High Suspended Sediment Concentration Research Articles

Medium resolution imaging spectrometer (MERIS) estimation of chlorophyll-a concentration in the turbid sediment-laden waters of the Changjiang (Yangtze) Estuary

Satellite estimation of chlorophyll-a (Chl-a) concentration in the Changjiang Estuary is challenging for ocean-colour retrieval algorithms. The Changjiang Estuary is characterized by suspended-sediment-dominated waters in the mouth and optically complex case 2 waters offshore of the mouth. Satellite ocean-colour products show that high-sediment loads of estuarine waters can cause overestimations or invalid flags of Chl-a concentration. A synthetic chlorophyll index (SCI) was designed for extracting chlorophyll information and for minimizing the influence of sediments on the remote-sensing reflectance spectrum. An SCI algorithm, a quadratic polynomial function of the SCI versus Chl-a concentration, was applied to the estimation of Chl-a concentration from Medium Resolution Imaging Spectrometer (MERIS) images. The overestimation of Chl-a concentration was corrected. The SCI algorithm has applications for MERIS estimation of Chl-a concentration in turbid waters with a moderate to high suspended-sediment concentration.

A simplified conception model for salt marsh sedimentation rate calculation: Response to changing suspended sediment concentration—A case study of Wanggang salt marsh, Jiangsu Province, China

Situated at the junction of land and sea, coastal salt marshes are very sensitive to global environmental changes. Existing numerical models simulating the vertical growth of coastal salt marshes are under the conditions of low suspended sediment concentration (SSC), but under high SSC, such as in the Jiangsu coast of China, the models need to be refined. In this paper, based on the mass balance principle, a refined zero-dimensional time-stepped model is applied after considering the average suspended sediment concentration (ASSC). The statistical results of in situ observation data show that there is an obvious positive linear relationship between ASSC and maximum inundation height, and an obvious positive exponential one between inundation time and inundation height. Using the short-term measurement values for the input parameters and incorporating the above-mentioned relationship between SSC and inundation height, the sedimentation rates are calculated, and the results are similar to previous data. In addition, some factors and hydrodynamic processes that were ignored or simplified could influence the calculated value.

Ocean color products retrieval and validation around China coast with MODIS

Waters along China coast are very turbid with high concentrations of suspended sediment nearly all the time, especially at the Hangzhou Bay, the Changjiang (Yangtze) River Estuary and the shoal along Jiangsu Province. In these turbid and optically complex waters, the standard MODIS ocean color products tend to have invalid values. Because the water-leaving radiances in the near-infrared (NIR) are significant resulting from the strong scattering of suspended particles, the standard MODIS atmospheric correction algorithm often gets no results or produces significant errors. And because of the complex water optical properties, the OC3 model used in the standard MODIS data processing tends to get extremely high chlorophyll-a (Chl-a) concentrations. In this paper, we present an atmospheric correction approach using MODIS short wave infrared (SWIR) bands based on the fact that water-leaving radiances are negligible in the SWIR region because of the extreme strong absorption of water even in turbid waters. A regional Chl-a concentration estimation model is also constructed for MODIS from in situ data. These algorithms are applied to MODIS Aqua data processing in the China coastal regions. In situ data collected in the Yellow Sea and the East China Sea in spring and autumn, 2003 are used to validate the performance. Reasonably good results have been obtained. It is noted that water-leaving reflectance in the NIR bands are significant in waters along the China coast with high sediment loadings. The satellite derived and in-situ reflectance spectra can match in the turbid waters along China coast, and there is relatively good linear relationship between satellite derived and in-situ reflectance. The RMSE value of Rrs(λ) is 0.0031 sr−1 for all the nine ocean color bands (412 to 869 nm). The satellite-derived Chl-a value is in the reasonable range and the root mean square percentage difference is 46.1%.

