Instantaneous Sediment Concentration Research Articles

Surf Zone Turbulence and Suspended Sediment Dynamics—A Review

The existence of sandy beaches relies on the onshore transport of sand by waves during post-storm conditions. Most operational sediment transport models employ wave-averaged terms, and/or the instantaneous cross-shore velocity signal, but the models often fail in predictions of the onshore-directed transport rates. An important reason is that they rarely consider the phase relationships between wave orbital velocity and the suspended sediment concentration. This relationship depends on the intra-wave structure of the bed shear stress and hence on the timing and magnitude of turbulence production in the water column. This paper provides an up-to-date review of recent experimental advances on intra-wave turbulence characteristics, sediment mobilization, and suspended sediment transport in laboratory and natural surf zones. Experimental results generally show that peaks in the suspended sediment concentration are shifted forward on the wave phase with increasing turbulence levels and instantaneous near-bed sediment concentration scales with instantaneous turbulent kinetic energy. The magnitude and intra-wave phase of turbulence production and sediment concentration are shown to depend on wave (breaker) type, seabed configuration, and relative wave height, which opens up the possibility of more robust predictions of transport rates for different wave and beach conditions.

Measurements of the sediment flocculation characteristics in the Three Gorges Reservoir, Yangtze River

AbstractThe flocculation of fine sediments in the Three Gorges Reservoir (TGR) was confirmed in previous studies, but the flocculation characteristics have yet to be fully clarified. In this study, field measurements were conducted in the TGR to investigate the sediment flocculation characteristics. First, the instantaneous flow velocity and sediment concentration were measured through Acoustic Doppler velocimeter and sediment sampling. Then, the effective settling velocity was calculated based on the sediment diffusion theory to deduce the floc size and flocculation degree. Finally, the influences of particle size, flow velocity, and sediment concentration on flocculation were analyzed. Results showed that flocculation occurred in more than half of the sediments in the TGR, and the maximum flocculation degree was between 10 and 30. Flocculation weakened as particle diameter increased, with the critical particle size being approximately 0.018 mm, meaning that flocculation was unlikely to occur when the particle size exceeded the critical value. As the flow velocity increased, the flocculation degree first increased and then decreased, with the critical flow velocity being approximately 0.7 m/s, but the critical flow velocity increased with an increase in sediment concentration and tended to be a constant. The flocculation degree also increased with increasing sediment concentration and tended to be constant when the sediment concentration exceeded approximately 0.5 kg/m3. The results provide new information on the flocculation characteristics of the TGR and should be useful for understanding and simulating fine sediment transport in the TGR.

Evaluación de los procesos superficiales de escorrentía en cárcavas originadas en olivares convencionales. Un apartado a tener en cuenta en la planificación territorial

Se analiza la respuesta hidrologica de una carcava efimera frente a un evento de lluvia equivalente de frecuencia “ordinaria” (≈1 ano) a partir de una simulacion de escorrentia en una cuenca experimental con olivar en Puente Genil, Cordoba (Espana). Mil litros de agua fueron bombeados durante 10 minutos y la descarga total, la velocidad superficial de avance de frente de flujo, la concentracion de sedimentos y los cambios morfologicos en el perfil transversal fueron calculados. Los resultados mostraron elevadas diferencias entre las tasas de escorrentia de hasta un 17% entre las condiciones “secas” y “humedas”. La concentracion de sedimentos instantanea llego a alcanzar valores de hasta 40 g l-1. Se concluye que las carcavas en olivares convencionales son capaces de registrar elevadas tasas de erosion y escorrentia, particularmente bajo condiciones de mayor humedad. Se requiere mas informacion y atencion a las medidas de control sobre la erosion por el flujo concentrado de la escorrentia en la planificacion agricola.

