ORCID Profile
0000-0002-2007-1717
Current Organisation
The University of Texas at Austin
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Publisher: Copernicus GmbH
Date: 17-07-2017
Abstract: Abstract. Debris flows such as avalanches and landslides are heterogeneous mixtures of solids and liquids but are often simulated as homogeneous non-Newtonian fluids using a Herschel-Bulkley model. By representing the heterogeneous debris as a homogeneous non-Newtonian fluid, it is possible to use standard numerical approaches for the Navier-Stokes equations where viscosity is allowed to vary in time and space (e.g. eddy-viscosity turbulence models). Common non-Newtonian models are time-independent so that the relationship between the time-space-varying effective viscosity and flow stress is unchanging. However, the complex behaviors of debris flows at flow initiation (jamming) and cessation (restructuralization) imply that the viscosity-stress relationships should have time-dependent behaviors, which is a feature of thixotropic non-Newtonian fluids. In this paper, both Herschel-Bulkley and thixotropic non-Newtonian fluid models are evaluated for simulating avalanches along a slope and subaqueous debris flows. A numerical solver using a multi-material level set method is applied to track multiple interfaces simultaneously. The numerical results are validated with analytical solutions and available experimental data using parameters selected based on the experimental setup and without post-hoc calibration. The thixotropic (time-dependent) fluid model shows reasonable agreement with all the experimental data. For most of the experimental conditions, the Herschel-Bulkley (time-independent) model results were similar to the thixotropic model, a critical exception being conditions with a high yield stress. Where the flow initiation is strongly dominated by the structural jamming and the initial yield behavior the time-independent model performed poorly.
Publisher: Copernicus GmbH
Date: 18-08-2020
DOI: 10.5194/HESS-24-4001-2020
Abstract: Abstract. The solution stability of river models using the one-dimensional (1D) Saint-Venant equations can be easily undermined when source terms in the discrete equations do not satisfy the Lipschitz smoothness condition for partial differential equations. Although instability issues have been previously noted, they are typically treated as model implementation issues rather than as underlying problems associated with the form of the governing equations. This study proposes a new reference slope form of the Saint-Venant equations to ensure smooth slope source terms and eliminate one source of potential numerical oscillations. It is shown that a simple algebraic transformation of channel geometry provides a smooth reference slope while preserving the correct cross-section flow area and the total Piezometric pressure gradient that drives the flow. The reference slope method ensures the slope source term in the governing equations is Lipschitz continuous while maintaining all the underlying complexity of the real-world geometry. The validity of the mathematical concept is demonstrated with the open-source Simulation Program for River Networks (SPRNT) model in a series of artificial test cases and a simulation of a small urban creek. Validation comparisons are made with analytical solutions and the Hydrologic Engineering Center's River Analysis System (HEC-RAS) model. The new method reduces numerical oscillations and instabilities without requiring ad hoc smoothing algorithms.
Publisher: Copernicus GmbH
Date: 21-05-2015
DOI: 10.5194/NHESS-15-1011-2015
Abstract: Abstract. New automated methods are developed for identifying narrow landscape features that cause hydrodynamic blocking and might have critical impacts for management models of river flooding, coastal inundation, climate change, or extreme event analysis. Lidar data processed into a fine-resolution raster (1 m × 1 m) can resolve narrow blocking features in topography but typically cannot be directly used for hydrodynamic modeling. For practical applications such data are abstracted to larger scales, which can result in a loss of hydrodynamic blocking effects. The traditional approach to resolving hydrodynamic blocking features is to represent them as cell boundaries within a customized unstructured grid that is tuned to the spatial features. A new automated edge-blocking approach is developed, which allows application of an arbitrarily structured (Cartesian) mesh at coarser scales and provides contiguous representation of blocking features along Cartesian cell boundaries. This approach distorts the shape of a blocking feature (i.e., making it rectilinear along grid cell faces) but retains its critical hydrodynamic blocking height characteristics and spatial continuity within the topographic model.