Sediment Fingerprinting to Determine the Source of Suspended Sediment in a Southern Piedmont Stream

Thousands of stream miles in the southern Piedmont region are impaired because of high levels of suspended sediment. It is unclear if the source is upland erosion from agricultural sources or bank erosion of historic sediment deposited in the flood plains between 1830 and 1930 when cotton farming was extensive. The objective of this study was to determine the source of high stream suspended sediment concentrations in a typical southern Piedmont watershed using sediment fingerprinting techniques. Twenty-one potential tracers were tested for their ability to discriminate between sources, conservative behavior, and lack of redundancy. Tracer concentrations were determined in potential sediment sources (forests, pastures, row crop fields, stream banks, and unpaved roads and construction sites), and suspended sediment samples collected from the stream and analyzed using mixing models. Results indicated that 137Cs and 15N were the best tracers to discriminate potential sediment sources in this watershed. The delta15N values showed distinct signatures in all the potential suspended sediment sources, and delta15N was a unique tracer to differentiate stream bank soil from upland subsurface soils, such as soil from construction sites, unpaved roads, ditches, and field gullies. Mixing models showed that about 60% of the stream suspended sediment originated from eroding stream banks, 23 to 30% from upland subsoil sources (e.g., construction sites and unpaved roads), and about 10 to 15% from pastures. The results may be applicable to other watersheds in the Piedmont depending on the extent of urbanization occurring in these watersheds. Better understanding of the sources of fine sediment has practical implications on the type of sediment control measures to be adopted. Investment of resources in improving water quality should consider the factors causing stream bank erosion and erosion from unpaved roads and construction sites to water quality impairment.

OBSERVATION OF CURRENTS AND SUSPENDED SEDIMENT CONCENTRATION OFF NORTHERN JIANGSU COAST, CHINA

The abandoned Yellow River delta off Jiangsu coast is an important sediment source area,where erosion occurs and large amounts of suspended materials are carried seaward. In order to understand the hydrodynamic process here,a 3-vessel simultaneous measurement was conducted with ADP (acoustic Doppler profiler) and water samplers in autumn of 2006. The analyses of survey data show suspended sediment fluxes,the changes of currents and suspended sediment concentrations. The net suspended sediment fluxes of a tidal cycle range from 103 kg m-1 to 104 kg m-1,mainly to the south. First order estimation of yearly fluxes through the area is similar to a long-term average estimation. The investigation shows the currents are controlled by tides. The suspended sediment concentrations change within a tidal cycle. There are peaks of suspended sediments concentrations corresponding approximately to the time just after the maximum current velocity. Data show that velocity gradients become larger when velocity is bigger. High velocity gradients mean high bed shear stress. The high bed shear stress in turn causes the erosion and resuspension of seabed sediments,resulting in a high suspended sediment concentration in the water column.

Sediment transport off the Huanghe (Yellow River) delta and in the adjacent Bohai Sea in winter and seasonal comparison

Based on the data on the current velocity, water temperature, salinity, turbidity and concentration of suspended sediment collected in November 2006 along three survey transects at three time-series, ship-based stations off the Huanghe (Yellow River) delta, and at twenty-four grid survey stations in the adjacent Bohai Sea, sediment transport off the Huanghe delta and in the adjacent Bohai Sea under winter regime were studied and compared with those from the summer season. The homogeneous distribution of salinity, temperature and sediment in the water column indicated a well-mixed body of water. A zone of high suspended sediment concentrations (SSC) >100 mg l −1 with a width of 35 km was identified along the delta coast. Two highest SSC centers were found in the surface and middle water layers around the abandoned Diaokou Huanghe river mouth and the present Huanghe river mouth, and the third one in the bottom layer around the abandoned Qingshuigou Huanghe river mouth. These three highest SSC centers become major sediment source areas in winter. Based on the location of the high bottom shear stress zone, the prevailing wave directions and the protuberant topography of the river mouths, we identified the resuspension origin of the three highest SSC centers caused mostly by the winter storm waves, partially by tidal currents. Suspended sediment flux (SSF) along the three coastal transects indicated that the sediment in the northwest part of the delta was transported northeastward and southeastward along the coast of the recent delta lobe. The SSC and SSF were higher in winter than summer by 1.7–27.1 and 2–122.5 times, respectively, except around the present river mouth. This indicates that the intensity of sediment transport in winter is much stronger than in summer due to the powerful effect of winter storms although the river water and sediment discharges to the sea were much greater in summer. The massive Huanghe sediment to the sea was accumulated in a narrow coastal area due to weak hydrodynamics in summer, and then strongly resuspended mostly by storm waves in winter and transported to the offshore area. The coastal area off the Huanghe delta acts as a sediment sink in summer and converted to be a sediment source in winter in response to the seasonal variation of the East Asian monsoon in this region. The erosion–accumulation pattern of the Huanghe subaqueous delta from 1976 to 2003 illustrates that river-laden sediment is deposited mainly in the southern part off the coastal area and it is also the result of intensive southward sediment transport in winter.

