Tidal Current Asymmetry Research Articles

Bi-layered spring-neap variability of water masses in estuaries and the impact of human activities

Estuaries are one of the most important ecosystems in the world, which are economically developed and densely populated. However, the intricate hydrodynamic environment and frequent human activities within estuaries have left the spatiotemporal variability of water properties in these areas inadequately understood. Recently, based on in situ observations and numerical simulations, we found significant spring-neap variability of water mass properties in the Yangtze River Estuary, which exhibited a bi-layered vertical structure. In the Yangtze River Estuary, salinity could decrease (increase) over 4 psu during spring (neap) tides in the upper layer, and satellite observations confirmed that both sea surface chlorophyll-a concentration and particulate organic carbon concentration also showed significant spring-neap variabilities. Decreasing salinity in the upper layer induced a shoreward pressure gradient force in the lower layer, which caused shoreward advection of high salinity water from the deep ocean and resulted in salinity increasing up to 2 psu in the lower layer of the Yangtze River Estuary. Dynamical diagnoses proved that spring-neap variability of water mass properties were caused by the asymmetry of tidal currents via modulating the ratio of freshwater to seawater. Similar situations also occurred in the Mississippi River Estuary. Furthermore, constructions of dams and other hydraulic projects in the watershed could greatly alter the locations with significant spring-neap water masses variability through reducing the riverine sediment flux and thus, leading to the erosion of the tidal flats in estuaries. The above results highlight the important roles of tidal asymmetry and human activities in affecting spring-neap variabilities of water mass properties in estuaries.

Identification and separation of baroclinic M4 current in estuaries

Tidal current asymmetry is a challenging issue in estuaries and is generally quantified based on the relative amplitude and phase of M4 constituents in semi-diurnal regions. Previous studies have focused on the barotropic M4 current (BTC), whereas baroclinic overtides are often ignored. Using field data and numerical simulation results, we established a separation method of the baroclinic M4 current (BCC), clarified its typical features in estuaries of large tidal range, and explored the mechanism of BCC from the perspective of frontogenesis. The results showed that a 180° phase reversal in the vertical direction and significant enhancement of the normalised amplitude were fundamental features of BBC. Meanwhile, a decrease in currents around the salt intrusion head may not be inevitable owing to the strong BTC induced by nonlinearity, especially in meso- or macro-tidal estuaries. In contrast, near the river plume, BCC may develop and be stronger than BTC. Furthermore, BCC leads to zero longitudinal net transport of water mass, as it is treated as an ‘oscillatory’ full-depth lock-exchange flow. Our findings present a framework for BCC in estuarine zones.

A framework for estuarine future sea-level scenarios: Response of the industrialised Elbe estuary to projected mean sea level rise and internal variability

In this study, we apply probabilistic estimates of mean sea level (MSL) rise and a sub-set of regional climate model ensemble simulations to force a numerical model of the southern North Sea, downscaling projected sea level variability to the Elbe estuary that serves as a prototype for an industrialised meso-tidal estuary. The specific forcing combination enables a localised projection of future estuarine hydrodynamics accounting for the spread of projected global sea level rise and the spread of the regional climate projection due to internal variability. Under the applied high-emission scenario, the Elbe estuary shows high decadal rates of mean water level (MWL) rise beyond 19 mm y-1, increase in the tidal range of up to 14 mm y-1 and increase in extreme water levels of up to 18 mm y-1. The bandwidth of the estuarine response is also high. For example, the range of average monthly extreme water levels is up to 0.57 m due to the spread of projected global sea level rise, up to 0.58 m due to internal variability whereas seasonal range attains 1.99 m locally. In the lower estuary, the spread of projected global sea level rise dominates over internal variability. Internal variability, represented by ensemble spread, notably impacts the range of estuarine water levels and tidal current asymmetry in the shallow upper estuary. This area demonstrates large seasonal fluctuations of MWLs, the M2 tidal amplitude and monthly extreme water levels. On the monthly and inter-annual time scales, the MWL and M2 amplitude reveal opposite trends, indicative of a locally non-linear response to the decadal MSL rise enforced at the open boundary. Overall, imposed by the climate projections decadal change and MSL rise enhance the horizontal currents and turbulent diffusivities whereas internal variability locally mitigates sea level rise–driven changes in the water column. This work establishes a framework for providing consistent regionalised scenario-based climate change projections for the estuarine environment to support sustainable adaptation development.

