Suspended Sediment Profiles Research Articles

Depth‐Partitioning of Particulate Organic Carbon Composition in the Rising and Falling Stages of the Amazon River

AbstractThe Amazon River mobilizes organic carbon across one of the world's largest terrestrial carbon reservoirs. Quantifying the sources of particulate organic carbon (POC) to this flux is typically challenging in large systems such as the Amazon River due to hydrodynamic sorting of sediments. Here, we analyze the composition of POC collected from multiple total suspended sediment (TSS) profiles in the mainstem at Óbidos, and surface samples from the Madeira, Solimões and Tapajós Rivers. As hypothesized, TSS and POC concentrations in the mainstem increased with depth and fit well to Rouse models for sediment sorting by grain size. Coupling these profiles with Acoustic Doppler Current Profiler discharge data, we estimate a large decrease in POC flux (from 540 to 370 kg per second) between the rising and falling stages of the Amazon River mainstem. The C/N ratio and stable and radiocarbon signatures of bulk POC are less variable within the cross‐section at Óbidos and suggest that riverine POC in the Amazon River is predominantly soil‐derived. However, smaller shifts in these compositional metrics with depth, including leaf wax n‐alkanes and fatty acids, are consistent with the perspective that deeper and larger particles carry fresher, less degraded organic matter sources (i.e., vegetation debris) through the mainstem. Overall, our cross‐sectional surveys at Óbidos highlight the importance of depth‐specific sampling for estimating riverine export fluxes. At the same time, they imply that this approach to sampling is perhaps less essential with respect to characterizing the composition of POC sources exported by the river.

Inclusive Hyper- to Dilute-Concentrated Suspended Sediment Transport Study Using Modified Rouse Model: Parametrized Power-Linear Coupled Approach Using Machine Learning

The transfer of suspended sediment can range widely from being diluted to being hyper-concentrated, depending on the local flow and ground conditions. Using the Rouse model and the Kundu and Ghoshal (2017) model, it is possible to look at the sediment distribution for a range of hyper-concentrated and diluted flows. According to the Kundu and Ghoshal model, the sediment flow follows a linear profile for the hyper-concentrated flow regime and a power law applies for the dilute concentrated flow regime. This paper describes these models and how the Kundu and Ghoshal parameters (linear-law coefficients and power-law coefficients) are dependent on sediment flow parameters using machine-learning techniques. The machine-learning models used are XGboost Classifier, Linear Regressor (Ridge), Linear Regressor (Bayesian), K Nearest Neighbours, Decision Tree Regressor, and Support Vector Machines (Regressor). The models were implemented on Google Colab and the models have been applied to determine the relationship between every Kundu and Ghoshal parameter with each sediment flow parameter (mean concentration, Rouse number, and size parameter) for both a linear profile and a power-law profile. The models correctly calculated the suspended sediment profile for a range of flow conditions (0.268 mm≤d50≤2.29 mm, 0.00105gmm3≤particle density≤2.65gmm3, 0.197mms≤vs≤96mms, 7.16mms≤u*≤63.3mms, 0.00042≤cˉ≤0.54), including a range of Rouse numbers (0.0076≤P≤23.5). The models showed particularly good accuracy for testing at low and extremely high concentrations for type I to III profiles.

Rouse revisited: The bottom boundary condition for suspended sediment profiles

A linearly depth-dependent diffusivity is commonly used to model the vertical distribution of dilute, near-bed, suspended sediment in marine fluid environments, i.e., the Rouse model; this simplified form predicts unfortunately infinite concentration at the bed. That singularity causes problems in linking bed models to suspended sediment models. A common remedy to this problem is to assume a reference concentration and height above the bed, but some studies point to the arbitrariness of that procedure and suggest, instead, the existence of a residual eddy diffusion near the bed, which removes the singularity. In these past studies, the residual diffusion, εo, has been assigned a value based on the Nikuradse roughness height, ks=2.5ds, where ds is the (mean) equivalent sand-grain diameter. We examine this assumption by fitting a residual diffusion model to 14 time-averaged suspended-sediment profiles available in the literature, without assuming the Nikuradse length. This procedure produces εo values from 0.6 to 13cm2s−1, which are much greater than both the (molecular) kinematic viscosity of water and the expected Brownian diffusion coefficient of the sediment grains of the reported sizes. These residual diffusion coefficients indicate mixing scale heights at the bed, εo/κu∗, where κ is von Kármán's constant and u∗ is the shear velocity, are 7.4–56.0× greater than the Nikuradse height, ks. Although defined by only limited data, we also find that the residual diffusion correlates with grain size and the roughness Reynolds number but not with the Rouse parameter, nor with the shear and settling velocities. In addition, the residual diffusion coefficient correlates with van Rijn's roughness scale, again based on the same limited data, suggesting that bedforms influence residual diffusion near the bed.