Estimation of local extreme suspended sediment concentrations in California Rivers

The total amount of suspended sediment load carried by a stream during a year is usually transported during one or several extreme events related to high river flow and intense rainfall, leading to very high suspended sediment concentrations (SSCs). In this study quantiles of SSC derived from annual maximums and the 99th percentile of SSC series are considered to be estimated locally in a site-specific approach using regional information. Analyses of relationships between physiographic characteristics and the selected indicators were undertaken using the localities of 5-km radius draining of each sampling site. Multiple regression models were built to test the regional estimation for these indicators of suspended sediment transport. To assess the accuracy of the estimates, a Jack-Knife re-sampling procedure was used to compute the relative bias and root mean square error of the models. Results show that for the 19 stations considered in California, the extreme SSCs can be estimated with 40–60% uncertainty, depending on the presence of flow regulation in the basin. This modelling approach is likely to prove functional in other Mediterranean climate watersheds since they appear useful in California, where geologic, climatic, physiographic, and land-use conditions are highly variable.

An observation of the three-dimensional structure of a cross-shelf penetrating front off the Changjiang mouth

Satellite images and in situ hydrography and nutrient observations are used to study the three-dimensional structure of a penetrating front off the Changjiang mouth in the East China Sea. The satellite images of MODIS/Terra on October 16, 2006 show anomalously high concentrations of chlorophyll and total suspended sediments extending offshore from the mean coastal front off the Chiangjiang mouth. However, the sea-surface temperature image of the same satellite shows no signature of this extension at all. The in situ hydrography measured during the same time as the satellite images discloses that the surface layer of the front is occupied by low-salinity waters from the nearshore area, which suggests a cross-shelf jet covering a distance of about 50 km. A counter intrusion underneath the surface offshore jet is indicated by an increase of the subsurface temperature and salinity during the offshore excursion of the front, which leads to a significant temperature inversion at the head of the front. The vertical temperature and salinity distribution shows propensity of double diffusive processes, which may contribute to the ensuing evolution of the front. The nutrient distribution of the front is consistent with the cross-shelf jet derived from the hydrographic data, with surface and bottom intensified concentrations and minimum mid-depth concentrations, suggesting that the primary transport of the surface nutrients is in the horizontal cross-shelf direction. The potential dynamics of the frontal intrusion is discussed in light of wind forcing and frontal instability. The intrusion is found to be very important to cross-shelf and cross-front exchanges in the East China Sea.

Can suspended sediment concentrations be estimated from multispectral imagery using only image-derived information?

Sampling for suspended sediment concentrations (SSC) in inland waters is traditionally based on collecting samples at sparse locations and in limited intervals. A number of investigators explored the utility of earth-observing satellites and air-borne sensors for monitoring of SSC over vast areas. Two approaches are commonly deployed: (1) empirical relationships between a chosen remotely sensed quantity and the actual in-situ SSC; and (2) bio-optical models founded on radiative transfer modeling. Unfortunately, in-situ measurements are often unavailable for direct image calibration, and inherent optical properties of optically active constituents (specific scattering and absorption coefficients) are usually unknown. This paper examines the possibility to retrieve SSC from multispectral satellite imagery without any in-situ data, i.e. using only image-derived information. The fundamental principle of image selfcalibration relies on the fact that in the visual domain of wavelengths (∼400–700 nm) the at-sensor reflectance becomes “saturated“ at high SSC, whereas the near-infrared domain (∼700–900 nm) remains almost perfectly linearly related to sediment concentrations. The core idea of the self-calibrating procedure is rather simple and is based on fitting an exponential function between reflectance and SSC, with SSC replaced by a linear relationship between SSC and reflectance in the near-infrared domain. As a first approximation of the non-linearity between reflectance and SSC levels in the 400–700 nm range, we used the equation proposed by Schiebe et al. (1992), although other equations, especially those arising from optical theory could be used as well. The technique is illustrated on a moderately sediment-laden reservoir and two scenes acquired from Landsat ETM+. The standard error of the estimated SSC was below 15 mg/L (i.e. ∼25 % relative error for the observed range of SSC). Although the proposed algorithm does not yield better results than other models mentioned in the literature, the primary advantage of the outlined methodology is that no in-situ measurements (water sampling nor spectral profiling) are needed — i.e. only image-derived information is used.

