Development of robust, physically-based numerical models for transport processes and geomorphodynamics changes

Bed changes in rivers may occur under several morphodynamics and hydrodynamics conditions. The modeling of this type of phenomena can be performed coupling the Shallow Water Equations (SWE) for the hydrodynamic part and the Exner equation for the morphodynamic part. The Exner equation states that the time variation of the sediment layer is due to the sediment transport discharge through the boundaries of the volume. Considering that sediment transport discharge are computed by means of sediment capacity formulae based on 1D experimental steady flows, the assessment of these empirical relations under unsteady 1D and 2D situations must be studied. In order to ensure the reliability of the numerical experimentation, the numerical scheme must handle correctly the coupling between the 2D SWE and the Exner equation under any condition. If possible, it is convenient to express the formulation of different empirical laws under a general framework. In consequence, a finite-volume numerical scheme that includes these two main features has been chosen as a benchmark for comparing the 1D and 2D results obtained when using several well known sediment transport formulae: Meyer-Peter and M\"uller, Ashida and Michiue, Engelund and Fredsoe, Fernandez Luque and Van Beek, Parker, Smart, Nielsen, Wong and Camenen and Larson. In addition, a new interpretation of the Smart empirical law is presented in order to cope with bed load transport over irregular beds of changing slope. Detailed results for this new modified empirical law together with the ones obtained with Meyer-Peter and M\"uller (which is the sediment capacity formula more used in hydraulic engineering) are provided for every test case analyzed. Furthermore, the Root Mean Square Error (RMSE) associated to every formula at each experimental condition is calculated with the purpose of evaluating quantitatively the overall behavior of each one. The results point out that the new interpretation of the Smart formula reaches the most accurate results in all cases, but in a genuinely 2D flow, that is, a situation involving more than one flow direction, the differences among sediment transport formulae are not as noticeable as in the 1D studied situations. Once the forecasting capacity of each sediment transport formula has been studied, another concern is the computational cost. The coupling between the SWE and the Exner equation by means of an augmented Jacobian matrix involves a high number of algebraic operations for computing the eigenvalues and the eigenvectors. Therefore, the computational cost is increased significantly, limiting the applicability of the numerical scheme to realistic situations where large domains are involved. In order to improve the computational efficiency, the coupling technique is modified, not decreasing the number of waves involved in the Riemann Problem but simplifying their definitions. The approach proposed in this thesis is a new strategy to combine concepts from hyperbolic conservation laws and conservative finite volume schemes. With the aim to control numerical stability in the most efficient form possible, a numerical eigenvalue is defined to control the discrete Exner equation in the explicit scheme. This bed wave celerity helps mainly to ensure conservation and to control automatically the numerical stability of the explicit scheme. The effects of the numerical coupling strategy proposed in this thesis are tested against exact solutions and 1D and 2D experimental data. The results emerging from this analysis show that efficiency and accuracy can be obtained when choosing an adequate sediment transport law and the stability condition is augmented by including a new celerity associated to the bed changes. On the other hand, in environmental and civil engineering applications, geomorphological changes are not only present in rivers but also in steep areas where massive mobilizations of poorly sorted material can occur. This sliding material is usually composed by a mixture of sand and water. For simplifying the phenomenon, dry granular flows have been considered as a starting point for the understanding of the physics involved within the landslides. The hypothesis of Saint-Venant equations are considered valid for modeling these land movements. Taking advantage of this approach, in this thesis approximate augmented Riemann solvers are formulated providing appropriate numerical schemes for mathematical models of granular flow on irregular steep slopes. Fluxes and source terms are discretized to ensure steady state configurations including correct modeling of start/stop flow conditions, both in a global and a local system of coordinates. The weak solutions presented involve the effect of bed slope in pressure distribution and frictional effects by means of the adequate gravity acceleration components. The numerical solvers proposed are first tested against 1D cases with exact solution and then are compared with 2D experimental data in order to check the suitability of the mathematical models described in this thesis. Comparisons between results provided when using global and local system of coordinates are presented. Both the global and the local system of coordinates can be used to predict faithfully the overall behavior of the landslides. The performance of the numerical scheme has been studied using novel experimental situations. These laboratory works include bidimensional configurations, the inclusion of obstacles in the flow path and a variable slope in the domain. Hence, a further step in mimicking realistic situations is obtained, since the behavior of the granular flow is affected by the presence of natural elements such as boulders or trees. Three situations have been considered. The first experiment is based on a single obstacle, the second one is performed against multiple obstacles and the third one study the influence of a dike when an overtopping situation takes place. Due to the impact of the flow against the obstacles, fast moving shocks appear, and a variety of secondary waves emerge. Comparisons between computed and experimental data are presented for the three cases. The computed results show that the numerical tool previously developed is able to predict faithfully the overall behavior of this type of complex dense granular flow

Application of a distributed 2d overland flow model for rainfall/runoff and erosion simulation in a mediterranean watershed

