Dieter Gutknecht

Hydraulic resistance of submerged flexible vegetation

The main research objective consisted in analysing the influence of roughness caused by aquatic vegetation (av), in particular submerged macrophytes, on the overall flow field. These plants are highly flexible and behave differently depending on the flow situation. They also react substantially to the flow field and thus, the roughness becomes variable and dynamic. Conventional flow formulas, such as the Manning or the Strickler formula, are one-dimensional and based on integral flow parameters. They are not suitable for quantifying the roughness of av, because the flow is complex and more dimensional due to the variable behaviour of the plants. Therefore, the present investigation concentrates on the definition of a characteristic hydraulic roughness parameter to quantify the resistance of av. Within this investigation laboratory experiments were carried out with three different types of av, chosen with respect to varying plant structures as well as stem lengths. Velocity measurements above these plants were conducted to determine the relationship between the hydraulic roughness and the deflected plant height. The deflected plant height is used as the geometric roughness parameter, whereas the equivalent sand roughness based on the universal logarithmic law modified by Nikuradse was used as hydraulic roughness parameter. The influence of relative submergence on the hydraulic roughness was also analysed. The analysis of the velocity measurements illustrates that equivalent sand roughness and zero plane displacement of the logarithmic law are correlated to the deflected plant height and are equally to this height.

Distributed Snowmelt Simulations in an Alpine Catchment: 1. Model Evaluation on the Basis of Snow Cover Patterns

This paper presents an attempt at deterministically modeling spatially distributed snowmelt in an alpine catchment. The basin is 9.4 km2 in area and elevations range from 1900 to 3050 m above sea level. The model makes use of digital terrain data with 25 m grid spacing. Energy balance components are calculated for each grid element taking topographic variations of solar radiation into account. For each grid element albedo and snow surface temperatures are simulated. Model performance is evaluated on the basis of snow cover depletion patterns as derived from weekly air photographs. The use of spatially distributed data allows for addressing individual model components. Results indicate that the basic model assumptions are realistic. Model inadequacies are shown to arise from processes not included in the model such as avalanching and long wave emission from surrounding terrain as well as inaccurate model parameters.

Linking flood frequency to long‐term water balance: Incorporating effects of seasonality

Derived flood frequency models can be used to study climate and land use change effects on the flood frequency curve. Intra-annual (i.e., within year) climate variability strongly impacts upon the flood frequency characteristics in two ways: in a direct way through the seasonal variability of storm characteristics and indirectly through the seasonality of rainfall and evapotranspiration which then affect the antecedent catchment conditions for individual storm events. In this paper we propose a quasi-analytical derived flood frequency model that is able to account for both types of seasonalities. The model treats individual events separately. It consists of a rainfall model with seasonally varying parameters. Increased flood peaks, as compared to block rainfall, due to random within-storm rainfall time patterns are represented by a factor that is a function of the ratio of storm duration and catchment response time. Event runoff coefficients are allowed to vary seasonally and include a random component. Their statistical characteristics are derived from long-term water balance simulations. The components of the derived flood frequency model are integrated in probability space to derive monthly flood frequency curves. These are then combined into annual flood frequency curves. Comparisons with Monte Carlo simulations using parameters that are typical of Austrian catchments indicate that the approximations used here are appropriate. We perform sensitivity analyses to explore the effects of the interaction of rainfall and antecedent soil moisture seasonalities on the flood frequency curve. When the two seasonalities are in phase, there is resonance, which increases the flood frequency curve dramatically. We are also able to isolate the contributions of individual months to the annual flood frequency curve. Monthly flood frequency curves cross over for the parameters chosen here, as extreme floods tend to mainly occur in summer while less extreme floods may occur throughout the year.

