Net Import Of Sediment Research Articles

Storm and tidal interactions control sediment exchange in mixed-energy coastal systems.

Storms can have devasting effects on shorelines, causing flooding and the destruction of property and infrastructure. As global warming and the frequency and magnitude of tropical storms increase, barrier islands comprising 10% of the world's coast may undergo significant change caused by beach erosion, loss of dunes, and formation of washovers and tidal inlets. Understanding how storms affect sediment transport at tidal inlets is an understudied subject that directly influences barrier island erosional-depositional processes and long-term sediment budgets. This study models hydrodynamics and sediment transport at a conceptualized mixed-energy, mesotidal inlet system using 10 synthetic storm tracks. We investigate the provenance and the role of various storm characteristics and timing between the peak storm surge and high tide on sediment fluxes for different grain sizes. We find that most storms (38 of 40) cause a net import of sediment into the basin that is sourced primarily from the updrift and downdrift nearshore and secondly from the ebb-delta. Very little sediment comes from inlet channel scour. Cumulative (net) transport correlates well with peak significant wave height because wave height influences bottom shear stresses and sediment suspension on the ebb-tidal delta and in the nearshore. The duration of the storm surge also correlates with net transport because it controls the period of flood-directed currents. Our findings help explain the formation of flood deltas inside tidal inlets and the formation of sand shoals in backbarrier regions. Storm-induced enlargement of these deposits represents a permanent long-term loss of sand from barrier islands that will lead to erosion.

Can Unleveed Agricultural Fields in Deltas Keep Pace With Sea‐Level Rise?

AbstractEffective coastal management requires a fundamental understanding of the impacts of anthropogenic activities on sediment dynamics, yet it is challenging to isolate individual impacts in heavily altered regions. The Ayeyarwady Delta, Myanmar has been extensively deforested for agriculture but has few levees/polders. In this study, the relative resilience to subsidence was compared between a 45‐year‐old agricultural field and a nearby mangrove‐forest preserve. At both sites, water velocity and turbidity were measured in tidal channels, topography was mapped, and sediment cores were collected during 2018–2019. There was net sediment import at both sites due to sediment trapping by vegetation. Relative elevations were equivalent, suggesting that the field has aggraded at rates similar to the forest (0.7 cm/year). Unleveed fields may be less vulnerable to subsidence than leveed fields. However, uncertainties remain and the decision to replace mangroves with agricultural fields should weigh all the benefits provided by each environment.

Small-scale benthic faunal activities may lead to large-scale morphological change- A model based assessment

A novel 3-dimensional numerical model resolving dynamic interactions between environmental drivers and benthic fauna was applied to an idealized domain as analogous to typical tidal embayments. The aim is to derive insights into the role of benthic fauna in guiding long-term (decadal to centennial) coastal morphological evolution at a system scale. Three major functions by benthic fauna on sediment dynamics, namely bio-destabilization, bio-deposition and bio-stabilization, were incorporated. Results indicate that each of the three functions is able to guide a unique and profound long-term change of the embayment morphology. Bioturbation-induced sediment mixing and bio-destabilization may result in net sediment export out of the embayment, whilst bio-deposition and bio-stabilization tend to alter the embayment toward a net sediment import environment. Benthic fauna is able to modify large-scale hydro-morphology toward a state favorable for living. A combined effect of the three functions is not just a simple neutralization of the opposing impacts between sediment stabilization and destabilization. Rather, it leads to a unique response of the embayment morphology due to interactions between different benthic functional groups. Comparison with a real tidal embayment (Jade Bay from the Wadden Sea) justified a general validity of the model results in terms of statistics in both morphology and benthic fauna, and suggested an equal importance of interactions between benthic fauna and bed morphology and between different benthic functional groups in guiding morphological development of complex coastal systems.

