Net Suspended Sediment Transport Research Articles

Net suspended sediment transport modulated by multiple flood-ebb asymmetries in the progressive tidal wave dominated and partially stratified Changjiang Estuary

In many estuaries, current velocity, suspended sediment concentration (SSC), and water depth have notable flood-ebb asymmetries in magnitude or duration. Such asymmetries can make marked impact on estuarine sediment transport and require in-depth analysis of the underlying mechanism. Based on field measurement over a neap-spring cycle in the Changjiang Estuary in January 2015, net suspended sediment transport modulated by multiple asymmetries were analyzed. The results showed that there were notable flood-ebb asymmetries in the current velocity, current duration, SSC, and water depth. The former two were ebb-dominated, while the latter two were flood-dominated. The net suspended sediment transport was landward and exhibited remarkable tidal fluctuation in magnitude. Ebb-dominated velocity and duration asymmetries favored seaward transport, but flood-dominated SSC and water depth asymmetries promoted landward transport. The relative contribution of these asymmetries to the net sediment transport was different and closely related to their asymmetry intensities. The magnitude of net sediment transport was highly modulated by the combined effects of these asymmetries. Further analyses revealed that the ebb-dominated asymmetries in velocity and duration were controlled by river flow, tidal wave deformation, and compensation flow of Stokes drift. The flood-dominated water depth asymmetry was related to the progressive tidal wave. The flood-dominated SSC asymmetry was mainly attributed to the lower SSCs in early ebbs, notable sediment supply limitation in late ebbs, wind wave height asymmetry, and gravitational circulation. Additionally, it is worth noting that the progressive tidal wave can yield strong current asymmetry, notable water depth asymmetry, and obvious wind wave height asymmetry during strong wind periods.

Importance of infragravity waves in a wave-dominated inlet under storm conditions

The processes associated with infragravity waves (IGW) and their role on the hydro-sedimentary dynamics of a wave-dominated inlet are investigated using field observations collected at the Albufeira Lagoon (Portugal) during storm Leslie in October 2018. During the storm, significant IGW heights reached up to 1.0 m in the surf zone and 0.4 m inside the lagoon. IGW frequencies were blocked by ebb currents at the flood-delta and this effect was stronger for higher IGW frequencies. Therefore, low-frequency IGW were able to propagate further into the inner lagoon. The application of a 1D energy balance equation suggests that depth-induced breaking and bottom friction contributed equally to dissipate IGW energy at the flood-delta and that this dissipation was stronger during the flood than during the ebb.Large horizontal velocities under IGW crests increased the instantaneous suspended sediment transport rate by up to two orders of magnitude during flood at the flood-delta. The net suspended sediment transport rate and its variability, associated with the wave motion, was positive (landward) during flood and negligible during ebb. These positive values were associated with large wave-induced orbital velocities, suggesting that waves were more effective than tidal currents in inducing suspended sediment transport. The oscillatory suspended sediment transport rate was directed towards the lagoon and dominated by IGW frequencies. The observed sediment accretion of the northern channel occurred during the most energetic IGW. The measurements presented in this study and their analyses demonstrate how IGW play a major role in the sedimentary dynamics of wave-dominated inlets during storm conditions.

