Net Seaward Transport Research Articles

Hydrodynamics and sediment transport patterns on intertidal flats along middle Jiangsu coast

Two field campaigns in Dafeng and Jianggang were organized to compare spatial variations of hydrodynamic characteristics and sediment transport patterns on intertidal flats of different types with distinct human interferences along middle Jiangsu coast, China. The major contributors to the different patterns of sediment dynamics between the two tidal flats were offshore tidal current field and human interference. Offshore tidal force provide the basic setup of tidal current patterns on tidal flats, which is then modified by local morphology. Seawalls parallel to coast reduce tidal flat width, forcing tidal energy to dissipate within a shorter distance and thus influencing tidal flat morphology. Seawalls vertical to coast and major tidal current significantly reduce tidal current speed, which favors sediment deposition on tidal flats. Two seawalls built on both sides of the observational tidal flat profile caused much reduced current speed at Dafeng tidal flat, comparing to the offshore station. Being exposed to offshore radial tidal currents, hydrodynamics at Jianggang was much stronger than that at Dafeng. Residual currents at both areas showed net landward transport at the lower flat and net seaward transport at the upper flat, in favor to sediment accumulation at the middle flat. Sediment flux over tidal cycles showed net landward sediment transport at Dafeng, and net seaward transport at Jianggang, consistent with the convex-up accretion-dominated profile observed at Dafenge, and concave-up erosion-dominated profile observed at Jianggang. The instantaneons sediment flux changed significantly due to variations in velocity and sediment concentrations, but these terms counteracted with each other within tidal cycles, leading to the dominant role of Eulerian flux in determining net sediment flux over tidal cycles.

Towards an Integrated Observational System to Investigate Sediment Transport in the Tidal Inlets of the Lagoon of Venice

An observation system integrating satellite images, in situ water parameters and hydrodynamic measurements was implemented in a tidal inlet of the Venice Lagoon (Northern Adriatic Sea, Italy). The experimental infrastructure was developed to autonomously investigate suspended sediment dynamics in the two channels of the Lido inlet in relation to the longshore currents in the littoral zone and the tidal circulation along the lagoon channel network. It provided time series of turbidity at the surface, water flow and acoustic backscatter, which was converted into turbidity along the vertical column during different tidal phases and meteo-marine conditions. Accurate turbidity maps were derived from Sentinel-2 (Copernicus) and Landsat 8 (NASA) satellites. Long-term in situ data from field surveys enabled the calibration and intercalibration of the instrumental setup and validation of satellite-derived products. Time series from the instrumental network were analyzed in order to evaluate the temporal variability of suspended sediment in relation to tidal phases and the different meteo-marine conditions. The integration of available datasets with satellite images also permitted the testing of the methodology for a 3-D reconstruction of the suspended sediment pattern in calm sea conditions, under the effect of the sole hydrodynamical forcing. Remotely sensed data provide a synoptic distribution of turbidity in the inlet area allowing the analysis of the surficial patterns of suspended sediment and the inferring of information on the transport processes at different spatial scales. In calm sea conditions, the results show that the transport is driven by tidal currents with a net seaward transport related to a larger export of materials from the northern basin of the Lagoon of Venice. During typical northeasterly storms, materials mobilized on the beaches and in the shoreface are transported into the inlet and distributed into the lagoon channel network, following the flood tidal currents and determining net import of materials. The multitude of information provided by this system can support research on aquatic science (i.e., numerical simulations) and address end-user community practices. The ecosystem management will also benefit operational purposes, such as the monitoring of morphological transformations, erosion processes and planning of coastal defense in the future scenarios of sea level rise. The developed approach will also help to understand how the regulation of the inlet flow introduced by the operation of the flood barriers will affect the fluxes of particles and, in the long term, the lagoon morphodynamics.

Modeling of Spacio-temporal Evolution of Salt Dispersion on Nokoue Lake

Abstract. The numerical modeling of spatio-temporal evolution of lagoon has an important role in predicting the behaviour of these systems. Knowing the concentration of the pollutant field distribution in time and space contributes significantly to the prediction of exceptional phenomena. The purpose of this paper is to simulate the transport and dispersion of salt at Nokoue Lake. To this end, the 2D hydrodynamic model SMS (Surface Water Modeling System) has been used. Results showed that in flood period the freshwater inflows produce a net seaward transport, while in low water period the tides lead to periodic seaward and landward transport. The developed numerical model is useful for predicting pollutants transport in this system, for water quality management of the Nokoue Lake, and therefore, fight against eutrophication.