Modelling the sensitivity of suspended sediment profiles to tidal current and wave conditions

Seawater turbidity due to suspended particulate material (SPM) is an important property of a marine ecosystem, determining the underwater light environment and many aspects of biological production and ecology. SPM concentrations are largely determined by patterns of sediment resuspension from the seabed due to shear stress caused by waves and currents. Hence planning for the construction of large scale offshore structures which will alter regional hydrodynamics needs to consider the consequences for SPM concentrations. Here we develop a one-dimensional (vertical) model of SPM dynamics which can be used to scope the effects of changes in wave and tidal current properties at a site. We implement the model for a number of sites off the east coast of Scotland where we have extensive data sets to enable numerical parameter optimisation. The model performs well at simulating fluctuations in turbidity varying from flood-ebb tidal cycles, spring-neap cycles, storm wave events, and an annual cycle of SPM concentration which is attributed to seasonal consolidation of seabed sediments. Sensitivity analysis shows that, for the range of seabed sediment types in the study (water depth 16–50 m; mud content 0.006–0.380 proportion by weight), relatively large (50%) attenuations of tidal current speed are required to produce changes in water column turbidity which would be detectable by observations given the variability in measurements. The model has potential for application to map the large scale sensitivity of turbidity distributions to the installation of wave and tidal energy extraction arrays.

3D Numerical Simulation of Current and Sediment Transport in Estuary and Coast

In estuarine and coastal area, due to the colligated function of tide, runoff, wind and wave, suspended sediment profile in main flow direction varies from the inland river. Thus, combined with massive data sets from experiments in flume and observations in field, the study on the profile of concentration of suspended sediment are carried out in the thesis. Finally, 3D numerical model of turbulent sediment is established in estuarine and coastal area to carry out a simulation research on current and sediment movement in Yangtze River estuary.

Scaling surf zone turbulence

Turbulence in the surf zone, the shallow region adjacent to the shoreline, has a key role in beach erosion, fertilization, dispersal, and larval settlement of marine invertebrates, and microbial contamination dilution in beach waters. Breaking‐wave generated (the dominant source) surf zone turbulence is understood poorly. A new surf zone turbulent dissipation rateϵ scaling is derived, that collapses new field surf zone ϵobservations with relatively high skill compared to other scalings. The vertically‐uniform length‐scale is 1/6 the water depth, and 15% of the wave‐energy flux gradient is dissipated below the mean surface. Field and laboratory surf zone turbulence observations are shown to be consistent using the scaling. The non‐dimensional surf zone diffusivity and suspended sediment profile can be applied to sediment transport and a range of biological processes including microbial pathogen contamination of beach waters.

Comparison of measurements and models of bed stress, bedforms and suspended sediments under combined currents and waves

This study presents an analysis of data collected in an embayment off a sandy beach protected by shore parallel breakwaters. Observations were carried out over a forty day period including spring and neap tidal cycles and low and high wave activity. Measurements were collected on the waves, currents, bedforms and suspended sediments and these were used to derive estimates of the bed shear-stresses, ripple dimensions, reference concentrations and suspended sediment profiles. The aim of the present study was to use the measurements to assess commonly employed empirical formulations applied to sediment modelling in the coastal marine environment. Assessments are made of four ripple predictors, two formulations of reference concentration and power law and exponential suspended sediment shape functions. In some cases predicted parameters are well represented during low wave conditions, whilst other parameters are better predicted under high wave events. The outcome from the work is an evaluation of the predictive capability of the selected empirical formulae at the sandy location and discussions on the comparison between the observations and predictions.