Estimating instantaneous concentration of suspended sediment using acoustic backscatter from an ADV

The measurement of instantaneous sediment concentration remains a challenging task. In this study, a three-step procedure is proposed to estimate instantaneous sediment concentration using acoustic backscatter from Acoustic Doppler Velocimeter (ADV). The influences of acoustic noise and particle diameter on an ADV׳s performance was first tested in the laboratory, then the three-step procedure was verified based on field measurements in the Zhongxian and Fengjie reaches in the Three Gorges Reservoir. The first step involves reconstructing the backscatter signal time series. Due to contamination from both the noise floor and spurious spikes, the denoising-despiking method was applied instead of the traditional velocity-despiking methods, and this approach performed well based on spectrum analysis. The second step involves calibrating the sediment concentration against the backscatter signal. A linear relation, whose slope and intercept were calibrated to be dependent on particle diameter, is proposed in double logarithmic coordinates. The third step involves calculating the instantaneous sediment concentration using reconstructed instantaneous backscatter based on the proposed relation. The accuracy of the proposed method was evaluated through consideration of the concentration spectrum and sediment flux, indicating that the proposed three-step procedure is effective for the measurement of instantaneous sediment concentration.

Tidal bore hydrodynamics and sediment processes: 2010–2016 field observations in France

ABSTRACT A tidal bore is a compressive wave, advancing upstream in an estuary when the flood tidal flow starts. It is observed when a macro-tidal flood flow enters the funnel shaped river mouth with shallow waters. Its upriver propagation impacts the natural system, with sediment scouring and suspension. The tidal bores of the Garonne and Sélune Rivers in France were extensively investigated between 2010 and 2016. Instantaneous velocity measurements were conducted continuously at high-frequency (50 to 200 Hz) during each bore event. In the Garonne River, instantaneous sediment concentration data were obtained and the sediment properties were systematically tested. The nature of the observations was comprehensive, regrouping hydrodynamics and turbulence, sedimentology and suspended sediment transport. The key outcomes show that the tidal bore occurrence has a marked effect on the velocity field and suspended sediment processes, including a sudden flow deceleration and flow reversal during the bore passage. The turbulent Reynolds stresses present large instantaneous amplitudes, with rapid fluctuations, during the tidal bore. The sediment flux data imply considerable mass transport rates during the first hour of flood tide. This unique review of field data further shows a number of common features, as well as the uniqueness of each individual event.

Field measurements of settling velocities of fine sediments in Three Gorges Reservoir using ADV

The Three Gorges Reservoir (TGR) is suffering from unexpected fine sediment deposition, to better understand the fine sediment transport processes, field measurements were conducted at the Zhongxian and Fengjie reaches. A method based on the sediment diffusion equation was proposed to measure the settling velocities using the Acoustic Doppler Velocimeter (ADV). The backscatter acoustic intensities (BSI) received from the ADV were calibrated against the sediment concentrations measured via water sampling, suggesting a linear relationship in double logarithmic coordinate system. The instantaneous sediment concentration was calculated using the derived relationship, and then the settling velocity was obtained through the proposed procedure. The settling velocities of the fine particles in the TGR were found to vary with the water depth. Most of the effective settling velocities were within the range of 0.1–10mm/s, which were larger than those of the primary particles, indicating that the flocculation was likely to occur in the TGR. Additionally, it is suggested that the turbulent motion played an important role in the flocculation in the TGR.

The spatio-temporal invariability of sediment concentration and the flow–sediment relationship for hilly areas of the Chinese Loess Plateau