Feedback between Basin Morphology and Sediment Transport at Tidal Inlets: Implications for Channel Shoaling

Increasing societal pressures (e.g., population growth and urbanization) are driving land use change practices in coastal areas that could potentially alter the hydrodynamics and sediment transport patterns near coastal inlets in ways that might exacerbate existing shoaling conditions. To investigate the potential impact of coastal development, a numerical model is used to predict the long-term evolution of an idealized lagoonal-type barrier island inlet under five different morphological conditions that transitioned from net sediment import to net sediment export. The simulations were designed to address the potential effect of inter-tidal placement and land reclamation on sediment transport and the resulting deposition/erosion patterns. Estuaries that were deeper and devoid of extensive tidal flats tended to promote sediment import and had a greater propensity to exacerbate channel shoaling. Simulations that were characteristic of inter-tidal placement showed net export, yet the likelihood of channel shoaling was increased because some of the material eroded from the tidal flats was deposited in the deeper channels as opposed to being carried out the inlet throat. Alternatively, it was found that regions in which the intertidal area was restricted to elevations higher in the tidal frame, which also showed a net export, produced greater sediment loss in the inter-tidal zone that tended to bypass the deeper sections, reducing the likelihood of channel shoaling.

Future sediment exchange between the Dutch Wadden Sea and North Sea Coast - Insights based on ASMITA modelling

The sediment exchange between the Dutch Wadden Sea and the North Sea coastal zone is of key importance to Dutch coastal management. Net sediment import from the coastal zone to the Wadden Sea results in coastal erosion which needs to be compensated through nourishments. At the same time net sediment import is the source of sediment for the intertidal flats in the Wadden Sea to adapt to sea level rise (SLR). Understanding the current and future sediment exchange is therefore essential for sustainable coastal management. Insights in the sediment exchange directly influence the coastal nourishment strategies applied to the Dutch coasts.Projections of the future sediment exchange between the Dutch Wadden Sea and the North Sea are established using the aggregated morphodynamic model ASMITA for five sea level rise scenarios, viz. the present rate of 2 mm/yr and accelerated rates of 4, 6, 8 and 17 mm/yr in 2100. The differences in the projected import rates between the five sea level rise scenarios until 2100 are not as large as the differences in sea level rise rates may suggest. For the Eastern part of the Dutch Wadden Sea, where the morphology is near its dynamic equilibrium, the projected import rate in 2100 varies with a factor 3 (300%), for sea level rise rates from 2 to 17 mm/yr (factor 8.5, 850%). In the western part of the Dutch Wadden Sea, where the morphology is still far from equilibrium due to the closure of the Zuiderzee, the projected import rate in 2100 varies a factor 1.45 (145%) for these sea level rise rates. For the total Dutch Wadden Sea this is a factor 1.7 (170%). The projected increase of the import rate until 2100 with respect to the present situation (2020) is up to a factor 1.45 (145%) for the highest sea level rise scenario, which is significant but not substantial.

Clariant significantly increased profitability in 2Q 2021 on the back of double-digit sales growth: 1H 2021 – Higher sales in all business areas and double-digit profitability improvement

This article contains data outlining the effects of increased storm intensity on estuarine salt marshes, previously evaluated in Pannozzo et al. (2021), using the Ribble Estuary, in North West England, as a case study. The hydrodynamic model Delft3D was used to simulate various surge height scenarios and evaluate the effects of increasing surge height on the sediment budget of the system. The data shows that an increase in storm intensity (i.e. surge height) promotes flood dominance and triggers a net import of sediment, positively contributing to the sediment budget of the marsh platform and the estuarine system. The timing of the storm surge relative to high or low tide, the duration of the surge and the presence of vegetation do not cause major changes in the sediment budget. This dataset could be used to evaluate how increased storm intensity might influence the sediment budget of estuaries in comparison to other types of coastal systems (e.g., bays) to illustrate how the response of salt marshes to increased storm intensity varies with a change in the hydrodynamics and sediment delivery dynamics of the system.

Salt marsh resilience to sea-level rise and increased storm intensity

Salt marshes are important ecosystems but their resilience to sea-level rise and possible increases in storm intensity is largely uncertain. The current paradigm is that a positive sediment budget supports the survival and accretion of salt marshes while sediment deprivation causes marsh degradation. However, few studies have investigated the combined impact of sea-level rise and increased storm intensity on the sediment budget of a salt marsh. This study investigates marsh resilience under the combined impact of various storm surge (0 m, 0.25 m, 0.5 m, 1.0 m, 2.0 m, 3.0 m and 4.0 m) and sea-level (+0 m, +0.3 m, +0.5 m, +0.8 m and +1.0 m) scenarios by using a sediment budget approach and the hydrodynamic model Delft3D. The Ribble Estuary, North-West England, whose salt marshes have been anthropogenically restored and have a high economic and environmental value, has been chosen as test case. We conclude that storm surges can positively contribute to the resilience of the salt marsh and estuarine system by promoting flood dominance and by triggering a net import of sediment. Conversely, sea-level rise can threaten the stability of the marsh by promoting ebb dominance and triggering a net export of sediment. Our results suggest that storm surges have a general tendency to counteract the decrease in sediment budget caused by sea-level rise. The timing of the storm surge relative to high or low tide, the duration of the surge, the change in tidal range and vegetation presence can also cause minor changes in the sediment budget.