Punctuated sand transport in the lowermost Mississippi River

[1] Measurements of sand flux and water flow in the Mississippi River are presented for a portion of the system 35–50 km upstream from the head of its subaerial delta. These data are used to provide insight into how nonuniform flow conditions, present in the lower reaches of large alluvial rivers, affect the timing and magnitude of sand transport near the river outlet. Field surveys during both low and high water discharge include (1) sequential digital bathymetric maps defining mobile river bottom topography which were used to estimate bed material flux, (2) multiple water velocity profiles, and (3) multiple suspended sediment profiles collected using a point-integrated sampler. These data show that total sand transport increases by two orders of magnitude over the measured range in water discharge (11,300 to 38,400 m3 s−1). During low water discharge no sand is measured in suspension, and sand discharge via bed form migration is minimal. During high water discharge 54% of the sand discharge is measured in suspension while 46% of the sand discharge is part of bed form migration. The component of boundary shear stress associated with moving this sediment is estimated using a set of established sediment transport algorithms, and values for the total boundary shear stress are predicted by fitting logarithmic velocity functions to the measured profiles. The estimates of boundary shear stress, using measurements of suspended sand transport, bed form transport, and downstream oriented velocity profiles are internally consistent; moreover, the analyses show that boundary shear stress increases by nearly 10-fold over the measured water discharge range. We show how this increase in shear stress is consistent with backwater flow arising where the river approaches its outlet. The hydrodynamic properties of backwater flow affect the timing and magnitude of sand flux and produce punctuated sand transport through the lowermost Mississippi River. Our field data are used to evaluate the influence of this sand transport style on development of the mixed bedrock alluvial channel for the lowermost Mississippi River.

The influence of bed forms on reference concentration and suspension under waves and currents

Hydrodynamic, suspension and bed-form measurements were made 2 km off the Dutch coast near Noordwijk aan Zee in ∼14 m water depth for a period of 32 days in 2003. Tidal currents were just able to suspend sand at the bed at peak spring tide but most suspension and transport occurred as a result of the combination of waves and currents. Burst-average (17 min) sand concentration profiles ( C¯-profiles) from an acoustic backscatter instrument were used to define the (varying) location of the sea-bed, following the method used by Green et al. [Green, M.O., Dolphin, T.J., Swales, A., Vincent, C.E., 1999. Transport of mixed-size sediments in a tidal channel. Coastal Sediments ‘99, edited by N.C. Kraus, and W.G. McDougal, ASCE, Long Island, New York, pp. 644–658]. Reference concentrations at the sea-bed ( C 0) and at 1 cm ( C 1) were examined in relation to both the hydrodynamic conditions and the type of bed forms present. The C 0 predictive equations of Green and Black [Green, M.O., Black, K.P., Suspended sediment reference concentration under waves: field measurements and critical analysis of two predictive models, Coastal Engineering, 38, 115–141, 1999](short-wave ripples) and Nielsen [Nielsen, P., Suspended sediment concentrations under waves, Coastal Engineering, 10, 23–31, 1986](all bed forms; includes ripple steepness), both of which require knowledge of the bed-form type, were not as successful in explaining the variance in our C 0 data as a regression of C 0 against the skin-friction Shields parameter θ′ cw that ignored bed-form type (73% of variance explained). The values of the reference concentration C 1 were compared with the Lee et al. [Lee, G.-H., Dade, W.B., Friedrichs, C.T., Vincent, C.E., Examination of Reference Concentration Under Waves and Currents on the Inner Shelf., Journal of Geophysical Research, 109, 1–10, 2004] equation which predicts C 1 from the product of the Shields parameter and the inverse Rouse parameter; 51% of the variance in C 1 was explained. The shapes of the C¯-profiles were very variable as reported by Dolphin et al. [Dolphin, T.J., Grasmeijer, B.T.,Vincent, C.E., Sand suspension due to waves and tidal flow over short and long-wave ripples and flat beds on the Dutch Coast, in Sand Transport and Morphology of Offshore Mining Pits, edited by L.C. van Rijn, R.L. Soulsby, P. Hoekstra, and A.G. Davies, Aqua Publications, Amersfoort, The Netherlands, 2005.], particularly over larger and steeper bed forms when the position of the measurements in relation to the bed-form geometry became important. However, a Rouse-shaped profile (in which the sediment diffusivity increases linearly with height) could be fitted ( R 2>95%) to the lowest 0.2 m of 89% of the suspended sand profiles. The apparent separation of C¯-profiles, when grouped by bed-form type, is due to the variation in the reference concentration and not to the dependence on the friction velocity, which shows large scatter and no systematic variation with bed-form type. It is concluded that, for most practical applications, reference concentration at 1 cm can be predicted by Lee et al. [Lee, G.-H., Dade, W.B., Friedrichs, C.T., Vincent, C.E., 2004. Examination of Reference Concentration Under Waves and Currents on the Inner Shelf., Journal of Geophysical Research, 109, 1–10.] and that the suspended sediment profiles are Rouse-shaped with a shape parameter p of 0.80±0.25; knowledge of the bed-form type does not, at the Noordwijk site, help to refine either the reference concentration or the shape of the suspended concentration profiles.