On the relationship between flow and suspended sediment transport over the crest of a sand dune, Río Paraná, Argentina

The links between large-scale turbulence and the suspension of sediment over alluvial bedforms have generated considerable interest in the last few decades, with past studies illustrating the origin of such turbulence and its influence on flow resistance, sediment transport and bedform morphology. In this study of turbulence and sediment suspension over large sand dunes in the Río Paraná, Argentina, time series of three-dimensional velocity, and at-a-point suspended sediment concentration and particle-size, were measured with an acoustic Doppler current profiler and laser in situ scattering transmissometer, respectively. These time series were decomposed using wavelet analysis to investigate the scales of covariation of flow velocity and suspended sediment. The analysis reveals an inverse relationship between streamwise and vertical velocities over the dune crest, where streamwise flow deceleration is linked to the vertical flux of fluid towards the water surface in the form of large turbulent fluid ejections. Regions of high suspended sediment concentration are found to correlate well with such events. The frequencies of these turbulent events have been assessed from wavelet analysis and found to concentrate in two zones that closely match predictions from empirical equations. Such a finding suggests that a combination and interaction of vortex shedding and wake flapping/changing length of the lee-side separation zone are the principal contributors to the turbulent flow field associated with such large alluvial sand dunes. Wavelet analysis provides insight upon the temporal and spatial evolution of these coherent flow structures, including information on the topology of dune-related turbulent flow structures. At the flow stage investigated, the turbulent flow events, and their associated high suspended sediment concentrations, are seen to grow with height above the bed until a threshold height (ca 0·45 flow depth) is reached, above which they begin to decay and dissipate.

Bacterial production in the Lower Mississippi River: importance of suspended sediment and phytoplankton biomass

The roles and metabolic activity of heterotrophic bacteria, along with factors controlling their activity, are poorly known for large, turbid rivers. The aim of this study was to evaluate temporal patterns in heterotrophic bacterial production (BP) in the main channel of the Lower Mississippi River (LMR) in relation to several seasonally dynamic environmental factors. We hypothesized that whole-water BP would vary with levels of temperature, as well as phytoplankton biomass and suspended sediment concentration. Further, we hypothesized that bacteria attached to suspended sediment would comprise an important component of whole-water BP, their importance varying with sediment concentration. Measurements were made at three locations on the LMR for up to 29 months. Bacterial production in whole-water ranged over an order of magnitude between summer and winter, with little variation among sites. Peaks in whole-water BP were associated with periods of high suspended sediment concentrations in spring, and elevated phytoplankton biomass in summer. Attached BP was correlated with all the measures of sediment concentration, especially particulate phosphorus, and accounted for a large majority of water-column BP. After temperature, the only positive correlate of free-living cells was with phytoplankton biomass. Rates of BP in the LMR during summer were much higher than measurements made previously in the Mississippi River plume, similar to the Hudson River, but lower than in three large tributaries of the LMR. Determination of bacterial population dynamics is an essential step in analysis of the food web structure and biogeochemical processes of large rivers. This is the first study of heterotrophic bacterial production in the main channel of the LMR.

Numerical simulation of tidal current and erosion and sedimentation in the Yangshan deep-water harbor of Shanghai

Yangshan near-shore sea area is the multi-island and multi-channel area with strong flow velocity and high suspended sediment concentration. Based on the characteristics of tidal currents, waves, and sediment in the Yangshan area, a two-dimensional numerical model of tidal currents, sediment transport, and sea bed deformation is developed. In the model, the effects of tidal currents and wind waves on sediment transport are considered. According to characteristics of the study area, unstructured grids are applied to fit the boundaries of the near-shore sea area. The results show that the calculated values are in good agreement with the measured data. The field of tidal currents, suspended sediment concentrations, and the deformation of the seabed can be successfully simulated.