Responses of hydrodynamics to changes in shoreline and bathymetry in the Pearl River Estuary, China

Numerous estuaries worldwide have been modified in the past decades by human interventions. The shoreline and bathymetry in the Pearl River Estuary (PRE) have changed greatly over the past 40 years due to the influence of land reclamation and waterway dredging, which have resulted in the corresponding adjustment of its hydrodynamics. Utilizing the ROMS numerical model, this paper studies the hydrodynamic responses to changes in shoreline and bathymetry in the PRE from 1971 to 2012. The results show that, on one hand, during neap tide, the change in the shoreline makes the residual current in the West Channel (WC) increase by 0.10 (0.05) m/s at maximum in the surface (near bottom) layer. Therefore, the exchange flow increases by 9.5% and the longitudinal circulation strengthens. The surface isohalines move southward up to 18 km, but the bottom isohalines move northward ∼2 km in the WC and East Channel (EC), which is different from the previous conclusion that the seaward extension of coastlines inhibits saltwater intrusion. The decrease in salinity in the upper layer reduces the upper seaward salt transport, resulting in a larger net landward salt transport from 25.21 × 103 kg/s to 35.44 × 103 kg/s. During spring tide, the changes are relatively weaker, but the direction of salt transport changes to seaward and the net seaward transport also increases. Moreover, the change in shoreline reduces the water area and volume in the PRE by 21.3% and 15.6% respectively, which causes a reduction of 11.3% (17.1%) in tidal prism during spring (neap) tide. The wave celerity is enhanced (>23%) and the amplification of semidiurnal tide is strengthened (>20%) in the WC and West Shoal (WS). The reduction in tidal prism together with the strengthened reflection of tidal waves with a larger phase lag between elevation and velocity of M2 tidal component cause a significant decrease of 19.0% in tidal energy flux entering the PRE. However, the tidal range increases by ∼0.30 m (mainly due to the increase in M2 tidal amplitude) in the EC, which is induced by lower tidal energy dissipation there. On the other hand, during neap tide, the change in bathymetry greatly enhances the bottom landward residual current whose peak value increases by 80% in the WC, thereby increasing approximately 14 km of the intrusion distance of saline water, enhancing the exchange flow by 27.5% and strengthening the longitudinal circulation. The net landward salt transport increases to 38.39 × 103 kg/s. The results during spring tide are similar to those during neap tide but with smaller changes, and the net seaward salt transport decreases. Moreover, the wave celerity is slightly reduced (<8%) and the amplification of semidiurnal tide is also decreased (<10%) in the WC and WS. Meanwhile, the change in bathymetry only decreases the water volume by 4.7%, leading to a relatively smaller effect on the tidal prism, and it strengthens (weakens) the reflection of tidal waves in the West Shoal and Middle Shoal (WC and EC), resulting in a slight reduction in tidal energy flux entering the bay. In the WC, the tidal range is basically unchanged since the increased tidal energy flux is offset by the increased dissipation. The quantitative results obtained in this study may provide some references for the development and protection of the PRE and other estuaries that are subject to strong human interventions.

Suspended Sediment Flux in the San Francisco Estuary: Part I—Changes in the Vertical Distribution of Suspended Sediment and Bias in Estuarine Sediment Flux Measurements

