The Di models method: geological 3-D modeling of detrital systems consisting of varying grain fractions to predict the relative lithological variability for a multipurpose usability

The coexistence of a wide variety of subsurface uses in urban areas requires increasingly demanding geological prediction capacities for characterizing the geological heterogeneities at a small-scale. In particular, detrital systems are characterized by the presence of highly varying sediment mixtures which control the non-constant spatial distribution of properties, therefore presenting a crucial aspect for understanding the small-scale spatial variability of physical properties. The proposed methodology uses the lithological descriptions from drilled boreholes and implements sequential indicator simulation to simulate the cumulative frequencies of each lithological class in the whole sediment mixture. The resulting distributions are expressed by a set of voxel models, referred to as Di models. This solution is able to predict the relative amounts of each grain fraction on a cell-by-cell basis and therefore also derive a virtual grain size distribution. Its implementation allows the modeler to flexibly choose both the grain fractions to be modeled and the precision in the relative quantification. The concept of information entropy is adapted as a measure of the disorder state of the clasts mixture, resulting in the concept of “Model Lithological Uniformity,” proposed as a measure of the degree of detrital homogeneity. Moreover, the “Most Uniform Lithological Model” is presented as a distribution of the most prevailing lithologies. This method was tested in the city of Munich (Germany) using a dataset of over 20,000 boreholes, providing a significant step forward in capturing the spatial heterogeneity of detrital systems and addressing model scenarios for applications requiring variable relative amounts of grain fractions.

The transport of sediment mixtures examined with a birth-death model for grain-size fractions

Bedload transport of sediment mixtures in mountain streams is challenging to predict, with implications for understanding how rivers form and respond to environmental change. Experimental work shows that collective particle entrainment is an important contributing mechanism of bedload transport, but questions remain. We use four different time series of experimental sediment flux for granular particles 4–32 mm in diameter to indirectly examine the role of collective mobilization. Flux was measured at a fixed position in space using an imaging light table. The light table provides a flux measurement that is sampled at a resolution of 1 Hz, and for total time durations ranging from 75 to 240 min. Experimental conditions include periods of statistical steady-state, and transient adjustments due to changes of the upstream supply of water and sediment. We find that despite the contrasting experimental conditions, the time series encode a consistent transport behaviour within the Fourier domain: the transport of finer grain size populations has increasing power density for decreasing frequency, whereas the transport of larger grain size populations has a near constant power density across all frequencies. Hence, smaller particle sizes dominate the power spectra. We seek an explanation for this result, and elaborate on a probabilistic birth-death model introduced to the field by Christophe Ancey and colleagues. Analysis using the expanded birth-death model provides two important results. The transport of smaller particles includes collective entrainment terms that represent grain mobilization due to smaller and larger particle sizes colliding with the streambed surface. In contrast, the transport of larger particles includes collective entrainment terms limited to larger particle sizes. The size-dependent collective controls on particle mobilization is an important finding, and we show that it offers a testable explanation for observed flux differences between smaller and larger particle sizes, common to gravel-bed mountain streams. As a result, our work motivates the need to better understand collective entrainment within the context of granular sediment transport along mountain stream beds.

