Variation Of Sediment Concentration Research Articles

Empirical Orthogonal Function Analysis of Suspended Sediment Concentration in Gulf of Kachchh, India, and Its Tidal Influence

Complexity of the temporal sediment variability within the Gulf of Kachchh is resolved into major modes of variations using empirical orthogonal function (EOF) analysis on sequential OCM derived suspended sediment concentration (SSC) images during spring intermonsoon period of 2011. Variance accounted collectively by the first four modes is around 80% and the principal component (PC) of each mode is correlated with different hydro-meteorological forces influencing the hydrodynamics of the gulf. Hydro-meteorological forces are classified in the order of precedence of influencing the sediment dynamics of the region. PC1 contributing 50% of the total variance is correlated with average of the simulated current velocities between consecutive images, indicating the variation of tides from neap-spring ranges (r2 = 0.86). The ocean state parameters like tidal height, wind speed, and current speed at the time of OCM pass contribute to around 20%, 6%, and 4%, respectively, to the total variability of the temporal dynamics of SSC. The interchange of tides from neap and spring is the major contributor toward the temporal variability of sediment concentration within the gulf, where the sediments are subjected to deposition and resuspension. The gulf being highly tide dominated, the suspended sediment variability also reverberates with the variability of tidal characteristics and subtly with the wind conditions.

Well-balanced central-upwind scheme for a fully coupled shallow water system modeling flows over erodible bed

Intense sediment transport and rapid bed evolution are frequently observed under highly-energetic flows, and bed erosion sometimes is of the same magnitude as the flow itself. Simultaneous simulation of multiple physical processes requires a fully coupled system to achieve an accurate hydraulic and morphodynamical prediction. In this paper, we develop a high-order well-balanced finite-volume method for a new fully coupled two-dimensional hyperbolic system consisting of the shallow water equations with friction terms coupled with the equations modeling the sediment transport and bed evolution.The nonequilibrium sediment transport equation is used to predict the sediment concentration variation. Since bed-load, sediment entrainment and deposition have significant effects on the bed evolution, an Exner-based equation is adopted together with the Grass bed-load formula and sediment entrainment and deposition models to calculate the morphological process. The resulting 5×5 hyperbolic system of balance laws is numerically solved using a Godunov-type central-upwind scheme on a triangular grid. A computationally expensive process of finding all of the eigenvalues of the Jacobian matrices is avoided: The upper/lower bounds on the largest/smallest local speeds of propagation are estimated using the Lagrange theorem. A special discretization of the bed-slope term is proposed to guarantee the well-balanced property of the designed scheme. The proposed fully coupled model is verified on a number of numerical experiments.

ANN modelling of sediment concentration in the dynamic glacial environment of Gangotri in Himalaya.

The present study explores for the first time the possibility of modelling sediment concentration with artificial neural networks (ANNs) at Gangotri, the source of Bhagirathi River in the Himalaya. Discharge, rainfall and temperature have been considered as the main controlling factors of variations in sediment concentration in the dynamic glacial environment of Gangotri. Fourteen feed forward neural networks with error back propagation algorithm have been created, trained and tested for prediction of sediment concentration. Seven models (T1-T7) have been trained and tested in the non-updating mode whereas remaining seven models (T1a-T7a) have been trained in the updating mode. The non-updating mode refers to the scenario where antecedent time (previous time step) values are not used as input to the model. In case of the updating mode, antecedent time values are used as network inputs. The inputs applied in the models are either the variables mentioned above as individual factors (single input networks) or a combination of them (multi-input networks). The suitability of employing antecedent time-step values as network inputs has hence been checked by comparative analysis of model performance in the two modes. The simple feed forward network has been improvised with a series parallel non-linear autoregressive with exogenous input (NARX) architecture wherein true values of sediment concentration have been fed as input during training. In the glacial scenario of Gangotri, maximum sediment movement takes place during the melt period (May-October). Hence, daily data of discharge, rainfall, temperature and sediment concentration for five consecutive melt periods (May-October, 2000-2004) have been used for modelling. High Coefficient of determination values [0.77-0.88] have been obtained between observed and ANN-predicted values of sediment concentration. The study has brought out relationships between variables that are not reflected in normal statistical analysis. A strong rainfall: sediment concentration and temperature: sediment concentration relationship is shown by the models which are not reflected in statistical correlation. It has also been observed that usage of antecedent time-step values as network inputs does not necessarily lead to improvement in model performance.