Sorted bed forms as self-organized patterns: 1. Model development

[1] Here we present a modified version of the exploratory numerical model originally presented by Murray and Thieler (2004) and use it to further investigate the development of sorted bed forms on the inner continental shelf. The new version of the model is based on widely accepted parameterizations for hydrodynamics and sediment transport that are more detailed and empirically tested than the parameterizations used in the original model. The shallow water wave equations are replaced by the general equations, and changes in water depth related to morphological development are accounted for. This model reproduces the main results of the previous one, supporting the hypothesis that sorted bed forms are a self-organized pattern and develop as a result of a coupling between bed composition and sediment flux. The sensitivity of the model predictions, evaluated in terms of the geometry of the sorted bed forms and their growth rate, with respect to external forcing variables and internal model parameterizations, shows increasing (decreasing) sorted bed form spacing (height) with increasing wave height and period. The opposite occurs for increasing water depths. More complicated behavior is observed when the ratio fine-to-coarse sediment diameter (or the percentage of fine and coarse material) is varied. The shape of the suspended sediment and current profiles and the formulation used to predict the geometry of wave-generated ripples can significantly affect (or even inhibit) growth rates of sorted bed forms. Results from a sensitivity analysis show that there are deficiencies in the description of small-scale processes that limit the ability to quantitatively predict sorted bed form evolution, highlighting gaps in our knowledge of sediment transport over mixed grain beds that need to be filled.

Sediment transport in the coastal ocean: a retrospective evaluation of the benthic tripod and its impact, past, present, and future

In-situ observations of near-bed flow and sediment transport have been carried out for approximately 50 years. The observational technique employs a benthic tripod which can remain submerged on the seafloor for an extended period of time upon which are mounted various oceanographic instruments. The instruments transmit their signals to shipboard via connecting cable or more frequently record their output in accompanying data loggers. During the early years of benthic tripod work, measurements emphasized nearbed current velocity and associated suspended sediment profiles over periods of tidal cycles. The objectives of these studies were to evaluate existing boundary-layer flow and sediment transport theories under natural marine conditions. More recently, instrument capabilities and tripod deployment times have increased dramatically. Research objectives have expanded to include documentation of the physical processes active in the coastal ocean and their contribution to along-and across-margin sediment transport. Tripod-based studies continue to be a significant component of many comprehensive field programs. These long-term studies provide a way to document seabed physical processes and sediment transport over the full range of environmental conditions and continue to shape our concepts of sediment transport on continental shelves. Additionally, these boundary-layer studies have provided critical input for boundary-layer-numerical model development. Benthic tripods and self-contained instrumentation also represent a base of expertise that will help with design and implementation of coastal observatories in the future.

Suspended sediment transport in the swash zone of a dissipative beach

Simultaneous high frequency field measurements of water depth, flow velocity and suspended sediment concentration were made at three fixed locations across the high tide swash and inner surf zones of a dissipative beach. The dominant period of the swash motion was 30–50 s and the results are representative of infragravity swash motion. Suspended sediment concentrations, loads and transport rates in the swash zone were almost one order of magnitude greater than in the inner surf zone. The vertical velocity gradient near the bed and the resulting bed shear stress at the start of the uprush was significantly larger than that at the end of the backwash, despite similar flow velocities. This suggests that the bed friction during the uprush was approximately twice that during the backwash. The suspended sediment profile in the swash zone can be described reasonably well by an exponential shape with a mixing length scale of 0.02–0.03 m. The suspended sediment transport flux measured in the swash zone was related to the bed shear stress through the Shields parameter. If the bed shear stress is derived from the vertical velocity gradient, the proportionality coefficient between shear stress and sediment transport rate is similar for the uprush and the backwash. If the bed shear stress is estimated using the free-stream flow velocity and a constant friction factor, the proportionality factor for the uprush is approximately twice that of the backwash. It is suggested that the uprush is a more efficient transporter of sediment than the backwash, because the larger friction factor during the uprush causes larger bed shear stresses for a given free-stream velocity. This increased transport competency of the uprush is necessary for maintaining the beach, otherwise the comparable strength and greater duration of the backwash would progressively remove sediment from the beach.