The spatio-temporal variation underlies and complicates studies related to earth surface processes. Subsequent to our previous studies reporting the temporal invariability of sediment concentration, this study further reports the spatial invariability of sediment concentration or the flow–sediment relationship in the hilly part of the Chinese Loess Plateau. The dense channel network dissects this area into numerous sub-watersheds, which are composed of entire slopes (a combination of the hill slope and the valley side slope). Our data mainly involves ten experimental sites of entire slope and sub-watershed and 580 flood events at three locations. Our analysis at the intra-event time scale shows that the instantaneous sediment concentrations in runoff can generally approach the detach-limited maximum sediment concentration since the active massing wasting ensures an abundant sediment supply. Consequently, the event mean sediment concentration does not only remain fairly constant among flow events but also among the experimental sites despite the possible differences in topography, soil management or land cover, water source, soil surface properties, etc. This observation also holds at the annual time scale. Our result implies that a single proportional function is adequate to fit the flow–sediment relationship for any entire slope or sub-watershed, where eroded sediments are primarily sourced, over a considerable areal extent unless soil property has changed or check dams are installed. Moreover, the proportionality coefficient, which is indicative of the maximum detachment capacity, can be approximated through flume experiments rather than necessarily through long-term field observations. Our finding can facilitate the obtainment of spatially distributed information on soil loss in the Chinese Loess Plateau and is also indicative of other locations where sediment supply is sufficiently available.

Near-Bed Sediment Condition in Oscillatory Sheet Flows

AbstractSeveral empirical formulas have been proposed to predict the near-bed conditions for sediment transport in steady channel flows; however, few are extendable to oscillatory flows. This study presents a method for evaluation of the reference concentration and the entrainment rate near the undisturbed bed under oscillatory sheet-flow conditions. A new expression of the erosion depth in terms of the Shields parameter is adopted in order to describe the conservation of sediment mass more accurately. Both the exponential law and the power law are used to represent the instantaneous sediment concentration profile. The newly obtained formulas explicitly express the reference concentration and the entrainment rate in terms of the near-bed flow conditions and the sediment properties. The maximum value of the reference concentration obtained has a similar trend as given by the classical formulas. However, the variation of the reference concentration within a period of the oscillatory flow shows better agreem...

Hidrogramas e sedimentogramas associados à erosão hídrica em solo cultivado com diferentes sequências culturais, com diferentes condições físicas na superfície

Estudos hidrossedimentológicos detalhados são de grande valia para o melhor entendimento e controle do processo de erosão hídrica e, por conseguinte, para se aprofundar a pesquisa no assunto e, assim, praticar a conservação do solo e da água de modo mais eficaz e duradouro. Considerando isso, realizou-se este trabalho com o objetivo de desenvolver hidrogramas (taxas instantâneas de descarga da enxurrada versus tempo) e sedimentogramas (concentrações instantâneas de sedimento na enxurrada e taxas instantâneas de perda de solo versus tempo) individualizados (cada repetição de tratamento), associados à erosão hídrica ocorrida em solo cultivado com diferentes sequências culturais (gramíneas e leguminosas de inverno e verão, implantadas em semeadura direta, nos modos de cultivo isolado e consorciado), com diferentes condições físicas na superfície (solo não mobilizado, solo recém-escarificado, solo previamente escarificado e solo recém-gradeado, com presença e ausência de crosta e de completa, pouca e nenhuma cobertura por resíduos culturais). O estudo foi desenvolvido em campo, na EEA/UFRGS, em Eldorado do Sul (RS), usando-se chuva simulada e um Argissolo Vermelho distrófico típico com textura franco-argiloarenosa na camada superficial e declividade média de 0,115 m m-1. No início da pesquisa esse solo encontrava-se bastante degradado. As chuvas simuladas (seis no total, sendo uma para cada teste de erosão) foram aplicadas com o aparelho simulador de braços rotativos, todas elas na intensidade planejada de 64 mm h-1 (neste trabalho usaram-se as intensidades observadas das chuvas, as quais variaram de 59,1 a 74,6 mm h-1) e com duração variando de 1 a 3 h (devido almejar-se atingir a condição de equilíbrio da enxurrada em todos os testes de erosão). Observou-se que os hidrogramas e sedimentogramas, de modo geral, resultaram coerentes com as condições das quais foram desenvolvidos. Assim, na maior parte das vezes, os hidrogramas diferenciaram-se mais entre os testes de erosão do que entre os tratamentos de sequência cultural, e o inverso ocorreu com os sedimentogramas. As superfícies de solo não mobilizadas e com pequena rugosidade e pouca cobertura, de modo geral, proporcionaram hidrogramas e sedimentogramas com menor tempo de partida e maior magnitude, independentemente da presença ou ausência de crosta e do tipo de sequência cultural. As superfícies de solo recém-mobilizadas e com média e grande rugosidade, de modo geral, proporcionaram hidrogramas e sedimentogramas com maior tempo de partida e menor magnitude, praticamente independentemente da cobertura e do tipo de sequência cultural. As superfícies de solo com completa cobertura tiveram suas enxurradas diminuídas e suas erosões eliminadas, independentemente da sua mobilização e do tipo de sequência cultural. A sequência cultural com teosinto proporcionou hidrogramas e sedimentogramas com os maiores tempos de partida e as menores magnitudes, seguida das com milho+feijão-miúdo e milheto, as quais diferenciaram pouco entre si. As repetições dos tratamentos proveram valores das taxas instantâneas de descarga da enxurrada que se pareceram mais e se distribuíram no tempo de modo mais uniforme do que os providos para as correspondentes concentrações instantâneas de sedimento e taxas instantâneas de perda de solo. Os dados obtidos foram valiosos no que diz respeito a poder entender melhor a variação comumente observada nos resultados de pesquisas de erosão hídrica realizadas em campo.

