Modelling sand–mud morphodynamics in the Friesche Zeegat

The objective of the study presented in this paper is to investigate the predictive capabilities of a process-based sand–mud model in a quantitative way. This recently developed sand–mud model bridges the gap between noncohesive sand models and cohesive mud models. It explicitly takes into account the interaction between these two sediment fractions and temporal and spatial bed composition changes in the sediment bed [Van Ledden (2002) 5:577–594, Van Ledden et al. (2004a) 24:1–11, Van Ledden et al. (2004b) 54:385–391]. The application of this model to idealized situations has demonstrated a good qualitative agreement between observed and computed bed levels and bed composition developments. However, in real-life situations, a realistic quantitative prediction of the magnitude and timescale of this response is important to assess the short-term and long-term impacts of human interventions and/or natural changes. For this purpose, the Friesche Zeegat in the Wadden Sea (the Netherlands) is used as a reference to hindcast the morphological response in the period 1970–1994. Due to the closure of the Lauwerszee in 1969, the tidal prism of this tidal basin was reduced by about 30%. Significant changes in the bed level and bed composition have occurred in the decades following the closure to adjust to the new hydrodynamic conditions. We modeled the long-term bed level and bed composition development in the Friesche Zeegat in the period 1970–1994 starting with the geometry of 1970 by using a research version of Delft3D, which incorporates the sand–mud formulations proposed by [Van Ledden (2002) 5:577–594].The computed total net deposition in the tidal basin in the period 1970–1994 agrees well with the observations, but the observed decrease of the import rate with time is not predicted. The model predicts net deposition in the deeper parts and at the intertidal area in the basin and net erosion in between, which resembles the observations qualitatively. Furthermore, the computed distribution of sand and mud in the basin of the Friesche Zeegat appears to be realistic. Analysis of the results shows that the absence of the decreasing import rate in the basin is caused by a poor quantitative prediction of the changes in the hypsometry of the basin. Because of this, the computed velocity asymmetry in the main channel tends toward flood dominance, whereas the observations indicate that the system is ebb-dominant in 1992. Although the sand–mud model needs to be further improved and verified, the results presented in this paper indicate that the model can be applied as a first step to estimate the effects of human interventions on the large-scale bed level and bed composition changes in tidal systems with sand and mud.     

Sand–mud morphodynamics in a short tidal basin

The influence of sand and mud transport on the morphological behaviour of a short tidal basin is investigated in this paper. For this purpose, a morphological model is applied in which sand and mud transport are included and the temporal and spatial bed composition variations are taken into account. Initially, the morphological development shows a sand wave near the entrance of the basin and a mud deposition wave more landward. A quasi equilibrium bed level profile is found after a long period (order century) with a sandy bed surface over almost the entire basin and only a small muddy area near the landward end. The dimensionless ratio between the deposition and erosion flux turns out to be a crucial parameter for the understanding of the observed behaviour. Comparison with previous studies on short tidal basins for sand indicates only that the presence of mud in a combined sand mud model does not change the equilibrium bed level profile considerably for the applied parameter settings herein, but drastically decreases the morphological time scale. Comparison between model results and field data of the Wadden Sea suggests that the obtained bed level and bed composition profile are realistic, indicating that the process-based sand mud model is a first step towards a better understanding of sand mud distributions in tidal basins.Responsible Editor: Jens Kappenberg     

Numerical modeling of bedload and suspended load contributions to morphological evolution of the Seine Estuary (France)