Soil erosion has reemerged as an environmental problem associated with climate change that requires the help of simulation tools for forecasting future consequences. This topic becomes even more relevant in Mediterranean catchments due to the highly variable and irregular rainfall regime. Hence, an approach that includes the rainfall/runoff and erosion phenomena is required for quantifying the amount of soil the catchments are transferring to the rivers. As the calibration process of the infiltration and erosion parameters can become cumbersome in terms of iterations to the optimal values to fit experimental data, a Simplified Catchment Model (SCM) is introduced as a first approach. The set of tuning constants that provides the best fit are used as input for re-calibrating the parameters by means of the simulation of the real catchment. The modeling effort here presented opens its application to the analysis of the hydro-sedimentary processes at larger temporal and spatial scales

EXPERIMENTAL RESULTS ON SEDIMENT ENTRAINMENT BY GRAVITY CURRENTS

Gravity currents are geophysical flows responsible of the distal transport of high volumes of sediments. In particular, turbidity currents, a form of gravity currents where sediments in suspension confer the buoyancy that ignites the flow, are the main mechanism for distal sediment transport within lakes and reservoirs. In maritime environment, submarine clay-rich gravity currents can impact and may endanger human made infrastructures such as submarine cables and platforms. It is thus important to understand the dynamics of sediment transport associated to gravity currents. In the present research, it is intended to experimentally investigate the mechanisms of entrainment, transport and deposition of fine sediments caused by the passage of a saline gravity current. Conservative saline currents, with varied initial density, are let to flow over an erodible bed sector where fine sediments, with three different grain sizes, are at rest. A detailed description of the gravity current dynamics is reported using 3D instantaneous velocities measurements over a certain profile. Video records obtained synoptically and laterally through a transparent wall, provide a visualization of the entrainment and resuspension processes which is further related to the flow hydrodynamics. The critical threshold conditions for initiating sediment motion is frequently related to the balance of boundary shear stress and the submerged weight of the particle. However boundary shear stress is just one of several impelling forces and the particle submerged weight is just one of several inertial forces. Here the attention is first focused on the complete description of the flow velocity, in term of instantaneous and mean flow. A deep analysis of the hydrodynamic of one gravity current reproduced in laboratory is here presented and its role in sediments’ entrainment discussed

LABORATORY EXPERIMENTS OF PULSED SUBAQEOUS SEDIMENT DENSITY FLOWS: INTERNAL STRATIFICATION AND LAYERING

This paper reports on an experimental study which focuses on reproducing small scale sediment density flows in a laboratory flume. The observations are aimed at improving the parametrization of the relations that govern the transition between different phases of the flows. Dense mixtures of plastic sediment of variable grain size (thermoplastic polyurethane) and water were fed into the water-filled flume by gravity from a supply tank. Different grain-size and slope combinations were examined and the resulting velocity fields were obtained using UVP (Ultrasonic Velocity Profile) at various points along the channel. The flow was imaged through the transparent sidewall to track the velocity of the flow head for each test case. Image analysis also allowed a qualitative characterization of the vertical density stratification and water entrainment during runout. The runout distance and flow thickness are used to highlight the differences in the dynamic behavior of the flow for each test case. Data gathered from the experiments will be exploited in the further development of predictive numerical models for slide-induced debris flows transitioning to turbidity currents

Transport of suspended sediments under the influence of bank macro-roughness

River restoration works often include measures to promote morphological diversity and enhance habitat suitability. One of these measures is the creation of macro-roughness elements, such as lateral cavities and embayments, in the banks of channelized rivers. However, in flows that are heavily charged with fine sediments in suspension, such as glacier-fed streams and very low-gradient reaches of large catchment rivers, these lateral cavities may trap these sediments. Consequently, the morphological changes may be affected, and the functionality of the restoration interventions may be compromised. Herein, we analyse the influence of these macro-roughness elements on the transport of fine sediments in the main channel. Laboratory tests with uniform flow charged with sediments in a channel with banks equipped with large-scale rectangular roughness elements were carried out. The laboratory experiments covered a wide range of rectangular cavity geometrical configurations and shallowness ratios. The influence of key parameters such as flow shallowness, geometric ratios of the cavities and initial sediment concentration was tested. Surface particle image velocimetry, sediment samples and temporal turbidity records were collected during the experiments. The amount of sediments captured by the cavities, the temporal evolution of the concentration of sediments in suspension and the flow hydrodynamics are cross-analysed and discussed. It is shown that the trapping efficiency of the macro-roughness elements is a clear function of the channel geometry and the shallowness of the flow