Experimental Study on Silt Transport Due to Waves for Prediction of Navigation Channel Deposition in Taihu Lake

The study on the sediment transport due to waves is necessary for prediction of deposition of waterway. Owing to the lack of appropriate formula for calculation of suspended sediment load in Taihu Lake under wave action, an experimental study on transport of silt collected from Taihu Lake is conducted by using a wave flume. In this paper, the process of entrainment and suspension of silt is quantitatively described, and detailed measurements of suspended sediment profiles are carried out for each run of the experiments. Based on the test results, an empirical relationship for calculation of the mean suspended load on waves is presented. The verification results of a case study show that the present equation is satisfying to be used in prediction of sediment deposition of waterway in Taihu Lake.

Assessment of Acoustic Coherent Doppler and Cross-Correlation Techniques for Measuring Near-Bed Velocity and Suspended Sediment Profiles in the Marine Environment

Abstract The simultaneous measurement of current flow and suspended sediment concentration in the marine environment is central to the study of sediment transport processes. In view of this, two acoustic approaches for measuring flow were tested in a tidal estuary to assess their capabilities in this environment. A coherent Doppler velocity profiler and a cross-correlation velocity profiler were assessed using conventional current meters and a commercially available acoustic Doppler velocimeter. Mean velocity profiles were obtained up to a range of 1.47 m in 0.046-m range bins over a number of flood tides. The measurements compared well with the reference instruments and regression analysis produced gradients close to unity. Turbulent velocities measured with the coherent Doppler profiler were comparable with turbulent fluctuations measured with the acoustic Doppler velocimeter. The cross-correlation velocity profiler was shown to be unable to measure turbulent velocities. The backscattered signals received on the cross-correlation transducers were also used to compute the sediment concentration profiles using an explicit solution to the acoustic backscatter equation. Combining the concentration and flow measurements enabled sediment flux profiles to be obtained, the results of which compared favorably with flux measurements obtained from the conventional current meters and pumped sampling.

Effect of wave breaking on sediment eddy diffusivity, suspended-sediment and longshore sediment flux profiles in the surf zone

Turbulence intensity estimates from a wave basin experiment of prototype scale provide description of an eddy viscosity profile above the bottom-boundary layer under unbroken and broken waves. Detailed suspended-sediment concentration data from the DUCK94 surf zone experiment show that the sediment eddy diffusion profile ( K s ) in the nearbed is consistent with a linear nearbed profile whose slope depends on the friction velocity. A full water column K s profile that reflects the wave-breaking state is formulated from the wave basin and DUCK94 data. A simple model shows that the vertical structure of the K s profile above the nearbed region is an important factor in predicting suspended-sediment profiles in the surf zone, in contrast to locations outside the surf zone where bottom-boundary layer friction is the primary mechanism in controlling suspension and distribution of suspended sediment. Under broken wave conditions, the difference between the predictions of suspended-sediment load in the lower 50 cm using a K s profile that reflects broken wave influences and one that does not, averages 28 percent in this study. The prediction of longshore sediment flux is even more strongly influenced by the water column K s profile because even small quantities of sediment that are suspended into the upper water column are subjected to higher velocities than in the nearbed. In this study, the ratios of measured longshore flux to prediction average 0.67 when the wave breaking state is not taken into account in the K s profile.

Comparing Calculated and Observed Vertical Suspended-Sediment Distributions from a Hudson River Estuary Turbidity Maximum

Suspended-sediment concentrations calculated using a vertical suspended-sediment distribution equation were compared to observations from a field study of the lower turbidity maximum of the Hudson River Estuary. At four stations, an instrumented tripod measured vertical profiles of suspended-sediment concentration, current velocity, salinity and temperature through a tidal cycle. Bed and suspended-sediment samples were also analysed to determine inorganic sediment size distributions. Velocities were as high as 1·3ms−1, with suspended-sediment concentrations up to 2000mgl−1. When a well-defined pycnocline existed, cross-isopycnal mixing was strongly damped (based on the gradient Richardson number). Suspended-sediment profiles were calculated with a stratification-modified Rouse equation, using (1) reference concentrations measured at 20cm above the bed, (2) estimates of shear velocity based on the quadratic stress law, and (3) a constant sediment settling velocity of 0·22cms−1. Differences between mean calculated and observed total suspended load for each station were −17, 7, 14 and 58%, respectively. An uncertainty analysis revealed that the two parameterizations most likely to account for differences of this magnitude were those used for settling velocity and stratification. Best results were found when substituting a power law relationship for settling velocity based on suspended-sediment concentration. This demonstrates the improvement which a power law formulation can provide over the commonly used constant wsparameterization in fine sediment environments.