This numerical modeling study (i) assesses the influence of the sediment erosion process on the sediment dynamics and subsequent morphological changes of a mixed-sediment environment, the macrotidal Seine estuary, when non-cohesive particles are dominant within bed mixtures (non-cohesive regime), and (ii) investigates respective contributions of bedload and suspended load in these dynamics. A three dimensional (3D) process-based morphodynamic model was set up and run under realistic forcings (including tide, waves, wind, and river discharge) during a 1-year period. Applying erosion homogeneously to bed sediment in the non-cohesive regime, i.e., average erosion parameters in the erosion law (especially the erodibility parameter, E₀), leads to higher resuspension of fine sediment due to the presence of coarser fractions within mixtures, compared to the case of an independent treatment of erosion for each sediment class. This results in more pronounced horizontal sediment flux (two-fold increase for sand, +30% for mud) and erosion/deposition patterns (up to a two-fold increase in erosion over shoals, generally associated with some coarsening of bed sediment). Compared to observed bathymetric changes, more relevant erosion/deposition patterns are derived from the model when independent resuspension fluxes are considered in the non-cohesive regime. These results suggest that this kind of approach may be more relevant when local grain-size distributions become heterogeneous and multimodal for non-cohesive particles. Bedload transport appears to be a non-dominant but significant contributor to the sediment dynamics of the Seine Estuary mouth. The residual bedload flux represents, on average, between 17 and 38% of the suspended sand flux, its contribution generally increasing when bed sediment becomes coarser (can become dominant at specific locations). The average orientation of residual fluxes and erosion/deposition patterns caused by bedload generally follow those resulting from suspended sediment dynamics. Sediment mass budgets cumulated over the simulated year reveal a relative contribution of bedload to total mass budgets around 25% over large erosion areas of shoals, which can even become higher in sedimentation zones. However, bedload-induced dynamics can locally differ from the dynamics related to suspended load, resulting in specific residual transport, erosion/deposition patterns, and changes in seabed nature.     

Effect of wave–bedform feedbacks on the formation of,and grain sorting over shoreface-connected sand ridges

The influence of wave–bedform feedbacks on both the initial formation of shoreface-connected sand ridges (sfcr) and on grain size sorting over these ridges on micro-tidal inner shelves is studied. Also, the effect of sediment sorting on the growth and the migration of sfcr is investigated. This is done by applying a linear stability analysis to an idealized process-based morphodynamic model, which simulates the initial growth of sfcr due to the positive coupling between waves, currents, and an erodible bed. The sediment consists of sand grains with two different sizes. New elements with respect to earlier studies on grain sorting over sfcr are that wave-topography interactions are explicitly accounted for, entrainment of sediment depends on bottom roughness, and transport of suspended sediment involves settling lag effects. The results of the model indicate that sediment sorting causes a reduction of the growth rate and migration speed of sfcr, whereas the wavelength is only slightly affected. In the case where the entrainment of suspended sediment depends on bottom roughness, the coarsest sediment is found in the troughs; otherwise, the finest sediment occurs in the troughs. Compared to previous work, modeled maximum variations in the mean grain size over the topography are in better agreement with field observations. Settling lag effects are important for the damping of high-wavenumber mode instabilities such that a preferred wavelength of the bedforms is obtained.     

The settling dynamics of flocculating mud and sand mixtures: part 2—numerical modelling

Estuarine and coastal sediment transport is characterised by the transport of both sand-sized particles (of diameter greater than 63?μm) and muddy fine-grained sediments (silt, diameter less than 63?μm; clay, diameter less than 2?μm). These fractions are traditionally considered as non-cohesive and cohesive, respectively, because of the negligible physico-chemical attraction that occurs between sand grains. However, the flocculation of sediment particles is not only caused by physico-chemical attraction. Cohesivity of sediment is also caused by biology, in particular the sticky extra-cellular polymeric substances secreted by diatoms, and the effect of biology in binding sediment particles can be much larger than that of physico-chemical attraction. As demonstrated by Manning (2008) and further expanded in part 1 of this paper (Manning et al., submitted), the greater binding effect of biology allows sand particles to flocculate with mud. In many estuaries, both the sand and fine sediment fractions are transported in significant quantities. Many of the more common sediment transport modelling suites now have the capability to combine mud and sand transport. However, in all of these modelling approaches, the modelling of mixed sediment transport has still essentially separated the modelling of sand and mud fractions assuming that these different fractions do not interact except at the bed. However, the use of in situ video techniques has greatly enhanced the accuracy and reliability of settling velocity measurements and has led to a re-appraisal of this widely held assumption. Measurements of settling velocity in mixed sands presented by Manning et al. (2009) have shown strong evidence for the flocculation of mixed sediments, whilst the greater understanding of the role of biology in flocculation has identified mechanisms by which this mud-sand flocculation can occur. In the first part of this paper (Manning et al., submitted), the development of an empirical flocculation model is described which represents the interaction between sand and mud particles in the flocculation process. Measurements of the settling velocity of varying mud-sand mixtures are described, and empirical algorithms governing the variation of settling velocity with turbulence, suspended sediment concentration and mud-sand content are derived. The second part of this paper continues the theme of examination of the effects of mud-sand interaction on flocculation. A 1DV mixed transport model is developed and used to reproduce the vertical transport of mixed sediment fractions. The 1DV model is used to reproduce the measured settling velocities in the laboratory experiments described in the part 1 paper and also to reproduce measurements of concentration of mixed sediments in the Outer Thames. In both modelling exercises, the model is run using the algorithms developed in part 1 and repeated using an assumption of no interaction between mud and sand in the flocculation process. The results of the modelling show a significant improvement in the ability of the 1DV to reproduce the observed sediment behaviour when the empirical equations are used. This represents further strong evidence of the interaction between sand and mud in the flocculation process.     