Sediment Budget Estimates for a Highly Impacted Embayment with Extensive Wetland Loss

External sediment supply is an important control on wetland morphology and vulnerability to storms, sea-level rise, and land use change. Constraining sediment supply and net budgets is difficult due to multiple timescales of variability in hydrodynamic forcing and suspended sediment concentrations, as well as the fundamental limitations of measurement and modeling technologies. We used two independent observational campaigns and one hydrodynamic modeling effort to estimate the sediment supply to Jamaica Bay, New York, USA, an urbanized embayment with a history of extensive wetland loss. We found that all three estimates indicate a net import to the system, ranging from 36 to 74 kt/year, with a mean estimate of 55 kt/year ± 31 kt/year, which is consistent with a prior estimate derived from radionuclide tracers. Net sediment import is controlled by flood-ebb asymmetry in bed shear stress, which results in higher suspended sediment concentrations on flood tide relative to ebb. This indicates a seaward source of sediment that is resuspended by waves in the coastal ocean, likely offshore marine deposits or potentially from the adjacent Hudson River estuary. Despite the net sediment import, a simple sediment budget suggests that the rate of supply is not sufficient to maintain the present geomorphic planform of the system relative to sea-level rise. The convergent estimates from independent methods provide reasonable guidance as context for sediment-based restoration efforts.

Sediment transport in a restored, river-influenced Pacific Northwest estuary

Predicting the success of future investments in coastal and estuarine ecosystem restorations is limited by scarce data quantifying sediment budgets and transport processes of prior restorations. This study provides detailed analyses of the hydrodynamics and sediment fluxes of a recently restored U.S. Pacific Northwest estuary, a 61 ha former agricultural area near the mouth of the Stillaguamish River in Washington, USA. Water level, flow velocity, and suspended-sediment concentration (SSC) were measured between 21 March 2014 and 1 June 2015 at breaches excavated in the former flood-protection levee to determine transport patterns and the net sediment budget of the restoration area. SSC within the restoration area was primarily controlled by SSC variability of the nearby main stem Stillaguamish, but coastal processes also played a major role in sediment delivery. Fluvial sediment loading was dominated by runoff events associated with rainfall that lasted hours to a few days. Additionally, the 22 March 2014 SR 530 (Oso) landslide elevated sediment supply to the restoration area and coastal region for several weeks, indicating the importance of distal geomorphic events to coastal sediment budgets in small mountainous river systems. Sediment fluxes were controlled by river SSC and tidal dynamics, which set the quantity of water transported into the restoration area. Peak water discharge at the restoration area was about 12% of the river discharge, and peak sediment flux at the restoration area was about 5% of the river sediment discharge, although net sediment import was <1% of the total river load. Although sediment was imported to the restoration area, and inferred rates of accretion appear sufficient to keep pace with present rates of local sea-level rise, full recovery is challenged by significant lost grade from historical subsidence and will likely take decades to centuries. These results have implications for estuary restoration planning globally and indicate the importance of understanding coupled fluvial–coastal processes.

Impact of a water–sediment regulation scheme on the hydrodynamics and sediment conditions in the Sheyang Estuary