Remarked morphological change in a large tidal inlet with low sediment-supply

Sediment transport within small tidal inlets is sensitive to natural processes, whilst large tidal inlets are relatively robust systems because of their large tidal prism. However, remarked morphological changes may be initiated even under the condition of low sediment supply, as illustrated by Jiaozhou Bay, a large coastal embayment on the Shandong Peninsula, eastern China. Jiaozhou Bay is characterized by its relatively slow rate of natural change, and while the embayment has a flood-dominated entrance channel and muddy seabed, the suspended sediment concentration is generally low due to the lack of abundant source material. Observations of sediment dynamics show that net suspended sediment transport is directed towards outside of the bay, with an order of magnitude of 103t during a tidal cycle. The export of sediment associated with this flood-dominated environment implies that the net transport pattern is controlled by tidal exchange processes rather than the strength of the seabed shear stress. Sediment budget calculations show that supply of artificial sediment into the bay can account for up to 72% of the total input, which is in agreement with the 210Pb and 137Cs radioisotope geochronologies, and this leads to accumulation rates of 100–101mmyr−1; without this, the deposition rate would be low under natural conditions. The flood tidal delta area is also influenced by the input of anthropogenic material, and acts as a depocenter with relatively high accumulation rates. Furthermore, although the inlet system has not yet reached its equilibrium state (i.e., the entrance cross-sectional area is still larger than the equilibrium cross-sectional area), land reclamation activities have resulted in a rapid reduction of the embayment area (by 37%) over the last 80 years. Our findings indicate that the rapid changes observed in the tidal basin area and seabed morphology are mainly the result of human activity rather than natural processes.

The use of modeling and suspended sediment concentration measurements for quantifying net suspended sediment transport through a large tidally dominated inlet

Sediment exchange at large energetic inlets is often difficult to quantify due complex flows, massive amounts of water and sediment exchange, and environmental conditions limiting long-term data collection. In an effort to better quantify such exchange this study investigated the use of suspended sediment concentrations (SSC) measured at an offsite location as a surrogate for sediment exchange at the tidally dominated Golden Gate inlet in San Francisco, CA. A numerical model was calibrated and validated against water and suspended sediment flux measured during a spring–neap tide cycle across the Golden Gate. The model was then run for five months and net exchange was calculated on a tidal time-scale and compared to SSC measurements at the Alcatraz monitoring site located in Central San Francisco Bay ~ 5 km from the Golden Gate. Numerically modeled tide averaged flux across the Golden Gate compared well (r2 = 0.86, p-value < 0.05) with 25 h low-pass filtered (tide averaged) SSCs measured at Alcatraz over the five month simulation period (January through April 2008). This formed a basis for the development of a simple equation relating the advective flux at Alcatraz with suspended sediment flux across the Golden Gate. Utilization of the equation with all available Alcatraz SSC data resulted in an average export rate of 1.2 Mt/yr during water years 2004 through 2010. While the rate is comparable to estimated suspended sediment inflow rates from sources within the Bay over the same time period (McKee et al., 2013-this issue), there was little variation from year to year. Exports were computed to be greatest during the wettest water year analyzed but only marginally so.

Suspended Sediment Transport in Rip Currents on a Macrotidal Beach

Thorpe, A., Miles, J., Masselink, G., Russell, P., Scott, T., Austin, M., 2013. Sediment Transport In Rip Currents on a Macrotidal BeachRip currents are offshore-directed flows in the surf zone that may be responsible for significant offshore transport of sediment. In this study high frequency surface elevation and current velocity data were collected alongside suspended sediment concentrations in a rip channel on a high energy, macro-tidal beach. At low tide (water depth over the rip channel (h) = 0.9 m), when the rip current was active (maximum offshore directed flow = ~ 0.4 m/s), net suspended sediment transport was directed offshore, and 88% of the flux was due to the mean flow component. The rip current was found to pulse at very low frequencies (VLF) and this contributed significantly to offshore sediment transport, accounting for 48% of total oscillatory flux (not considering direction). Infra-gravity (IG) frequency oscillations had a marginal contribution to offshore sediment transport within the rip (10% of total oscillatory flux) and incident wave transport was onshore (42% of total oscillatory flux). The relative influence of the wave component increased as tidal elevations increased and the rip current became inactive. For comparison, at mid-tide (h = 3.5 m) the total suspended sediment flux was in the offshore direction and four times less than during low tide. The mean flux accounted for 34% of the total transport at mid-tide. At high tide (h = 6.1 m) the total sediment transport was in the onshore direction due to the incident waves, with a 0% contribution from the mean flux.

