Ilhan Özgen-Xian

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• PhD
• Professor (Associate) at Technische Universität Braunschweig

Advances in high-performance parallel computing have significantly enhanced the speed of large-scale hydrological simulations. However, the diversity and rapid evolution of available computational systems and hardware devices limit model flexibility and increase code maintenance efforts. This paper introduces SERGHEI-RE, a three-dimensional, variab...

Anthropogenic climate change has increased the probability, severity, and duration of heat waves and droughts, subsequently escalating the risk of wildfires. Mathematical and computational models can enhance our understanding of wildfire propagation dynamics. In this work, we present a simplified Advection-Diffusion-Reaction (ADR) model that accoun...

It is important to study the ecohydrological dynamics in tropical dry forests to understand drought impacts. This study aims to model and understand how plants extract water from various soil depths and the ecohydrological strategies of different plant species for adapting to dry periods in a tropical dry forest in Costa Rica. Fair agreement was fo...

Advances in high-performance parallel computing have significantly enhanced the speed of large-scale hydrological simulations. However, the diversity and rapid evolution of available computational systems and hardware devices limit model flexibility and increase code maintenance efforts. This paper introduces SERGHEI-RE, a three-dimensional, variab...

Anthrosols and technosols are urban soils that have been heavily influenced by anthropogenic activities. We hypothesise that such soils store information that can give insights into the system's co-evolution. In a case study of the urban green space Gaußbergpark, Braunschweig, Germany, we demonstrate how an interdisciplinary study of anthrosols yie...

Tree mortality is expected to increase in many regions as weather extremes tend to become more frequent and intense with global warming. In this study, tree water stress is assessed by hydraulic modeling along the soil-plant-atmosphere-continuum (SPAC) for different tree species in a secondary tropical dry forest in northwestern Costa Rica. Prevail...

We explore numerically the current water stress on urban trees (pedunculate oak; Quercus robur) at Jasperallee, Braunschweig, Lower Saxony, Germany. Ecohydrological signatures of this tree (and variations of it) are computed using climate data for the years 2012–2021. These exploratory numerical simulations do not aim for prediction, but instead in...

Understanding the dynamics of wildfire is crucial for developing management and intervention strategies. Mathematical and computational models can be used to improve our understanding of wildfire processes and dynamics. This paper presents a systematic study of a widely used advection-diffusion-reaction wildfire model with non-linear coupling. The...

The ecohydrology of the Urban Critical Zone is characterised by strong heterogeneity and the entangling of hydrological and human time scales (Sivapalan & Blöschl, 2015). This not only poses a challenge to field measurements and the transfer of insights to other urban systems, but consequently limits the development of universal theoretical approac...

We computationally explore the relationship between surface–subsurface exchange and hydrological response in a headwater-dominated high elevation, mountainous catchment in East River Watershed, Colorado, USA. In order to isolate the effect of surface–subsurface exchange on the hydrological response, we compare three model variations that differ onl...

The Simulation EnviRonment for Geomorphology, Hydrodynamics, and Ecohydrology in Integrated form (SERGHEI) is a multi-dimensional, multi-domain, and multi-physics model framework for environmental and landscape simulation, designed with an outlook towards Earth system modelling. At the core of SERGHEI's innovation is its performance-portable high-p...

The Simulation Environment for Geomorphology, Hydrodynamics and Ecohydrology in Integrated form (SERGHEI) is a multi-dimensional, multi-domain and multi-physics model framework for environmental and landscape simulation, designed with an outlook towards Earth System Modelling. It aims to provide a modelling environment for hydrodynamics, ecohydrolo...

An accurate characterization of the water content of snowpack, or snow water equivalent (SWE), is necessary to quantify water availability and constrain hydrologic and land surface models. Recently, airborne observations (e.g., lidar) have emerged as a promising method to accurately quantify SWE at high resolutions (scales of ∼100 m and finer). How...

Climate change and other disturbances significantly impact hydrogeochemical exports from mountainous headwater catchments such as the Upper Colorado River Basin. Developing a mechanistic understanding of how the physical and chemical processes interact in time and space in an integrated manner is key to quantifying the future impacts of such distur...

In this paper, an automated calibration tool for shallow water models (ActSwm) was developed utilizing the open-source Python library SciPy. The developed tool links the hms model for rainfall-runoff simulations to global optimization algorithms as a single executable. Three fast global optimization algorithms (Differential Evolution, Dual Annealin...