Tidal asymmetry (i.e., unequal ebb and flood intensities and durations) is a dominant factor in the dynamic equilibrium of current-related sediment transport in estuaries. The evolution of estuarine tides and sediment transport is of great importance to the development and utilization of channels in estuaries. Based on field hydrological and sedimentological data collected over multiple years, the hydrodynamic characteristics, suspended sediment concentration (SSC) and grain size, and median grain size of seabed sediments were analyzed to investigate the effects of the construction of a water–sediment regulation scheme (WSRS) in the Sheyang Estuary in China. Our results showed that small grain sizes (fine sand, silt, and clay) dominated and that suspension was the dominant transport mechanism in the area where tidal asymmetry was greatest. Tidal distortion occurred due to the construction of the WSRS. In this flood-dominant system (i.e., before and after the construction of phase I of the WSRS), the flood current velocity was significantly greater than the ebb current velocity, while it also had a shorter duration that caused higher SSCs during the flood current. This likely induced net sediment import to the estuary, which was the main cause of estuary infilling. Furthermore, the net transport of sediment into the estuary caused by the high flood current velocities resulted in the gradual deposition of sediments in the shipping channel of Sheyang Port. Ebb-dominant conditions (i.e., after the construction of phase II of the WSRS) induced net sediment export from the estuary, and sedimentation was significantly reduced. Since the ebb–flood current velocity ratio (Rv) and ebb–flood mean SSC ratio (Rs) could reflect tidal asymmetry and the direction of residual sediment transport with and without the WSRSs, they were used as important and sensitive indices to investigate hydraulic and sedimentological conditions. Results from this study can be used to improve the monitoring and management of navigation channels by providing insights into new methods of preventing or reducing sediment deposition in estuarine channels (i.e., by employing Rv and Rs).

Intense Storms Increase the Stability of Tidal Bays

AbstractCoastal bays and, specifically, back‐barrier tidal basins host productive ecosystems, coastal communities, and critical infrastructure. As sea level continues to rise and tropical cyclones increase in intensity, these coastal systems are increasingly at risk. Developing a sediment budget is imperative to understanding how storm events affect the system's resilience, where net import of sediment indicates growth and resilience against sea level rise, and net export of sediment indicates deterioration. Using high‐resolution numerical simulations, we show that intense storms import sediment into a system of bays in Virginia, USA. Duration and magnitude of storm surge are among the most important factors in sediment import, suggesting that intense storms increase the stability of tidal bays by providing the sediment necessary to counteract sea level rise. Since climate models project that tropical cyclones will increase in intensity in coming decades, our results have significant implications for the resilience of tidal bays and the future of coastal communities worldwide.

In situ measurements of shear stress, erosion and deposition in man-made tidal channels within a tidal saltmarsh

A field study was conducted in man-made ditches in a tidal saltmarsh in Lewes, Delaware, USA. Ditches are prevalent throughout tidal marshes along the Atlantic US coast, and influence hydrodynamics and sediment transport. The field study focused on measuring near-bed velocity, shear stress, sediment concentration, and bed level variability at 5 stations over a 3-week period. Velocities in the ditch (2–5 m wide, 1 m deep) peaked between 0.4 and 0.6 m/s and were slightly ebb dominated. Velocity and shear stress were maximum during a storm event, with peak shear stresses of 2 N/m2. Bed levels were estimated from acoustic amplitude return of a downward-looking velocity profiler. The bed level in the ditch at the landward locations increased ∼ 0.03 m over 3 weeks, while there was ∼ 0.01 m bed level decrease at the most seaward site suggesting a net import of sediment into the channel. At all sites, erosion (∼ 0.005–0.015 m) occurred during the accelerating phase of the flood tide, and accretion of a similar magnitude occurred during the decelerating phase of the ebb tide. This erosion-deposition sequence resulted in small net changes in bed level at the end of each tidal cycle. The intratidal behavior of the bed level was simulated using erosion and deposition flux equations based on shear stress, critical shear stress, and suspended sediment concentration. Erosion was predicted well with RMS errors on the order of 2⋅10−3 m. The bed level during the deposition phase could not be reproduced using the simple approach. Model inaccuracies for deposition were attributed to advection and variations in fall velocity due to flocculation that were not modeled due to lack of ground-truth observations.

Flocculation and floc break-up related to tidally induced turbulent shear in a low-turbidity, microtidal estuary