Dynamic development of sediment infiltration in an artificial river bed

<div> <div>The infiltration and accumulation of fine sediments in gravel-bed rivers leads to a reduction of the existing pore space and may lead in a worst case to a complete clogging of the river bed. To understand the highly dynamic process of sediment infiltration, measurements with high temporal and spatial resolution are required. Within this study, the development of sediment accumulations in an artificial river bed is investigated to gain further understanding on the  process of colmation. The artificial river bed, implemented in a research flume, is made of spheres with two different diameters and in different packing arrangements. Three sediment mixtures with different grain size distributions are supplied to observe the dynamic infiltration process, and to get information on the distribution over depth. In addition, supply rates and supply masses are varied during the experiments.</div> <div> </div> <div>To observe the dynamic development of sediment accumulation, the gamma-ray attenuation method is used, which provides the opportunity of non-intrusive and undisturbed continuous measurements during the experiments at a certain position. Additionally, the accumulated sediment masses are obtained right after the supply of sediments and 28 minutes later, with a high vertical resolution to detect changes as result of consolidation within the pores.</div> <div> </div> <div>From the measured amount of infiltrated sediments can be seen that the accumulated sediment mass is strongly particle size-dependent. The measurements of the fine sediment mixture show that the filling started from the bottom until the accumulation reach the surface of the artificial river bed. The experiments with the coarse sediment mixture resulted in a clogging layer in the upper section of the river bed, and subsequently less sediments reached the flume bed. By varying the supply rate, it can be seen that a higher supply rate leads to an earlier start of the infiltration and a rapid filling, while the lower supply rate resulted in a later infiltration and slow filling process. The measurements 28 minutes after the end of the experiments show, in addition, that dynamic changes happen mainly in the upper layers due to the washing of surface sediments by the flow, and only to a smaller extent by further settlements due to solidification within the pores. The feeding mass itself has no considerable effect on the infiltration behavior of the current setup, as once the pores are filled, almost no additional particles penetrate the bed.</div> <div> </div> <div>The use of a high sophisticated measurement method made it possible to investigate the infiltration process of sediments in an artificial river bed with high temporal and spatial resolution. Due to the use of different sediment mixtures, and different supply conditions, further insight on the process of fine sediment infiltration could be gained within this study.</div> </div><p> </p>

Sediment transport for nonuniform sediment mixtures: the Egiazaroff equation and its application in a canyon river in Brazil

Understanding the sediment yield and transport is one of the major topics of hydrosedimentology today. Although there are several methods to evaluate the incipient movement, the Shields model is mostly-used. However, the model proposed by Shields assumes that the sediments are uniform in an homogeneous and non-cohesive mixture. As such conditions are not easily found in nature, Egiazaroff proposed a sheltering coefficient, which considers the sediments in a non-homogeneous mixture in a wide range of granulometry. This coefficient allows correcting the Shields critical shear stress for nonuniform mixtures and can be used in mountain rivers, where the sediment size varies in several orders of magnitude. The equation proposed by Egiazaroff has been neglected for so many years in Brazil, and it is not mentioned in hydraulics or hydrosedimentology books . Thus, the present paper aimed to introduce the model proposed by Egiazaroff. Applying this equation to one natural river in a canyon, southern Brazil, the paper shows the good performance of this equation and recommends its use in mountain rivers in Brazil.

Experimental study on incipient shear stress of consolidated cohesive sediment

This paper analyzes the incipient motion mechanism of consolidated cohesive sediment. An experimental device based on previous studies was designed to investigate the influencing factors of the incipient shear stress, including the consolidation time, the density of dry bulk, cohesive particles content, and the composition of sediment mixtures. The experimental results indicated that the incipient shear stress of cohesive sediment increased with the increase of consolidation time, dry bulk density, and content of cohesive particles. The incipient motion mechanism of cohesive particles was further investigated using experimental data and theoretical analysis. A formula of the incipient shear stress for cohesive sediment was proposed herein, which is related to both the content of cohesive particles and the relative dry bulk density. The proposed formula was validated by the experimental data, and the calculated values of incipient shear stress using the formula were in good agreement with the experimental results.