Publisher: American Society of Civil Engineers (ASCE)
Date: 11-2023
Publisher: American Society of Civil Engineers (ASCE)
Date: 07-2022
Publisher: Copernicus GmbH
Date: 22-01-2018
DOI: 10.5194/NHESS-18-303-2018
Abstract: Abstract. Avalanches and subaqueous debris flows are two cases of a wide range of natural hazards that have been previously modeled with non-Newtonian fluid mechanics approximating the interplay of forces associated with gravity flows of granular and solid–liquid mixtures. The complex behaviors of such flows at unsteady flow initiation (i.e., destruction of structural jamming) and flow stalling (restructuralization) imply that the representative viscosity–stress relationships should include hysteresis: there is no reason to expect the timescale of microstructure destruction is the same as the timescale of restructuralization. The non-Newtonian Herschel–Bulkley relationship that has been previously used in such models implies complete reversibility of the stress–strain relationship and thus cannot correctly represent unsteady phases. In contrast, a thixotropic non-Newtonian model allows representation of initial structural jamming and aging effects that provide hysteresis in the stress–strain relationship. In this study, a thixotropic model and a Herschel–Bulkley model are compared to each other and to prior laboratory experiments that are representative of an avalanche and a subaqueous debris flow. A numerical solver using a multi-material level-set method is applied to track multiple interfaces simultaneously in the simulations. The numerical results are validated with analytical solutions and available experimental data using parameters selected based on the experimental setup and without post hoc calibration. The thixotropic (time-dependent) fluid model shows reasonable agreement with all the experimental data. For most of the experimental conditions, the Herschel–Bulkley (time-independent) model results were similar to the thixotropic model, a critical exception being conditions with a high yield stress where the Herschel–Bulkley model did not initiate flow. These results indicate that the thixotropic relationship is promising for modeling unsteady phases of debris flows and avalanches, but there is a need for better understanding of the correct material parameters and parameters for the initial structural jamming and characteristic time of aging, which requires more detailed experimental data than presently available.
Publisher: Copernicus GmbH
Date: 18-12-2018
Publisher: MDPI AG
Date: 17-06-2020
DOI: 10.3390/W12061727
Abstract: Piping systems (e.g., storm sewers) that transition between free-surface flow and surcharged flow are challenging to model in one-dimensional (1D) networks as the continuity equation changes from hyperbolic to elliptic as the water surface reaches the pipe ceiling. Previous network models are known to have poor mass conservation or unpredictable convergence behavior at such transitions. To address this problem, a new algorithm is developed for simulating unsteady 1D flow in closed conduits with both free-surface and surcharged flow. The shallow-water (hydrostatic) approximation is used as the governing equations. The artificial compressibility (AC) method is implemented as a dual-time-stepping discretization for a finite-volume solver with timescale interpolation used for face reconstruction. A new formulation for the AC celerity parameter is proposed such that the AC celerity matches the equivalent gravity wave speed for the local hydraulic head—which has some similarities to the classic Preissmann Slot used to approximate pressurized flow in conduits. The new approach allows the AC celerity to be set locally by the flow (i.e., non-uniform in space) and removes it as a free parameter of the AC solution method. The derivation of the AC method provides for only a minor change in the form of the solution equations when a computational element switches from free-surface to surcharged. The new solver is tested for both unsteady free-surface (supercritical, subcritical) and surcharged flow transitions in a circular pipe and is implemented in an open-source Python code available under the name “PipeAC.” The results are compared to laboratory experiments that include rapid flow changes due to opening/closing of gates. Results show that the new algorithm is satisfactory for 1D representation of unsteady transition behavior with two caveats: (i) sufficient grid resolution must be applied, and (ii) the shallow-water equation approximations (hydrostatic, single-fluid) limit the accuracy of the solution with regards to the celerity of the turbulent unsteady bore that propagates upstream. This research might benefit any piping network model that must smoothly handle unsteady transitions from free surface to surcharged flow.