Impacts of river discharge, coastal geomorphology, and regional sea level rise on tidal dynamics in Pearl River Estuary

IntroductionRecently, accelerated sea-level rise (SLR) has raised major concerns on estuarine management and restoration because it modifies estuarine hydrodynamics, such as tidal inundation extent and amplitude, tidal current asymmetry, flood frequency, etc. Understanding underlying processes which regulate estuarine tidal dynamics is critical for mitigating potential impacts on the coastal population and development.MethodsWe selected the Pearl River Estuary (PRE), an estuary with a higher regional SLR compared to the global SLR as the study area. To identify the effects of regional SLR on tidal dynamics, we analyzed long-term records of four tidal variables from 19 gauge stations between 1952 and 2020, including annual highest tidal levels (AHTLs), annual lowest tidal levels (ALTLs), annual mean tidal ranges (AMTRs), and annual mean tidal levels (AMTLs).ResultsResults suggested that variability in tidal variables was site-specific. The AMTRs decreased gradually from offshore regions to the upper reaches of rivers with a maximum at the mouth of the estuary. In contrast, the magnitudes of the AHTLs, AMTLs, and ALTLs decreased when approaching the coastline. Distance to the coastline had the highest correlations with the AHTLs, ALTLs, and AMTLs, followed by river width and river depth. The local SLR was responsible for the temporal variability in AHTLs, ALTLs, and AMTLs at the estuarine mouth with correlation coefficients from 0.43 to 0.85 (P < 0.001) but showed low correlations with these three tidal variables at the upper reaches of rivers (P > 0.001). Instead, the tidal variables were highly correlated with river discharge at the upper reaches of rivers, especially for the AHTLs (correlation coefficient = 0.83, P < 0.0001).ConclusionsOur study demonstrated that the tidal dynamics demonstrate significant spatial-temporal variability. Geomorphology, river discharge, and sea levels have jointly affected estuarine hydrodynamics.

On the Mechanism behind the Variation of the Tidal Current Asymmetry in Response to Reclamations in Lingding Bay, China

Tidal current asymmetry, an important factor for waterborne substance transport, is strongly affected by the changing shoreline. Lingding Bay (LDB), located in the center of the Guangdong-Hong Kong-Macao Greater Bay Area, has suffered great shoreline changes since the 1970s. A well-validated numerical model is built to investigate how the changing shoreline impacts tidal current asymmetry, which is quantified by the skewness parameter. Two types of tidal current asymmetries, the flow velocity asymmetry (FVA) and the flow duration asymmetry (FDA), are considered here. Despite their different dominant combinations, the two asymmetries display a similar tendency from the mouth to the head of the LDB. When the shoreline changes due to successive land reclamation are taken into consideration, the FVA and the FDA alter correspondingly. The results suggest that the shoreline change shifts the FVA in Deep Bay (a side embayment of LDB) from ebb dominance to flood dominance, because of the changes of its dominant constituent combinations (K1/O1/M2 and the residual current). As the shoreline propagates seaward, the increasing contributions of M2/M4 and M2/S2/MS4 enlarge the water area of positive FDA (i.e., shorter duration of low water slack) in LDB. Analyses of tidal current asymmetries in Lingding Bay show that tidal current asymmetry has been enhanced by sustained land reclamation and will further impact sediment transport in the bay.