The role of biostabilisation in controlling microplastic flux in rivers

<p>Microplastic burden in aquatic environments is now recognised as a potential threat to human and environmental health.  Although microplastic transfers to the ocean from the terrestrial river network contributes up to 90% of the plastics in the oceans the factors controlling that transfer remain largely unconstrained. In rivers microplastics are stored within sediment beds and whilst they are there both the microplastic particles and the sediment grains can become colonised by biofilms.  Biofilm growth on river sediments has been shown to increase a particles resistance to entrainment but the effects of such biostabilisation on microplastic flux has not yet been considered.  This is despite the fact that biofilm growth can change the buoyancy, surface characteristics and aggregation properties of the plastic particles  such as to cause them to be deposited rather than transported and hence increase their residence time.</p><p>In order to quantify biostabilisation processes on microplastic flux a two stage experimental programme was run.  During the first stage, bricks were submerged in a gravel-bed stream and biofilms allowed to colonise the bricks for 4 weeks.  The biofilm covered bricks were then extracted and placed within a re-circulating ‘incubator’ flume which had been divided into 9 smaller channels.  Within each of the 9 channels either a uniform sand, uniform gravel or a bimodal gravel mix were placed in Perspex boxes in the flume channels.  Each sediment type was seeded with either high density PVC microplastic nurdles (D<sub>50</sub> of 3mm, density of 1.33g/cm<sup>3</sup>) or polyester fibres (5 mm long, 0.5-1 mm wide, density of 1.38 g cm<sup>3</sup>), both at a concentration of 1%.  Blanks were also run where the sediment mixtures did not contain any micropalstics.  The flume was left to run with representative day/night cycles of lighting in order to let the biofilms colonise the test sediments for either 0 (control), 2, 4 or 6 weeks.  At the end of the chosen colonisation periods the persepx boxes containing the sediment were removed from the incubator flume and placed within a glass-sided, flow-recirculating flume (8.2m x 0.6m x 0.5m); this constituted the second stage of the experiment. During this stage the samples were exposed to a series of flow steps of increasing discharge designed to establish the entrainment threshold of the D<sub>50</sub> sediment grains. Entrainment thresholds were calculated for each of the growth stages such as to establish the effect of biostabilisation on sediment and microplastic flux.  Bedload and microplastic transport rates were also measured at every flow step to establish biostabilisation effects on overall fluxes. Finally, photographs of the sediment surface were taken at each flow step in order to estimate the percentage loss of biofilm from the surface. </p><p>Discussion concentrates on linking the changes in the degree of biofilm colonisation with the entrainment threshold of the sediment and the links between biofilm colonisation and the character of the bedload and microplastic flux.   The outcome of this research is pertinent to developing understanding surrounding the role biostabilisation has to play in the residence times of microplastics within fluvial systems.</p>

The difference of sediment sources between high flows and low flows in the Hailuogou Glacier stream

<p>       In the area of Mt Gongga (Hengduan range, China) most glaciers are experiencing considerable retreat and mass loss since the early 20th century. Drainage of recently deglaciated surfaces delivers fine sediments thus affecting patterns of sediment delivery with impacts on water quality. Research in the area indicates significant differences between sediment at high flows and low flows in the same river during different seasons. High level flows were usually caused by heavy rainfall events or continuous rainfall that erode the slopes by sheet, rill and gully erosion and transport important amounts of sediments to streams leading to significant increases in river sediment flux. During low flows subsurface soil flux during spring and the direct discharge at the outlet of the glacier result in much less sediment load and mean suspended sediment concentration compared with high flows. The runoff volume, hydrograph peak, sediment load and mean suspended sediment concentration in high flows are as much as an order of magnitude higher than in low flows. Therefore, it is of great significance exploring the provenance of fine sediment during high flows and low flows to assess if there are differences in the contributing sources of sediments.</p><p>          For this purpose during a 2 weeks field campaign in May 2016 in the frame of IAEA INT5153 project, source sediment samples and channel bed sediment mixtures were collected along the river valley of the Hailuogou Glacier. Three main sources were identified: surface glacier materials, old moraines and recent moraines. Composite surface samples (2 cm) were created of 10 subsamples in each representative site for surface glacier materials. Following the same scheme on old lateral moraines 10 sites were selected from the more mineral blocky deposits to the most vegetated parts at higher altitudes. On recent moraines 12 sites with different stages of vegetation cover were sampled. Starting from the glacier tongue a total of 7 fine sediment mixtures were collected along the river of which 3 composite samples corresponded to the dry season with low flow and 4 samples corresponded to high flow.  A new consensus test method and an unmixing model were used to estimate the apportionments of the sediment sources to the sediment loads. The results showed that the contribution of different sources to the sediment mixture deposits varied along the river showing different provenance for the low and high flow suggesting different mechanisms of sediment generation during melting and dry seasons. This study is of interest for gaining knowledge on changing dynamics of sediment in regions were the rapid disappearance of glaciers and snow as in Mt. Gongga, has increased the mobilization and transport of sediment loads with consequences for the local population.</p>