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.

An Alternative Algorithm for Simulating Flash Flood

Most of the approaches in numerical modeling techniques are based on the Eulerian coordinate system. This approach faces difficulty in simulating flash flood front propagation. This paper shows an alternative method that implements a numerical modeling technique based on the Lagrangian coordinate system to simulate the water of debris flow. As for the interaction with the riverbed, the simulation uses an Eulerian coordinate system. The method uses the conservative and momentum equations of water and sediment mixture in the Lagrangian form. Source terms represent deposition and erosion. The riverbed in the Eulerian coordinate system interacts with the flow of the mixture. At every step, the algorithm evaluates the relative position of moving nodes of the flow part to the fixed nodes of the riverbed. Computation of advancing velocity and depth uses the riverbed elevation, slope data, and the bed elevation change computation uses the erosion or deposition data of the flow on the moving nodes. Spatial discretization is implementing the Galerkin method. Furthermore, temporal discretization is implementing the forward difference scheme. Test runs show that the algorithm can simulate downward, upward, and reflected backward 1-D flow cases. Two-D model tests and comparisons with SIMLAR software show that the algorithm works in simulating debris flow.

Triggering and propagation of exogenous sediment pulses in mountain channels: insights from flume experiments with seismic monitoring

In the upper part of mountain river catchments, large amounts of loose debris produced by mass-wasting processes can accumulate at the base of slopes and cliffs. Sudden destabilizations of these deposits are thought to trigger energetic sediment pulses that may travel in downstream rivers with little exchange with the local bed. The dynamics of these exogenous sediment pulses remain poorly known because direct field observations are lacking, and the processes that control their formation and propagation have rarely been explored. Here we carry out flume experiments with the aims of investigating (i) the role of sediment accumulation zones in the generation of sediment pulses, (ii) their propagation dynamics in low-order mountain channels, and (iii) the capability of seismic methods to unravel their physical properties. We use an original setup wherein we supply liquid and solid discharge to a low-slope storage zone acting like a natural sediment accumulation zone that is connected to a downstream 18 % steep channel equipped with geophones. We show that the ability of the self-formed deposit to generate sediment pulses is controlled by the fine fraction of the mixture. In particular, when coarse grains coexist with a high content of finer particles, the storage area experiences alternating phases of aggradation and erosion strongly impacted by grain sorting. The upstream processes also influence the composition of the sediment pulses, which are formed by a front made of the coarsest fraction of the sediment mixture, a body composed of a high concentration of sand corresponding to the peak of solid discharge, and a diluted tail that exhibits a wide grain size distribution. Seismic measurements reveal that the front dominates the overall seismic noise, but we observe a complex dependency between seismic power and sediment pulse transport characteristics, which questions the applicability of existing seismic theories in such a context. These findings challenge the classical approach for which the sediment budget of mountain catchments is merely reduced to an available volume, since not only hydrological but also granular conditions should be considered to predict the occurrence and propagation of such sediment pulses.

Genesis and propagation of exogenous sediment pulses in mountain channels: insights from flume experiments with seismic monitoring

In the upper part of mountain river catchments, large amounts of loose debris produced by mass wasting processes can accumulate at the base of slopes and cliffs. Sudden destabilizations of these deposits are thought to trigger energetic sediment pulses that may travel in downstream rivers with little exchange with the local bed. The dynamics of these exogenous sediment pulses remain poorly known because direct field observations are lacking, and the processes that control their formation and propagation have rarely been explored experimentally. Here we carry out flume experiments with the aims of investigating (i) the role of sediment accumulation zones in the generation of sediment pulses, (ii) their propagation dynamics in low-order mountain channels, and (iii) the capability of seismic methods to unravel their physical properties. We use an original set-up where we supply with liquid and solid discharge a low slope storage zone acting like a natural sediment accumulation zone, and connected to a downstream 18 % steep channel equipped with geophones. We show that the ability of the self-formed deposit to generate sediment pulses depends on the sand content of the mixture. In particular, when a high fraction of sand is present, the storage area experiences alternating phases of aggradation and erosion strongly impacted by grain sorting. The upstream processes also influence the composition of the sediment pulses, which are formed by a front made of the coarsest fraction of the sediment mixture, a body composed of a high concentration of sand corresponding to the peak of solid discharge, and a diluted tail that exhibits a wide grain size distribution. Seismic measurements reveal that the front dominates the overall seismic noise, but we observe a complex dependency between seismic power and sediment pulses’ transport characteristics, which questions the applicability of existing simplified theories in such context. These findings challenge the classical approach for which the sediment budget of mountain catchments is merely reduced to an available volume, since not only hydrological but also granular conditions should be considered to predict the occurrence and propagation of such sediment pulses.

Suitability of phytoliths as a quantitative process tracer for soil erosion studies

<p>Phytoliths are a plant microfossil commonly used as qualitative archive markers in archaeological and paleoecological studies. Their potential uniqueness to the vegetation cover, robustness to weathering, and lack of chemical alteration along the paths make them a potentially suitable tracer for quantitative erosion studies.<br>In this pilot study, we explore the potential of phytoliths in a sediment fingerprinting study in the Ceguera catchment (28 km2) in NE Spain. The phytolith concentrations and morphologies of four land cover classes (agricultural land, badland, forest, and shrubland) were analyzed, and their contributions to four sediment mixture samples along the river course were modelled. Phytoliths concentrations allowed us to discriminate sources sufficiently, albeit with limited sample size. The performance of the phytoliths as the tracer was tested by reproducing the sources of artificial sediment mixture samples with satisfactory recall ratio. Results identified badlands to be the main contributor, with 84–96% of the sediment load to the sinks, followed by shrublands (median 5%) and agricultural lands (median 2%). Additionally, an intensively used agricultural area in the SW of the catchment was well indicated. These major findings can be reproduced by other conventional erosion studies from this area, indicating that phytoliths are suited to quantifying erosion patterns in mesoscale catchments.</p>

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>

Experimental Study of Hysteresis Characteristics of Water-Sediment Mixture Seepage in Rock Fractures

The hysteresis of water-sediment mixture seepage in rock fractures is one of the critical factors which affect the determination of the timing of coal mine water inrush disasters prevention and control. In this paper, a mechanical model was established to study the hysteresis whose criteria were also put forward. The area of the hysteresis loop and the maximum pressure gradient were selected as characterization parameters of hysteresis. On this basis, an experimental system was established to study influences of different sand particle size, sand mass concentration, and fracture opening on water-sediment mixture seepage in rock fractures. The results indicated that the increase in the sand particle size and sand mass concentration could effectively enhance hysteresis characteristics of specimen fractures. While hysteresis characteristics decreased significantly with the increase of fracture opening. The research results are useful to prevent and control water inrush disasters of coal mine.