Comparative calculation of suspended sediment loads with respect to hysteresis effects (in the Petzenkirchen catchment, Austria)

Streams in intensively used agricultural catchments are frequently characterised by high transport of suspended solids during rainfall events. Due to a high variability in runoff, the sediment concentration relationship during and between different events and various hysteresis effects, instantaneous sediment concentrations and event loads are difficult to calculate. We tested the applicability of turbidity measurements for calculating instantaneous sediment concentrations and loads in a small agricultural catchment in Austria. We calibrated quasi-continuous turbidity measurements using additional water sampling and employed these calibrated sediment concentrations as benchmark sediment concentrations. Four different methods to calculate instantaneous sediment concentrations were tested on 19 events. A generalized rating curve approach resulted in a considerable bias for both event specific sediment concentrations and total sediments loads. Fitting of event-specific rating curves still misrepresented instantaneous sediment concentrations for the different events, but gave load estimations that were in a range of 5% of the benchmark values. Two approaches accounting explicitly for hysteresis exhibited the best fit and provided load estimations that were in a range of 0–1% deviation to the benchmark sediment concentrations. Nevertheless, several limitations to the hysteresis model approach were identified. Testing the various hysteresis effects against other event parameters such as total rainfall amount, maximum rainfall intensity and initial soil water content revealed interactions to these parameters that could predefine parameter values of the hysteresis model approach.

Modeling dilute sediment suspension using large-eddy simulation with a dynamic mixed model

Transport of suspended sediment in high Reynolds number channel flows [Re=O(600 000)] is simulated using large-eddy simulation along with a dynamic-mixed model (DMM). Because the modeled sediment concentration is low and the bulk Stokes’ number (Stb) is small during the simulation, the sediment concentration is calculated through the use of the Eulerian approach. In order to employ the DMM for the suspended sediment, we formulate a generalized bottom boundary condition in a finite-volume formulation that accounts for sediment flux from the bed without requiring specific details of the underlying turbulence model. This enables the use of the pickup function without requiring any assumptions about the behavior of the eddy viscosity. Using our new boundary condition, simulations indicate that the resolved component of the vertical flux is one order of magnitude greater than the resolved subfilter-scale flux, which is in turn one order of magnitude greater than the eddy-diffusive flux. Analysis of the behavior of the suspended sediment above the bed indicates the existence of three basic time scales that arise due to varying degrees of competition between the upward turbulent flux and downward settling flux. Instantaneous sediment concentration and velocity fields indicate that streamwise vortices account for a bulk of the resolved flux of sediment from the bed.

