Suspended Sediment Particle Size Distribution Research Articles

Multifractal features of the particle-size distribution of suspended sediment in the Three Gorges Reservoir, China

The Three Gorges Reservoir (TGR) in China is the largest hydroelectric project in the world, but the threat of sediment affecting ecological sustainability of the reservoir is a topic of concern. Sediment particle-size distribution (PSD) is informative in understanding sediment transport dynamics and biochemical functions. It is, therefore, important to quantitatively characterize the distribution of sediment particles. In the current study, fractal theory is applied to determine the PSD of suspended sediment in the TGR. The results show that the volumetric fractal dimension (Dv) exhibits a significant seasonal difference (p < 0.05), reflecting sediment source and hydrodynamic sorting control the granularity of suspended sediment in the TGR. More specifically, suspended sediment particles are coarser in the wet season than in the dry season for the Yangtze River, and the opposite is true for the Ruxi River, an important tributary. The generalized dimension spectrum, D(q)–q, and multifractal singularity spectrum, f[α(q)]–α(q), were calculated for each suspended sediment sample. Thereafter, the parameters, D(0), D(1), D(2), α(0), Δα(q), and Δf[α(q)], were determined to characterize the PSD. As a result, the coarser suspended sediment during the wet season is characterised by a more complex PSD pattern, with a wider range of particle sizes, greater heterogeneity, and greater homogeneity of distribution over the measurement interval. However, the multifractal structure of the PSD of suspended sediment is more complex during the dry season than during the wet season, with higher local dispersion and variability. The findings of the current study highlight that multifractal analysis provides important insight for understanding the PSD of suspended sediment in the TGR.

Changes in particle size distribution of suspended sediment affected by gravity erosion: a field study on steep loess slopes

Gravity erosion (mass movement) generates an enormous volume of sediments on steep slopes throughout the world, yet its effect on the particle size distribution of suspended sediments (PSDSS) remains poorly understood. The objective of this study is to quantify the effects of gravity erosion on soil loss, especially the changes of particle size distribution of the suspended sediment. In this study, experiments were conducted in a field mobile laboratory in which mass movements were triggered on steep slopes under simulated rainfall. During the experiments, water-sediment samples were collected in polyethylene bottles directly from the gully and channel flows before and during the mass movements. The volumes of each mass failure during and 20 min after the rainfalls were measured by a topography meter. The particle fractions of the samples were analyzed by combining the sieving and photoelectric sedimentometer techniques. A suite of indexes such as median sediment size (d50), sediment heterogeneity (H), fractal dimension (D), and enrichment/dilution ratio (Red) were then used to evaluate the effect of mass movement on PSDSS. (1) Gravity erosion had a significant influence on PSDSS. After the mass movements occurred, the proportion of sand-sized particles was decreased from 71 to 51%, whereas the proportions of clay and silt were increased from 1 to 7% and 28 to 42%, respectively. (2) The d50, H, and D were significantly correlated with slope failures. The d50 was decreased from 0.084 to 0.051 mm, whereas the H and D were increased from 5.6 to 26.8 and from 2.60 to 2.78, respectively. This implies that mass movements make the PSDSS more nonuniform and irregular. (3) The suspended sediment tended to be enriched in the silt and clay fractions and diluted in the sand fractions after mass movements. Reds for clay, slit, and sand fractions were 13.9, 1.4, and 0.7 respectively. The changes of PSDSS after mass movements reflected a combined complex effect of soil sources, erosion types, selective detachment, and deposition processes. Mass movements led to a drastic increase of sediment concentration and the enrichment of fine particles, which developed into hyperconcentrated flows.

Spatiotemporal variability of suspended sediment particle size in a mixed-land-use watershed