Distributed Snowmelt Simulations in an Alpine Catchment 2. Parameter Study and Model Predictions

A distributed grid-based model is used (1) to analyze the importance of selected model parameters, (2) to simulate spatial distributions of snow cover properties in a small basin and (3) for a comparison with less sophisticated models as typically used in operational applications. Results indicate that variations of water equivalent with slope and local relief are of utmost importance for realistic distributed simulations but more moderately influence mean basin melt. Snow cover variables of which spatial distributions are simulated include the thermal and hydraulic state of the pack and hourly melt water release. All variables exhibit substantial variations in space and time. They are primarily controlled by topography and the delay of melt water in deep packs. The grid model is compared with a snow band model and a parametric model. The latter estimates the snowpacks areal extent from water equivalent. Simulated snow-covered areas suggest the grid model to be the most realistic. Differences in terms of mean basin melt derive from different assumptions associated with model structure.

Three-Dimensional Simulation of Free-Surface Flows Using Polyhedral Finite Volumes

Abstract The issue of mesh quality has become a topic of increasing interest among scientists and professionals working in the field of computational fluid dynamics, as it has a major impact on model performance. Here we introduce a mesh generation method that is based on a Voronoi decomposition of the flow domain, resulting in polygonal regions in 2D and polyhedral cells in 3D. The discretised Reynolds equations for arbitrary cells are derived using a fmite volume approach and the required boundary conditions are discussed. Both the grid generation process and a solver for the turbulent flow equations have been implemented in the simulation model RSim-3D. The model has been applied to several test cases and the results have been satisfactory. Polyhedral cells have the potential to delivering more accurate results under certain flow conditions than hexahedral cell types.

Das Katastrophenhochwasser vom 7. August 2002 am Kamp — Eine erste Einschätzung

ZusammenfassungIn dieser Arbeit wird das Katastrophenhochwasser am Kamp vom 7. August 2002 aus hydrologischer Sicht analysiert. Niederschlags- und Abflussdaten der letzten 100 Jahre wurden ausgewertet, um dieses Ereignis einzuordnen. Die Niederschläge waren um 70% höher als die größten bisher beobachteten Werte. Die Abflüsse betrugen rund das Dreifache des größten bisher beobachteten Hochwassers. Die rechnerische Jährlichkeit wurde mit mehreren tausend Jahren abgeschätzt. Nur bei einem Eisstoß im 17. Jahrhundert wurde ein ähnlicher Wasserstand erreicht. Die bei diesen Untersuchungen gewonnenen Erkenntnisse ermöglichen auch eine neue Einschätzung, wie Hochwasserschutz und Warnung in Österreich mit derart extremen Ereignissen umgehen kann. Überflutbare Dämme, Definition von Risikoflächen für Extremereignisse und Installation von Frühwarnsystemen können dazu beitragen.AbstractIn this paper the hydrologic situation of the flood in the Kamp catchment northwest of Vienna (August 7th 2002) is analyzed. Rainfall and discharge data from the past 100 years are examined. The rainfall associated with the 2002 flood was 70% higher than the second largest storm and peak discharges were three times those of the second largest flood in the past 100 years. A return period on the order of 2000–10000 years is estimated for the peak flows of this event. Current water resources management practices are discussed in the context of this event and a need for dams that can be overtopped, hazard zone mapping and early flood warning systems is pointed out.

Modelling snowmelt in a mountainous river basin on an event basis

Abstract A snowmelt model for short time flood forecasting of mixed rain-snowmelt floods in a high alpine watershed has been developed. The model is based on a subdivision of the basin into elevation bands. The energy input into the snowpack is computed by the energy-balance approach, using physically based preset parameters. The internal processes are parameterized by introducing heat and water storage capacities. The state of the snow cover throughout the basin is charactacterized by distinguishing three zones with different melt and drainage conditions. The lowest zone is saturated and runoff-producing. Above it, there is a transition zone of partly soaked snow. In the uppermost portion of the basin, no liquid water is stored in the snow. Snow line, saturation line and dry-snow line form the boundaries between the respective zones. Time variations of snow cover conditions are described by the altitudinal fluctuations of the lines. In performing simulation runs, the three boundary lines are found to follow different patterns during a six-day test period. Due to the prevailing melt conditions, the snow line rises monotonically and is only slightly influenced by different weather conditions. The saturation line and consequently the band width of the soaked zone, however, are controlled by day-to-day and diurnal changes in meteorological variables and exhibit a significant increase on rainy days and pronounced fluctuations during fair weather. The dry-snow line shows minor fluctuations. A sensitivity analysis indicates that the influence of model parameters on simulated melt rates is moderate or small when simulation periods of several days are considered so that parameters may be pre-set without inducing much additional uncertainty. The influence of model parameters on simulated melt and dynamics of basin snow cover conditions is discussed. The snowmelt routine was developed with the intention of starting it during the ablation period. Thus, initial conditions for the above mentioned boundary lines are required. Based on sensitivity analyses, it is found that the elevation of the snowline must be derived from current observations. For saturation line and dry-snow line, simple relations to air temperature are given. Simulation results indicate that the areal extent of the saturated snow cover must be considered, if proper model performance for the first hours after model start is desired.

