Gomdi, Harinbhanga, and Raimangal. At all stations or time-series locations (TSLs), the water level was measured every 15 min and water samples were collected every hour for estimating salinity. We report the observed spatio-temporal variations of salinity in this paper. The mean salinity over the six tidal cycles decreased upstream and the mean range of salinity over a tidal cycle increased upstream. In addition to this along-channel variation, the mean salinity also varied zonally across the SES. Salinity was lowest in the eastern SES, with the lowest value occurring at the TSLs on the Raimangal. Though higher than at the Raimangal TSLs, the mean salinity was also low at Mahendranagar, the westernmost TSL located on the West Gulley of the Saptamukhi. Salinity tended to be higher in the central part of the SES. CTD (conductivity–temperature–depth) measurements at three stations on the Matla show a well-mixed profile. Only the Raimangal has a freshwater source at its head. Therefore, the upstream decrease of salinity in the SES is likely to be the effect of the preceding summer monsoon, which would have freshened the estuary, and the ingress of salt from the seaward end due to the tide following the cessation of of the monsoon rains. The freshwater inflow from the Raimangal leads to the lowest salinities occurring in the eastern SES. The lower salinity in the western SES also suggests inflow from the Hoogly estuary, whose freshwater source is regulated via the Farakka Barrage. At 20 of the 30 TSLs, the salinity varied semi-diurnally, like the water level, and the maximum (minimum) salinity tended to occur at or around high (low) water. The temporal variation was more complex at the other 10 TSLs. Even at the TSLs at which a tidal stand exceeding 75 min was seen in the water level, the salinity oscillated with a semi-diurnal period. Thus, the salinity variation was unaffected by the stand of the tide that has been reported from the SES.

Tidal currents at the sills of the Northern Gulf of California

Current observations at the four main sills in the Northern Gulf of California: San Esteban (SE), San Lorenzo (SL), Delfin (DE) and Ballenas channel (BC), are analyzed to investigate their tidal structure. Tidal currents are mainly semidiurnal (M2, S2, and N2) at all of the four sills. M2 vertically integrated tidal currents at the three most energetic sills (SE, SL and BC) are at least 0.5 m/s, but there are significant vertical variations of the tidal amplitudes and phases of the measured currents. At the SL and DE sills there is a ~20○-30○ phase reduction in the lower 100 m of the water column (depths ~400 m) of the M2 tidal currents, and this is, at least in part, related to near-bottom overflows that are present at those sills. At the SL sill, the phase reduction is also accompanied by a significant reduction in amplitude. Baroclinic tidal currents were estimated using empirical orthogonal functions of the high-passed current vectors, and they typically account for about 10% of the observed variance. At the most energetic sill (SE) the two dominant baroclinic components are the nonlinear MS4 and M4. Complex demodulation of the currents at the semidiurnal band shows that tidal amplitudes and phases decrease with increasing low-frequency (subtidal) currents and with increasing vertical shear of the low-frequency current at the sills with the overflows (SL and DE). We also found that at two of the most energetic sills (SE and SL) which have a strong stratification, the annual satellites of the M2 barotropic tidal currents reach amplitudes up to 0.1 m/s. This modulation produces a 0.2 m/s and up to a 15○ annual range of the barotropic M2 tidal current amplitudes and phases, respectively, with maxima in August.