This chapter aims to introduce the basic background of shallow water flow models and robust numerical methods as well as to represent different examples of applications. After briefly explaining the main characteristics of shallow water flow problems and pointing out the specific numerical requirements to appropriately simulate those processes, one...

We numerically study how the interactions between surface, subsurface, and rainfall create complex runoff behavior in a headwater-dominated high elevation, mountainous catchment in East River Watershed, Colorado, USA. In order to understand the effect of surface–subsurface interactions on the hydrological response, we compare model variations with...

The Boussinesq flow equation describes depth-averaged saturated groundwater flow in nearly horizontal aquifers. In this contribution, Cattaneo's relaxation approach is applied to reformulate the parabolic Boussinesq equation as a first-order hyperbolic system. The reformulation allows to compute discharges as primary variables with the same accurac...

Hydrological (dis)connectivity, in the sense of surface and subsurface runoff (dis)connectivity, emerges from the formation of spatial flow patterns across scales. As such, hydrological connectivity can not be directly measured, but often manifests itself in observable hydrological signatures, such as hydrographs and water balance. These signatures...

Microtopography is the small scale deviation of Earth's topography, such as humps and depressions. The hydrological literature agrees that the presence of microtopography can affect infiltration patterns by creating local pressure head differences that drive water into the soil at increased or decreased rates. Runoff generation is a non-linear proc...

Runoff generation results from the non-linear interaction between rainfall, climate, soil properties, topography, and vegetation. In particular, the effect of micro-topography on runoff partitioning and runoff generation is widely documented. In this work, we aim to understand the control that micro-topography exerts on rainfall generation through...

This work is aimed at investigating flash floods in the region of El Gouna, Egypt, by using a 2D robust shallow-water model that incorporates the Green-Ampt model to find the most realistic infiltration setting for this desert area. The results of different infiltration settings are compared to inundation areas observed from LANDSAT 8 images as wel...

We apply Cattaneo's relaxation approach to the one-dimensional coupled Boussinesq groundwater flow and advection-diffusion-reaction equations, commonly used in engineering applications to simulate contaminant transport in the subsurface. The diffusion-type governing equations are reformulated as a hyperbolic system, augmented by an equation that ca...

In this paper, different optimization algorithms (Basin-Hopping, Dual-Annealing, SHGO) were applied to automatically calibrate a shallow water model for rainfall-runoff simulation in two real case studies. The automated calibration approach showed a very good agreement with measurements and performed better than a conventional manual calibration, p...

The predictor-corrector-type (P-C) numerical solution to the 1D Richards equation only requires one matrix inversion operation per time step, making it attractive in terms of computational cost. However, the mass conservation could be violated at the saturated-unsaturated interface. A new post-allocation procedure is designed for the P-C method, wh...

Hyperbolic systems under nonconservative form arise in numerous applications modeling physical processes, for example from the relaxation of more general equations (e.g. with dissipative terms). This paper reviews an existing class of augmented Roe schemes and discusses their application to linear nonconservative hyperbolic systems with source term...

Microtopography is recognised as one of the morphological features which controls runoff generation, surface hydrodynamics, and surface runoff hydrological response. The spatial scales of microtopography are orders of magnitude smaller than typical hydrological domains such as hillslopes or catchments. The hydrodynamic response in the presence of m...

A wavelet-based local mesh refinement (wLMR) strategy is designed to generate multiresolution and unstructured triangular meshes from real digital elevation model (DEM) data, for efficient hydrological simulations at catchment-scale. The wLMR strategy is studied considering slope-and curvature-based refinement criteria to analyze DEM inputs: the sl...

Within this work, the impact of mitigation measures and infiltration on flash floods is investigated by using a 2D robust shallow water model including infiltration with the Green-Ampt model. The results show the combined effects of infiltration and mitigation measures as well as the effectiveness of bypass channels in addition to retention basins....

A large amount of topography data is collected using LIDAR. While the individual components involved in the process have a very high accuracy, the typical vertical accuracy of the collected data is between 5-15 cm. We suggest that hydrological model results are significantly sensitive to the measurement accuracy, since the water depths in the domai...

The high computational cost of large-scale, process-based hydrological simulations can be approached using variable resolution meshes, where only the region around significant topographic features is refined. However, generating quality variable resolution meshes from digital elevation data is non-trivial. In literature, usually a slope or curvatur...