Flocculation and floc break-up dynamics were studied in two field campaigns with calm winds in the northernmost part of the Danish Wadden Sea. The studies were carried out using a LISST-100C together with CTD-instruments and a current meter. A quasi-Lagrangian profiling method was used to assess the dynamics on a short temporal scale (<tidal cycle) in the frame of the entire water column in the first campaign. In the second campaign the instruments were moored (Eulerian deployment) to get information on a longer temporal scale (>tidal cycle). The quasi-Lagrangian measurements showed strong flocculation and floc break-up dynamics in the lower part of the water column in the period around slack water. These dynamics were confirmed in the Eulerian deployments and were reoccurring in every tidal cycle. The dynamics were mostly governed by changes in turbulent shear. Strong microflocs with a lower mean threshold diameter of 50–60μm present at high turbulent shear flocculated to form fragile macroflocs with sizes of several hundred microns and mean diameters above 80μm around slack water periods. A hysteresis in floc break-up and flocculation was found at high water slack (HWS), as flocs formed at a root mean square velocity gradient (G) around 1s−1 were not broken up until the shear reached G>4s−1. As an effect, the flocculation time (time for flocculation from constituent microflocs to fragile macroflocs) was roughly 100min while the equivalent floc break-up time was roughly 200min. The asymmetrical, flood-dominated tidal wave showed how longer periods of low turbulent shear enhanced the hysteresis. The study demonstrates how the combination of the two deployment types will give a more adequate description of the flocculation dynamics over tidal cycles. Moreover, it shows the importance of incorporating specific flocculation aspects in numerical models, as a failure to incorporate the flocculation hysteresis results in an underestimation of the net import of sediment by about 30%.

Sediment Exchange between a River and Its Groyne Fields: Mobile-Bed Experiment

Experiments have been carried out in a mobile-bed laboratory flume in order to study the sediment exchange process between the main channel and the groyne fields. The flume represented half the width of a schematized river reach with a series of groynes. The experiment was designed to represent typical dimensions of the Dutch River Waal at a geometrical scale of 1:100. The conditions were set to guarantee bed load as well as suspended load sediment transport. Conditions with submerged and emerged groynes were investigated. In addition to traditional measurements, viz., bed-level changes, suspended sediment concentrations, and flow velocities, bed-form propagation was measured in two dimensions using a the particle image velocimetry technique. The results were analyzed with focus on sediment exchange mechanisms and sediment transport patterns. The results demonstrate that under all flow conditions there is a net import of sediment into the groyne fields. The prevailing transport mechanisms vary with the fl...

Modeling equilibrium bed profiles of short tidal embayments

In many tidal embayments, bottom patterns, such as the channel-shoal systems of the Wadden Sea, are observed. To gain understanding of the mechanisms that result in these bottom patterns, an idealized model is developed and analyzed for short tidal embayments. In this model, the water motion is described by the depth- and width-averaged shallow water equations and forced by a prescribed sea surface elevation at the entrance of the embayment. The bed evolves due to the divergence and convergence of suspended sediment fluxes. To model this suspended-load sediment transport, the three-dimensional advection–diffusion equation is integrated over depth and averaged over the width. One of the sediment fluxes in the resulting one-dimensional advection–diffusion equation is proportional to the gradient of the local water depth. In most models, this topographically induced flux is not present. Using standard continuation techniques, morphodynamic equilibria are obtained for different parameter values and forcing conditions. The bathymetry of the resulting equilibrium bed profiles and their dependency on parameters, such as the phase difference between the externally prescribed M2 and M4 tide and the sediment fall velocity, are explained physically. With this model, it is then shown that for embayments that are dominated by a net import of sediment, morphodynamic equilibria only exist up to a maximum embayment length. Furthermore, the sensitivity of the model to different morphological boundary conditions at the entrance of the embayment is investigated and it is demonstrated how this strongly influences the shape and number of possible equilibrium bottom profiles. This paper ends with a comparison between the developed model and field data for the Wadden Sea’s Ameland and Frisian inlets. When the model is forced with the observed M2 and M4 tidal constituents, morphodynamic equilibria can be found with embayment lengths similar to those observed in these inlets. However, this is only possible when the topographically induced suspended sediment flux is included. Without this flux, the maximum embayment length for which morphodynamic equilibria can be found is approximately a third of the observed length. The sensitivity of the model to the topographically induced sediment flux is discussed in detail.