Conceptual model of sedimentgraph from flood events in a small agricultural watershed

Conceptual model of sedimentgraph from flood events in a small agricultural watershed A procedure for predicting the sediment graph (i.e. the suspended sediment flux), from a small river catchment by heavy rainfall, has been developed using the concept of an instantaneous unit hydrograph (IUH) and dimensionless sediment concentration distribution (DSCD). A formula for instantaneous unit sedimentgraph (IUSG) is presented, and a procedure for estimating the sediment routing coefficient, which is a key parameter of the IUSG, based on measured data of rainfall-runoff-suspended sediment is applied. Field data from a small, field sized agricultural basin, lacated in center of Illinois has been used for analizing lag times for runoff (LAG) and sediment yield (LAGs). Assumptions about sediment generated during rainfall events are discussed.

Reduction of Stream Sediment Concentration by a Riparian Buffer: Filtering of Road Runoff in Disturbed Headwater Basins of Montane Mainland Southeast Asia

We determined the extent that a riparian buffer reduces stream suspended sediment concentrations by filtering road runoff during 18 rain events in a 2.5-ha, multi-use watershed in northern Thailand. The dominant buffer species was the perennial sedge Fimbristylis aphylla Zoll. ex Steud. (Cyperaceae). We monitored stream sediment concentration for situations where road runoff either flowed into the riparian buffer or was diverted directly into the stream (buffer and no buffer scenarios). These data were used to develop the following relationships between instantaneous stream sediment concentration (Ci) and discharge (Qi): Ci= 28.329Qi(0.851) (buffer scenario) and Ci= 22.265Qi(1.579) (no buffer scenario). Using these functions to calculate total event suspended concentrations, we determined that the buffer reduced suspended sediment concentration by 34 to 87%, for the range of events monitored. Removal of sediment from runoff generated on a 2.4-m-wide, 165-m-long unpaved road section was achieved principally via ponding, which reduced the transport capacity as flow entered the relatively flat, saturated buffer. Sediment deposition occurred primarily within the first 10 m of the buffer. Some sediment was also deposited on the fillslope leading to the buffer. Maximum road sediment concentration during the largest buffer event approached 100,000 mg L(-1). Meanwhile, the corresponding maximum stream suspended sediment concentration was <4000 mg L(-1). In contrast, maximum stream concentrations when flow bypassed the buffer during smaller events were commonly 4000 to 7000 mg L(-1). Naturally occurring buffers represent an economical means of mitigating road-related impacts in upland basins in Southeast Asia, particularly if combined with measures limiting sediment and runoff production on contributing road sections.

Toward modeling turbulent suspension of sand in the nearshore

We present two depth‐ and phase‐resolving models, based on single‐ and two‐phase approaches for suspended sediment transport under water waves. Both models are the extension of a wave hydrodynamic model Cornell Breaking Wave and Structure (COBRAS). In the two‐phase approach, dilute two‐phase mass and momentum equations are calculated along with a fluid turbulence closure based on balance equations for the fluid turbulence kinetic energy kf and its dissipation rate εf. In the single‐phase approach the fluid flow is described by the Reynolds‐Averaged Navier‐Stokes equations, while the sediment concentration is calculated by an advection‐diffusion equation for the conservation of sediment mass. The fluid turbulence is calculated by kf‐εf equations that incorporate the essential influence of sediment, which can also be consistently deduced from the two‐phase theory. By adopting a commonly used sediment flux boundary condition near the bed the proposed models are tested against laboratory measurements of suspended sediment under nonbreaking skewed water waves and shoaling broken waves. Although the models predict wave‐averaged sediment concentrations reasonably well, the corresponding time histories of instantaneous sediment concentration are less accurate. We demonstrate that this is due to the uncertainties in the near‐bed sediment boundary conditions. In addition, we show that under breaking waves the near‐bed sediment pickup cannot be solely parameterized by the bottom friction, suggesting that other effects may also influence the near‐bed sediment boundary conditions.

