Suspended Sediment Concentration Gradient Research Articles

Mechanisms of sediment trapping in coastal embayments off the Shandong Peninsula in summer—A case study in Weihai Bay

This paper investigates sediment trapping in coastal embayments during summer seasons based on observation data obtained from two instrumented tripods that were placed at the mouth of Weihai Bay on north coast of Shandong Peninsula, China, over a 15-day period between 5 and August 20, 2018. The data consisted of time series of water depths, current profiles, wave parameters, near-bed flow velocities, turbidity, water salinity and temperature. Distributions of suspended sediment concentration (SSC) and water temperature in the bay and adjacent sea areas were also investigated at 87 grid stations from July 31 to August 4, 2018. From these data the bed shear stresses, residual currents and suspended sediment fluxes (SSF) were calculated. The results indicated that the variability in the SSC at the northern mouth was mainly controlled by local sediment resuspension while at the southern mouth horizontal advection due to the SSC gradient was the dominant mechanism. To identify the main drivers of the sediment transport, the SSF was analyzed using the decomposition method. The seaward advective transport due to residual flow and landward tidal pumping effect dominated the total near-bed sediment transport at the northern and southern mouths, respectively. Therefore, trapping processes occurred mainly through the southern mouth. The suspended sediment fluxes (SSF) during neap and spring tides at the southern mouth were 0.053 and 0.321 g m−2 s−1, respectively, while during moderate wave conditions (Hs > 0.8 m), the SSF was two times larger than the spring tide. In summer, the fine-grained sediments trapped in the bay were mainly derived from resuspension off the eastern tip of the Shandong Peninsula and thus is indirectly from the Yellow River. During such sediment transport and trapping processes, upwelling and winds can also play important roles.

Marsh channel morphological response to sea level rise and sediment supply

A 2D tidal-dispersion model is implemented to simulate the morphodynamics of marsh channels and their response to changes in Sea Level Rise (SLR) rate and sediment supply. The model is applied to the mesotidal marshes of Plum Island Estuary (MA, USA). The peak tidal flow predicted by the model agrees with that measured in a channel for different spring-neap conditions. The model also reproduces the present channel geometry, which is characterized by a spatially variable width to depth ratio. The model suggests that an increase in sediment supply during colonial times (17th century) narrowed the channels of the existing fringing marshes. This narrowing was however temporary, and channels returned to their pre-disturbed width by the end of the 19th century. The model highlights that Suspended Sediment Concentration (SSC) within the channels did not respond uniformly to the changes in sediment supply, and that gradients in SSC can be used to infer sediment input to or export from the marsh. The model also reproduces the spatial patterns of channel widening that occurred since the beginning of the 20th century as a result of the increase in SLR rate from ∼1 to ∼2.6 mm/yr. Under the present SLR rate and sediment supply, the largest channels (>20 m wide) in Plum Island Estuary are predicted to widen by 5–10 m over the next centuries. Both an increase in SLR rate and a drop in sediment supply will increase the amount of widening, but will create different widening fingerprints: the latter will cause more spatially variable widening than the former. As such, spatial patterns of channel widening might theoretically be used to infer whether marsh loss is mainly driven by an increase in SLR rate or a decrease in sediment supply.

Distributions of suspended sediment concentration in the Yellow Sea and the East China Sea based on field surveys during the four seasons of 2011

The seasonal and spatial variation of the suspended sediment concentration (SSC) in the Yellow Sea and the East China Sea (YSECS) is studied using turbidity, temperature and velocity field data from all seasons of 2011. The important hydrodynamic factors affecting the SSC distribution are discussed, based on the field data. The data provide a picture of the seasonal evolution of the SSC in the YSECS in a single year. The results show that the highest surface layer SSC was measured in spring and winter, while the summer had the lowest surface layer SSC. In the bottom layer, the SSC distribution patterns were similar in all seasons except that a much higher SSC water plume extended from the Subei Shoal to the East China Sea shelf in winter survey. The vertical SSC gradient was stronger in the shallow water area than the deep water area, and it was stronger in summer and autumn than in winter and spring. We argue that wind-induced vertical mixing dominated the SSC distribution in the surface layer while the tidal currents played an important role in the bottom layer and sub-tidal currents dominated the horizontal pattern of the SSC distribution pattern over the shelf area. In particular, the presence of the summer thermocline limited the vertical mixing in summer, decreasing the SSC in the surface layer. The front between the East China Sea Coastal Current and the Taiwan Warm Current appears to prevent the high SSC water from being transported to the shelf. The sediment discharged by the Changjiang River was found to be concentrated in the estuary area and influenced the shelf SSC distribution only through the action of the subtidal currents.