Impact of earthquake-triggered landslides on catchment sediment yield

Abstract. This study explores the role of seismic activity in explaining spatial and temporal variation in sediment export from the Siret basin in Romania. Based on long-term (>30 years) sediment export measurements for 38 subcatchments, we found that spatial variation in sediment yield (SY) is strongly correlated to the degree of seismic activity and catchment lithology. Combined, these factors explain 80% of the variation in SY. To investigate the role of earthquake-triggered landslides in explaining these correlations, we studied the temporal variability in sediment concentrations before and after the 7.4 Mw earthquake of 1977 for ten subcatchments. Despite the fact that this earthquake triggered many landslides, only one subcatchment showed a clear (3-fold) increase in sediment concentration per unit discharge after the earthquake. This shows that, although prolonged seismic activity strongly controls average SY, individual earthquakes do not necessarily affect sediment export at short timescales.

Dynamic processes of soil erosion by runoff on engineered landforms derived from expressway construction: A case study of typical steep spoil heap

Soil erosion on newly engineered landforms severely threatens the ecological security of construction sites and their surrounding areas. A field scouring-erosion experiment was conducted on a typical spoil heap along the Shenmu–Fugu Expressway in China to investigate the processes of soil erosion by simulated runoff. A 2.5×12m2 runoff plot with 72.7% gradient was established on the spoil heap. Clean water was applied at the top of the plot with five different inflow rates of 0.167×10−3, 0.25×10−3, 0.33×10−3, 0.417×10−3 and 0.5×10−3m3/s to simulate surface runoff processes. The variation of sediment concentration was greatly influenced by gravitational erosion which occurred at the critical inflow rates ranging from approximately 0.33×10−3 to 0.417×10−3m3/s. The process of sediment yield on the spoil heap, in terms of sediment transport rate, was characterized by three stages: abruption, fluctuation and stabilization. The spatial distribution of sediment yield in different slope segments at different inflow rates showed two trends: steady decreasing and drastically fluctuated decreasing. The flow over the spoil heap belonged to supercritical flow and the critical inflow rate controlling the transformation from transitional flow to turbulent flow for rill flow was between 0.33×10−3 and 0.417×10−3m3/s. Statistically, power, exponential and logarithmic-linear equations can be used to describe the interrelations of main hydraulic parameters including flow velocity, Reynolds Number, Froude Number and Darcy–Weisbach roughness coefficient. The three forms of equations were also appropriate for describing sediment concentration in relation to the hydraulic parameters mentioned. Slope length was more significant than inflow rate in affecting hydraulic parameters, which showed an exponential relation with the increase in distance from the top of the plot for all the parameters except Reynolds Number. Stream power was the best hydrodynamic parameter to simulate the changing trend of soil detachment rate. As for different control equations, mechanic parameters and energy parameters had some relative advantages in depicting the dynamic processes of sheet erosion and rill erosion, respectively.

Sediment supply and channel morphology in mountain river systems: 2. Single thread to braided transitions

AbstractDifferences in sediment supply between single‐thread and braided channel types provide a long‐recognized, though difficult to quantify, pattern discrimination. Building on the results from our preceding paper, we present a multiscale assessment of the sediment supply, geomorphology, and sediment transport characteristics of braided, gravel‐bedded reaches in the northern Rocky Mountains, USA. First, we present a quantitative, theoretically based discriminant function that stratifies single‐thread and braided reaches on the basis of variations in bankfull sediment concentration and dimensionless discharge following the work of Millar (2005) and Eaton et al. (2010). This function correctly classifies 50 of the 53 channel types where bed load concentrations are known. Second, while channel pattern transitions are often linked to changes in slope, field studies along Sunlight Creek, Wyoming, show that downstream transitions between single‐thread and braided reaches are instead caused by valley constrictions and changes in the grain size of sediment from tributaries. Finally, we demonstrate that the two‐dimensional variability in flow properties in braided reaches produces locally high values of shear stress and bed load transport. Yet bed load measurements and sediment transport calculations also show that sediment transport rates between adjoining braided and single‐thread reaches may be approximately equal where channels are near the pattern threshold and downstream variations in bank fortitude and channel constriction force pattern transitions. Taken together, these results indicate that high bed load concentrations are fundamental to the braided channel planform and that braided channels likely reflect a quasi‐equilibrium state within watersheds with persistent high sediment supply.