Publisher: Copernicus GmbH
Date: 28-10-2018
Publisher: Copernicus GmbH
Date: 09-06-2015
DOI: 10.5194/NHESS-15-1163-2015
Abstract: Abstract. Moraine dam collapse is one of the causes of glacial lake outburst floods. Available models seek to predict both moraine breach formation and lake outflow. The models depend on hydraulic, erosion, and geotechnical parameters that are mostly unknown or uncertain. This paper estimates the outflow hydrograph caused by a potential erosive collapse of the moraine dam of Lake Palcacocha in Peru and quantifies the uncertainty of the results. The overall aim is to provide a simple yet hydraulically robust approach for calculating the expected outflow hydrographs that is useful for risk assessment studies. To estimate the peak outflow and failure time of the hydrograph, we assessed several available empirical equations based on lake and moraine geometries each equation has defined confidence intervals for peak flow predictions. Complete outflow hydrographs for each peak flow condition were modeled using a hydraulic simulation model calibrated to match the peak flows estimated with the empirical equations. Failure time and peak flow differences between the simulations, and the corresponding empirical equations were used as error parameters. Along with an expected hydrograph, lower and upper bound hydrographs were calculated for Lake Palcacocha, representing the confidence interval of the results. The approach has several advantages: first, it is simple and robust. Second, it evaluates the capability of empirical equations to reproduce the conditions of the lake and moraine dam. Third, this approach accounts for uncertainty in the hydrographs estimations, which makes it appropriate for risk management studies.
Publisher: Copernicus GmbH
Date: 19-09-2014
DOI: 10.5194/NHESSD-2-5971-2014
Abstract: Abstract. Moraine dam collapse is one of the causes of Glacier Lake Outburst Floods. Available models seek to predict both moraine breach formation and lake outflow. The models depend on hydraulic, erosion, and geotechnical parameters that are mostly unknown or uncertain. This paper estimates the outflow hydrograph caused by a potential collapse of the moraine dam of Lake Palcacocha in Peru and quantifies the uncertainty of the results. The overall aim is to provide a simple and robust method of calculation of the expected outflow hydrographs that is useful for risk assessment studies. To estimate the peak outflow and failure time of the hydrograph, we assessed several available empirical equations based on lake and moraine geometries each equation has defined confidence intervals for peak flow predictions. Complete outflow hydrographs for each peak flow condition were modeled using a~hydraulic simulation model calibrated to meet the peak flows estimated with the empirical equations. Failure time and peak flow differences between the simulations and the corresponding empirical equations were used as error parameters. Along with an expected hydrograph, lower and upper bound hydrographs were calculated for Lake Palcacocha, representing the confidence interval of the results. The method has several advantages: first, it is simple and robust. Second, it evaluates the capability of empirical equations to reproduce the conditions of the lake and moraine dam. Third, this method accounts for uncertainty in the hydrographs estimations, which makes it appropriate for risk management studies.
Publisher: Informa UK Limited
Date: 03-07-2021
Publisher: Copernicus GmbH
Date: 13-02-2015
DOI: 10.5194/NHESSD-3-1427-2015
Abstract: Abstract. New automated methods are developed for identifying narrow landscape features that cause hydrodynamic blocking and might have critical impacts for management models of river flooding, coastal inundation, climate change, or extreme event analysis. Lidar data processed into a fine-resolution raster (1 m × 1m) can resolve narrow blocking features in topography, but typically cannot be directly used for hydrodynamic modeling. For practical applications such data are abstracted to larger scales, which can result in a loss of hydrodynamic blocking effects. The traditional approach to resolving hydrodynamic blocking features is to represent them as cell boundaries within customized unstructured grid that is tuned to the spatial features. A new automated edge-blocking approach is developed, which allows application of an arbitrary structured (Cartesian) mesh at coarser scales and provides contiguous representation of blocking features along Cartesian cell boundaries. This approach distorts the shape of a blocking feature (i.e., making it rectilinear along grid cell faces), but retains its critical hydrodynamic blocking height characteristics and spatial continuity within the topographic model.