Estimation of Tidal Current Asymmetry in an Archipelagic Region: The Zhoushan Islands

Tidal current asymmetry (TCA) often occurs in coastal regions. It can significantly influence bedload sediment transport. Recently, the statistical skewness of the tidal current velocity was calculated to represent the TCA. In archipelagic region, the tidal current directions vary temporally and spatially from channel to channel. This creates complexity in finding the flood–ebb axis about which to discuss the axial dissymmetry of tidal currents. In the present work, a method that involves taking the main flood direction (MFD) as the axis to split the tidal current was suggested. The MFD is the most probable direction of the strongest flood flow during each tidal cycle. The method was applied in an archipelagic region: The Zhoushan Islands. The results show that the calculated skewness well represented the TCA in waters around islands, and the degree of the TCA was mainly determined by the residual current. When the direction of the residual current was the same as the MFD, the skewness was positive, which indicated flood dominance. On the contrary, when the direction of the residual current was opposite to the MFD, the skewness was negative, which indicated ebb dominance. The stronger a residual current is, the more significant the TCA will be. Islands play an important role in forming residual circulations. Large ones force flows to move offshore around headlands or along curved channels, because of centrifugal forcing, while small ones often cause segregated flood/ebb conduits and form residual circulations. In the waterways between the Zhoushan Islands, the ebb current generally carries more sediment than the flood current. Therefore, ebb dominance always means sediment is more likely to be deposited, and vice versa. Further research into sediment transport modeling is suggested.

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.

Tidal current-dominated depositional environments in the central-northern Yellow Sea as revealed by heavy-mineral and grain-size dispersals

This study confirms that the fine sand-sized detritus in the central-northern Yellow Sea is chiefly derived by the Yellow River and explains the Holocene Yellow River dispersal system and the preservation of transgressive deposits by heavy-mineral and grain-size analyses. Micas, which are sensitive to hydraulic sorting, offer an effective key for identifying the dispersal of Holocene Yellow River-derived fine-grained sediments. The heavy-mineral compositional changes are attributed to hydraulic sorting by different marine dynamics and various mixing with additional local transgressive deposits of miscellaneous provenance. The effects of hydraulic sorting are dominated by Holocene tidal currents and by foreshore waves during the rapid transgression, respectively. The former results in obvious enrichments of garnet and common stable species such as Ti oxides, tourmaline, and titanite, whereas the latter seems more remarkable for the compositional changes, with further depletion of epidote, actinolite/tremolite and zoisite/clinozoisite. The central Yellow Sea mud deposit and the alongshore muddy zone off the southern Liaodong Peninsula are opposite to each other as a pair of structurally symmetric coalesced terminal lobes far away from the high-energy strait between Cape Chengshan and Changsan-got. The Haiyang subaqueous clinoform as another terminal lobe is revealed by a heavy-mineral index, which represents an extremity of deposits accumulated by the modern Yellow River-derived sediments off the southwestern Shandong coast. A small mud patch is revealed, roughly corresponding to the central regime of the amphidromic system of principal M2 constituent. The tidal current-dominated depositional pattern proposed here emphasizes the importance of time–velocity asymmetry of tidal currents. Such a dynamic mechanism is primarily responsible for the dispersal of the modern Yellow River-derived sediments when they are entrained from the Bohai Sea to the South Yellow Sea through the Bohai Strait and the strait between Cape Chengshan and Changsan-got.

Asymmetry of tidal currents off the W.Brittany coast and assessment of tidal energy resource around the Ushant Island