In this study, we investigate how changes in the vertical distribution of suspended sediment affect continuous suspended sediment flux measurements at a location in the San Francisco Estuary. Current methods for measuring continuous suspended sediment flux estimates relate continuous estimates of suspended-sediment concentration (SSC) measured at-a-point (SSCpt) to discrete cross-section measurements of depth-averaged, velocity-weighted SSC (SSCxs). Regressions that compute SSCxs from continuous estimates of SSCpt require that the slope between SSCpt and SSCxs, controlled by the vertical distribution of SSC, is fixed. However, in tidal systems with suspended cohesive sediment, factors that control the vertical SSC profile—vertical turbulent mixing and downward settling of suspended sediment mediated by flocculation of cohesive sediment—constantly vary through each tide and may exhibit systematic differences between flood and ebb tides (tidal asymmetries in water velocity or particle size). We account for changes in the vertical SSC profile on estimates of SSCxs using time series of the Rouse number of the Rouse-Vanoni-Ippen equation combined with optical turbidity measurements, a surrogate for SSCpt, to predict SSCxs from 2009 to 2011 and 2013. Time series of the Rouse number were estimated by fitting the Rouse-Vanoni-Ippen equation to SSC estimated from optical-turbidity measurements taken at two elevations in the water column. When accounting for changes in the vertical SSC profile, changes in not only the magnitude but also the direction of cumulative sediment-flux measurements were observed. For example, at a mid-depth sensor, sediment flux estimates changed from − 319 kt (± 65 kt, negative indicating net seaward transport) to 482 kt (± 140 kt, positive indicating net landward transport) for 2009–2011 and from − 388 kt (± 140 kt) to 1869 kt (± 406 kt) for 2013–2016. At the study location, estimation of SSCxs solely from SSCpt resulted in sediment flux values that were underestimates on flood tides and overestimates on ebb tides. This asymmetry is driven by covariance between water velocity and particle settling velocity (Ws) with larger Ws on flood compared to ebb tides. Results of this study indicate that suspended-sediment-flux measurements estimated from point estimates of SSC may be biased if systematic changes in the vertical distribution of SSC are unaccounted for.

Circulation and transport in short, low-inflow estuaries under anthropogenic stresses

Data from field experiments were used to assess, for the first time, hydrodynamic and transport processes in three shallow low-inflow estuaries of Northeast Brazil (Ceará State). The estuaries of the Cocó, Pacoti and Pirangi rivers exhibit anthropogenic stresses related to freshwater withdrawals and sewage discharges and morphological changes. Sampling was carried out during complete spring tidal cycles at each estuary during consecutive days on November 6, 7 and 8, 2010. November is the end of the dry period, and the results represented expected extreme low inflow conditions. All estuaries showed well mixed water columns because of frictional influences. Cocó and Pacoti displayed ebb dominance while the Pirangi exhibited flood dominance, a result from morphological changes in the intertidal storage areas. Each estuary showed distinct longitudinal salinity distributions related to different water budgets, which were actually altered by human activities. The Cocó estuary was hyposaline because of waste water discharge from the city of Fortaleza. This estuary represented a source of materials to the coastal sea as it showed net export of Suspended Particulate Matter (SPM) and chlorophyll. The Pacoti estuary was slightly hypersaline but nearly neutrally buoyant as temperature compensated for salinity variations. The Pacoti also exhibited net seaward transport of SPM and chlorophyll. The Pirangi estuary showed strong hypersalinity and inverse estuarine circulation as a result of multiple river dams. The negative water budget combined with mangrove depletion led to magnifying trapping processes, thus resulting in net import of salt, SPM and chlorophyll. The three cases exemplified how estuaries on the order 10 km long can be sensitive to human-induced changes. These human changes affect their circulation structure and their function as source or trap for materials at the land–ocean interface.

Cross‐shelf exchange in the shelf of the East China Sea

AbstractA high‐resolution, 3‐dimensional primitive equation model is used to investigate the cross‐shelf exchange in the East China Sea (ECS). Favorable comparisons between field data and model simulations from both climatological run and hindcast run for 2006 indicate that the model has essential skills in capturing the key physics of the ECS. Temporal and spatial variations of the cross‐shelf exchanges are further analyzed. It was demonstrated from both observations and simulations that in 2006 high saline water could be delivered to the north of the Changjiang River mouth (near 32°N) as a result of stronger than typical cross‐shelf exchanges at the shelf break and flows through the Taiwan Strait with an annual mean rate of 2.59 and 1.83 Sv, respectively. A few new places at the shelf break were also identified where persistent and vigorous onshore or offshore exchanges occur throughout the year. Cross‐shelf exchange is largely determined by the along‐shelf geostrophic balance with weak seasonality, which is modulated in upper layers by northeasterly monsoon from early‐fall to late‐spring and at seabed by bottom friction during December–January, May, and August–September. Nonlinear effect, with strong spatial variations and intraseasonal variability, is a secondary but persistent contributor to the net seaward transport, except for northeast of Taiwan where the nonlinear effect becomes significant but more varied.

