G.S. Stelling

Hydrodynamic and sediment transport modeling with emphasis on shallow-water, vegetated areas (lakes, reservoirs, estuaries and lagoons)

Modeling capabilities for shallow, vegetated, systems are reviewed to assess hydrodynamic, wind and wave, submersed plant friction, and sediment transport aspects. Typically, ecosystems with submersed aquatic vegetation are relatively shallow, physically stable and of moderate hydrodynamic energy. Wind-waves are often important to sediment resuspension. These are open systems that receive flows of material and energy to various degrees around their boundaries. Bed shear-stress, erosion, light extinction and submersed aquatic vegetation influence each other. Therefore, it is difficult to uncouple these components in model systems. Spatial changes in temperature, salinity, dissolved and particulate material depend on hydrodynamics. Water motions range from wind-wave scales on the small end, which might be important to erosion, to sub-tidal or seasonal scales on the large end, which are generally important to flushing. Seagrass modifies waves and, therefore, affects the relationships among the non-dimensional scaling parameters commonly used in wave analysis. Seagrass shelters the bed, often causing aggradation and changes in grain size, while increasing total resistance to flow. Hydrodynamic friction can not be well characterized by a single-parameter equation in seagrass beds, and models need appropriate enhancement when applied to these systems.Presently, modeling is limited by computational power, which is, however, improving. Other limitations include information on seagrass effects expressed in frictional resistance to currents, bed-sheltering, and wave damping in very shallow water under conditions of both normal and high bed roughness. Moreover, quantitative information on atmospheric friction and shear stress in shallow water and seagrass areas are needed. So far, various empirical equations have been used with wind or wave forcing to describe resuspension in shallow water. Although these equations have been reasonably successful in predicting suspended sediment concentrations, they require site-specific data. More detailed laboratory and field measurements are needed to improve the resuspension equations and model formulation pertaining to seagrass beds.

An interactive simulation and visualization tool for flood analysis usable for practitioners

Developing strategies to mitigate or to adapt to the threats of floods is an important topic in the context of climate changes. Many of the world’s cities are endangered due to rising ocean levels and changing precipitation patterns. It is therefore crucial to develop analytical tools that allow us to evaluate the threats of floods and to investigate the influence of mitigation and adaptation measures, such as stronger dikes, adaptive spatial planning, and flood disaster plans. Up until the present, analytical tools have only been accessible to domain experts, as the involved simulation processes are complex and rely on computational and data-intensive models. Outputs of these analytical tools are presented to practitioners (i.e., policy analysts and political decision-makers) on maps or in graphical user interfaces. In practice, this output is only used in limited measure because practitioners often have different information requirements or do not trust the direct outcome. Nonetheless, literature indicates that a closer collaboration between domain experts and practitioners can ensure that the information requirements of practitioners are better aligned with the opportunities and limitations of analytical tools. The objective of our work is to present a step forward in the effort to make analytical tools in flood management accessible for practitioners to support this collaboration between domain experts and practitioners. Our system allows the user to interactively control the simulation process (addition of water sources or influence of rainfall), while a realistic visualization allows the user to mentally map the results onto the real world. We have developed several novel algorithms to present and interact with flood data. We explain the technologies, discuss their necessity alongside test cases, and introduce a user study to analyze the reactions of practitioners to our system. We conclude that, despite the complexity of flood simulation models and the size of the involved data sets, our system is accessible for practitioners of flood management so that they can carry out flood simulations together with domain experts in interactive work sessions. Therefore, this work has the potential to significantly change the decision-making process and may become an important asset in choosing sustainable flood mitigations and adaptation strategies.