Dynamics of photosynthetic picoplankton in a subtropical estuary and adjacent shelf waters

Dynamics ofSynechococcus(Syn),Prochlorococcus(Pro) and picoeukaryotes (Euk) was studied at 92 stations in the Pearl River Estuary and adjacent shelf waters in the winter of 2006 using flow cytometry. The average cell abundance of photosynthetic picoplankon (Pico) was 104cells/ml. Picoeukaryotes was the dominant group in the inner estuary whereProwas absent.Synechococcuswas dominant in the coastal and open waters. The proportion ofProwas higher in the open than in the coastal waters. The change of Pico community structure from the inner estuary to the open waters is related to the changes in hydrochemical dynamics. The vertical variations in the abundance and community composition of Pico were not significant. The abundances ofProandSynwere positively correlated with temperature, salinity and pH, whereas they were negatively correlated with dissolved oxygen (DO), suspended sediment concentrations (SSC) and nutrients (nitrogen, phosphorus and silicon) concentrations. But the correlations between %Euk (as of total pico) and these environmental factors were opposite to those between the abundance of prokaryotic cells (ProandSyn) and the environmental factors. %Euk had significant negative correlations with temperature, salinity and pH, and positive correlations with DO, SSC and nutrients. There were differences in the niches between prokaryotic cells and picoeukaryotes.ProchlorococcusandSynwere dominant in the warm waters with high salinity, low SSC and low nutrient concentrations, while %Euk was higher in the cold waters with low salinity, high SSC and high nutrient concentrations. Results of principal component analysis and regression analysis indicated that the relationship between the abundances of Pico and the environmental factors was a compound function (y = b0(b1)x), and that between the abundances ofPro,Synor %Euk and the environmental factors was a linear function.

A snapshot of suspended sediment and fluid mud occurrence in a mixed-energy embayment, Tijucas Bay, Brazil

Along the southern Brazilian coast, Tijucas Bay is known for its unique muddy tidal flats associated with chenier plains. Previous field observations pointed to very high suspended sediment concentrations (SSCs) in the inner parts of the bay, and in the estuary of the Tijucas River, suggesting the presence of fluid mud. In this study, the occurrences of suspended sediments and fluid mud were examined during a larger-scale, high-resolution 2-day field campaign on 1–2 May 2007, encompassing survey lines spanning nearly 80 km, 75 water sampling stations for near-bottom density estimates, and ten sediment sampling stations. Wave refraction modeling provided qualitative wave energy estimates as a function of different incidence directions. The results show that SSC increases toward the inner bay near the water surface, but seaward near the bottom. This suggests that suspended sediment is supplied by the local rivers, in particular the Tijucas. Near-surface SSCs were of the order of 50 mg l−1 close to the shore, but exceeded 100 mg l−1 near the bottom in the deeper parts of the bay. Fluid mud thickness and location given by densimetry and echo-sounding agreed in some places, although being mostly discordant. The best agreement was observed where wave energy was high during the campaign. The discrepancy between the two methods may be an indication for the existence of fluid mud, which is recorded by one method but not the other. Agreement is considered to be an indication of fluidization, whereas disagreement indicates more consolidation. Wave modeling suggests that waves from the ENE and SE are the most effective in supplying energy to the inner bay, which may induce the liquefaction of mud deposits to form fluid mud. Nearshore mud resuspension and weak horizontal currents result in sediment-laden offshore flow, which explains the higher SSCs measured in the deeper parts of the bay, besides providing a mechanism for fine-sediment export to the inner shelf.