Late Holocene Geomorphology of the Columbia River Estuary, Oregon and Washington, USA

Abundant river sediment supply and an open-water central bay area characterize the geomorphology of the large Columbia River estuary (~ 100 km in length). Lateral floodplains and marsh islands do constrict the uppermost reaches of the estuary, but the central axes of the lower estuary are dominated by shallow sand shoals (0–4 m water depth). A total of 58 vibracores are used to document the grain size and age (0–2,500 14CyrBP) of late Holocene deposits in the estuary. Sedimentation rates in stable floodplains (1.1 m ka-1) reflect rates of relative sea level rise (0.75 m ka-1). Sedimentation rates of muddy sand accretionary banks and prehistoric sand shoals (1.5–7 m ka-1) greatly exceed coeval rates of sea level rise, so they must represent short–term rates of vertical accretion resulting from channel lateral migration and associated cut and fill processes. The apparent paradox of unfilled accommodation space in the estuary is resolved by 1) winter wind–wave erosion of sand shoals to -3 m NAVD88 elevation and 2) asymmetric fluvial-tidal advection that results in net seaward transport of bed load in shallow tidal channels (&gt; – 10 m NAVD88) and shallow subtidal shoals (&gt; – 4 m NAVD88) during spring river flooding.

Morphologic and facies trends through the fluvial–marine transition in tide-dominated depositional systems: A schematic framework for environmental and sequence-stratigraphic interpretation

Most tide-dominated estuarine and deltaic deposits accumulate in the fluvial-to-marine transition zone, which is one of the most complicated areas on earth, because of the large number of terrestrial and marine processes that interact there. An understanding of how the facies change through this transition is necessary if we are to make correct paleo-environmental and sequence-stratigraphic interpretations of sedimentary successions. The most important process variations in this zone are: a seaward decrease in the intensity of river flow and a seaward increase in the intensity of tidal currents. Together these trends cause a dominance of river currents and a net seaward transport of sediment in the inner part of the transition zone, and a dominance of tidal currents in the seaward part of the transition, with the tendency for the development of a net landward transport of sediment. These transport patterns in turn develop a bedload convergence within the middle portion of all estuaries and in the distributary-mouth-bar area of deltas. The transport pathways also generate grain-size trends in the sand fraction: a seaward decrease in sand size through the entire fluvial–marine transition in deltas, and through the river-dominated, inner part of estuaries, but a landward decrease in sand size in the outer part of estuaries. A turbidity maximum (i.e., a zone of significantly elevated suspended-sediment concentrations) is developed within estuaries and the delta-plain region of deltas as a result of flocculation and density-driven water-circulation patterns. This leads to an area within the estuary or delta plain where the abundance and thickness of the mud drapes are greatest, including the potential for the development of fluid-mud deposits (i.e., structureless mud layers more than 0.5–1 cm thick that were deposited in a single slack-water period). A monotonic seaward increase in salinity characterizes both estuaries and deltas. The brackish-water conditions in the transition zone, accompanied by the high turbidity and physically harsh conditions, produce a biologically stressed environment, in which bioturbation is generally not pervasive. The ichnofossil assemblage in this zone is characterized by the low diversity of ichnogenera, small size of the individual burrows (typically smaller than their open-marine counterparts), and highly variable population densities, ranging from unbioturbated to very high-density mono-specific assemblages in local areas. This review begins with a survey of how and why each depositional process varies through the fluvial-to-marine transition and then examines the sedimentological responses to these processes, focussing on the observable, longitudinal variations in the development and/or abundance of each deposit characteristic (e.g., sand grain size, paleocurrent patterns, mud drapes, and biological attributes). The review ends with a summary of the characteristics of each major facies zone through the transition, with separate discussions for both estuaries and deltas. It must be noted that any attempt to generalize, as is done here, will undoubtedly contain statements that are inappropriate for individual deposits or systems. Exceptions must be expected and the concepts must be applied with due consideration for the local context.

