Browsing by Subject "Sediment transport"

Now showing 1 - 4 of 4
  • Thumbnail Image
    Item
    Interpretable Machine Learning to Improve Predictions of Fluvial Sediment Transport
    (2021-08) Lund, J.
    Fluvial sediment-transport contributes to many environmental concerns including flooding, nutrient loading, aquatic habitat degradation, reservoir sedimentation impacting dam operation, filling in of navigable waterways, and degrading of streams requiring costly restorations. However, there is a general lack of fluvial sediment-transport predictive power as it is difficult to comprehensively understand due to its complex process controlled by many factors such as hydrology, geology, land-use, and sediment supply. Field data can be used to better understand fluvial sediment-transport but is often limited due to it being expensive, technical, and labor intensive to collect. Minnesota provides a unique opportunity to study sediment-transport due to a complex glacial history that produced diverse landforms which combine with varying land uses and land covers to yield surface water conditions. Fortunately, Minnesota has a large sediment-transport dataset available with which to build and test a statewide predictive model in order to increase the knowledge of sediment-transport and help solve environmental concerns. XGBoost machine learning models were developed and trained to predict suspended-sediment concentration (SSC) and bedload transport rates at unsampled rivers and streams by using SSC samples collected from 56 sites and bedload samples collected from 43 sites by the U.S. Geological Survey from 2007-2019. Basin (full upstream area), catchment (nearby landscape), near-channel, and in-channel feature variables were compiled from available state and national datasets (NHDPlusV2, StreamCat, U.S. Stream Classification System, and StreamStats). The 2-year recurrence interval statistic for each site was used to normalize streamflow. The slope of the dimensionless hydrograph was calculated to teach the model the rate of rising or falling streamflow conditions before and after each sample was collected. Models for both bedload and SSC transport explained roughly 70% of the variance in the dataset. Shapley additive explanation values (SHAP) facilitated model interpretation and connected important model features to their roles with the sediment-transport process. Cumulative suspended sediment loads were calculated from model output and compared to in-situ surrogate loads from four sites in the study area to show model utility, and test model improvements. Results show that these models can inform sediment loads and stream-restoration activities across Minnesota by providing estimates of suspended-sediment concentrations and bedload rates where samples have not been collected.
  • Thumbnail Image
    Item
    Non-local theories of geomorphic transport: from hillslopes to rivers to deltas to the stratigraphic record
    (2012-09) Ganti, Naga Vamsi Krishna
    Landscapes are shaped by the interplay between tectonics and climate. The mass fluxes associated with the physical and chemical processes acting across the landscape involve the production and transport of sediment and solutes from the uplands to the lowlands. The processes operating on the Earth's surface dictate the selective long-term preservation of the history of these processes in the geological record. Acknowledging the stochastic nature of the processes that drive the evolution of the landscapes at various time scales involved is essential for building predictive models of sediment transport on the Earth's surface. However, traditional models often do not acknowledge the high variability of the driving forces, broad scales of motion involved and the heavy-tailed nature of events that shape the landscapes. This thesis research challenges existing thinking and puts forth a new class of macroscopic sediment transport models which take into account the probabilistic structure of the processes that shape the landscapes. A new class of macroscopic sediment flux models that are based on non-local theories, where sediment flux is not only a function of local hydro-geomorphic quantities but is a linear function of the space-time history of the system, are introduced. The unifying goal underlying this work is to develop sediment transport models that capture the extreme heterogeneity of the involved processes over a large range of scales, consider the presence of extreme fluctuations that arise due to the climatic forcing, and the spatial heterogeneity of landscapes that affects sediment production, storage, movement and delivery and to study how these surface dynamics are preserved in the Earth's geological record.