The effects of flow stratification by non-cohesive sediment on transport in high-energy wave-driven flows

The two-way effects of the time-varying suppression of turbulence by gradients in suspended sediment concentration have been investigated using a modified form of the Generalized Ocean Turbulence Model (GOTM). Field measurements of fluid velocities and sediment concentrations collected under high-energy conditions (mobility number ≈ 900) have been simulated both including and neglecting the feedback between sediment and turbulence. The results show that, when present, this feedback increases the wave-coherent component of transport relative to the mean component of transport, which can even change the direction of transport. Comparisons between measured and simulated time series of near-bed sediment concentrations show great coherence (0.95 correlation) and it is argued that the differences in net transport rates may be partially explained by the use of a uniform grain size in the simulations. It is seen that the effects of sediment stratification scale with orbital velocity divided by sediment setting velocity,um/ws, for all grain sizes.

Estimating suspended sediment concentration using backscatterance from an acoustic Doppler profiling current meter at a site with strong tidal currents

Since 1998, ferry observations have been carried out in the Marsdiep tidal inlet (Dutch Wadden Sea), using amongst other instruments a vessel-mounted acoustic Doppler current profiler (ADCP). Besides 32 cross-sections a day of current velocity data, the instrument also records the echo intensity, which has been interpreted in terms of suspended sediment concentration (SSC) before (Thorne and Hanes, Cont Shelf Res 22:603–632, 2002). However, we show herein that the random phase scattering model as outlined by Thorne and Hanes (Cont Shelf Res 22:603–632, 2002), predicts unrealistically high values of SSC if the depth-averaged current velocity exceeds approximately 0.7 m/s. Therefore, we extended the random phase scattering model by including scattering by turbulence-induced variability in SSC. The important mechanism is that when SSC fluctuations are present at length, scales of the order of the acoustic wavelength, the phase of the returned signal is no longer random and causes stronger backscatter. Such SSC fluctuations occur as a result of turbulent eddies in combination with an SSC gradient. The extended model was compared with data of two field surveys carried out in the Marsdiep inlet. The extended model, when compared with the classical random phase model, showed a large improvement of accuracy of the estimated SSC, which allows us to apply the model to the ferry data set to analyse suspended sediment transports through the Marsdiep tidal inlet.

Discussion: a critique of “Modeling suspended sediment distribution in continental shelf upwelling/downwelling settings”

A recent one-dimensional numerical model, the so-called GJ model, neglected horizontal advection and diffusion terms and used vague physical definition for its boundary conditions. An alternative two-dimensional numerical model, which corrects the GJ model’s deficiencies, is proposed to evaluate the physical effects of upwelling/downwelling on suspended sediment concentration (SSC) on continental shelves. The general SSC patterns derived from the two models are similar. However, in the case where settling velocity is larger than the maximum upwelling/downwelling velocity, the GJ model shows a homogeneous distribution of SSC, whereas the two-dimensional model identifies a distinct maximum SSC in the middle of the water column in combined upwelling and downwelling systems, and a SSC gradient in upwelling settings. Furthermore, when the maximum upwelling velocity is larger than the settling velocity, the GJ model shows a maximum SSC at the water surface, whereas the SSC is lower at the water surface when using the two-dimensional model.

The Interpretation of Vertical Sequences in Turbidite Beds: The Influence of Longitudinal Flow Structure

Because turbidite beds aggrade progressively beneath a moving current, the vertical grain-size profile of a bed is generally an indication of the longitudinal velocity structure of the flow, and longitudinal gradients in suspended sediment concentration (density). A current is more likely to show a simple waning flow history farther from its source; this is because faster-moving parts of the flow overtake slower moving parts, and the flow organizes itself over time so that the fastest parts are at the front. Thus distal (e.g., basin plain) turbidites commonly show simple, normally graded profiles, whereas more proximal turbidites often show complex vertical sequences within a bed, related to unsteadiness. A turbidity current may deposit a structureless, poorly sorted bed where the capacity of the current is exceeded, i.e., where there is insufficient turbulent kinetic energy to maintain the entire suspended mass. Capacity-driven deposition may occur where the flow decelerates. Where flow nonuniformity is the cause of capacity-driven deposition, a massive interval will form the lowest part of the bed, and will have a flat base. Where flow unsteadiness is the cause, a normally graded massive interval may overlie erosional features or traction structures at the base of the bed. Based on the assumption of longitudinal gradients in velocity, density, and grain-size distribution, the longitudinal density structure of a current may induce a switch, at any given point, from capacity-driven deposition to either (1) bypass and resuspension, (2) bypass with traction, or (3) competence-driven deposition, each resulting in a characteristic upward change in deposit character. The temporal evolution of the flow at a point varies systematically in a streamwise sense. Taking account of these longitudinal variations permits predictions of complex vertical sequences within beds, and of their downstream relations.