Fine sediment dynamics in the Río de la Plata

Abstract. This paper investigates the main characteristics of the fine sediments dynamics in a very complex and extensive inner shelf area located in South America, the Río de la Plata. The results derive from a 5 yr international research program that allowed intensive data acquisition throughout the area, as well as the development of the first numerical model investigating sediment dynamics at a regional scale in the area. The model is based on the MARS3D suite: it computes the regional tide- and wind-induced circulation, as well as sediment mixtures dynamics (erosion, advection and deposition of several sedimentary variables). Erosion due to wave action is taken into account thanks to the use of SWAN. The hydrodynamic and wave models were calibrated against several measurements in different zones of the Río de la Plata. Turbidity data (time series and vertical profiles) were then used in order to calibrate the sediment dynamics model. The model is shown to accurately reproduce the sediment concentration variability during both fair and stormy weather conditions in the intermediate and in the outer Río de la Plata. The paper also shows that the suspended sediment dynamics in the Río de la Plata is mainly driven by water/bottom exchanges.

3D Flow Velocity Pattern in a Circular Section Within River Reach: An Experimental Study

Circular sections utilize vortex flow within the chamber to separate sediment particles from the river flow. The structure of flow inside the circular basins is complicated and fully three-dimensional. In this paper, three-dimensional flow velocity was measured to understand the flow structure and to distinguish secondary currents above and below a deflector which was installed inside the basin. Within the vortex extractor, the variation of sediment concentration is greatly affected by variation in the velocity components, in vertical, radial and tangential directions of flow inside the basin. The three-dimensional flow velocity was measured using an Acoustic Doppler Velocity meter. The velocity of flow was measured at 448 different nodal points, at eight different layers within the flow depth. Of these 448 nodal points, 336 nodal points were below the deflector and 112 nodal points were above it. The measurements were taken at eight different degrees. From this study, it was found that the tangential velocity under a deflector decreases, compared to the tangential velocity in a vortex basin without a deflector, and as a result the secondary currents under a deflector also decrease. Furthermore, as the secondary currents under a deflector decrease, these currents approach the central orifice. Secondary currents under a deflector are useful to impinge deposited sediment particles towards the flushing orifice.

Empirical Formulation of Sediment Pickup Rate in Terms of Wave Energy Flux Dissipation Rate

The rate of sediment pickup for suspension by breaking waves is shown to be proportional to the dissipation rate of wave energy flux within the surf zone. A database of mean concentration of suspended sediment has been compiled by collecting various data of field measurements and large-scale laboratory tests and by calculating the depth-averaged mean concentration. The wave energy flux dissipation rate is computed with the PEGBIS model for random wave breaking on beaches of arbitrary profile. By comparing the mean sediment concentration and the energy flux dissipation rate, a sediment pickup coefficient is assessed at a value of 0.0045 on the average. Use of this coefficient makes it possible to estimate the cross-shore variation of mean sediment concentration. Peak values of mean sediment concentration within measured cross-shore profiles agree well with the prediction based on the wave energy flux dissipation rate.

Assessment of suspended solids in the Guadalquivir estuary using new DEIMOS-1 medium spatial resolution imagery