Frontal observations of the Delaware Coastal Current source region

The Delaware Coastal Current is formed by the outflow of buoyant estuarine water from Delaware Bay. Its source region is concentrated near the Delaware coast at the estuary mouth in a narrow region typically only 4–6 km in width. We used a combination of moored current, temperature, and conductivity records together with CTD profiles, underway surface thermosalinograph records, and repeated acoustic Doppler profiling to study the source region hydrographic and current fields with emphasis on its frontal boundaries and their tidal frequency variability. Our central finding is that twin frontal boundaries exist for the source region over much of the tidal period, that is, both its offshore and onshore boundaries are fronts. Between the twin fronts lies the buoyant water of the source region, a conduit for the net seaward transport of buoyancy. These frontal boundaries are strongly modulated in horizontal density gradient at the M 2 tidal frequency. Maximum gradients occur during early flood current phase. Weaker gradients characterize the ebb current phase. The density gradient modulation is partly explained at both fronts by the occurrence of the lowest salinity and density water in the source region near the time of slack water before flood. Near the offshore front, furthermore, horizontal convergence of the flood tidal currents provides subsequent frontogenesis, so that peak frontal density gradients are reached there during the flood current phase.

Inputs, Transformations, and Transport of Nitrogen and Phosphorus in Chesapeake Bay and Selected Tributaries

In this paper we assemble and analyze quantitative annual input-export budgets for total nitrogen (TN) and total phosphorus (TP) for Chesapeake Bay and three of its tributary estuaries (Potomac, Patuxent, and Choptank rivers). The budgets include estimates of TN and TP sources (point, diffuse, and atmospheric), internal losses (burial in sediments, fisheries yields, and denitrification), storages in the water column and sediments, internal cycling rates (zooplankton excretion and net sediment-water flux), and net downstream exchange. Annual terrestrial and atmospheric inputs (average of 1985 and 1986 data) of TN and TP ranged from 4.3 g TN m−2 yr−1 to 29.3 g TN m−2 yr−1 and 0.32 g TP m−2 yr−1 to 2.42 g TP m−2 yr−1, respectively. These rates of TN and TP input represent 6-fold to 8-fold and 13-fold to 24-fold increases in loads to these systems since the precolonial period. A recent 11-yr record for the Susquehanna River indicates that annual loads of TN and TP have varied by about 2-fold and 4-fold, respectively. TN inputs increased and TP inputs decreased during the 11-yr period. The relative importance of nutrient sources varied among these estuaries: point sources of nutrients delivered about half the annual TN and TP load to the Patuxent and nearly 60% of TP inputs to the Choptank; diffuse sources contributed 60–70% of the TN and TP inputs to the mainstream Chesapeake and Potomac River. The direct deposition of atmospheric wet-fall to the surface waters of these estuaries represented 12% or less of annual TN and TP loads except in the Choptank River (37% of TN and 20% of TP). We found direct, although damped, relationships between annual rates of nutrient input, water-column and sediment nutrient stocks, and nutrient losses via burial in sediments and denitrification. Our budgets indicate that the annual mass balance of TN and TP is maintained by a net landward exchange of TP and, with one exception (Choptank River), a net seaward transport of TN. The budgets for all systems revealed that inorganic nutrients entering these estuaries from terrestrial and atmospheric sources are rapidly converted to particulate and organic forms. Discrepancies between our budgets and others in the literature were resolved by the inclusion of sediments derived from shoreline erosion. The greatest potential for errors in our budgets can be attributed to the absence of or uncertainties in estimates of atmospheric dry-fall, contributions of nutrients via groundwater, and the sedimentation rates used to calculate nutrient burial rates.