A method of tidal stream energy resource assessment around the Ushant Island in the Iroise Sea, using surface velocity time series from High Frequency radars (HFR) and ADCP measurements, is presented. Remotely sensed velocities provided by the radars allow to augment the industry standard approach of 3D numerical modeling and in-situ ADCP surveying to make a large-scale quantification of tidal stream resource. They capture the real ocean dynamics and thus provide context on the complex spatial variability of tidal currents that are so often feature at potential tidal energy sites. The observations show current velocities of 4 m/s northwest of the Island and in the Fromveur Strait, with 1 m/s value exceeded 60% and 70% of time respectively. Emphasis is given to the peculiarities in tidal flow asymmetry and to the study of the variation of vertical velocity profiles during different tidal stages. Radar derived velocities reveal a pronounced asymmetry between the flood and ebb flow around the Ushant Island, quantified by dimensionless number a - velocity asymmetry. The largest range of asymmetry variation, from 0.5 to 2.5, is observed in the Fromveur Strait. Harmonic analysis demonstrated that a joint variation of phase of the principal semi-diurnal (M2) and quarter-diurnal (M4) tidal velocity component accounts for flow asymmetry variation in the strait. Asymmetry in current direction is also quantified. ADCP measurements show that the vertical velocity profiles follow a 1/α power law with a power law exponent mostly depending on geographic location than on tidal stage. It was demonstrated that, in the Fromveur Strait, the 1/7 power law is appropriate to characterize the velocity profile. The combination of two sources of data enables characterization of the velocity variations in three spatial dimensions and in time thus increasing accuracy of the hydrokinetic resource assessment from HF radar observations. The estimation shows that the mean technical resource is 50% smaller in the lower half than in the upper half of the water column. The theoretical resource on average is three times higher than the technical resource and appears to be more sensitive to variations in the shape of the velocity profile in the lower layer.

Dune-associated sand fluxes at the nearshore termination of a banner sand bank (Helwick Sands, Bristol Channel)

Sand dunes typically migrate in opposing directions along the two sides of sand banks, reflecting a circulation driven by tidal current asymmetry, but it has been less clear how this pattern is distorted where banks intersect the coastline. The nearshore end of Helwick Sands, a banner bank in the Bristol Channel, was surveyed three times over three years, twice with a high-resolution multibeam echo-sounder. In both multibeam surveys, an unusual geometry was found over the crest of the bank, whereby dunes connect continuously with the dunes on the flanks, despite the flank dunes migrating in opposite directions. The crestal dunes thus appear to realign rapidly. We suggest that this morphological behaviour arises here because of vigorous wave-driven transport and because surface waves propagate almost exactly parallel to the crestal dunes. Sand transported parallel to the crestal dunes ensures that efficient reconnection occurs with dunes migrating along the flanks, particularly at low tide when wave currents are more strongly felt at the bed.Dunes were tracked between the surveys to assess the bedload transport fluxes associated with dune migration. Fluxes on the two flanks of the bank were found to be similar. The coarse-scale pattern of deposition and erosion was calculated from divergence of the fluxes using the continuity relation. That pattern differs strongly compared with the deposition/erosion pattern derived more directly from the bathymetry change between successive surveys. This implies that the dune-associated fluxes fail to represent all the sediment movements in this shallow area; significant contributions from storm wave-assisted transport, suspended load and/or sand transport during the reversed tide also probably affect transport flux besides dune-associated bedload.

Persistent upwelling and front over the Sulu Ridge and their variations

The persistent upwelling and front isolated over the Sulu Ridge are newly detected by a suite of satellite measurements and confirmed by historical cruise observations. A three‐dimensional tide‐circulation coupled model is employed to investigate the dynamic mechanisms. The intense vertical velocity shear induced by the vigorous internal tidal mixing is identified as the vital dynamic factor to trigger the upwelling and nutrient pumping, through the strong tide‐topography interactions. Satellite observations indicate that the pronounced surface cooling and phytoplankton bloom are isolated over the Sulu Ridge throughout the year. The steady front characterizes 10–20 km width and strength over 0.3°C/km near the southern edge. The feature is further confirmed by the numerical simulation and in situ hydrological observation. Modeling experiment suggests that a quasi‐symmetrical vertical circulation bifurcation across the Sulu Ridge due to the asymmetry of tidal currents is the main mechanism to maintain the persistent upwelling and front. Moreover, the local wind forcing plays an important regulating role in the seasonal and interannual variations. The surface cooling and thermal front intensity reach the maximum in winter under the forcing of strong northeasterly monsoons. Due to prolonged El Niño–Southern Oscillation (ENSO) effects, the surface cooling was significantly weakened during 1998 and 2010 summer. Further analysis indicates the fact that the southwesterly monsoon is weakened dramatically as part of the anticyclonic atmospheric circulation anomalies over the northwest Pacific. The latter results from the significant influence of tropical Indian Ocean capacitor effect associated with El Niño–Southern Oscillation.