Measurements of Total Suspended concentrations (TSS) are critical for monitoring geomorphological processes as well as ensuring the “good status” of transitional and coastal waters. Here, we explore the potential of the new DEIMOS-1, multi-spectral, medium-resolution satellite sensor for quantifying TSS dynamics within the large, turbid Guadalquivir estuary (SW Iberian Peninsula). Eight scenes were atmospherically corrected using various image-based procedures (apparent reflectance; Dark Object Subtraction, DOS; and solar zenith angle approximation, COSTZ). In-situ measurements (100 to 1400mgL−1) from three campaigns coinciding (±15min) with satellite overpasses were used to assess the semi-analytical algorithm from Nechad et al. (2010) relating water-leaving reflectance (ρw) to TSS. All bands were very sensitive to TSS, however saturation of the visible wavelengths was observed at the highest sediment loads. The best correlation was obtained for the NIR band (755–906nm) using the simple band-reflectance model at the top of atmosphere reflectance (ρw(TOA)), TSS (mgL−1)=21428.77 ρw(TOA)/(1−ρw(TOA)/0.21)−346.17 (r2=0.864, p<0.001, εr=25.411%, n=53), which was used to generate maps. This approach revealed detailed spatio-temporal variability in sediment concentrations within the river plume, near to shore and far up the river channel. We demonstrate that DEIMOS-1 has a high capability as a valuable tool for the frequent, synoptic monitoring of water-quality parameters needed for the optimum management of coastal and transitional waters.

Toward representing wave‐induced sediment suspension over sand ripples in RANS models

Parameterizations of near‐bed sediment processes are commonly associated with the poor predictive skill of coastal sediment transport models. We implement a two‐dimensional Reynolds‐averaged Navier‐Stokes model to directly assess these parameterizations by reproducing measurements obtained in large‐scale wave flume experiments. A sediment transport model has been coupled to wave hydrodynamics and turbulence, and numerical experiments provide temporal and spatial variations of free surface, flow velocity, sediment concentration, and turbulence quantities. Model‐data comparisons enable the direct assessment of how key suspension processes are represented and of the inherent variability of the sediment transport model. We focus on the different processes occurring above rippled beds versus dynamically flat beds. Numerical results show that increasing roughness alone is not sufficient to have good predictive capability above steep ripples. Some parameterization of the vortex entrainment process is necessary and a simple modification, which leads to constant sediment diffusivity above steep‐rippled beds, is sufficient to obtain good predictions of wave‐averaged suspended concentrations. Model‐data comparisons for the turbulent kinetic energy are also presented and highlight the need to account for the effect of vortex entrainment on near‐bed turbulence and transfer of momentum.

Suspended sediment concentration–discharge relationships in the (sub-) humid Ethiopian highlands

Abstract. Loss of top soil and subsequent filling up of reservoirs in much of the lands with variable relief in developing countries degrades environmental resources necessary for subsistence. In the Ethiopia highlands, sediment mobilization from rain-fed agricultural fields is one of the leading factors causing land degradation. Sediment rating curves, produced from long-term sediment concentration and discharge data, attempt to predict suspended sediment concentration variations, which exhibit a distinct shift with the progression of the rainy season. In this paper, we calculate sediment rating curves and examine this shift in concentration for three watersheds in which rain-fed agriculture is practiced to differing extents. High sediment concentrations with low flows are found at the beginning of the rainy season of the semi-monsoonal climate, while high flows and low sediment concentrations occur at the end of the rainy season. Results show that a reasonably unique set of rating curves were obtained by separating biweekly data into early, mid, and late rainfall periods and by making adjustments for the ratio of plowed cropland. The shift from high to low concentrations suggests that diminishing sediment supply and dilution from greater base flow during the end of the rainfall period play important roles in characterizing changing sediment concentrations during the rainy season.

Using remotely sensed suspended sediment concentration variation to improve management of Poyang Lake, China

This study applied Moderate-Resolution Imaging Spectroradiometer (MODIS) images from 2000 to 2010 to obtain and analyze the spatiotemporal variation of suspended sediment concentration (SSC) and discussed factors affecting it in Poyang Lake, China. Results showed that (1) the mean SSC was lower in the south, higher in the north, and moderate in the central lake region; (2) the mean SSC in the south was lower than or close to 20 mg/L, with no clear annual trend; (3) the mean SSC in the north was slightly higher than 20 mg/L in 2000 and increased from 2001, with the highest value >60 mg/L in 2006; (4) the mean SSC in the central lake region, except for 2009, ranged from 20 to 40 mg/L and had an annual pattern similar to that in the southern lake region; (5) for the entire lake, the mean SSC declined from January to March, increased from September to December, and fluctuated from April to August; and (6) several higher SSC values were found in the central or southern lake regions. The spatiotemporal variation of SSC was controlled by natural and human factors, in which dredging was dominant. Limiting the area of dredging and reducing dredging intensity would decrease SSC and maintain sustainable development of Poyang Lake. Remote sensing can obtain the spatiotemporal information of some water quality parameters, which will help managers understand the lake dynamics and mechanisms to make better decisions for lake management.