Model analyses of the future water quality of the eutrophicated Ghar El Melh lagoon (Northern Tunisia)

Ghar El Melh (Northern Tunisia) is a shallow eutrophicated lagoon (total surface area 35.6 km2) which was one of the primary sites within the MELMARINA project. It is connected to the Mediterranean through a dredged opening and receives untreated sewage from two towns and a catchment of 131 km2. During the past decade the lagoon’s water quality has deteriorated and vegetation has changed. Ruppia sp. has diminished and green macroalgae (mainly Cladophora sp.) has increased. Frequent resuspension of fine alluvial sediment has reduced both water transparency and angiosperm (Ruppia sp.) vegetation cover. Flash floods during winter deliver most of the load of sediment and nutrients from the catchment to the lagoon. Options for restoring the angiosperm cover in the lagoon have been analysed using a model system developed for the main part of the lagoon consisting of a 2-D hydrodynamic model coupled to an ecological model. The ecological model includes a combined description of eutrophication and sediment transport with rooted vegetation (Ruppia sp.), benthic macroalgae, benthic microalgae, phytoplankton and suspended matter and nutrients. A separate sediment module includes pools of nutrients, organic carbon and fine sediment. Deposition and resuspension of the sediment pools are dependent on current and wind-generated shear stress. The load of nutrients and fine sediment was estimated through calibrating the model against measured water quality data and sediment deposition rates. The anthropogenic load from July 2003 to July 2004 was 33,000 tonnes fine sediment, 182 tonnes total N and 26 tonnes total P. A net import of sediment and nutrients from the sea was simulated, driven by the relatively high concentrations of suspended matter and particulate-bound N and P measured in coastal Mediterranean water outside the lagoon. Scenario simulations indicate that the lagoon acts as a deposition area for sediment N and P for boundary concentrations down to 50% of the measured concentrations at the coast outside the lagoon. A range of simulations with decreasing load of sediment and nutrients indicate that angiosperm vegetation will slowly recover with loads of 25% of current loads, combined with increased filtration by bivalves which would decrease suspended matter concentrations and increase transparency. Elevated concentrations of suspended matter and nutrients in the sea outside the lagoon are discussed in relation to a 30-year flash flood of the Mejerda River in February 2003. Coastal erosion as well as nutrients and sediment delivered by this river probably account for a general elevation of these concentrations close to the coast.

Morphological evolution of the Dee Estuary, Eastern Irish Sea, UK: A tidal asymmetry approach

Asymmetry in the tide (unequal ebb and flood duration) is a dominant factor in causing residual sediment transport and morphological changes in estuaries. The evolution of estuarine morphology is a process of dynamic equilibrium in the short-term, while these features are ephemeral in the long-term. In this study we investigate the spatial distribution of tidal distortion and asymmetry in the Dee estuary, UK, by 3-dimensional numerical modelling methods. High resolution LIDAR surveys are used to underpin and explain our numerical modelling results in terms of basin hypsometry and areas of recent erosion and deposition. Harmonic analysis of the numerical modelling results showed that the shallower intertidal areas (sand and mud banks) were the most tidally asymmetric, showing flood dominance. The main navigation channels showed some ebb dominance but the tides here were relatively undistorted. This overall flood dominance is likely to induce net sediment import to the Dee, which explains known historical morphological changes (large scale accretion over the last two centuries) and also recent morphological changes as seen from the LIDAR surveys (which show predominantly net accretion between 2003 and 2006). Hypsometrical analysis suggests the Dee may be approaching equilibrium, and that the flood dominance and sedimentation rate may therefore decrease in the future. In an infilling estuary, an increase in the area and elevation of tidal flats can eventually shift an estuary towards ebb dominance, as shown by previous research and by ‘idealised estuary’ modelling results presented in this study. The large tidal amplitude to hydraulic depth ratio of the Dee, however, suggests that the tidal flats would have to be very extensive indeed for this to occur.

Annual sediment flux estimates in a tidal strait using surrogate measurements

Annual suspended-sediment flux estimates through Carquinez Strait (the seaward boundary of Suisun Bay, California) are provided based on surrogate measurements for advective, dispersive, and Stokes drift flux. The surrogates are landward watershed discharge, suspended-sediment concentration at one location in the Strait, and the longitudinal salinity gradient. The first two surrogates substitute for tidally averaged discharge and velocity-weighted suspended-sediment concentration in the Strait, thereby providing advective flux estimates, while Stokes drift is estimated with suspended-sediment concentration alone. Dispersive flux is estimated using the product of longitudinal salinity gradient and the root-mean-square value of velocity-weighted suspended-sediment concentration as an added surrogate variable. Cross-sectional measurements validated the use of surrogates during the monitoring period. During high freshwater flow advective and dispersive flux were in the seaward direction, while landward dispersive flux dominated and advective flux approached zero during low freshwater flow. Stokes drift flux was consistently in the landward direction. Wetter than average years led to net export from Suisun Bay, while dry years led to net sediment import. Relatively low watershed sediment fluxes to Suisun Bay contribute to net export during the wet season, while gravitational circulation in Carquinez Strait and higher suspended-sediment concentrations in San Pablo Bay (seaward end of Carquinez Strait) are responsible for the net import of sediment during the dry season. Annual predictions of suspended-sediment fluxes, using these methods, will allow for a sediment budget for Suisun Bay, which has implications for marsh restoration and nutrient/contaminant transport. These methods also provide a general framework for estimating sediment fluxes in estuarine environments, where temporal and spatial variability of transport are large.