Sediment transportation and deposition by rain-triggered lahars at Merapi Volcano, Central Java, Indonesia

Abstract A lahar is a general term for a rapidly flowing mixture of rock debris and water (other than normal streamflow) from a volcano and refers to the moving flow. Located in the populated area of Central Java, the stratovolcano Merapi (2965 m) is prone to lahar generation, due to three main factors: (1) millions of cubic meters of pyroclastic deposits are the product of frequent pyroclastic flows, which have occurred on 2- to 4-year intervals; (2) rainfall intensity is high (often 40 mm in 2 h on average) during the rainy season from November to April; and (3) drainage pattern is very dense. Following the 22 November 1994 eruption of Merapi, 31 rain-triggered lahar events were recorded in the Boyong River between December 1994 and May 1996. On Merapi's slopes, instantaneous sediment concentration at any given time of the lahars varies widely over time and space. Lahars are transient sediment-water flows whose properties are unsteady, so that the sediment load fluctuates during the flow. The boundary between the flow types (debris flow, with sediment concentration >60% volume, or hyperconcentrated flow, with sediment concentration ranges from 20% to 60% volume) may fluctuate within the flow itself. Grain-size distribution, physical composition of sediments, shear stress, yield stress, and water temperature play each a role on this boundary. Natural self-damming and rapid breakout are partly responsible for the sediment variations of the flows. Debris-flow phases at Merapi typically last a few minutes to 10 min, and are often restricted to the lahar front. Debris-flow surges are sometimes preceded and always followed by longer hyperconcentrated flow phases. As a result, mean sediment concentration of the lahars is low, commonly from 20% to 50% volume. Besides, transient normal streamflow phases (sediment concentration Low sediment load and frequent transient flows in the Merapi channels may result from at least three factors: (1) several breaks-in-slope along the channel increase the deposition rate of sediment, and hinder the bulking capacity of the lahars; (2) source material is mainly coarse debris of “Merapi-type” block-and-ash flows. Consequently, the remobilization of coarse debris is more difficult and the clast deposition is accelerated; (3) variations of rainfall intensity over time and space, common in tropical monsoon rainfall, also influence the sediment load variations of the lahars. Sedimentologic analyses of the lahar deposits in the Boyong River at Merapi encompass clast-supported and matrix-supported debris-flow deposits, hyperconcentrated flow deposits, and streamflow deposits. The stratigraphic succession of massive and stratified beds observed immediately after any given lahar event in the Boyong River indicates that the sediment concentration varies widely over time and space during a single lahar event. Sedimentation rate varies from 3 to 4.5 cm/min during relatively long-lived surges to as much as 20 cm/min during short-lived surges. These results indicate that the sediment load fluctuates during lahar flow, further demonstrating that lahars are transient sediment-water flows with unsteady flow properties.

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.

Sediment suspension in oscillatory flow: measurements of instantaneous concentration at high shear

Different syphon type suspended load probes were used together with a specially developed “carousel” sampler for measurements of the instantaneous sediment concentration in turbulent oscillatory flow over a sand bed. Shields parameters were well above the ripple/flat bed transition region, resulting in intense sediment transport over a flat bed. The measurements were performed at different levels in a large oscillating water tunnel. They showed some characteristic features of the temporal concentration variation at fixed levels, including a pronounced effect of the orientation of the suction tube relative to the flow. The variation with height of the average concentration is reasonably well described assuming a constant turbulent diffusivity, and the magnitude of this can be predicted relatively well using simple turbulence arguments. Two empirical formulae for the extrapolated bed concentration are also presented. The analysis is partly based on Wilson's finding that in sheet flow the equivalent bed roughness is nearly proportional with the Shields parameter, resulting in a roughness/grain diameter ratio much larger than 2.5, the figure often adopted for a flat bed, but without sheet flow.