Holocene evolution of bottom sediment distribution on the continental shelves of the Bohai Sea, Yellow Sea and East China Sea

Coastline shifts due to transgression and regression can remodel tidal fields of continental shelves, and this can control transport of sediments and modulate sediment distribution accordingly. Tidal currents have become the dominant hydrodynamic processes on the continental shelves of the Bohai Sea, Yellow Sea and East China Sea (BYECS) since the transgression after the Last Glacial Maximum (LGM). To examine the evolution of the bottom sediment distributions on the continental shelves of the BYECS, we simulated patterns of tides and tidal currents, sediment transport, and bottom sediment types (sand, mud and mixed sediments) for five periods, corresponding to sea level lowstands of 80m, 52m and 30m below present, the Holocene transgression maximum (HTM), and the present. The simulation shows that both sediment transport and shelf sediment distribution patterns were controlled by the strength, type and asymmetry of tidal currents in the BYECS since the LGM. Evolution of shelf sediment distribution patterns occurred in two stages: (1) sediment emplacement and formation stage before the HTM, and (2) local adjustment after the HTM. The marked changes in coastline configuration since the LGM are the dominant factor controlling tide and tidal current evolution. Distribution of shelf sediment types in the BYECS is closely related to tidal current fields during transgression after the LGM.

Sedimentary characterization of bed types along the Guadiana estuary (SW Europe) before the construction of the Alqueva dam

The Guadiana River estuary is one of the most important mesotidal fluvio-marine systems of the south-western Iberian Peninsula. The river mouth was formed as a narrow channel that was excavated by fluvial incision during the Pleistocene lowstand and was then flooded 6500 years ago. The estuary is in an advanced state of sediment infilling in its proximal part, due to its narrow morphology, which prevents passage of sediment through to the open coast. Consequently, sediment is accumulating in the river mouth, which causes progradation. The analysis of the sediments into the estuarine channel (grain size, organic carbon content) and bedform distribution (Side-Scan Sonar) allows a distinction between two types of sedimentation to be made: a fluvial–marine sedimentation (favoured by tidal action) and an autochthonous sedimentation that is related to water mixing. The net transport of sediments is towards the sea because of the tidal current asymmetry. The autochthonous deposits and the sediments from the extreme fluvial floods that tides are unable to rework are preserved on the meandering convex margins under low tidal velocities. These areas acquire a lateral tidal-bar morphology with cohesive beds because the narrow geometry of the estuary inhibits the presence of longitudinal tidal bars, as may be expected in tide-dominated systems. The higher energy zones (deeper zones of the channel) become bypassing channels where the flocculated material cannot be settled on the bottom; this part of the channel then develops sandy beds with mesoforms as ebb-oriented two-dimensional and three-dimensional dunes (sand waves and megaripples).

Evidence of a Mid-Holocene Sea Level Highstand from the Sedimentary Record of a Macrotidal Barrier and Paleoestuary System in Northwestern Australia

This study addresses the mid- to late Holocene stratigraphy and sea level history of a macrotidal barrier and paleoestuary system located along the relatively unstudied northwest coast of Australia. Thirty-nine shallow cores were obtained from three transects perpendicular to the barrier and paleoestuary axis. Seven sedimentary facies were identified on the basis of sediment texture, carbonate content, and foraminifera assemblages: slope, upper intertidal mud, upper intertidal sand flat, lower intertidal sand flat, barrier, estuarine beach, and flood tide delta. The sedimentary infill reveals a fining upward succession of marine sediments up to 6 m thick, mostly along a regressive sequence. All facies are of Holocene age and started to be laid down when sea level was approximately 3 m below present elevation. A radiocarbon date from the topmost sedimentary facies (upper intertidal mud) indicates that relative sea level was at least 1 m higher than today by 2720 years BP. At this time, the estuary was at the final stages of sedimentary infill, with tidal inundation reduced to a minimum. Further evidence of a higher relative sea level during the Holocene is in the form of estuarine beach deposits found at the back of the paleoestuary at an elevation above the present day beach/dune interface. Net nearshore transport in the area, driven by tidal current asymmetry, is northward, and it is proposed that this has significantly influenced the alongshore component of the barrier progradation and evolution of the barrier estuary.