Modeling profile shape evolution for accreting tidal flats composed of mud and sand: A case study of the central Jiangsu coast, China

The evolution of the shore-normal profile shape of accreting tidal flats is controlled mainly by tidally induced mud and sand transport. To understand the evolution processes, a model is developed to simulate the tidal flat profile changes in response to spring-neap tidal cycles. The model treats both sand and mud transport patterns over the tidal flats and adopts an algorithm to deal with the areas near the high water (HW) level on springs. The model is applied to an accreting tidal flat on the central Jiangsu coast, to investigate the relationship between the equilibrium profile shape of the tidal flat and the various influencing factors (e.g. initial profile shape of tidal flat, tidal range and sediment supply). Based on the modeling results the following conclusions are derived: (1) the accreting tidal flat tends to be convex in profile shape when it reaches an equilibrium state; (2) sediment supply is a key factor affecting the width and accretion-erosion status of the tidal flat; (3) filling the area close to high water (HW) on spring tides is essential for reproducing the shape evolution and the morphodynamic behavior of tidal flats; (4) after an equilibrium shape is formed, a tidal flat with abundant sediment supply can steadily prograde to seaward, at the same time maintaining the equilibrium shape; and (5) the modeled width and the slope of the tidal flat are consistent with those of the central Jiangsu coast when the parameters adopted in the model are appropriate for the local conditions.     

Calibration of an estuarine sediment transport model to sediment fluxes as an intermediate step for simulation of geomorphic evolution

Modeling geomorphic evolution in estuaries is necessary to model the fate of legacy contaminants in the bed sediment and the effect of climate change, watershed alterations, sea level rise, construction projects, and restoration efforts. Coupled hydrodynamic and sediment transport models used for this purpose typically are calibrated to water level, currents, and/or suspended-sediment concentrations. However, small errors in these tidal-timescale models can accumulate to cause major errors in geomorphic evolution, which may not be obvious. Here we present an intermediate step towards simulating decadal-timescale geomorphic change: calibration to estimated sediment fluxes (mass/time) at two cross-sections within an estuary. Accurate representation of sediment fluxes gives confidence in representation of sediment supply to and from the estuary during those periods. Several years of sediment flux data are available for the landward and seaward boundaries of Suisun Bay, California, the landward-most embayment of San Francisco Bay. Sediment flux observations suggest that episodic freshwater flows export sediment from Suisun Bay, while gravitational circulation during the dry season imports sediment from seaward sources. The Regional Oceanic Modeling System (ROMS), a three-dimensional coupled hydrodynamic/sediment transport model, was adapted for Suisun Bay, for the purposes of hindcasting 19th and 20th century bathymetric change, and simulating geomorphic response to sea level rise and climatic variability in the 21st century. The sediment transport parameters were calibrated using the sediment flux data from 1997 (a relatively wet year) and 2004 (a relatively dry year). The remaining years of data (1998, 2002, 2003) were used for validation. The model represents the inter-annual and annual sediment flux variability, while net sediment import/export is accurately modeled for three of the five years. The use of sediment flux data for calibrating an estuarine geomorphic model guarantees that modeled geomorphic evolution will not exceed the actual supply of sediment from the watershed and seaward sources during the calibration period. Decadal trends in sediment supply (and therefore fluxes) can accumulate to alter decadal geomorphic change. Therefore, simulations of future geomorphic evolution are bolstered by this intermediate calibration step.     