  • Thumbnail Image
    Item
    Three-dimensional unsteady modeling of clear-water scour in the vicinity of hydraulic structures: Lagrangian and Eulerian perspectives
    (2008-07) Escauriaza, Cristian
    The complex interaction between turbulence and sediment dynamics in aquatic environments is the most important mechanism of sediment transport and bed erosion in multiple geophysical, environmental, and engineering flows. Scour around hydraulic structures is an example in which this relation acquires great relevance. The bed erosion in the vicinity of bridge foundations is controlled by the dynamically rich horseshoe vortex system (HSV) that develops in front of the structures and increases the near-bed turbulent stresses by one order of magnitude compared to the approaching turbulent boundary layer flow. Advances in numerical simulations designed to understand the physical mechanisms of sediment transport and bed erosion in turbulent flows, however, have been limited by the ability of the models to capture the large-scale coherent vortical structures with adequate resolution, and by the level of description and assumptions of the sediment transport models utilized to predict the sediment flux. In this thesis we develop an advanced computational fluid dynamics (CFD) model to simulate the flow, bed-load transport, and scour in the vicinity of hydraulic structures. To handle arbitrarily complex multi-connected geometries, the numerical solver employs domain decomposition techniques with structured Chimera overset grids. An Arbitrary Lagrangian-Eulerian (ALE) approach is also incorporated to consider the effects of moving boundaries in the flowfield solution. We carry out numerical simulations of the turbulent flow past a cylindrical pier using the detached-eddy simulation (DES) approach as the turbulence model. DES is a hybrid method that combines an unsteady Reynolds-averaged Navier-Stokes (URANS) model in regions of the computational domain near the wall, with large-eddy simulation (LES) in regions away from solid boundaries This numerical method is capable of capturing the dynamics of the HSV and reproducing for the first time, along with the recent study of Paik, Escauriaza, and Sotiropoulos [Phys. Fluids 19, 045107, 2007], all the experimental trends observed in junction flows at high Reynolds numbers. Two models of sediment transport are developed in the present investigation to study the initiation of motion, transport processes, and clear-water scour by the large-scale vortical structures of the HSV system: (1) A Lagrangian model for sediment grains to simulate the transport of individual particles. The trajectory and momentum of the sediment particles are computed to evaluate the effects of the instantaneous hydrodynamic forces induced by the HSV system. Since the magnitude of the particle stresses are near the threshold of motion, the transport is characterized by intermittent displacement events of varying magnitudes. Groups of sediment grains move continuously, saltating or sliding on the bed, and streaks aligned with near-wall vortices are formed around the cylindrical pier. The global transport of particles past the cylinder is studied by performing a statistical analysis of the flux to reveal scale-invariance of the process and multifractality of particle transport as the overall effect of the flow around the pier. (2) A new unsteady bed-load transport model based on the momentum equation of the sediment in an Eulerian framework. The evolution of scour is obtained from the solution of the Exner equation, computing the bed elevation from the instantaneous flowfield. The model reproduces scour in non-equilibrium conditions, giving information of the spatial distribution and time evolution of erosion and deposition in the vicinity of the pier. A remarkable process captured for the first time by our model is the development of bed-forms along the legs of the HSV system. The interaction of the vortical structures with the wall produces the bed instability that grows and propagates, generating ripples that travel and merge in the downstream direction showing the same dynamic features observed in experiments.^ The model constitutes a powerful simulation tool to investigate the relation between sediment and bed processes with coherent structures in turbulent flows, and it can also serve as a general framework for developing three-dimensional non-equilibrium sediment transport models that can be used in the future for engineering design and optimization. The model also highlights the importance of integrating high-resolution numerical simulations with laboratory experiments to understand and be able to predict the complex physics of sediment transport in nature.
  • Thumbnail Image
    Item
    Variation in vegetation establishment, hydrologic regime, and sediment transport within the Minnesota River Basin
    (2014-01) Triplett, Laura Jean
    This study investigates the relationships between hydrologic regime and riparian vegetation establishment; specifically the impact of changes in hydrologic regime on the establishment of riparian vegetation in addition to exploration of associated sediment transport patterns. Recent flow increases within the Minnesota River basin have been associated with reductions in woody riparian vegetation establishment as a result of decreased point bar exposure time and increased scour at high flow. Reductions in riparian vegetation establishment may contribute to reduced sediment deposition; further promoting river widening and sediment loading. Field, geo-spatial, and stream flow data collection were completed within the Elm Creek and lower Minnesota River watersheds to further demonstrate and characterize the eco-hydrologic relationships between stream flow, vegetation establishment, and sediment transport within the Minnesota River basin.