In brief: We study the influence of locally refined static meshes on the S-curve of a mountainous catchment. Local mesh refinement is steered by pre-calculated river networks based on analysis of the digital elevation model using different flow routing methods. The S-curve is found to be sensitive to mesh design. All applied methods fail to predict...

This paper presents novel flux and source term treatments within a Godunov-type finite volume framework for predicting the depth-averaged shallow water flow and sediment transport with enhanced the accuracy and stability. The suspended load ratio is introduced to differentiate between the advection of the suspended load and the advection of water....

This paper describes an improved vector manipulation multislope monotone upstream-centred scheme for conservation laws (MUSCL) reconstruction for solving the shallow water equations on unstructured grids. This improved MUSCL reconstruction method includes a bigger stencil for the interpolation and saves time for determining the geometric relations...

Complex transport mechanism and interaction between fluid and sediment make the mathematical and numerical modeling of sediment transport very challenging. Different types of models can lead to different results. This paper investigates a non-equilibrium sediment transport model based on the total load. In this type of model, it is assumed that a b...

Current topographic survey technology provides high-resolution (HR) datasets for urban environments. Incorporating this HR information in models aiming to provide flood risk assessment is desirable because the flood wave propagation is depending on the urban topographic features, i.e. buildings, bridges and street networks. Conceptual, numerical an...

In shallow water flow and transport modeling, the monotone upstream-centered scheme for conservation laws (MUSCL) is widely used to extend the original Godunov scheme to second-order accuracy. The most important step in MUSCL-type schemes is the MUSCL reconstruction, which calculate extrapolates the values of independent variables from the cell cen...

The integral porosity shallow water model is a type of porous shallow water model for urban flood modeling, that defines two types of porosity, namely a volumetric porosity inside the computational cell and a conveyance porosity at each edge. Porosity terms are determined directly from the underlying building geometry, hence buildings do not need t...

After three hours of intense rainfall, the city of Nice was flash flooded on October 3, 2015, resulting in casualties and severe damages in property. This study presents a porous shallow water-model based numerical simulation of the flash flood event in a district of Nice, and compares the results with a high-resolution conventional shallow water m...

In urban flood modeling, so-called porosity shallow water equations (PSWEs), which conceptually account for unresolved structures, e.g., buildings, are a promising approach to addressing high CPU times associated with state-of-the-art explicit numerical methods. The PSWE can be formulated with a single porosity term, referred to as the single poros...

This paper presents numerical simulations of dam-break flow over a movable bed. Two different mathematical models were compared: a fully coupled formulation of shallow water equations with erosion and deposition terms (a depth-averaged concentration flux model), and shallow water equations with a fully coupled Exner equation (a bed load flux model)...

Developments in survey technology such as light detection ranging and laser-scanning are able to provide high-resolution topography data sets. In shallow water equations based modeling, the use of high-resolution topography data is generally desirable, because it is considered to be a more accurate representation of the “real world”. Indeed, high-r...

In urban flood modeling, so-called porosity shallow water equations (PSWEs), which conceptually account for unresolved structures, e.g., buildings, are a promising approach to addressing high CPU times associated with state-of-the-art explicit numerical methods. The PSWE can be formulated with a single porosity term, referred to as the single poros...

This study presents a novel high-resolution simulation of free-surface flow and tracer retention over a streambed with ripples based on varying ripple morphologies, surface hydraulics and the transport of a tracer pulse from surface water to surface dead zone. For the simulations, the computational fluid dynamics (CFD) model OpenFOAM was used to so...

Flood events in urbanized areas are usually caused by localized rainfall events with very high intensity that occur in catchments located at the upstream of the city. The typical model chain for forecasting this type of event is a hydrological model that generates input for a two-dimensional hydraulic model. However, in recent years, the depth-aver...

In a broad range of applications of hydro-and environmental system modeling, the flow field is computed by the shallow water equations. The flow field is then used to compute the physical processes that depend on it, e.g., transport of a passive tracer, sediment transport and morphodynamics or infiltration. Thus, a scientific code for the computati...

Current topographic survey technology provides high-resolution (HR) datasets for urban environments. Incorporating this HR information in models aiming to provide flood risk assessment is desirable because the flood wave propagation is depending on the urban topographic features, i.e. buildings, bridges and street networks. Conceptual, numerical an...