Distribution and transport of suspended sediments off the Yellow River (Huanghe) mouth and the nearby Bohai Sea

Hydrological and sedimentological investigations were carried out off the Yellow River (Huanghe) mouth and the nearby Bohai Sea during August–September 2007 to examine the distribution and transport of suspended sediments. Our data witnessed that suspended sediments from the Yellow River mouth primarily extend southerly and southeasterly during the flood season. High suspended sediment concentration (SSC) occurs as turbid plume on both sides of the river mouth, where it can reach >20 mg l −1 in the surface water and >40 mg l −1 in the bottom water. Accordingly, salinity varies from 25 to 29 psu, mean grain size (Dmz) is >200 μm, and volume concentration (VC) is >100 μl l −1. High SSC was concentrated in the river mouth area as two muddy patches, i.e. the North Mud and South Mud, consisting mostly of fine-grained silt and clayey silt sediments. Our results have verified the larger extent of the South Mud than that of the North one, implicating south- and southeastward sediment dispersal paths that prevail near the river mouth area. The existing anti-clockwise and clockwise flows in the river mouth area were synthesized as the major hydrodynamics responsible for forming the two muddy patches. Tidal and residual currents are considered as the key hydrodynamic factors controlling the sediment distribution and transport in the region.

Seasonal patterns of suspended sediment transport in an abandoned farmland catchment in the Central Spanish Pyrenees

AbstractThe suspended sediment response of a small catchment subjected to farmland abandonment and subsequent plant recolonization was studied in relation to its hydrological functioning. The analysis of data over a seven‐year period demonstrated that suspended sediment yield was greatly influenced by the occurrence of intense, low‐frequency events. Greater amounts of suspended sediment were exported during spring, when the catchment was hydrologically more active. Rainfall intensity and baseflow at the start of a flood event had a strong influence on the sediment response, suggesting that several hydrological processes were active within the catchment. SSC (suspended sediment concentration)‐Q hysteretic loop analysis at the event scale aided understanding of the sedimentological and hydrological behaviour of the catchment. During the study period the SSC‐Q loops showed a high degree of seasonality and two main patterns strongly related to catchment wetness were distinguished. When the catchment was dry (mainly during summer and the beginning of autumn) the predominant process was infiltration excess runoff over sparsely vegetated areas close to the main channel. Under these conditions, floods exhibited a counter‐clockwise hysteretic loop and were characterized by a small streamflow response, short duration and high SSC. Under wet conditions (mainly during winter and spring), saturation excess runoff was increasingly dominant over vegetated areas. Under these conditions, floods exhibited a clockwise hysteretic loop, and were characterized by a larger streamflow response, longer duration and higher suspended sediment yield. The lower SSC during the falling stage of the hydrograph is likely to be due to dilution effects related to the contribution of clean water resulting from enlargement of the saturated areas, together with an increase in the baseflow discharge. Copyright © 2009 John Wiley & Sons, Ltd.

Particulate suspended matter concentrations in the Bahía Blanca Estuary, Argentina: Implication for the development of phytoplankton blooms

The inner zone of the Bahía Blanca Estuary is shallow, nutrient-rich and turbid. Tidal energy and water turbulence strongly affect the water column resulting in a well-mixed structure and high concentrations of suspended sediment. The phytoplankton community is mostly dominated by diatoms and the annual pattern has been characterized by a recurrent winter-early spring bloom. Here, we investigated to what extent the temporal variations of suspended particulate matter (SPM) regulate the phytoplankton blooms in the head of the estuary by light-limitation. Sampling was done on a fortnightly basis (weekly during the blooming season) at a fixed station in the inner zone of the estuary from January 2007 to February 2008. SPM concentrations and light extinction coefficients ( k) in the water column were significantly correlated and showed relatively lower values during the phytoplankton maximal biomass levels. During winter, SPM and k reached values of 23.6 mg l −1 and 0.17 m −1 which were significantly lower than the annual means of 77.6 mg l −1 and 2.94 m −1, respectively. The particulate organic matter (POM) concentration was significantly correlated with the calculated phytoplankton biomass although the contribution of the latter to the total POM was rather low. Both, POM and biomass, had maximal values during winter (21.8 mg l −1 and 393.5 μg C l −1) and mid summer (24.3 mg l −1 and 407.0 μg C l −1), with cell densities up to 8 × 10 6 cells l −1 and chlorophyll a up to 24.6 μg l −1. Our results suggest that the decrease of SPM concentrations in the water column with a concomitant increase in the penetration of solar radiation seems to be one of the main causes for the development of the phytoplankton winter bloom in the Bahía Blanca Estuary.