Sediment pathways and transport rates through a tide‐dominated entrance, Rangaunu Harbour, New Zealand

ABSTRACTBed load sediment traps were deployed at two sections across channels in Rangaunu Harbour entrance. Traps were inspected and emptied by divers at hourly intervals through both spring and neap tidal cycles for a total of 292 trap deployments. Current velocities were measured simultaneously with the trap inspections. Transport is concentrated in sandy megaripple fields on the channel banks and sub‐tidal platforms flanking the channels. There, transport is almost continuous throughout the tidal cycle, increasing with flow velocity but lagging by approximately one hour. The channel floors are lined with shell‐gravel lag across which bedload transport rates are low and discontinuous. Tidal asymmetry produces a net seaward transport through the channel troughs and a net landward transport across the channel banks and flanking sub‐tidal platforms. Sediment leaving the harbour recirculates in anticlockwise gyres across the ebb‐tide delta to re‐enter the harbou and maintain the supply of sand to the megaripple field. Transport during spring tides is typically 25–30 times that during neaps. Predictions of transport rates, from a method developed by Black &amp; Healy utilizing the Yalin bedload equation, produced transport rates similar to the traps over sand beds. Transport over shell lag surfaces appears independent of near‐bed velocity and more dependent on the passage of ribbons of sand across the lag surface.

Seaward Nutrient Transport in the Lower St. Lawrence Estuary

A transect across the lower St. Lawrence estuary was occupied to estimate volume transport and nutrient distributions (total dissolved nitrogen and phosphorus, and silicate) during February and May through September. Seaward transport was restricted to the upper 50 m and varied from 17 × 103 m3/s (August) to 129 × 103 m3/s (May). Nutrient and volume transport data are combined to estimate seasonal variability in monthly surface nutrient transport towards the Gulf of St. Lawrence. The estimates vary as follows: 0.72–6.72 × 103 tons N/mo, 0.05–0.48 × 103 tons P/mo and 0.50–6.73 tons Si/mo. Ratios between monthly freshwater input into the estuary and the corresponding values for net seaward transport, which varied from 1.1 to 9.4, are combined with annual data (1960–73) on monthly freshwater input into the estuary to estimate potential annual variability in seaward transport for each of the above months. Observed nutrient data, with the above transport estimates, suggest the possible range in monthly seaward nutrient transport that would be associated with the annual variability in monthly freshwater input into the estuary. Seaward nitrogen transport from May through September, estimated at 2.3 × 104 tons, is considered in relation to published data on the primary production in the western part of the Gulf of St. Lawrence. The estuary supplied a smaller amount of nutrients to the Gulf than has previously been assumed.

Distribution and Transport of Suspended Particulate Matter in the Gulf of St. Lawrence

The distribution of suspended matter was studied in the Gulf of St. Lawrence. A three-layer distribution was found. In the surface layer, which was less than 50 m deep, concentrations of suspended matter ranged from 0.10 mgl−1 to 2.9 mgl−1. A relationship to the surface circulation pattern existed with the highest concentrations occurring in the low salinity surface outflow from the estuary. In the intermediate layer, which extended down to about 50 m above bottom, the concentrations of suspended matter ranged from 0.05 mgl−1 to 0.1 mgl−1. The intermediate layer, characterized by these concentrations, was found in all the deep areas of the Gulf. Concentrations of 0.1 mgl−1 to 0.4 mgl−1 were found in the bottom layer, with highest values occurring near the estuary.Budget calculations indicate that 5 × 106 tons/yr of suspended matter enter the Gulf from the rivers, and that erosion of the coastline and reworking of sediments are significant processes which contribute about 10.5 × 106 tons/yr of suspended matter. The net seaward transport at Cabot Strait is estimated as 5.5 × 106 tons/yr.

Suspended sediment transport from the Gironde estuary (France) onto the adjacent continental shelf

Current velocity and suspended sediment concentration measurements at anchor stations in the downstream extremity of the Gironde estuary indicate that during periods of high river discharge, a significant amount of suspended sediment is transported out of the estuary onto the adjacent continental shelf. The vertical profile of the residual (non-tidal) suspended sediment flux is similar to that of the residual current velocity, with a net upstream flux near the bottom and an overlying seaward-directed transport. The overall, depth-integrated result is a net seaward transport of suspended sediment out of the estuary. It appears that this net seaward transport varies directly with tidal amplitude. Aerial photography and water sampling indicate that during high river inflow, the downstream extremity of the turbidity maximum extends onto the continental shelf at ebb tide. The tidal and coastal current patterns of the inlet and inner shelf induce a northward transport of the turbid estuarine water, and at each tidal cycle, a certain amount of suspended sediment leaves the estuary; part of this sediment is deposited in a silt and clay zone on the continental shelf.