Effect of Grass Coverage on the Sediment Concentration in Overland Flow in the Hillslope-Gully Side Erosion System

By scouring experiments, the changeable process and characteristics of sediment concentration in sloping surface with different coverage degrees and spatial locations of grass were studied. Five grass coverage degrees of 0, 30%, 50%, 70%, 90%, three spatial locations of grass (upslope, mid-slope, low-slope) and two water inflow rates of 3.2L/min, 5.2L/min were applied to a 0.5 by 8 m soil bed in scouring experiments. Results showed that in the hillslope–gully side erosion system, the grass coverage has great effect on reducing the sediment concentration in the overland flow when the water inflow rate is small, but when the water inflow rate is large this kind of effect is very limited. When the grass located at the low-slope on the hillslope the sediment concentration is relatively smaller. The sediment concentration in the overland flow is gradually decreasing with the duration of scouring, and the effect of spatial locations of grass on the temporal variation of sediment concentration is very complicated because of gully erosion.

The development of a new optical total suspended matter algorithm for the Chesapeake Bay

Sediment loading is one of the primary threats to the health of the Chesapeake Bay. We have developed a high resolution (250m) ocean color satellite tool to monitor sediment concentrations in the Bay. In situ optical and sediment sampling is used to develop a total suspended matter (TSM) algorithm for the Chesapeake Bay. The Coastal Optical Characterization Experiment (COCE) is part of an ongoing effort to optically characterize processes and to develop regional remote sensing ocean color algorithms in the coastal waters. The goal is to characterize sediment concentrations and to develop a tool to track plumes cascading down the Bay following heavy rainfall events. Background TSM concentrations in the Chesapeake Bay Watershed can also be characterized. The plumes can have potentially devastating effects on the Chesapeake Bay's fragile ecosystem by increasing nutrient loads, depositing sediments, and decreasing salinity and light levels. Sampling took place throughout 2006 to 2008 in the upper and mid portions of the Chesapeake Bay. Measurements of TSM, chlorophyll a (Chl), and hyperspectral optics were collected. The optical measurements included above water surface irradiance (Es(λ)), in-water downwelling irradiance (Ed(λ)) and in-water upwelling radiance (Lu(λ)). These optical data were used to analyze the performance and utility of the Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua Band 1 (645nm) for use as a TSM monitoring tool. From the optical measurements we have derived a 3rd order polynomial regression of TSM to normalized water-leaving radiance (r2=0.79) to form an algorithm that quantitatively relates TSM to the MODIS 250m resolution band 1 (645nm). The algorithm performance was validated (a mean percent difference of −4.2%) against 270 total suspended solids samples collected by the Chesapeake Bay Program during routine water quality monitoring of the Chesapeake Bay environment. The TSM algorithm tool is then used to demonstrate monitoring of significant runoff events that occurred in June, 2006 and March, 2008. In addition, the utility of the Chesapeake Bay TSM product is demonstrated by describing regional and seasonal variations in sediment concentrations throughout the Chesapeake Bay for 2009. Mean concentrations ranged from 11.55mg/l in the upper Chesapeake Bay winter season to 6.37mg/l in the middle Chesapeake Bay spring season. These remote sensing tools can be valuable instruments in the detection and tracking of runoff events and background concentration for monitoring the health and recovery of the Chesapeake Bay.

Wavelet Analysis on Suspended Sediment Concentration Variations over a Tidal Cycle in the Lianyungang Nearshore Area, China

To investigate the characteristics of the suspended sediment concentration (SSC) temporal variation, one representative SSC profile and its relationship with current velocity profile in calm weather in the Lianyungang nearshore area were analyzed using wavelet analysis. Results indicate that the variations of the SSC had multi-time scale characteristics and the fluctuations varied in different time periods. The correlation of periodic oscillation in the upper layers between SSC and current velocity was much more significant than the corresponding lower layer correlation. For the scale more than 4 hours, the curves of SSC and current turned out to be out phase, while this phenomenon was not obvious for the scale less than 4 hours. Wavelet analysis provided further insights into the characteristics of the suspended sediment concentration.