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 25-03-2022
DOI: 10.36227/TECHRXIV.19393892
Abstract: System partitioning for effective simulation of civil infrastructure flow networks on parallel processors is a nontrivial problem. Arbitrary partitioning focused only on balancing processor workload can lead to a large interprocessor communication burden that limits parallel speedup. Thus, there is a need for intelligent partitioning algorithms that balance the estimated computational load while minimizing the number of connections between partitions. Graph theory provides widely used partitioning methods, but these are applicable to networks with power-law connectivity and where the computational workload is proportional to the number of system nodes—conditions that do not hold for finite-volume solution of water drainage networks (e.g., river systems, stormwater drainage systems). This paper presents the novel BIPquick algorithm, which is shown to be an effective approach to identifying network partitions with reduced connectivity for systems that are directed acyclic graphs (DAGs) and have a physical limit on the number of connections per network node. Novel developments include (1) a node-cut approach that allows a partitioning workload function to be exactly balanced in systems where the computational work is proportional to the link length between nodes, (2) a finite-pass approach to partitioning that ensures a partitioning solution in a known time, and (3) a new connectivity scaling metric that allows simple evaluation and comparison of different partitioning results. The BIPquick model is tested on a large river network with up to 10,000 partitions. /
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 25-03-2022
DOI: 10.36227/TECHRXIV.19393892.V1
Abstract: System partitioning for effective simulation of civil infrastructure flow networks on parallel processors is a nontrivial problem. Arbitrary partitioning focused only on balancing processor workload can lead to a large interprocessor communication burden that limits parallel speedup. Thus, there is a need for intelligent partitioning algorithms that balance the estimated computational load while minimizing the number of connections between partitions. Graph theory provides widely used partitioning methods, but these are applicable to networks with power-law connectivity and where the computational workload is proportional to the number of system nodes—conditions that do not hold for finite-volume solution of water drainage networks (e.g., river systems, stormwater drainage systems). This paper presents the novel BIPquick algorithm, which is shown to be an effective approach to identifying network partitions with reduced connectivity for systems that are directed acyclic graphs (DAGs) and have a physical limit on the number of connections per network node. Novel developments include (1) a node-cut approach that allows a partitioning workload function to be exactly balanced in systems where the computational work is proportional to the link length between nodes, (2) a finite-pass approach to partitioning that ensures a partitioning solution in a known time, and (3) a new connectivity scaling metric that allows simple evaluation and comparison of different partitioning results. The BIPquick model is tested on a large river network with up to 10,000 partitions. /
Publisher: American Geophysical Union (AGU)
Date: 04-2020
DOI: 10.1029/2019WR026619
Abstract: Tidal freshwater zones (TFZs) are transitional environments between terrestrial and coastal waters. TFZs have freshwater chemistry and tidal physics, and yet are neither river nor estuary based on classic definitions. Such zones have been occasionally discussed in the literature but lack a consistent nomenclature and framework for study. This work proposes a measurable definition for TFZs based on three longitudinal points of interest: (1) the upstream limit of brackish water, (2) the upstream limit of bidirectional tidal velocities, and (3) the upstream limit of tidal stage fluctuations. The resulting size and position of a TFZ is transient and depends on the balance of tidal and riverine forces that evolves over event, tidal, seasonal, and annual (or longer) timescales. The concept, definition, and transient analysis of TFZ position are illustrated using field observations from the Aransas River (Texas, USA) from July 2015 to July 2016. The median Aransas TFZ length was 59.9 km, with a late summer maximum of 66.0 km and a winter minimum of 53.6 km. The TFZ typically (annual median) began 11.8 km upstream from the river mouth (15.4 km winter/11.2 km summer medians) and ended 71.7 km upstream (69.0 km/77.2 km). Seasonally low baseflow in the Aransas River promoted gradual coastal salt encroachment upstream, which shortened the TFZ. However, sporadic large rainfall/runoff events rapidly elongated the TFZ. The TFZ definition establishes a quantifiable framework for analyzing these critical freshwater systems that reside at the nexus of natural and human‐influenced hydrology, tides, and climate.
