High-performance simulation technologies for water-related natural hazards

PhD ThesisWater-related natural hazards, such as flash floods, landslides and debris flows, usually happen in chains. In order to better understand the underlying physical processes and more reliably quantify the associated risk, it is essential to develop a physically-based multi-hazard modelling system to simulate these hazards at a catchment scale. An effective multi-hazard modelling system may be developed by solving a set of depth-averaged dynamic equations incorporating adaptive basal resistance terms. High-performance computing achieved through implementation on modern graphic processing units (GPUs) can be used to accelerate the model to support efficient large-scale simulations. This thesis presents the key simulation technologies for developing such a novel high-performance water-related natural hazards modelling system. A new well-balanced smoothed particle hydrodynamic (SPH) model is first presented for solving the shallow water equations (SWEs) in the context of flood inundation modelling. The performance of the SPH model is compared with an alternative flood inundation model based on a finite volume (FV) method in order to select a better numerical method for the current study. The FV model performs favourably for practical applications and therefore is adopted to develop the proposed multi-hazard model. In order to more accurately describe the rainfallrunoff and overland flow process that often initiates a hazard chain, a first-order FV Godunovtype model is developed to solve the SWEs, implemented with novel source term discretisation schemes. The new model overcomes the limitations of the current prevailing numerical schemes such as inaccurate calculations of bed slope or friction source terms and provides much improved numerical accuracy, efficiency and stability for simulating overland flows and surface flooding. To support large-scale simulation of flow-like landslides or debris flows, a new formulation of depth-averaged governing equations is derived on the Cartesian coordinate system. The new governing equations take into account the effects of non-hydrostatic pressure and centrifugal force, which may become significant over terrains with steep and curved topography. These equations are compatible with various basal resistance terms, effectively leading to a unified mathematical framework for describing different type of water-related natural hazards including surface flooding, flow-like landslides and debris flows. The new depthaveraged governing equations are then solved using an FV Godunov-type framework based on the second-order accurate scheme. A flexible and GPU-based software framework is further designed to provide much improved computational efficiency for large-scale simulations and ease the future implementation of new functionalities. This provides an effective codebase for the proposed multi-hazard modelling system and its potential is confirmed by successfully applying to simulate flow-like landslides and dam break floods.Newcastle University and China Scholarship Council, Henry Lester Trust and Great Britain China Education Trus

Blended numerical schemes for the advection equation and conservation laws

In this paper we propose a method to couple two or more explicit numerical schemes approximating the same time-dependent PDE, aiming at creating new schemes which inherit advantages of the original ones. We consider both advection equations and nonlinear conservation laws. By coupling a macroscopic (Eulerian) scheme with a microscopic (Lagrangian) scheme, we get a new kind of multiscale numerical method

Probabilistic tsunami hazard assessment: quantifying uncertainty in landslide generated waves

Landslide generated waves (LGWs) have many associated uncertainties that need to be ac- counted for during a hazard analysis. The work presented in this thesis developed and applied numerical modelling techniques to investigate and quantify these sources of uncertainty. Firstly, to model the LGW source as a deformable slide, a smoothed particle hydrodynamics (SPH) simulator was improved and adapted. The simulator was tested using lab scale bench- marks and an idealised full scale LGW scenario. The effects of landslide source parameters on the wave at increasing scales were then investigated. In order to make use of the findings regarding complex LGW source models, a probabilistic sensitivity analysis on the full range of source parameters and their effect on the generated wave was performed using the SPH simulator. This showed that the geometric landslide parameters (such as volume and submergence depth) contributed more to uncertainty in the resulting wave characteristics near the source than the rheological parameters. By coupling different wave propagation models to the results from the near-field SPH simulator, it was revealed that the choice of mathematical formulation for propagation made a significant difference to which parameters affected the inundation level the most. These findings have important implications for the design of future LGW modelling studies and which parts of the model workflow should have more computational cost dedicated to them. Near the source the landslide geometry outweighs the complexity of the rheological model in terms of influence on the wave characteristics. During propagation the mathematical formulation chosen can have a large influence on results, so dedicating extra computational cost to this phase would be worthwhile.Open Acces