Field Observation of Fine Sediment Resuspension Process at Tama River Mouth in Tokyo Bay

A field observation, comprising bottom mounted instruments for near-bottom currents and suspended sediment concentrations, was carried out for 1-month at Tama River mouth in Tokyo Bay. The measurements have captured sediment resuspension during extreme wave conditions associated with typhoon that passed over Kanto district in the early September of 2007. The sediment resuspension was caused primarily by waves at the monitoring site and combined current-wave bottom shear stress reached 0.9 Pa. Based on the SS-flux analysis during the recording period, the net suspended sediment transport at the site was towards the SW or outer Tokyo Bay.

2003년 3월 대조기 광양만 부유퇴적물의 유 &amp;#x00B7; 출입에 관한 연구

As a preliminary study on the sediment flux, concentrations of suspended particulate matter and current speeds were measured at three inlets of Gwangyang Bay during one tidal cycle of a spring tide of March 2003. The suspended sediment flux rate <TEX>$(g/m^{2}/s)$</TEX> at the mouth of Seomjin River (St. K1) was observed to be higher throughout surface layer during ebb tide <TEX>$(14.3\;g/m^{2}/s)$</TEX> and throughout near-bottom layer during the flood tide <TEX>$(23.2\;g/m^{2}/s),$</TEX> resulting in a net upstream-ward transport of<TEX>$0.9{\times}10^{3}kg/m$</TEX> during 13 period. At the inlet toward Yeosu Bay (St. K2), a relatively low rate (<TEX>$(5.0-6.7\;g/m^{2}/s)$</TEX> of sediment flux occurred throughout the water column compared to St. K1, with a depth-integrated net transport of <TEX>$5.6{\times}10^{3}kg/m$</TEX> toward the outer reaches of Gwangyang Bay inlet. At St. K3 located at Gwangyang Bay-side of Noryang Strait, the outward flux toward the Jinju Bay was observed to be dominant during the flood tide <TEX>$(16.2-23.2\;g/m^{2}/s)$</TEX>, especially through the mid and near bottom layer, compared to the inward flux throughout the whole water column during the ebb tide <TEX>$(13.1-19.7\;g/m^{2}/s).$</TEX> The net transport at St. K3 was calculated to be <TEX>$4.0{\times}10^{3}kg/m$</TEX> toward the outside of Gwangyang Bay. The outward net transport of suspended sediment at all three inlets seems to be consistent with a trend of bottom sediment texture, which suggests a net movement of sediment from a relatively coarse and poorly sorted inner-bay toward a relatively fine and better sorted outer-bay environment.

Sediment transport in Yalu River estuary

Tidal cycle measurements of tidal currents, salinity and water temperature, and suspended sediment concentrations were measured at four stations, together with surveys along two profiles short core collection within the Yalu River estuary. Grain size analysis of the three core sediment showed that: 1) the sediment from B1 to B3 became finer, worse sorting and positively skewed; 2) the diversification of matter origin became more and more evident from east to west; 3) the sediments over the region were of the same origin, as indicated by their similar colors and grain sizes. The data indicated that stratification occurred in the flood season, from upstream to downstream, and a salt wedge was formed. The water column was well mixed, but the longitudinal gradient of the salinity was larger on spring tide. The results also showed that the dominating mechanism of suspended sediment transport in the Yalu River estuary was T1, T2, T3 and T5. The non-tidal steady advection transport was restricted by the net transport of suspended sediment induced by mass Stoked drift directed to landwards, then the net sediment transport rate were decreased and the turbidity maxima was also favored to forming and extending.