A model of inter-annual variability in beach levels

Beach profile data, collected twice per year at 19 stations over a 25 km length of coastline in Tremadoc Bay, have been analysed to quantify the inter-annual variability in beach levels over a 7 year period and the results compared against the output of a numerical model. Using hourly wind data as forcing, the morphological development of northern Tremadoc Bay was simulated by wave, tidal, longshore transport, total transport and bed level change models. The modelling methodology was efficient and innovative, allowing realistic simulations of long duration with a time step of 1 h, hence capturing the high frequency nature of wind events. The model was run for each of the 7 autumn/winter periods (generally November–April) and the modelled net change in beach levels compared with the data from all 19 stations. The model results had reasonable agreement with the beach profile surveys. However, the observed magnitude of bed level change in the bay lagged the model output by 1 year, indicating that sediment processes acting over a larger area are important in a relatively localised study of inter-annual variability.     

Sediment-transport modeling on Southern Californian shelves: A ROMS case study

Suspended sediment-transport processes in Santa Monica and San Pedro Bay are analyzed using the sediment-transport capabilities of the Regional Oceanic Modeling System (roms). A one-month simulation for December 2001 has been carried out with a set of nested domains. The model inputs include tides, winds, surface waves, and idealized initial sediment conditions for sand and non-cohesive silt. Apart from the control run, the sensitivity of the results to surface waves, ripple roughness and bed armoring has been analyzed. From the control experiment, the horizontal transport of sand turns out to be limited to within a few km of the nearshore erosion zones. During high wave events, silt is transported over further distances and also partly offshelf in distinct plumes. The effectiveness of horizontal silt transport depends strongly on vertical mixing due to both surface wind stress and wave-enhanced bottom stress. High wave events coincident with strong winds (hence strong vertical mixing) are the most optimal conditions for sediment-transport. Excluding wave effects in the simulation shows that surface waves are the dominant factor in resuspending bed material on the Southern Californian shelves. The sensitivity experiments also show that the direct influence of additional ripple roughness on erosion and deposition is relatively weak. Switching off bed armoring locally results in increases of near-bottom concentrations by a factor of 20 for silt and a factor of 5 for sand as well as stronger spatial gradients in grain size.     

Sediment dispersal and deposition due to sand mining in the coastal waters of Korea

Understanding the impact of marine sand mining operations in a complex coastal environment requires a combined observational and modeling approach. Here, we use field measurements collected during mining operations in Kyunggi Bay, Korea to develop sediment parameters and source conditions for a three-dimensional (3D) sediment transport model built on the Regional Ocean Modeling System (ROMS). The model is run with realistic forcing obtained from a 9 km meteorological model, tides, and river discharges. The resulting vertical and horizontal distributions of sediment show encouraging agreement with the field data, demonstrating markedly different dispersal patterns due largely to the differential settling of the various sand classes. The resulting depositional patterns suggest that only the coarser size classes (500 and 250 μm) particles remain close to the mined site, while finer size classes are widely dispersed. These results suggest that this new methodology of multi-size class, 3D sediment transport modeling is quite promising, and further work is ongoing to include more realistic representation of sediment resuspension processes.     

Modeling the tidal channel morphodynamics in a macro-tidal embayment,Hangzhou Bay,China

The Hangzhou Bay is a macro-tidal bay located to the south of the Changjiang estuary in China. Along its northern shore, a large-scale tidal channel system has developed, which includes a main northern tidal channel, with a length of more than 50 km and a width up to 10 km, and a secondary southern tidal channel. A process-based morphodynamic model, incorporating the cohesive sediment transport module of Delft3D, is used to analyze the physical processes and mechanisms underlying the formation and evolution of this tidal channel system. The results show that spatial gradients of flood dominance, caused by boundary enhancement via current convergences, is responsible for the formation of the channel system, due to a combination of the various factors such as funnel-shaped geometry hindering associated with the presence of islands, and flow deviation by the southern tidal flat and so on. The model results agree well with the real morphological features. This study also indicates that the reclamation of the southern tidal flat imposes a profound influence on the morphological evolution of the tidal channel system in the Hangzhou Bay. It is feasible to use the model to simulate long-term estuarine morphological changes with cohesive sediment settings.     