Modelling of cohesive sediment transport in a tidal lagoon—an annual budget

The annual net sediment flux for an enclosed estuary is usually calculated as a residual value in sediment budget studies based mainly on sediment core dating. In this study, the numerical model MIKE 21 MT was used to directly model the annual net transport of cohesive sediment for the Lister Dyb tidal area in the northern part of the European Wadden Sea. The model was calibrated by use of a combination of field data and calibration parameters derived from existing literature. The model reproduced the hydrodynamics satisfactorily as well as the concentration level and variation of suspended cohesive sediment concentration. Net import for the year 2002 was found to be 45 000 t of cohesive sediment, corresponding to a mean tidal period import of 64 t which compares to an approximate deposition of 0.1 mg l −1 of the tidal prism. Gross annual transport through a control cross section of the tidal inlet was 1 million ton meaning that 4–5% of the sediment transported in suspension was on average deposited within the tidal lagoon. This net transport was found to be rather constant from tidal period to tidal period thus supporting the theory that the processes of settling and scour lag, tidal velocity asymmetry together with sediment floc formation are the controlling processes for import of fine-grained sediment to shallow coastal plain estuaries. Temporary export of sediment from the area occurred during two major wind events in January and February when the maximum loss of sediment during one single tidal period was computed to be 40 000 t which is of the same order as the yearly net import of sediment. This indicates that net annual import of cohesive sediment to shallow estuaries may be much more variable than inferred from core dating studies found in literature.

Fluxes and exchange of suspended sediment in tidal inlets draining a degraded mangrove forest in Kenya

This study focuses on sediment exchange in the degraded Mwache mangrove forest wetland located in southern Kenya. It involved measurement of total and particulate organic suspended sediment concentrations (TSSC and POSC), tidal water elevation and current velocities. Results showed that in the heavily degraded backwater zone mangrove forest, the ebb and flood tide total sediment fluxes were of same order of magnitude, however, flood tide sediment fluxes were slightly higher than the ebb ones. In the moderately degraded frontwater zone mangrove forest, the flood tide sediment fluxes were more than 50% higher than the ebb tide fluxes. The peak net sedimentation in the highly degraded backwater zone was 4 g m −2 tide −1 but that in the moderately degraded frontwater zone was 63 g m −2 tide −1. In the frontwater zone of the mangrove forest, the peak instantaneous ebb tide sediment flux was 3206 kg tide −1 equivalent to 35.6 g m −2 tide −1 and the flood one 8574 kg tide −1 (95 g m −2 tide −1). The peak instantaneous flood and ebb tide particulate organic sediment (POS) fluxes in the frontwater zone mangrove forest were 1316 kg tide −1 (15 g m −2 tide −1) and 587 kg tide −1 (6.5 g m −2 tide −1), respectively. The peak ebb and flood tide sediment fluxes in the backwater mangrove forest were 3206 kg tide −1 (36 g m −2 tide −1) and 3305 kg tide −1 (36.7 g m −2 tide −1), respectively. In case of POS fluxes in the backwater zone mangrove forest, the peak flood period POS flux was 969 kg tide −1 (10.7 g m −2 tide −1) while the ebb period one was 484 kg tide −1 (5.4 g m −2 tide −1). In both highly degraded backwater and moderately degraded frontwater zone of the mangrove forest, there is net import of sediments. However, the net import is relatively lower in the backwater zone forest where the trapping efficiency is 27%. In the moderately degraded frontwater zone of the mangrove forest, the sediment trapping efficiency is 65%. The net sediment import occurs mainly in periods of high river discharge in both neap and spring tides, but occurs only in spring tides during dry season. The net accretion rates in the backwater and frontwater zone mangrove forests are 0.25 and 3.5 cm year −1, respectively.