Settling Properties of Cohesive Sediments in Lakes and Reservoirs

The fall velocity of fine particles was measured in a column of height 2.50 m and diameter of 0.30 m to model the settling properties of cohesive sediments in lakes and large reservoirs. In contrast to the traditional particle-size-based methods, an approach relying on time and depth variation of sediment concentration was employed to estimate the mean value of fall velocity at any depth and time. Particles at greater depths, particularly for the samples with a higher rate of sediment concentration, accelerated faster and remained at higher velocities for a longer duration owing to a higher rate of flocculation. As a particular property of cohesive sediments, particles for all concentrated samples and in all depths reached their maximum fall velocity at around the same time—15 min after the start of the tests. The low concentration samples reached higher maximum velocities because of the lower rates of hindering, but for a much lower duration in comparison to the higher concentrated samples. An empirical ...

Numerical simulation on seasonal transport variations and mechanisms of suspended sediment discharged from the Yellow River to the Bohai Sea

Based on sediment and discharge flux data for the Yellow River, realistic forcing fields and bathymetry of the Bohai Sea, a suspended sediment transport module is driven by a wave-current coupled model to research seasonal variations and mechanisms of suspended load transport to the Bohai Sea. It could be concluded that surface sediment concentration indicates a distinct spatial distribution characteristic that varies seasonally in the Bohai Sea. Sediment concentration is rather high near the Yellow River estuary, seasonal variations of which are controlled by quantity of sediment from the Yellow River, suspended sediment concentration reaches its maximum during summer and fall. Furthermore, sediment concentration decreases rapidly in other seas far from the Yellow River estuary and maintains a very low level in the center of the Bohai Sea, and is dominated by seasonal variations of climatology wind field in the Bohai Sea. Only a small amount of sediments imported from the Yellow River are delivered northwestward to the southern coast of the Bohai Bay. Majority of sediments are transported southeastward to the Laizhou Bay, where sediments are continuously delivered into the center of the Bohai Sea in a northeastward direction, and part of them are transported eastward alongshore through the Bohai Strait. 69% of sediments from the Yellow River are deposited near the river delta, 31% conveyed seaward, within which, 4% exported to the northern Yellow Sea through the Bohai Strait. Wind wave is the most essential contributor to seasonal variations of sediment concentration in the Bohai Sea, and the contribution of tidal currents is also significant in shallow waters when wind speed is low.

An unstructured grid hydrodynamic and sediment transport model for Changjiang Estuary

An unstructured grid hydrodynamic and sediment transport model for Changjiang Estuary is developed in the current paper. The model employs finite volume method to discretize the governing equations. Semi-implicit method originally developed by Casulli is utilized to remove the stability limitations associated with the surface gravity wave. A wetting and drying (WAD) scheme is proposed to account for the moving boundary at the shoals and tidal flats. The model is used to investigate the hydrodynamics and sediment transport in the Changjiang Estuary. Comparisons with the measured data show that the model can predict water level and tidal current very well. The variations of sediment concentration are also reasonably captured by the model.

Variation of Sediment Concentration and Its Drivers Under Different Soil Management Systems

In order to prevent soil erosion in southern China, a study was performed to determine the drivers of sediment concentration variation using simulated rainfall and four soil management systems under field condition. Four soil management systems, i.e., forest and grass coverage (FG), forest coverage with disturbed soil surface (FD), contour tillage (CT) and downslope tillage (DT), were exposed to two rainfall intensities (40 and 54 mm h −1) using a portable rainfall simulator. The drivers of sediment concentration variation were determined by the variations of runoff rate and sediment concentration as well as their relationships. The effects of the four soil management systems in preventing water and soil losses were compared using runoff rates and sediment concentrations at steady state. At runoff initial stage, sediment concentration variation was mainly driven by rainfall and management. The degree of sediment concentration variation driven by flow varied with different soil management systems. Three best relationships between runoff rate and sediment concentration were identified, i.e., reciprocal (CT), quadratic (FG and FD) and exponential (DT). At steady state, runoff rates of the four soil management systems varied slightly, whereas their sediment concentrations varied greatly. FG and CT were recommended as the best soil management systems for preventing water and soil losses.