One of the approaches to flood modelling is numerical simulation of the diffusive wave approximation of the shallow water equations. Improving these models in various aspects is still an open area of research. In this study, a new diffusive wave model with explicit time integration was developed which includes some novel features: (1) time steps ar...

This paper presents a novel mass conservative, positivity preserving wetting and drying treatment for Godunov-type shallow water models with second-order bed elevation discretization. The novel method allows to compute water depths equal to machine accuracy without any restrictions on the time step or any threshold that defines whether the finite v...

Flash floods are localized, sudden events characterized by a high rise of the water table in very short time usually caused by localized rainfall events with very high intensity. The short time period and relative small scale of flash flood events rise challenges in flood protection measures as well as the model-based forecasting of these events. H...

The shallow water model with anisotropic porosity conceptually takes into account the unresolved subgrid-scale features, e.g. microtopography or buildings. This enables computationally efficient simulations that can be run on coarser grids, whereas reasonable accuracy is maintained via the introduction of porosity. This article presents a novel num...

This study presents a comparison of two different model concepts for the simulation of sediment transport, namely the capacity model with the Exner equation and the non-capacity model with suspended sediment transport and erosion and deposition terms. The Exner equation is modified to account for momentum loss due to sediment particle movement. Bot...

A shallow water model with anisotropic porosity (AP) is used to solve the shallow water equations on coarse resolutions in order to reduce the computational cost. The AP model enhances the accuracy of the solution with the help of two types of porosities to account for topographical features which cannot be resolved by the computational mesh: (1) t...

The shallow water model with anisotropic porosity conceptually takes into account the unresolved subgrid-scale features, e.g. microtopography or buildings. This enables computationally efficient simulations that can be run on coarser grids, whereas reasonable accuracy is maintained via the introduction of porosity. This article presents a novel num...

Coarse grid shallow water simulations of rainfall-runoff in small catchments with modified friction law to account for unresolved microtopography

This paper derives a novel formulation of the depth-averaged shallow water equations with anisotropic porosity for computational efficiency reasons. The aim is to run simulations on coarser grids while maintaining an acceptable accuracy through the introduction of porosity terms, which account for subgrid-scale effects. The porosity is divided into...

This paper investigates the relationship between the rainfall and runoff in idealised catchments, either with or without obstacle arrays, using an extensively- validated fully-dynamic shallow water model. This two- dimensional hydrodynamic model allows a direct trans- formation of the spatially distributed rainfall into the flow hydrograph at the c...

This study presents a novel roughness formulation to conceptually account for microtopography and compares it to four existing roughness models from literature. The aim is to increase the grid size for computational efficiency, while capturing subgrid scale effects with the roughness formulation to prevent the loss in accuracy associated with coars...

This study presents a novel roughness formulation to conceptually account for microtopography and compares it to four existing roughness models from literature. The aim is to increase the grid size for computational efficiency, while capturing subgrid scale effects with the roughness formulation to prevent the loss in accuracy associated with coars...

This paper studies the usage of roughness formulations to account for subgrid-scale effects based on microtopographic features in the context of upscaling overland flow models. The aim is to increase the cell size in order to reduce the computational cost of shallow water models applied to surface runoff in small catchments. The integral discharge...

Wind-induced flow and transport processes were investigated in Icó-Mandantes bay, a branch of the São Francisco river, Brazil. Aim of the study was to analyse the effects of the wind on the water body and on the interaction between the São Francisco river and the bay, for different scenarios and different wind conditions. It was found out that whil...

In hydro- and environmental systems modelling, there are several application cases where very small water depths occur, for example rainfall and runoff in natural or urban catchments, possibly associated with tracer transport. In these cases, the water depth may be in the range of millimeters to a few centimeters. The numerical simulation of the as...

This work investigates the interaction between Icó-Mandantes bay and the São Francisco river. Flow conditions and spreading of tracer in the main stream are simulated for mean discharge and a moderate flooding event using the Hydroinformatics Modelling System (HMS). It was found out that the bottom roughness has a considerable influence on velociti...

This paper presents a numerical model suitable for a broad range of surface flow problems such as overland flow, wetting and drying processes, varying flow conditions and shock waves. It is based on solution of two-dimensional fully dynamic shallow water equations using a cell-centred finite-volume method. Numerical fluxes are computed with a Harte...