Transport of Fine Sands by Currents and Waves. III: Breaking Waves over Barred Profile with Ripples

This paper presents an analysis of a series of laboratory measurements of horizontal velocity and suspended sediment concentration (arrayed vertically) at a number of locations across a simulated shallow water bar built in fine sands (D50 = 95 μm) and subjected to irregular waves of varying significant wave height but with a constant spectral shape (JONSWAP spectrum). Near the bar crest the net suspended transport rates, integrated between the lowest measurement point and the water surface (measured zone), are dominated by the time-averaged (current-induced) transport resulting from undertow. Away from the bar crest the net suspended sediment transport in the measured zone resulting from high-frequency oscillatory currents is important (and dominant) and directed offshore everywhere. Comparison of the measured suspended transport rates and those derived from bed elevation changes over time reveals significant fluxes in the unmeasured zone near the bed. These are inferred as being driven by high-frequency ...

The systematic contribution of transporting mechanisms to the cross-shore sediment transport in water depths of 3 to 9 m

The systematic contributions of short waves, infragravity waves and mean flows to the cross-shore sediment transport were studied with direct measurements of instantaneous near-bed sediment concentrations and velocities as well as with an energetics-based sediment transport model using measured time series of near-bed cross-shore flow as input. The study was conducted at four cross-shore positions in the multiple bar system of Terschelling, Netherlands during three field campaigns, each with a five-week duration. The data were collected in 3- to 9-m water depth during a wide range of conditions, from low-energy non-breaking conditions to almost fully saturated breakers. The height over depth ratio H s/ h was applied at all four measuring stations as a local conditional parameter. All model predictions were grouped into H s/ h classes with a width of 0.02 to highlight the generality of the data. The energetics approach predicted that the suspended load dominated over the bedload at all four stations. Both the sediment flux measurements as well as the model predictions showed that the largest contributors to the gross suspended sediment transport were made by the short waves and the undertow, inducing an onshore and offshore transport, respectively. Their contributions to the gross suspended transport were about 40 to 50% under surf zone conditions. Bound infragravity waves were observed and predicted to result in an offshore directed transport that was of subordinate magnitude compared to the observed and estimated magnitudes by the short waves and the undertow. However, as these transports almost cancelled out because of their about equal magnitude but opposite sign, the suspended transport by bound infragravity waves may not be ignored and may, rather paradoxically, have a relatively large influence on the net suspended sediment transport. Hydrodynamical processes that do not seem to be of importance to the onshore and offshore sediment transport in 3- to 9-m water depth in the long run are mean flows under non-breaking conditions and free infragravity motions. A direct comparison between measured sediment fluxes and the model predictions suggests that simple energetics models are suitable for predicting cross-shore sediment transport in 3- to 9-m water depth.

Suspended sediment transport and the role of infragravity waves in a barred surf zone

Collocated optical backscatterance suspended solids sensors and electromagnetic flowmeters were deployed at Stanhope Lane Beach, P.E.I., Canada, to assess the contributions of wind waves, infragravity waves and mean currents to the cross-shore flux of suspended sediment across a barred surf zone. The local net transport of suspended sediment (magnitude and direction) was highly variable both spatially and temporally. With respect to the relative magnitudes of the suspended sediment transport: (1) that induced by “quasi-steady” mean currents and oscillatory currents at both incident and infragravity frequencies were almost equal in magnitude, when the total surf zone was averaged over the storm event; (2) waves in the near-infragravity band were quantitatively more important during the waning of the storm, especially where (when) surf persisted well after the initial decay of the offshore wave field; (3) incident waves were important where (when) the waves were not saturated i.e. during the early stages of the storm at all locations and persistently throughout the storm on the seaward slope of the bar, where “wave saturation” was limited to a very short period. With respect to the direction of net sediment transport: (1) that attributable to mean currents was directed offshore in the inner surf zone, probably reflecting the presence of an undertow, while further seaward, an onshore transport occurred, probably resulting from mass transport induced by surf bores, or drift velocities induced by the infragravity wave boundary layer; (2) that induced by oscillatory currents at incident wave frequencies was generally onshore at any one location throughout the storm event. In contrast, oscillatory transport at low frequencies changed direction (and also varied in magnitude) at any given location; in part these changes reflect the existence of significant oscillatory sediment transport at discrete infragravity frequencies. The standing infragravity waves consistently transported suspended sediment towards antinodes in the water surface elevation.