The effect of suspension on mean velocity profile

The vertical gradient of suspended sediment concentration often exists in estuarine and coastal regions. The existence of such concentration gradient, particular that of cohesive suspended sediment concentration gradient, affects the flow velocity profile. Based on the wave-current full depth combined model, k-ɛ turbulent model is employed in this paper to simulate the effect of sediment concentration on the wave-current flow. A quantitative analysis of change mechanism of mean velocity profile is presented. The results show that the change mechanism of Eulerian mean current velocity under clear water is different from that under sediment-water mixed flow.

Swash zone sediment suspension and transport and the importance of bore‐generated turbulence

A study of swash zone sediment transport was conducted at Gleneden Beach, Oregon during February 25–28, 1994. The data collected included suspended sediment concentration (SSC), sea surface elevation, and velocity (initially 4 and 8 cm above the bed) at three cross‐shore locations within the swash zone spanning high tides. Ensemble averages of 6, 9, and 12 s duration swash events showed that the uprush suspension was high, concentrated in the leading edge, and nearly vertically uniform above the lower 1–2 cm of the water column. Shortly after the sensors were inundated by run‐up, the sediment rapidly settled out of the water. During flow reversal the SSC was small but increased again in the backwash. Backwash vertical profiles were markedly different from uprush profiles with much of the suspension being confined to very near the bed where strong vertical gradients in SSC existed. These marked differences show that the backwash is not simply the reverse of the uprush, implying significant differences in the underlying fluid dynamics and sediment transport mechanisms. Backwash sediment suspension increased with flow duration. However, ensemble‐averaged SSC profiles of varying duration showed that the backwash concentrations were not consistent at the same temporal phases, which suggests that water depth, in addition to flow duration, may be a controlling factor. Strong cross‐shore gradients in SSC suggest that bore‐derived turbulence may affect local sediment transport. Specifically, our data show this bore‐generated turbulence (turbulent kinetic energy) directly influences local sediment suspension, hence, standard bed shear (Bagnold‐type) sediment transport models may no longer be valid in the vicinity of the bore. In the vicinity of the bore a higher correlation between bore‐generated turbulence and suspended sediment transport was found than between a Bagnold‐type formulation and suspended sediment transport.

Vertical suspension profile in the Changjiang Estuary

By using an acoustic suspended sediment monitor (ASSM), four patterns of vertical suspension profile have been obtained at spring tide (28–29 May 1994) in the Changjiang Estuary. Type I is L-shaped at flood tide. The vertical gradients in suspended sediment concentration (SSC) were small (< 1 g l −1). Type II is jet-like at max flood. SSC at the head of the jet reaches or exceeds 6 g l −1. Vertical gradients in SSC are large. The suspension profile just above the cohesive bed is discontinuous. Type III is characterized by an exponential increase from surface to bottom at ebb tide. Type IV is discontinuous and jet-like in the mid-water depth. Vertical gradients in SSC are large. There are upper lutocline and lower lutocline in vertical suspension profiles. In general, the turbid bottom suspensions occupy 20% of the whole water column at max flood and ebb tides. They correspond to high salinity, low velocity, but with large current velocity gradient (shear stress).

Modeling Sediment-Turbulent Flow Interactions

Models of widely differing complexity have been used in recent years to quantify sediment transport processes for engineering applications. This paper presents a review of these model types, from simple eddy viscosity models involving the “passive scalar hypothesis” for sediment predication, to complex two-phase flow models. The specific points addressed in this review include, for the suspension layer, the bottom boundary conditions, the relationship between the turbulent eddy viscosity and particle diffusivity, the damping of turbulence by vertical gradients in suspended sediment concentration, and hindered settling. For the high-concentration near-bed layer, the modeling of particle interactions is discussed mainly with reference to two-phase flow models. The paper concludes with a comparison between the predictions of both a classical, one-equation, turbulence k-model and a two-phase flow model, with “starved bed” experimental data sets obtained in steady, open-channel flow.

Fluidization of mud in estuaries

The South Alligator River, located in the Northern Territory, Australia, is a macrotidal estuary with suspended sediment concentration values reaching 10 g 1−1 In September 1986, in the dry season, the estuary was well mixed in temperature and salinity. While the vertical gradients in suspended sediment concentration were small at flood tides, for most of the ebb tide duration a lutocline separated a clear upper layer from an extremely turbid bottom layer, both layers being of comparable thickness. The tidal evolution of the suspended sediment concentration is consistent with that computed by a numerical model based on the equation of conservation of mass of suspended sediment. In this model, sediment is entrained from the bottom and mixed vertically upward by eddy diffusion, but through a Richardson number dependence, sediment‐induced buoyancy effects inhibit vertical mixing. The final depth of the turbid layer can be readily estimated analytically as a result of a balance between the rate of kinetic energy input and the buoyancy flux determined by the particle fall velocity. The presence of a lutocline helps form mud banks on the inner side of a meander.