Publisher: American Geophysical Union (AGU)
Date: 06-2021
DOI: 10.1029/2020WR028760
Abstract: Total dissolved gas supersaturation (TDGS) produced by flood flows over a high dam has negative consequences on downstream fish. Reducing the total dissolved gas (TDG) levels and/or providing adequate low‐TDGS fish shelter areas remains a challenge. The presence of a low‐TDG‐saturation (LTS) zone at river confluences can protect the fish from harmful high TDG levels, but the effects of confluence hydrodynamics on the TDGS distribution are not well understood. In this study, three‐dimensional (3D) hydrodynamic simulations are performed to understand the influence of flow ratio and junction angle on the TDGS distribution downstream of river confluences. The simulation results show that the TDGS distribution is mainly affected by (a) inflow TDGS that determines the supply, (b) secondary flow that leads to rapid renewal of surface water for degassing, and (c) increased turbulence intensity that promotes dissipation of TDGS. Relationship between the LTS area, the flow ratio and the junction angle is constructed via regression analysis. The regression model is validated against a 3D numerical simulation that is compared to field observations at two river confluences along the Jinshajiang River (China). This study provides scientific basis for potential hydraulic modifications at confluences to enlarge an LTS zone and reduce fish fatality.
Publisher: Informa UK Limited
Date: 05-11-2019
Publisher: Informa UK Limited
Date: 02-01-2020
Publisher: Copernicus GmbH
Date: 19-03-2020
DOI: 10.5194/HESS-2020-75
Abstract: Abstract. The solution stability of river models using the one-dimensional (1D) Saint–Venant equations can be easily undermined when source terms in the discrete equations do not satisfy the Lipschitz smoothness condition for partial differential equations. Although instability issues have been previously noted, they are typically treated as model implementation issues rather than as underlying problems associated with the form of the governing equations. This study proposes a new reference slope form of the Saint–Venant equations to ensure smooth source terms and eliminate potential numerical oscillations. It is shown that a simple algebraic transformation of channel geometry provides a smooth reference slope while preserving the correct cross-sectional flow area and the total Piezometric pressure gradient that drives the flow. The reference slope method ensures the slope source term in the governing equations is Lipschitz-continuous while maintaining all the underlying complexity of the real-world geometry. The validity of the mathematical concept is demonstrated with the open-source SPRNT model in a series of artificial test cases and simulation of a small urban creek. Validation comparisons are made with analytical solutions and the HEC-RAS model. The new method reduces numerical oscillations and instabilities without requiring ad hoc smoothing algorithms.
Publisher: MDPI AG
Date: 17-08-2021
DOI: 10.3390/W13162236
Abstract: A new sweep-search algorithm (SSA) is developed and tested to identify the channel geometry transitions responsible for numerical convergence failure in a Saint-Venant equation (SVE) simulation of a large-scale open-channel network. Numerical instabilities are known to occur at “sharp” transitions in discrete geometry, but the identification of problem locations has been a matter of modeler’s art and a roadblock to implementing large-scale SVE simulations. The new method implements techniques from graph theory applied to a steady-state 1D shallow-water equation solver to recursively examine the numerical stability of each flowpath through the channel network. The SSA is validated with a short river reach and tested by the simulation of ten complete river systems of the Texas–Gulf Coast region by using the extreme hydrological conditions recorded during hurricane Harvey. The SSA successfully identified the problematic channel sections in all tested river systems. Subsequent modification of the problem sections allowed stable solution by an unsteady SVE numerical solver. The new SSA approach permits automated and consistent identification of problem channel geometry in large open-channel network data sets, which is necessary to effectively apply the fully dynamic Saint-Venant equations to large-scale river networks or for city-wide stormwater networks.
No related grants have been discovered for Ben Hodges.