Comparison between ADCP and transmissometer measurements of suspended sediment concentration

For more than a decade, acoustic Doppler current profilers, ADCPs, have been in common use measuring current profiles. It has been recognised over this period that the backscattered ADCP signal could be used to not only evaluate the Doppler shift, but also offered the possibility to extract information on the scatterers. The present work reports on an analysis of opportunistic backscatter measurements collected using a 1 MHz ADCP system, to assess the potential of ADCPs to measure suspended sediment concentration quantitatively. The data were gathered during a water monitoring campaign which deployed ADCPs, near-bed and profiling transmissometers, and in situ bottle samplers. Although the original study was not specifically designed to test the capability of ADCPs to evaluate suspended sediment concentration, sufficient data were collected to examine the use of ADCPs for such measurements. The backscattered amplitude from one ADCP beam was recorded for quality control to assess the accuracy of velocity measurements. However, in this study these data have also been used to examine the potential of ADCPs for suspended sediment measurements. To investigate ADCPs in this role, the backscattered signals from one range cell has been calibrated against in situ bottle samples of the suspended material. Using this calibration, the backscattered signals have been inverted to give time series profiles of suspended particulate matter. To assess these profiles, comparisons have been made with in-situ calibrated profiling and moored transmissometers. The outcome from the present study shows ADCP results which are comparable with the transmissometer observations, and clearly demonstrate the potential of ADCPs for directly measuring suspended sediment profiles.

The effect of sea breeze on beach morphology, surf zone hydrodynamics and sediment resuspension

Beach morphodynamic processes were investigated over part of a sea breeze cycle on a microtidal, low wave energy sandy beach in southwestern Australia. Prior to the onset of the sea breeze, offshore winds with speeds less than 5 m/s prevailed. During the sea breeze, alongshore winds with speeds higher than 10 m/s were experienced. The sea breeze induced pronounced changes to the nearshore morphodynamics which were similar to that of a storm event: (1) root mean square wave height increased from 0.3 to 0.5 m; (2) zero-upcrossing wave period decreased from 8 to 4 s; (3) mean cross-shore flows reached velocities of 0.2 m/s directed offshore; and (4) the longshore current increased in strength from 0.05 to 1.0 m/s. Before the sea breeze, sediment resuspension typically occurred during isolated high-wave events associated with the passage of wave groups. Flux coupling between the wave-oscillatory currents and the suspended sediment (Jaffe, B.E., Sternberg, R.W., Sallenger, A.H., 1984. The role of suspended sediment in shore-normal beach profile changes. Proc. 19th Int. Conf. Coastal Engineering, ASCE, pp. 1983–1996) induced net onshore suspended sediment transport, resulting in beachface accretion and a steepening of the foreshore profile. In contrast, during the sea breeze, sediment resuspension was almost continuous. The suspended sediment load increased six-fold and, as a consequence, the longshore suspended sediment transport rate increased by a factor of 100. During the sea breeze, cross-shore transport was directed offshore and primarily associated with mean offshore flows. This resulted in erosion of the beachface and deposition in the surf zone and, consequently, a flattening of the beach profile. The suspended sediment load was strongly related to the Shields parameter defined for the combined action of waves and currents. Investigation of the suspended sediment profiles indicated that the degree of vertical mixing, parameterised by the sediment diffusion coefficient, increased with distance from the bed. In addition, the sediment diffusion coefficient exhibited a large temporal variation, ranging between 0.005 and 0.018 m 2/s. The variability in the sediment diffusion coefficient is possibly due to changes in the sea bed morphology.

OBSERVATION AND NUMERICAL ANALYSIS OF TURBIDITY IN A RESERVOIR DURING FLOOD

It is an important subject to understand the flow and sediment transport field in reservoir management works. Continuous measurements of suspended sediment, turbidity, and velocity profiles are conducted at Nibutani Dam during a large scale flood caused by a typhoon in August of 1997. Based on the results of the observation, we carried out a numerical analysis of two dimensional model. Comparing the calculation results with field data, The turbidity, and velocity profiles are also reappeared

OBSERVATION OF TURBIDITY IN A RESERVOIR DURING FLOOD

It is an important subject to understand the flow and sediment transport field in reservoir management works. Continuous measurements of suspended sediment, turbidity, and velocity profiles are conducted at Nibutani Dam during a large scale flood caused by a typhoon in August of 1997. Using the flow and sediment transport data of upstream, downstream and inside of the reservoir, sediment transport budget of the reservoir is investigated. From the sediment quantity and velocity profile data, it is suggested that a turbidity current in the bottom layer of the reservoir was taking place.