Frequency dependent cross-shore suspended sediment transport. 1. A non-barred shoreface

The local, time-varying and time-averaged suspended sediment transports across a marine, non-barred shoreface were obtained from field measurements of the near-bed velocity and sediment concentration vectors using electromagnetic current meters and optical backscatterance suspended solids sensors. Co-spectral analyses of velocity and sediment concentration revealed the contributions of waves of different frequencies to the total transport; the transport attributable to quasi-steady currents was determined from the product of the time-averaged cross-shore velocity and sediment concentration. Estimates of the local, time-integrated sediment volume flux (total and net) and the associated erosion or accretion were determined using depth-of-activity rods. Suspended sediment transport was associated with: (1) local wind-forced waves; (2) swell; (3) low frequency waves (group-bound long waves) and (4) offshore-directed mean currents (undertow). These transport components varied spatially and temporally in response to changes in the wave height to water depth ratio. The local net oscillatory transport rate at wind-wave and swell-wave frequencies was directed onshore predominantly, and increased as water depth decreased in association with wave shoaling. The local mean sediment transport rate was predominantly offshore and dominated the net transport rate wherever wave-induced components were weak relative to the mean currents. Under breaking waves the suspended sediment transport contribution associated with wind-waves and swell-waves was reduced as a result of dissipation. The net suspended sediment transport rate exhibited a distinct vertical structure, reflecting the balance at each elevation between the opposing mean and oscillatory components of transport. The local time-averaged total and net sediment volume flux and resulting erosion and accretion patterns support the hypothesis of a near balance in sediment flux for the complete storm event.

Depth‐Integrated Suspended‐Load Calculations

The net transport of sediment by suspension is calculated by integrating the product of the sediment concentration and velocity over the water depth. For many flows, accurate calculations must include effects of density stratification by suspended sediment, as well as effects due to bed forms and a mixture of sediment sizes. Each of these effects can cause differences of up to two orders of magnitude for some cases and therefore often must be included for reasonable accuracy. However, even though stratification and bed forms tend to decrease the entrainment capacity of a flow, and treating the sediment as a single size usually underpredicts the actual sediment concentration, the interactions among these processes are complex and it is nearly impossible to generalize the net effect. Results are presented for the depth‐integrated product of sediment concentration and velocity for a wide range of flow and sediment conditions. Plots arc provided so that more accurate predictions of net transport of suspended sediment can be made.

Suspended sediment transport across a tidal flat

Many investigations have shown that most of the suspended sediment supplied to the Wadden Sea originates from the North Sea. Based on measurements of current velocity, depths and suspended sediment concentrations in the Danish part of the Wadden Sea, computations indicate, however, that suspended sediment is in fact exported to the North Sea. The reason for this discrepancy is twofold: Firstly, measurements have been carried out mainly in tidal channels where the largest amounts of water and suspended sediment are in the main transported during ebb periods. Secondly, computation of net transport in an estuarine environment such as the Wadden Sea always suffers from the difficulty that net transport within each single tidal period is of the same order as the uncertainty of the transport rates during ebb and flood periods respectively. The first problem can be dealt with if measurements are carried out in both tidal channels and over the tidal flats. The second problem can be solved by collecting time series instead of discrete measurements because accumulation of the transport rates found for successive tidal periods will neutralise the relatively large random error. In this paper, time series of suspended sediment flux across a tidal flat covering a full spring neap tidal cycle are presented. They showed net transport of suspended sediment in a landward direction which is mainly caused by flocculation of fine-grained sediment during periods of high concentration level. During such periods the field settling velocities of suspended sediment are so large that the settling- and scour-lag processes can be effective.