High-performance tsunami modelling with modern GPU technology

PhD ThesisEarthquake-induced tsunamis commonly propagate in the deep ocean as long waves and develop into sharp-fronted surges moving rapidly coastward, which may be effectively simulated by hydrodynamic models solving the nonlinear shallow water equations (SWEs). Tsunamis can cause substantial economic and human losses, which could be mitigated through early warning systems given efficient and accurate modelling. Most existing tsunami models require long simulation times for real-world applications. This thesis presents a graphics processing unit (GPU) accelerated finite volume hydrodynamic model using the compute unified device architecture (CUDA) for computationally efficient tsunami simulations. Compared with a standard PC, the model is able to reduce run-time by a factor of > 40. The validated model is used to reproduce the 2011 Japan tsunami. Two source models were tested, one based on tsunami waveform inversion and another using deep-ocean tsunameters. Vertical sea surface displacement is computed by the Okada model, assuming instantaneous sea-floor deformation. Both source models can reproduce the wave propagation at offshore and nearshore gauges, but the tsunameter-based model better simulates the first wave amplitude. Effects of grid resolutions between 450-3600 m, slope limiters, and numerical accuracy are also investigated for the simulation of the 2011 Japan tsunami. Grid resolutions of 1-2 km perform well with a proper limiter; the Sweby limiter is optimal for coarser resolutions, recovers wave peaks better than minmod, and is more numerically stable than Superbee. One hour of tsunami propagation can be predicted in 50 times on a regular low-cost PC-hosted GPU, compared to a single CPU. For 450 m resolution on a larger-memory server-hosted GPU, performance increased by ~70 times. Finally, two adaptive mesh refinement (AMR) techniques including simplified dynamic adaptive grids on CPU and a static adaptive grid on GPU are introduced to provide multi-scale simulations. Both can reduce run-time by ~3 times while maintaining acceptable accuracy. The proposed computationally-efficient tsunami model is expected to provide a new practical tool for tsunami modelling for different purposes, including real-time warning, evacuation planning, risk management and city planning

A coupled hydrodynamic and discrete element method for modelling flash flood debris

PhD ThesisFloating debris transported during ash ooding damages structures, blocks bridges and alters channel hydraulics. In recent years, a number of high pro le ash ood events have exhibited these processes. Recreating ood events through hydrodynamic modelling is an essential means by which engineers understand ood risk. However, there exists relatively little research focused on oating debris as a ash ood process and until now there have been limited attempts to incorporate oating debris processes into hydrodynamic ood modelling. In this work, a new coupled oating debris modelling tool is developed for 1D and 2D applications. The new tool combines a nite volume Godunov-type hydrodynamic scheme that solves the governing shallow water equations with the discrete element method for solving object contact and motion. A balanced force coupling procedure is used to calculate the hydraulic forces acting on oating objects and the corresponding shear stress imparted to uid cells. Hydrodynamic and hydrostatic force components are derived from the uid momentum principle and overcome problems associated with an empirically derived drag force used elsewhere. Balanced force coupling enables the new tool to predict both the transport dynamics of oating objects and their resulting backwater e ects. Debris dimensions are approximated using the multi-sphere method for shape representation. This ensures collisions are realistically modelled and application is not restricted by debris shape and size. The new modelling tool is extensively validated for dam break experimental test cases performed in a hydraulic ume. Predicted values for water depth and oating object position compare well with their observed counterparts for both 1D and 2D validation cases. Additionally, the coupled numerical modelling approach is applied to investigate ash ooding, including oating debris impacts in Boscastle, 2004. The Boscastle event was signi cant as 116 vehicles were washed downstream, some of which blocked bridges, altering ood hydraulics. Model predictions of water depth, depth averaged velocity and Froude number demonstrate the localised e ects of two debris blockages during the ood. Predicted water levels compare well to evidence of maximum depths collected after the event. Application of the new debris modelling tool to investigate the transport of ooded vehicles predicts vehicle transport pathways consistently with eye witness and post event observations. Application of the oating debris modelling tool to the Boscastle event demonstrates that the new tool can perform well for real world applications. However, i high computational costs require further model development to accelerate the long simulation process. This work demonstrates that a combined nite volume, discrete element approach to hydrodynamic modelling provides a greater understanding of ood hazard than purewater hydrodynamic modelling alone. Model outputs are valuable for quantifying ood risk, assessing ood damage and planning remediation measures. Furthermore, the new tool will enable a multitude of future applications and improve understanding of oating debris processes. Though the coupled approach has here been applied to ash ooding, the modelling methodology is applicable to a number of other natural hazards. Object transport by tsunami inundation, storm surge and river ice may all be simulated using the modelling methodology presented in this work.Natural Environment Research Council (NERC) and falls within the Susceptibility of Catchments to Intense Rainfall and Flooding (SINATRA) consortium project.6 months of funding through the SINATRA project budget

Advances in Modelling and Prediction on the Impact of Human Activities and Extreme Events on Environments

YesThis book is an edition of the Special Issue Advances in Modelling and Prediction on the Impact of Human Activities and Extreme Events on Environments that was published in Water journal