Calculation of Sediment Transport Capacity of Flowing Water in Rivers with Machine Learning

Sediment Transport and Morphodynamic

The application of terrestrial laser scanning to measure small scale changes in aeolian bedforms

Traditional methods used to measure aeolian sediment transport rely on point based sampling, such as sand traps or saltation impact sensors, which ignore the spatial heterogeneity displayed in the transport system. Obtaining an accurate transport rate is important to parameterise predictive models, which currently show large deviations between measured and predicted rates.Terrestrial laser scanning (TLS) is a tool that is rapidly emerging in the field of geomorphology. It provides the ability to capture surface elevation data of in-situ bedforms at the spatial and temporal scale necessary to link change, such as ripple migration, to the processes that drive them. Repeat scans provide digital elevation models which can be differenced to provide volumetric rates of change, in a process known as the morphologic method. However, utilising data at such high resolutions requires an accurate estimation of error in order to provide meaningful results.Typically the morphologic method documents surface change between geomorphic events. However, due to the high temporal variability displayed by the aeolian transport system, measuring topographic change during a transport event would be beneficial. Using TLS during active transport removes the ability to take multiple convergent scans. Therefore current methods of approximating error in TLS derived surfaces by using convergent scans from multiple scan locations cannot be applied.The influence of scanning geometry and survey set up is explored in order to quantify and reduce errors when scanning small scale bedforms from a single location. This is then applied to an active transport event to measure wind ripple migration, and derive a sediment transport rate using the morphologic method. The results suggest TLS is a viable tool for capturing in-situ aeolian ripples. Scan incidence angle is shown to significantly affect point density and therefore point cloud accuracy. The influence of incidence angle is different depending on the extent of the bedform studied. Ripples were measured during an active transport event in the Great Sand Dunes National Park, Colorado. Ripple morphologies and migration rates were within previously observed ranges. Applying the morphologic method highlighted ripple migration patterns, surface change and enabled an overall sediment budget to be calculated

Exploring Explanations of Subglacial Bedform Sizes Using Statistical Models

Sediments beneath modern ice sheets exert a key control on their flow, but are largely inaccessible except through geophysics or boreholes. In contrast, palaeo-ice sheet beds are accessible, and typically characterised by numerous bedforms. However, the interaction between bedforms and ice flow is poorly constrained and it is not clear how bedform sizes might reflect ice flow conditions. To better understand this link we present a first exploration of a variety of statistical models to explain the size distribution of some common subglacial bedforms (i.e., drumlins, ribbed moraine, MSGL). By considering a range of models, constructed to reflect key aspects of the physical processes, it is possible to infer that the size distributions are most effectively explained when the dynamics of ice-water-sediment interaction associated with bedform growth is fundamentally random. A ‘stochastic instability’ (SI) model, which integrates random bedform growth and shrinking through time with exponential growth, is preferred and is consistent with other observations of palaeo-bedforms and geophysical surveys of active ice sheets. Furthermore, we give a proof-of-concept demonstration that our statistical approach can bridge the gap between geomorphological observations and physical models, directly linking measurable size-frequency parameters to properties of ice sheet flow (e.g., ice velocity). Moreover, statistically developing existing models as proposed allows quantitative predictions to be made about sizes, making the models testable; a first illustration of this is given for a hypothesised repeat geophysical survey of bedforms under active ice. Thus, we further demonstrate the potential of size-frequency distributions of subglacial bedforms to assist the elucidation of subglacial processes and better constrain ice sheet models

Incipient transport of silt-sized sediments

Laboratory experiments were conducted to determine the influence of stream bed shear stress and water chemistry on the sediment transport rate for silt-sized particles near the critical threshold for motion. Experiments were conducted in two large recirculating laboratory flumes, 40 m and 12 m long, with a small sediment bed 40 cm long. The sediment transport rate was determined from the volume of sediment eroded from this sediment bed per unit time. The smaller flume was filled with deionized water, to which specific electrolytes were added to vary the water chemistry. Dimensional analysis predicted the sediment transport rate of non-cohesive material can be described by two dimensionless groups, one for transport and one for bed shear stress. A new transport model was developed on physical considerations for particles smaller than the thickness of the viscous sublayer, and supported this conclusion. Sediment transport rates were measured in experiments using carefully cleaned glass beads (15 μm to 69 μm) in low ionic strength (10^-4 M) solution by measuring the elevation of the sediment bed along transects with a laser displacement meter every 10 to 30 minutes. The results supported the prediction that the dimensionless transport rate is solely a function of the dimensionless shear stress (Shields parameter) and the water composition, and not of the bed Reynolds number, when the latter is less than one. Experiments were conducted with NaCl and CaCl2 electrolytes at differing concentrations up to 10^-2 M, which reduced the transport rate by up to 2 to 3 orders of magnitude for the finest particles. Calcium was more effective at reducing the sediment transport rate than sodium. These trends were captured by the transport model, but additional work is required in estimating the inter-particle forces. A new criterion for initiation of motion is proposed based on a small dimensionless transport rate qs* = 0.01, corresponding to about 2% of the surface grains in motion. For bed Reynolds numbers u*d/v < 1, the equivalent Shields parameter for critical shear becomes [Greek tau]* = 0.075 for non-cohesive sediment. With cohesion, a new model is used to predict the change in the Shields curve for various dimensionless interparticle forces