Effect of Suspended-Sediment Concentration on Nitrification in River Water: Importance of Suspended Sediment−Water Interface

High suspended sediment (SPS) concentrations exist in many Asian river systems. In this research, the effects of SPS concentration on nitrification in river water systems were studied. With orwithout introducing ammonium-oxidizing bacteria isolated from the water and sediment samples of the Yellow River, the microbially mediated nitrification rate increased with SPS concentration as described by the power function y = a x x(b), where y is the nitrification rate, x is the SPS concentration, and a and b are constants. With an indigenous ammonium-oxidizing bacteria, nitrification rate constants, i.e., K4 (micromax/Ks) values obtained from the Monod model, were 0.0016, 0.0036, 0.0040, 0.0063, 0.0066, 0.0071, and 0.0077 day(-1) microM(-1) for the systems with SPS concentrations of 0, 0.2 1.0, 5.0, 10, 20, and 40 g/L, respectively. The sorption percentage of NH4+-N increased with SPS concentration as a power function. Bacteria tend to attach onto SPS, and the maximum specific growth rate at the SPS-water interface was about twice that in the water phase. An increase of bacterial population and nitrification rate constant with SPS as a power function resulted in an increase of nitrification rate with SPS as a power function. Therefore, the high SPS concentration caused by erosion and bottom sediment resuspension and other factors will accelerate ammonium oxidation in many turbid river systems. This has useful implications for nitrogen removal from river systems.

Remote sensing of temporal and spatial patterns of suspended particle size in the Irish Sea in relation to the Kolmogorov microscale

Abstract Suspended sediments form an integral part of shelf sea systems, determining light penetration for primary production through turbidity and dispersion of pollutants by adsorption and settling of particles. The settling speed of suspended particles depends on their size and on turbulence. Here a method of determining particle size via remote sensing measurements of ocean colour and brightness has been applied to a set of monthly satellite images of the Irish Sea covering a full year (2006). The suspended sediment concentration was calculated from the ratio between green (555 nm) and red (665 nm) wavelengths in MODIS imagery. Empirical formulae were employed to convert suspended sediment concentrations and irradiance reflectance in the red part of the spectrum into specific scattering by mineral particles and floc size. A geographical pattern was evident in all images with shallow areas with fast currents having high year-average suspended sediment concentrations (7.6 mg l −1 ), high specific scattering (0.225 m 2 g −1 ) and thus small particle sizes (143 μm). The reverse is true for deeper areas with slower currents, e.g. the Gyre southwest of the Isle of Man where turbidity levels are lower (3.3 mg l −1 ), specific scattering is lower (0.081 m 2 g −1 ) and thus particle sizes are larger (595 μm) on average over a year. Temporal signals are also seen over the year in these parameters with minimum seasonal amplitudes (a factor 3.5) in the Turbidity Maximum and maximum seasonal amplitudes twice as large (a factor 7) in the Gyre. In the Gyre heating overcomes mixing in summer and stratification occurs allowing suspended sediments to settle out and flocs to grow large. The size of aggregated flocs is theoretically proportional to the Kolmogorov scale. This scale was calculated using depth, current, and wind speed data and compared to the size of flocculated particles. The proportionality changes through the year, indicating the influence of biological processes in summer in promoting larger flocs.

Modelling hyperconcentrated flow in the Yellow River

AbstractA large amount of the total sediment load in the Chinese Yellow River is transported during hyperconcentrated floods. These floods are characterized by very high suspended sediment concentrations and rapid morphological changes with alternating sedimentation and erosion in the main channel, and persistent sedimentation on the floodplain. However, the physical mechanisms driving these hyperconcentrated floods are still poorly understood. Numerical modelling experiments of these floods reveal that sedimentation is largely caused by large vertical concentration gradients, both in the channel during the rising stage of the flood, as well as on the floodplains, during a later stage of the flood. These vertical concentration gradients are large because the turbulent mixing rates are reduced by the increased sediment‐induced density gradients, resulting in a positive feedback mechanism that produces high deposition rates. Erosion prevails when the sediment is largely held in suspension due to hindered settling, and is strengthened by the reduced wetted cross‐section caused by massive sedimentation on the floodplain. Observed patterns of erosion and sedimentation during these floods can be qualitatively reproduced with a numerical model in which sediment‐induced density effects and hindered settling are included. Copyright © 2009 John Wiley & Sons, Ltd.