Undular tidal bore dynamics in the Daly Estuary, Northern Australia

Measurements in the macro-tidal Daly Estuary show that the presence of an undular tidal bore contributed negligibly to the dissipation of tidal energy. No recirculation bubble was observed between a trough and the following wave crest in the lee waves following the undular bore. This differs to stationary undular bores in laboratory experiments at larger Froude numbers where a recirculation bubble exists. Secondary motions and the turbulence generated by the undular bore had no measurable influence on the sediment transport. This situation contrasts with the intense sediment resuspension observed in breaking tidal bores. The tidally averaged sediment budget in the Daly Estuary was controlled by the asymmetry of tidal currents. The undular bore may widen the river by breaking along the banks that it undercuts, leading to bank slippage. A patch of river-wide macro-turbulence of 3-min duration occurred about 20 min after the passage of the bore during accelerating tidal currents.

Undular tidal bore dynamics in the Daly Estuary, Northern Australia

Measurements in the macro-tidal Daly Estuary show that the presence of an undular tidal bore contributed negligibly to the dissipation of tidal energy. No recirculation bubble was observed between a trough and the following wave crest in the lee waves following the undular bore. This differs to stationary undular bores in laboratory experiments at larger Froude numbers where a recirculation bubble exists. Secondary motions and the turbulence generated by the undular bore had no measurable influence on the sediment transport. This situation contrasts with the intense sediment resuspension observed in breaking tidal bores. The tidally averaged sediment budget in the Daly Estuary was controlled by the asymmetry of tidal currents. The undular bore may widen the river by breaking along the banks that it undercuts, leading to bank slippage. A patch of river-wide macro-turbulence of 3-min duration occurred about 20 min after the passage of the bore during accelerating tidal currents.

Sediment transport on macrotidal flats in Garolim Bay, west coast of Korea: significance of wind waves and asymmetry of tidal currents

A self-recording instrument, named Tidal Sediment Dynamics Observational System (TISDOS), was built to monitor transport characteristics of nearbed sediments on tidal flats. It was deployed on a tidal flat in the semi-enclosed Garolim Bay, west coast of Korea, over a 15-day period between 5 and 20 January 2002 to examine sediment-transport processes during winter seasons. The measurements involved brief durations of high waves allowing for observation of wave effects upon the sediment transport on the tidal flat. Time series of various hydrodynamic parameters (water depth, current velocities, wave height, suspended sediment concentration, and bed level) from point measurements show characteristic interrelationships between parameters on both temporal and spatial scales. The tidal flat is dominated by flooding currents up to 2 times stronger than ebb currents. The current speeds measured simultaneously at two stations along a cross-shore transect varied in harmony with water depth, reaching the maximum during spring tide that was steadily decreased onshore. The onshore decrease in current speed was compatible with a fining textural trend from sand on the lower flat to mud toward the upper flat. Both the maximum water depth and current speed during individual tidal cycles also show semi-diurnal asymmetry that was highlighted during spring tide. Waves were of critical importance in resuspending bed material and thus yielding higher suspended sediment concentrations. On the middle flat, the suspended sediment concentrations were highest, exceeding 400 mg/l at 0.5 m above the seabed during large waves (relative wave height, 0.33) under weakest neap currents. In this wavy climate, the suspended sediment concentration increased over time during ebb, in strong contrast with a gradual decrease through time after mid-flood peaks under tidal currents without waves. The daily vertical flux of suspended sediments trapped in a plastic bottle also indicates the significance of wave effects in terms of enhancing resuspension of bottom sediments. As a whole, the suspended silts as well as seabed sands should be transported onshore by dominant flooding currents despite occasional offshore increase in suspension load by waves.