Given existing knowledge gaps, there is a need for research that quantitatively characterizes spatiotemporal variation of suspended sediment particle size distribution (PSD) in contemporary watersheds. A five-year study was conducted in a representative watershed of the central United States utilizing a nested-scale experimental watershed study design, comprising five gauging sites partitioning the catchment into five sub-watersheds. Streamwater grab samples were collected four times per week, at each gauging site, for the duration of the study period (Oct. 2009–Feb. 2014). Samples were analyzed using laser particle diffraction. Significantly different (p<0.05) suspended sediment PSDs were observed at monitoring sites throughout the course of the study. For example, results indicated greater proportions of silt at site #5 (65%), relative to other sites (41, 32, 29, and 43%, for sites #1–#4, respectively). Likewise, results showed greater proportions of sand at sites #2 and #3 (66 and 68%, respectively), relative to other sites (57, 55, and 34%, for sites #1, #4, and #5, respectively). PSD spatial variability was not fully explained by hydroclimate or sub-watershed land use/land cover characteristics. Rather, results were strengthened by consideration of surficial geology (e.g. supply-controlled spatial variation of particle size). PSD displayed consistent seasonality during the study, characterized by peaks in the proportion of sand (and aggregates) during the winter (i.e. 70–90%), and minimums during the summer (i.e. 12–38%); and peaks in the proportion of silt particles in the summer (i.e. 61–88%) and minimums in the winter (i.e. 10–23%). Likely explanations of results include seasonal streamflow differences. Results comprise distinct observations of spatiotemporal variation of PSD, thereby improving understanding of lotic suspended sediment regimes and advancing future management practices in mixed-land-use watersheds.

Event-based total suspended sediment particle size distribution model

One of the most challenging modelling tasks in hydrology is prediction of the total suspended sediment particle size distribution (TSS–PSD) in stormwater runoff generated from exposed soil surfaces at active construction sites and surface mining operations. The main objective of this study is to employ gene expression programming (GEP) and artificial neural networks (ANN) to develop a new model with the ability to more accurately predict the TSS–PSD by taking advantage of both event-specific and site-specific factors in the model. To compile the data for this study, laboratory scale experiments using rainfall simulators were conducted on fourteen different soils to obtain TSS–PSD. This data is supplemented with field data from three construction sites in Ontario over a period of two years to capture the effect of transport and deposition within the site. The combined data sets provide a wide range of key overlooked site-specific and storm event-specific factors. Both parent soil and TSS–PSD in runoff are quantified by fitting each to a lognormal distribution. Compared to existing regression models, the developed model more accurately predicted the TSS–PSD using a more comprehensive list of key model input parameters. Employment of the new model will increase the efficiency of deployment of required best management practices, designed based on TSS–PSD, to minimize potential adverse effects of construction site runoff on aquatic life in the receiving watercourses.

Seleção de modelos para o traçado de curvas granulométricas de sedimentos em suspensão em rios

O conhecimento da distribuição granulométrica dos sedimentos em suspensão em cursos d'água é fundamental para a realização de estudos hidrossedimentológicos. As técnicas geralmente utilizadas para a avaliação da distribuição granulométrica de amostras de sedimentos resultam em valores pontuais, dependendo de posterior interpolação para o traçado da curva granulométrica e para a obtenção de diâmetros característicos específicos. A transformação de valores pontuais em funções contínuas pode ser realizada por meio de modelos matemáticos; entretanto, são poucos os estudos desenvolvidos com a finalidade de determinar o melhor modelo para o ajuste de curvas granulométricas. Neste trabalho, objetivou-se a seleção de modelos para o traçado de curvas granulométricas de sedimentos em suspensão em rios; utilizando-se 30 amostras contendo de 8 a 10 pontos medidos, testaram-se 14 diferentes modelos. A comparação entre os modelos foi baseada na diferença da soma do quadrado dos erros entre os valores observados e os ajustados, cujos resultados indicaram que os modelos Haverkamp &amp; Parlange (1986) e Skaggs et al. (2001), ambos com 3 parâmetros de ajuste, são os melhores para o traçado das curvas granulométricas de amostras de sedimentos em suspensão em rios.

Effects of Sediment‐Laden Flow on Channel Bed Clogging

In open channel systems the phenomenon commonly referred to as clogging is caused by settling and straining of suspended sediment as water moves through the channel bed material, as well as by microbial transport, colonization, and other biological and chemical action. This paper summarizes a three‐year investigation of the clogging process due to accumulation of fine soil particles as it occurs in sediment‐laden open channel flow. Significant results include experimental relationships developed between major independent variables (i.e., flow velocity, suspended sediment concentration, and suspended sediment particle size distribution) and the infiltration reduction occurring through a sand bed filter in the bottom of a recirculating laboratory flume. Factors affecting both magnitude and temporal variation of channel bed infiltration are explained as well as observed differences in the physical nature of the clogged layer developed atop the filter bed. Significant features of sedimentary clogging in open ...