The impact of wind-waves and sea level rise on the morphodynamics of a sandy estuarine shoal

Intertidal shoals are pronounced morphological features found in many estuaries worldwide. Apart from maintaining an ecologically unique intertidal environment, shoals also protect adjacent dyke systems by attenuating waves. The fate of sandy shoals under anticipated sea level rise (SLR) scenarios is underexplored. The current research investigates the long-term morphodynamic evolution of estuarine sandy shoals under forcing by short fetch, locally generated wind-waves, tides, and SLR by means of a numerical, process-based model (Delft3D). The focus lies on a sheltered shoal complex in the Western Scheldt, the Netherlands. Starting from the initial, 1963 bathymetry, we model 50-year morphodynamic development with schematized wind-wave forcing. We analyze in detail the impact of locally generated wind-waves on shoal formation. Finally, we impose regional SLR of 1.10 m and 1.95 m for 100 years. Model results show that, on the spatial scale of intertidal flats, small, locally generated wind-waves lower and widen the shoals while the adjacent channels deepen. However, on the estuarine system scale, wind-waves do not lead to fundamentally different channel–shoal patterns and morphodynamic evolution trends. This suggests that channel–shoal formation is mainly due to tide residual sediment transports, with wind-waves playing a secondary role. SLR leads to a notable intertidal area loss, despite a continuous heightening of the shoals, implying that morphodynamic adaptation lags behind SLR. The inclusion of wind-waves does not fundamentally change the reaction of the estuarine shoal to SLR. Future research may focus on exploring the impact of including multiple sediment classes.     

Suspended sediment dynamics at high and low storm flows in two small watersheds

A record spanning almost 20 years of suspended sediment and discharge measurements on two reaches of an agricultural watershed is used to assess the influence of in‐channel sediment supplies and bed composition on suspended sediment concentrations (SSC). We analyse discharge‐SSC relationships from two small streams of similar hydrology, climate and land use but widely different bed compositions (one dominated by sand, the other by gravel). Given that sand‐dominated systems have more fine sediment available for transport, we use bed composition and the relative proportion of surface sand and gravel to be representative of in‐channel sediment supply. Both high flow events and lower flows associated with onset and late recessional storm flow (‘low flows’) are analysed in order to distinguish external from in‐channel sources of sediment and to assess the relationship between low flows and sediment supply. We find that SSC during low flows is affected by changes to sediment supply, not just discharge capacity, indicated by the variation in the discharge‐SSC relationship both within and between low flows. Results also demonstrate that suspended sediment and discharge dynamics differ between reaches; high bed sand fractions provide a steady supply of sediment that is quickly replenished, resulting in more frequent sediment‐mobilizing low flow and relatively constant SSC between floods. In contrast, SSC of a gravel‐dominated reach vary widely between events, with high SSC generally associated with only one or two high‐flow events. Results lend support to the idea that fine sediment is both more available and more easily transported from sand‐dominated streambeds, especially during low flows, providing evidence that bed composition and in‐channel sediment supplies may play important roles in the mobilization and transport of fine sediment. In addition, the analysis of low‐flow conditions, an approach unique to this study, provides insight into alternative and potentially significant factors that control fine sediment dynamics. Copyright © 2007 John Wiley & Sons, Ltd.     

Net sediment transport in tidal basins: quantifying the tidal barotropic mechanisms in a unified framework

Net sediment transport in tidal basins is a subtle imbalance between large fluxes produced by the flood/ebb alternation. The imbalance arises from several mechanisms of suspended transport. Lag effects and tidal asymmetries are regarded as dominant, but defined in different frames of reference (Lagrangian and Eulerian, respectively). A quantitative ranking of their effectiveness is therefore missing. Furthermore, although wind waves are recognized as crucial for tidal flats’ morphodynamics, a systematic analysis of the interaction with tidal mechanisms has not been carried out so far. We review the tide-induced barotropic mechanisms and discuss the shortcomings of their current classification for numerical process-based models. Hence, we conceive a unified Eulerian framework accounting for wave-induced resuspension. A new methodology is proposed to decompose the sediment fluxes accordingly, which is applicable without needing (semi-) analytical approximations. The approach is tested with a one-dimensional model of the Vlie basin, Wadden Sea (The Netherlands). Results show that lag-driven transport is dominant for the finer fractions (silt and mud). In absence of waves, net sediment fluxes are landward and spatial (advective) lag effects are dominant. In presence of waves, sediment can be exported from the tidal flats and temporal (local) lag effects are dominant. Conversely, sand transport is dominated by the asymmetry of peak ebb/flood velocities. We show that the direction of lag-driven transport can be estimated by the gradient of hydrodynamic energy. In agreement with previous studies, our results support the conceptualization of tidal flats’ equilibrium as a simplified balance between tidal mechanisms and wave resuspension.     