Flow asymmetry associated with astronomical tides: Implications for the residual transport of sediment

Tidal current asymmetry is usually associated to nonlinear tidal interactions in shallow water, generating compound tides and overtides. The present paper demonstrates that tidal asymmetry is not only caused by nonlinear tidal interaction, but is also generated by astronomical tides in absence of shallow‐water effects. In particular, the K1, O1, and M2 constituents give an asymmetrical periodic flow pattern, with the largest peak velocities persistently in the same direction. A theoretical explanation for this phenomenon is presented, and an inventory is made of other, subordinate constituents that may contribute to repetitive asymmetric flow patterns. Tidal current asymmetry may induce residual transport of sediment. Analytical expressions are derived which quantify the residual transport of sediment due to the K1, O1, and M2 tides, as a function of the phases and amplitudes of these constituents, and the time lag between variation in suspended sediment transport and flow velocity. The residual transport induced by the K1, O1, and M2 tides is compared to the residual transport due to the asymmetry associated with the M2 and M4 tides. The former mechanism is particularly relevant in diurnal, or mixed, mainly diurnal tidal regimes, where the K1 and O1 tides are dominant.

A modeling study of the Satilla River estuary, Georgia. I: Flooding-drying process and water exchange over the salt marsh-estuary-shelf complex

The flooding-drying process over the intertidal zone of the Satilla River estuary of Georgia was examined using a three-dimensional (3-D) primitive equations numerical model with Mellor and Yamada's (1982) level 2.5 turbulent closure scheme. The model was forced by the semi-diurnal M2, S2, and N2 tides and freshwater discharge at the upstream end of the estuary. The intertidal salt marsh was treated using a 3-D wet-dry point treatment technique that was developed for the σ-coordinate transformation estuary model. Good agreement was found between model-data comparison at anchor monitoring sites and also along the estuary that suggested that the model provided a reasonable simulation of the temporal and spatial distribution of the 3-D tidal current and salinity in the Satilla River estuary. Numerical experiments have shown that the flooding-drying process plays a key role in the simulation of tidal currents in the main river channel and in water transport over the estuarine-salt marsh complex. Ignoring this process could lead to a 50% under-estimation of the amplitude of tidal currents. The model results also revealed a complex spatial structure of the residual flow in the main channel of the river, with characteristics of multiple eddy-like cell circulations. These complicated residual currents are formed due to tidal rectification over variable topography with superimposition of inertial effects, asymmetry of tidal currents, and baroclinic pressure gradients. Water exchanges over the estuary-intertidal salt marsh complex are asymmetric across the estuary, and tend to vary periodically on the northern side while quickly washing out of the marsh zone on the southern side. Strong Stokes’ drifting velocity was predicted in the estuary, so that the Lagrangian trajectories of particles were characterized by strong nonlinear processes that differ significantly from those estimated by the Eulerian residual currents.

Tidal current asymmetry in shallow estuaries and tidal creeks

We examine tidal asymmetry in a 100-km long coastal plain estuary, a 15-km long tidal creek closed at the end, and a small side channel of a coastal plain estuary closed at low water. Current meter and water depth data were harmonically analyzed for M2 and the M4 and M6 overtide. The results are displayed in a plot of tidal current versus water level and the effects of the M2, M2+M4, and M2+M4+M6 are compared in order to show how overtides affect tidal asymmetry in the different systems.The long estuary showed a gradual increase in tidal current asymmetry. The current takes on a more progressive nature as it goes into the estuary, as indicated by increasing lags between high or low water and the corresponding slack water. On the other hand, the tidal creek has its reflection point at the head, a relatively short distance from the observations, which reflects the incident tidal wave more efficiently.The tidal creek has a high salt marsh which is covered only an hour or so before high water. Since its intertidal areas flood and drain near the time of high water, maximum flood and ebb currents occur a short time before and after high water slack throughout the tidal creek. Most of the tidal energy is dissipated at the end of the creek where M6/M2 of the tidal current almost doubles.The small side channel has tidal flats that become covered soon after the time of low water. The relative time of maximum flood and ebb currents occurs much closer to the time of low-water-slack, which contrasts significantly to the timing in the tidal creek channel surrounded by high salt marsh.