The science behind scour at bridge foundations : a review

Foundation scour is among the main causes of bridge collapse worldwide, resulting in significant direct and indirect losses. A vast amount of research has been carried out during the last decades on the physics and modelling of this phenomenon. The purpose of this paper is, therefore, to provide an up-to-date, comprehensive, and holistic literature review of the problem of scour at bridge foundations, with a focus on the following topics: (i) sediment particle motion; (ii) physical modelling and controlling dimensionless scour parameters; (iii) scour estimates encompassing empirical models, numerical frameworks, data-driven methods, and non-deterministic approaches; (iv) bridge scour monitoring including successful examples of case studies; (v) current approach for assessment and design of bridges against scour; and, (vi) research needs and future avenues

Debris flow susceptibility mapping for initiation areas at medium scale: a case study in Western Norway

In recent years, rapid mass movements such as debris flow and debris avalanches resulted in a significant impact on Norwegian society and economy. The need for dispelling the uncertainty inherent in landslide risk assessment has encouraged the development of hazard and susceptibility maps. Different statistically-based modelling methods, in combination with geographic information systems (GIS), have been extensively used to ascertain landslide susceptibility in quantitative terms. This thesis proposes a bivariate statistical method (Weights of Evidence) for assessing the spatial proneness of debris flows within Førde and Jølster municipalities (Western Norway), where emphasis is put on the critical conditions of initiation. Since no feasible landslide database could be exploited for susceptibility mapping at medium scale, this thesis addressed the realisation of a new inventory. By coupling pre-existing data from remote sensing and field observations, circa 1100 debris flow initiation areas were outlined and differentiated in four categories with geomorphological repeatable features. Simple topography-based parameters such as slope, upslope contributing area, curvature and roughness were used to find significant statistical differences between the initiation areatypes. Moreover, they were employed together with other thematic maps as informative layers for landslide modelling. In order to test the model fitting performance, the ROC curves method is used in this thesis. The evaluation of different discretization schemes and combinations of the above-mentioned variables led to individuate models with different performances in terms of success rates. The best model is obtained by using only a combination of slope, flow accumulation and elevation (82% true positive rate), while the manual adjustment of the classification scheme did not lead to significant improvements

Sediment Dynamics and Channel Connectivity on Hillslopes

The pattern, magnitude, and frequency of hillslope erosion and deposition are spatially varied under the influence of micro-topography and channel geometry. This research investigates the interrelationships between erosion/deposition, micro-topography, and channel connectivity on a hillslope in Loudon, Tennessee using the centimeter (cm) level temporal Digital Elevation Models collected using laser scanning. This research addressed (1) the effect of spatial resolution on the erosion/deposition quantification, and rill delineation; (2) the influences of micro-topographic factors (e.g. slope, roughness, aspect) on erosion and deposition; (3) the relationship between the structural connectivity -- depressions and confluence of rills -- and the sedimentological connectivity. I conducted (1) visual and quantitative assessments for the erosion and deposition, and the revised automated proximity and conformity analysis for the rill network; (2) quantile regression for micro-topographic factors using segmented rill basins; and (3) cross-correlation analysis using erosion and deposition series along the channels.Overall, rills are sedimentologically more dynamic than the interrill areas. A larger grid size reduces the detectable changes in both areal and volumetric quantities, and also decreases the total length and number of rills. The offset between delineated rills and the reference increases with larger grid sizes. A larger rill basin has higher erosion and deposition with the magnitude of erosion greater than deposition. The slope has a positive influence on erosion and a negative one on deposition; roughness has a positive influence on deposition and a negative one on erosion. Areas that are more north-facing experience higher erosion and lower deposition. Rill length explains 46% of the variability for erosion and 24% for deposition. The depressions are associated with higher erosion in the downslope direction. The correlations between the erosion and the confluence are positive; the correlation between the deposition and the sink is positive. Overall, the influence of structural connectivity on the sedimentological connectivity is within 25 cm in both upstream and downstream directions. This research contributes to the understanding in how the sediment movement on hillslopes is governed by topographic variations and channel connectivity, and future work may explore hillslope channels at broader geographical and temporal scales