Growth of large-scale bed forms due to storm-driven and tidal currents: a model approach

An idealized morphodynamic model is used to gain further understanding about the formation and characteristics of shoreface-connected sand ridges and tidal sand banks on the continental shelf. The model consists of the 2D shallow water equations, supplemented with a sediment transport formulation and describes the initial feedback between currents and small amplitude bed forms. The behaviour of bed forms during both storm and fair weather conditions is analyzed. This is relevant in case of coastal seas characterized by tidal motion, where the latter causes continuous transport of sediment as bed load.The new aspects of this work are the incorporation of both steady and tidal currents (represented by an M₂ and M₄ component) in the external forcing, in combination with dominant suspended sediment transport during storms. The results indicate that the dynamics during storms and fair weather strongly differ, causing different types of bed forms to develop. Shoreface-connected sand ridges mainly form during storm conditions, whereas if fair weather conditions prevail the more offshore located tidal sand banks develop. Including the M₄ tide changes the properties of the bed forms, such as growth rates and migration speeds, due to tidal asymmetry. Finally a probabilistic formulation of the storm and fair weather realization of the model is used to find conditions for which both types of large-scale bed forms occur simultaneously. These conditions turn out to be a low storm fraction and the presence strong tidal currents in combination with strong steady currents during storms.     

Effect of self-weight consolidation on a hydro-sedimentological model for the Río de la Plata estuary

The effect of the consolidation process on the morphodynamics and fine sediment dynamics of the Río de la Plata estuary is explored through a circulation-wave-sediment transport model. The consolidation model is calibrated based on settling column experimental data. Different simulations are done in order to initialize the mud layer distribution and to investigate the impact of different erosion parameter assumptions on the modeled sediment dynamics. Finally a two-year simulation is done with and without the consolidation process and realistic hydrodynamic forcings. Considering the consolidation process, the model correctly reproduces measured vertical density profiles in the Montevideo Bay access channel. The simulated suspended sediment dynamics behavior in Montevideo Bay with the consolidation process provides a more realistic deposition pattern in regard to the dredging activities.     

Flocculation,heavy metals (Cu,Pb, Zn) and the sand–mud transition on the Adriatic continental shelf,Italy

Across a limited depth range (5–10 m) on many continental shelves, the dominant sediment size changes from sand to mud. This important boundary, called the sand–mud transition (SMT), separates distinct benthic habitats, causes a significant change in acoustic backscatter, represents a key facies change, and delimits more surface-reactive mud from less surface-reactive sand. With the goal of improving dynamical understanding of the SMT, surficial sediments were characterized across two SMTs on the Adriatic continental shelf of Italy. Geometric mean diameter, specific surface area (SSA), mud fraction (<63 μm) and heavy metal concentrations were all measured. The SMT related to the Tronto River is identified between 15 and 20 m water depth while the SMT associated with the Pescara River varies between 15 and 25 m water depth. The sediment properties correlate with a new, process-based sedimentological parameter that quantifies the fraction of the sediment in the seabed that was delivered as flocs. These correlations suggest that floc dynamics exert strong influence over sediment textural properties and metal concentrations. Relative constancy in the depth of the SMT along this portion of the margin and its lack of evolution over a period during which sediment input to the margin has dramatically decreased suggest that on the Adriatic continental shelf energy is the dominant control on the depth of the SMT.     