Sediment Transport under Sheet-Flow Conditions

Experiments on sediment transport under sheet-flow conditions were conducted by tracing individual and grouped light plastic particles. The measurements based on individual particles yielded the vertical and temporal variations of the instantaneous sediment concentration and transport velocity from which the sediment-transport rate per unit width was obtained. In addition, moving areas of painted particle groups were measured to estimate instantaneous sediment transport rates per unit width directly. Measured local sediment-transport rates were the largest at one or two particle diameters above the initial bed surface. Instantaneous sediment-transport rates per unit width exhibited a π/6 phase lead relative to the free-stream velocity. The measured sediment-transport rates averaged over a half-cycle were compared with forcing functions Shields and mobility parameters. The present data as well as most available existing data in terms of the normalized transport rate without the settling velocity were well arranged by the mobility number to the power of 1.5.

The effect of cross-sectional velocity and concentration variations on suspended sediment transport rates in tidal creeks

Instantaneous suspended sediment transport rates over numerous tidal cycles have been determined for two tidal creeks in South Carolina: Town Creek, North Inlet, and Bass Creek, Kiawah Island. Three different calculation methods were employed: (1) area-integrated product of sediment concentration, velocity, and sample subarea (Method I, the true transport rate); (2) product of instantaneous channel cross-section area, cross-sectionally averaged velocity, and cross-sectionally averaged concentration (Method II); and (3) product of instantaneous discharge and sediment concentration at a single, near-surface, mid-channel sample location (Method III). Logistical and financial constraints at times dictate use of Method III rather than the more rigorous Method I. Deviations between Method II (and/or III) and Method I as a percentage of the Method I transport rates can be large (up to 120%). Fortunately the largest percent deviations occur at times of low Method I transport. As a result, linear regressions of Method I transport rates against those of Methods II or III yield correlation coefficients r2 > 0·9. Thus Method II or III transport rates can be corrected by regression analysis to give reliable estimates of the Method I transport rate. Accordingly, it may be possible to calculate tidal residual suspended sediment transport rates for the Long-Term Ecological Research (LTER) site at North Inlet using daily water sample data, providing an accurate hypsometric model can be developed for estimating instantaneous discharge from continuous tide data.

WAVE-INDUCED FLOW AND NEARSHORE SUSPENDED SEDIMENT

It has been realized for nearly one hundred years that the transport of sediment is related to the characteristics of a wave, in particular its shape. Cornish (1898) noticed that the shoreward velocity associated with a wave crest was more effective at moving coarse sediment than was the seaward velocity associated with the wave trough. Cornish's observation was consistent with the theory of Stokes (1847), which predicts the onshore velocity associated with the wave crest is stronger and of shorter duration than the offshore velocity associated with the wave trough. This horizontal asymmetry of the cross-shore flow, which is a reflection of the wave shape, is known as velocity skewness. It has been suggested that "the existence of the beach depends on small departures from symmetry in the velocity field balancing the tendency for gravity to move material offshore"(Bowen, 1980). Although the concept of velocity skewness has been incorporated into detailed predictors of sediment transport (Bowen, 1980; Bailard and Inman, 1981) it is only one of many facets that needs to be understood in order to make the accurate prediction of sediment transport realizable. A comprehension of sediment transport is hampered by both an incomplete knowledge of the hydrodynamics and a lack of instrumentation to directly measure instantaneous sediment concentration and the accurate prediction of sediment transport is probably the most enigmatic problem in coastal engineering. Occasionally, suspended sediment concentration has been inferred from in situ pumps and hand-held tubes, but these methods lack the temporal and spatial resolution necessary to elucidate the details of the interaction between the waveinduced flow and the sediment. Recently, a miniature optical backscatter sensor (MOBS), which provides a time series of suspended sediment concentration at a "point", was developed by Downing et al. (1981). During a recent field experiment a vertical array of 5 of these optical backscatter sensors and a colocated flow meter was deployed close to the sea bed. These colocated measurements provide a unique opportunity to investigate the response of near-bed suspended sediment concentration to the wave-induced flow.