Numerical Modeling of Wave Propagation, Breaking and Run-Up on a Beach

A numerical method for free-surface flow is presented to study water waves in coastal areas. The method builds on the nonlinear shallow water equations and utilizes a non-hydrostatic pressure term to describe short waves. A vertical boundary-fitted grid is used with the water depth divided into a number of layers. A compact finite difference scheme is employed that takes into account the effect of non-hydrostatic pressure with a small number of vertical layers. As a result, the proposed technique is capable of simulating relatively short wave propagation, where both frequency dispersion and nonlinear shoaling play an important role, in an accurate and efficient manner. Mass and momentum are strictly conserved at discretelevel while themethod only dissipates energy in the case of wave breaking.A simple wet-dry algorithm is applied for a proper calculation of wave run-up on the beach.The computed results show good agreement with analytical and laboratory data for wave propagation, transformation, breaking and run-up within the surf zone.

Accurate vertical profiles of turbulent flow in z-layer models

Three-dimensional hydrodynamic z-layer models, which are used for simulating the flow in rivers, estuaries, and oceans, suffer from an inaccurate and often discontinuous bottom shear stress representation, due to the staircase bottom. We analyze the governing equations and clearly show the cause of the inaccuracies. Based on the analysis, we present a new method that significantly reduces the errors and the grid dependency of the results. The method consists of a near-bed layer-remapping and a modified near-bed discretization of the k − e turbulence model. We demonstrate the applicability of the approach for uniform channel flow, using a schematized two-dimensional vertical model and for the flow over a bottom sill using the Delft3D modeling system.

A domain decomposition method for the three-dimensional shallow water equations

Abstract This paper deals with the implementation of a domain decomposition method for the three-dimensional shallow water equations. The domain decomposition method belongs to the class of generalized additive Schwarz methods. It is based on a local coupling mechanism that allows for a flexible formulation of subdomain interfaces. For a real-life application in the Netherlands numerical results are presented. The experiments are carried out on a HP workstation and on a CRAY C90.

A multi block method for the three-dimensional shallow water equations

This paper deals with the implementation of a multi block method for the three-dimensional shallow water equations. A multi block datastructure is incorporated into the Alternating Operator Implicit (AOI) method. In this research the computational domain is partitioned into a limited number of blocks c.q. subdomains. Application of the multi block technique leads to a reduction of the memory requirements, especially if irregular geometries are used. Moreover, the multi block implementation of the AOI method is efficient on vector computers. For a real-life application in the Netherlands results will be presented.

Efficient Non Hydrostatic Free Surface Models

Abstract: In this paper we present a new vertical approximation o f the non-hydrostatic pressure based on a Box method. It makes it possible to take into account the effect o f the nonhydrostatic pressure with a very small number of layers (1-3). Also for a one layer depthaveraged model the results improve and are for short free surface waves comparable with a Boussinesq type o f model. For stratified flows the number of layers is dependent on the vertical density gradients and the method presented here is less efficient. The stratified case needs further research.

River computations: artificial backwater from the momentum advection scheme

ABSTRACT The established method for determining dike heights and dimensioning river training structures is to assess the resulting backwater by numerical modelling. The common consensus is that bottom friction determines the backwater and that momentum advection only has a local effect. We demonstrate that the numerical/artificial backwater contribution from the momentum advection approximation can be of the same order of magnitude as the bottom friction contribution, depending on the advection scheme. This is realized using a one-dimensional analysis and verified using a set of one- and two-dimensional test problems including a wavy bed case, flow over emerged and submerged groynes and finally an actual river. We compare first- and second-order accurate advection schemes and compute their artificial contribution to the backwater, for a range of practically-feasible grid resolutions. The tests demonstrate that the conservation/constancy properties of the scheme determine the size of this contribution, rather than the order of the scheme.

Parallelisation of inundation simulations

Publisher Summary The chapter presents parallelization of an inundation model. The chapter focuses on parallelization of the matrix solver that combines a direct Gaussian elimination method with the iterative conjugate gradient algorithm. It is similar to the reduced system conjugate gradient method, and it is applicable not only to the problems that allow a red-black ordering of the equations but to a more general class of unstructured sparse matrices. These arise in the discretization of the flow equations on the combined 1D-2D grid of channel networks and flooded land that are used in the inundation model. With a simple parallelization strategy, the elimination process remains strictly local, and good scaling behavior is achieved. The most important part of parallelization is that of the applied matrix solver.