Morphological and sedimentary features of sandy-muddy transitional beaches in estuaries and bays along mesotidal to macrotidal coasts

Sandy-muddy transitional beaches (SMT-Beaches), representing the transition from sandy beaches to tidal mudflats, should theoretically develop very different morphological and sedimentological characteristics in river estuaries and in semi-enclosed bays due to their contrasting dynamic sedimentary environments. Evidence, however, is rare in the scientific literature. To reveal these morphological and sedimentary differences, the sand–mud transition (SMT) boundary distribution, beach profiles, and surface and downcore sediment grain-size compositions of 27 SMT-Beaches located along mesotidal to macrotidal coasts of the western Taiwan Strait, southeastern China, were investigated. The results show that typical estuarine SMT-Beaches are mainly characterized by an ambiguous SMT, a long distance between the SMT and the coastline (31–302 m), lower SMT and inflection point altitudes (average –0.76 m and –0.04 m), and lower upper beach gradients (~0.068) with fine sand. Estuarine SMT-Beach sediments display clear interbedded mud and sand layers, implying potential SMT migrations over various timescales. By contrast, typical bay SMT-Beaches are characterized by distinct SMT, a short distance between the SMT and the coastline (11–52 m), higher SMT and inflection point altitudes (~0.24 m and ~0.35 m), and narrower upper beaches with higher gradients (~0.095) and coarse sand. Bay SMT-Beaches present relatively stable sedimentary sequences and a narrow gravel belt surrounding the inflection point and/or SMT. These morphological and sedimentary differences between the two SMT-Beach types are initially constrained by sediment supply and transport and are further affected by tide conditions and wave climate. Sediment supply and transport predominately control the sediment structures, while the tidal range strongly influences spatial variations in SMT distances. Wave climate normally drives SMT altitude variations. This study highlights the morphological and sedimentary differences in SMT-Beaches in estuaries and bays, providing important knowledge for further revealing their morphodynamic processes and potential future nourishment. © 2020 John Wiley & Sons, Ltd.     

Field measurement and modeling of near-bed sediment transport processes with fluid mud layer in Tokyo Bay

Tokyo Bay is one of the estuaries in Japan with a high population of almost 26 million people in the basin area. One of the major concerns for the environment in this water area is the decreasing ecosystem functions including the deterioration of water and sediment qualities caused by various anthropogenic activities. Since the bottom sediments around almost the entire area of the inner bay consist of fine materials with a high organic content, which cause the deterioration of water quality through processes such as hypoxia, an understanding of the fine sediment dynamics in the Bay is crucial for an environmental assessment of the water area. This paper proposes a model for the key processes of fine sediment dynamics, which reflects field data about muddy bed structures and their dynamics obtained during the monitoring campaign in 2007. One of the specific features of the sediment in the Bay at present is the persistent existence of fluid mud layers (water content over 300?%) with a thickness of around a few decimeters, which might be caused by deposition of abundant organic particles due to eutrophication. The present study shows that diffusion flux model delivers quite reliable results for estimating erosion flux from the top of fluid mud layers after calibrating the model parameter against the time series data of vertical flux measured by an acoustic Doppler velocimeter system. This study also derives analytical solutions, based on the Bingham fluid concept, of advection flux in the fluid mud layer on which external shear stress force is applied.     

Field measurement and modeling of near-bed sediment transport processes with fluid mud layer in Tokyo Bay

Tokyo Bay is one of the estuaries in Japan with a high population of almost 26 million people in the basin area. One of the major concerns for the environment in this water area is the decreasing ecosystem functions including the deterioration of water and sediment qualities caused by various anthropogenic activities. Since the bottom sediments around almost the entire area of the inner bay consist of fine materials with a high organic content, which cause the deterioration of water quality through processes such as hypoxia, an understanding of the fine sediment dynamics in the Bay is crucial for an environmental assessment of the water area. This paper proposes a model for the key processes of fine sediment dynamics, which reflects field data about muddy bed structures and their dynamics obtained during the monitoring campaign in 2007. One of the specific features of the sediment in the Bay at present is the persistent existence of fluid mud layers (water content over 300 %) with a thickness of around a few decimeters, which might be caused by deposition of abundant organic particles due to eutrophication. The present study shows that diffusion flux model delivers quite reliable results for estimating erosion flux from the top of fluid mud layers after calibrating the model parameter against the time series data of vertical flux measured by an acoustic Doppler velocimeter system. This study also derives analytical solutions, based on the Bingham fluid concept, of advection flux in the fluid mud layer on which external shear stress force is applied.