INFLUENCE OF LATERAL RECTANGULAR EMBAYMENTS ON THE TRANSPORT OF SUSPENDED SEDIMENTS IN A FLUME

Systematic experimental investigations have been performed under uniform flow conditions in a channel whose banks are equipped with large scale rectangular roughness elements. The practical motivation of this project is to see how restoration of banks, such as lateral cavities, has an influence on the transport of fine sediments. The implementation of lateral cavities may affect the sediment and morphological equilibrium of the river since these may trap sediments. This work aims to study the influence of the lateral cavities on the transport of fine sediments in the main channel. A set of laboratory experiments are done which covers a wide range of rectangular cavity configurations and includes aspect ratios (lateral cavity depth divided by cavity length) between 0.2 and 0.8. Key parameters such as the flow discharge and the initial sediment concentration are tested. Surface PIV, sediment samples and turbidity temporal records are collected during the experiments. The trapping efficiency of the cavities and the associated flow patterns are calculated and discussed. The resulting conclusions provide useful information for the future design of river restoration projects

Two-Dimensional Numerical Simulation of Bed-Load Transport of a Finite-Depth Sediment Layer: Applications to Channel Flushing

Numerical modeling of bed-load transport in shallow flows, particularly oriented toward environmental flows, is an active field of research. Nevertheless, other possible applications exist. In particular, bed-load transport phenomena are relevant in urban drainage systems, including sewers. However, few applications of coupled two-dimensional (2D) shallow-water and bed-load transport models can be found, and their transfer from environmental applications-usually river and floodplain-into sewer applications requires some adaptation. Unlike to river systems, where there is a thick layer of sediment that constitutes a movable riverbed, sewer systems have thin layers of sediment that need to be removed, thus exposing a rigid, nonerodible surface. This problem requires careful numerical treatment to avoid generating errors and instability in the simulation. This paper deals with a numerical approach to tackle this issue in an efficient way that allows large-scale studies to be performed and provides empirical evidence that the proposed approach is accurate and applicable for sewage and channel-flushing problems. (C) 2017 American Society of Civil Engineers

Feinsedimentdynamik in revitalisierten Flüssen

Im Rahmen von Flussrevitalisierungen werden oftmals seitliche Aufweitungen geschaffen, in denen sich Feinsedimente ablagern können. Insbesondere in Flüssen, die von Schwall und Sunk betroffen sind, stellt sich die Frage, wie der Ablagerungsprozess von Feinsedimenten in solch lokalen Aufweitungen verläuft. Werden die Ablagerungen bei Hochwasser- oder Schwallabfluss wieder ausgewaschen oder verlanden die Aufweitungen? Die durchgeführten systematischen Experimente geben Hinweise, welche Geometrien der Aufweitungen die Sohlenstrukturen nachhaltig erhalten können

NUMERICAL SIMULATION OF SEDIMENT ENTRAINMENT BY LOCK-EXCHANGE GRAVITY CURRENTS

Gravity currents are flows driven by buoyancy differences between two contacting fluids caused by differences in temperature, salinity, or by the presence of suspended particles. Such flows can reach high velocities near the bed, especially on the area behind the front of the current. As a result, rapid morphological changes may take place in river and estuarine beds due to the passage of these flows. Essential to determine the erosion induced by the current, are the spatial and temporal distributions of the bed shear stress. However, these are troublesome to measure in laboratory or in the field. To bridge this difficulty, the eddy-solving numerical simulations may be used. This study presents here the three-dimensional numerical simulations of lock-exchange salinity currents flowing over a mobile bed. It is aimed at the characterization of the sediment entrainment capacity of the current. The large eddy simulation technique is employed for analyzing the evolution and the structure of the current. For the sediment simulation, an Euler-Euler methodology based on a single phase approach is used. The main features of the current are compared with experimental data obtained in the laboratory. Velocity fields and bed shear stress distributions for different initial current densities are analyzed and linked to entrainment scenarios. The influence of small variations in particle size of the mobile bed is also discussed

Evaluation of long-term changes in precipitation over Bolivia based on observations and Coupled Model Intercomparison Project models

Using observations and model simulations from the 5th and 6th phases of the Coupled Model Intercomparison Project (CMIP5 and CMIP6, respectively), this study evaluated changes in monthly, seasonal, and annual precipitation over Bolivia from 1950 to 2019. Results demonstrate that observed precipitation is characterized by strong interannual and decadal variability. However, long-term precipitation trends were not identified on the annual scale. Similarly, changes in seasonal precipitation were almost nonsignificant (p > .05) for the study period. Spatially, albeit with its complex orography, no substantial regional variations in observed precipitation trends can be identified across Bolivia. In contrast, long-term precipitation trends, based on CMIP5 and CMIP6 models, suggest a dominance of negative trends, mainly during austral winter (JJA) (−10%) and spring (SON) (−15%). These negative trends were more pronounced in the lowlands of Bolivia (−20%). Overall, these contradictory results highlight the need for validating precipitation trend outputs from model simulations, especially in areas of complex topography like Bolivia.This work was sup-ported by the research projects CGL2017-82216-R,PCI2019-103631, and PID2019-108589RA-I00, financedby the Spanish Ministry of Science and FEDER, theCROSSDRO project financed by the AXIS (Assessment ofCross(X)-sectorial climate impacts and pathways for Sus-tainable transformation), the JPI-Climate co-funded callof the European Commission, and the LINCGLOBAL-CSIC project (INCGLO0023, RED-CLIMA)