Ein Mehr-Standbeine -Ansatz zur Ermittlung von Bemessungshochwässern kleiner Auftretenswahrscheinlichkeit

KurzfassungDie Ermittlung des Bemessungswertes nach dem hier vorgestellten Ansatz erfolgt auf Basis einer Kombination der Ergebnisse nach verschiedenen Methoden, die einander ergänzende Information einbringen. Sie nutzen unterschiedliche Daten und unterscheiden sich in Hinblick auf Voraussetzungen, Annahmen und Einflussfaktoren. Es ist zu erwarten, dass dadurch die Gesamtunschärfe des Bemessungswertes im Vergleich zu den Einzelmethoden reduziert wird. Die Umsetzung des Konzepts wird für die Ermittlung des Bemessungshochwassers von Talsperren gezeigt. Zur Anwendung kommen dabei die folgenden Methoden: (1) Lokale Hochwasserstatistik; (2) Regionale Hochwasserstatistik; (3) Niederschlag-Abfluss-Modellierung; (4) Gradex-Verfahren; (5) Hüllkurven-Verfahren.SummaryThe design flood estimation methodology proposed in this paper combines the results from a number of methods that are based on complementary information. The methods use different types of data, and differ in terms of their assumptions and controls. One would therefore expect that the uncertainly resulting from the combined approach is smaller than that from each of the individual methods. The opplication of the proposed approach is illustrated for the case of estimating design floods of dams. The individual methods used are (1) at site (local) flood frequency analysis, (2) regional flood frequency analysis, (3) rainfall runoff modelling, (4) the Gradex method, and (5) an envelope curve analysis.

Three-dimensional (3D) modeling of non-uniform sediment transport in a channel bend with unsteady flow

A fully three-dimensional non-hydrostatic model was applied to compute the water flow and morphodynamic processes in a laboratory flume. The experiments provided data for bed deformation and grain sorting processes in a 180? channel bend. The data were used to investigate the performance of different sediment transport approaches. The original van Rijn formula for uniform sediments was compared with two advanced nonuniform sediment formulas that considered the interaction between individual grain sizes by defining hiding-exposure approaches. The results showed that the default configuration of the model was able to predict bed deformations and grain sorting processes with satisfying magnitude and trend. However, the default approach considerably overestimated the bar evolution. The computed results improved significantly when applying any of the introduced hidingexposure approaches, representing better the physical mechanisms of the hiding-exposure effect. This study shows that the present numerical model is able to predict the morphodynamic bed changes in a flume of nonuniform sediments with good agreement if appropriate sediment formulations are used.

Bayesian Estimation of Design Floods under Regional and Subjective Prior Information

Usually design floods are estimated as certain quantiles of a cumulative distribution function (CDF) fitted to a sample of yearly maxima of floods observed at a gauging station. If such observations do not exist at a site where a hydraulic structure is to the built, the hydrologist can collect flood data for several years during the planning phase. This small new sample, however, will not be sufficient for estimating the design flood by means of common flood statistics but it fits as one source of information to be combined with some prior information within the BAYESIAN estimation procedure. Prior information can be taken from long time flood records observed at stations of the same region to be incorporated in a data-based a-priori density function of the parameters.