Adaptation of an Existing Intake Structure Caused by Increased Sediment Level

An unexpected and massive redistribution of fine sediment in a large Alpine reservoir was triggered by a further lowering of the water level to conduct maintenance work. This caused the need of a total redesign of the existing head race intake for a high head power plant in the Austrian Alps. Two main geometry options for the trash rack support structure are compared with numerical simulations (ANSYS-CFX) as well as with a scale model test (scale 1:20). The laboratory experiment substantially benefited from the preliminary numerical investigation in respect of the location and implementation of the model boundaries. In return was the confidence in the numerics strengthened by the successful validation of the local head loss and the velocity distribution for the main operation cases. This allowed that the main inputs for the structural design and the further optimisation is conducted only with the 3D-numerical tool. The paper presents the interlaced concept as well as the main finding of the investigation, which lead to a successful adaptation of the intake structure

Submerged Wall Instead of a Penstock Shutoff Valve—Alternative Protection as Part of a Refurbishment

Hydropower is an important source of renewable energy. Due to ageing infrastructure, more and more existing hydropower plants have to be refurbished and modernised. This includes a complete review of the design parameters as well as the change of specific parts. Investments should be targeted to improve the overall performance of hydropower plants and ensure a long lasting life extension. This paper presents the concept of the submerged wall as a local high point in the headrace tunnel, which can—in combination with the intake gates—replace existing penstock shutoff valves. Such a replacement was conducted for the hydropower plant Schneiderau in Austria, which also allowed us to prove the concept based on measurements including a simulated break of the penstock. The presented solution can help to reduce investment costs and also minimise maintenance efforts and therefore is an attractive option for classic penstock shutoff valves for comparable projects

Engineering-Geological Analysis of a Subaerial Landslide in Taan Fiord, Alaska

On 17 October 2015, a large-scale subaerial landslide occurred in Taan Fiord, Alaska, which released about 50 Mm3 of rock. This entered the water body and triggered a tsunami with a runup of up to 193 m. This paper aims to simulate the possible formation of a weak layer in this mountainous slope until collapse, and to analyze the possible triggering factors of this landslide event from a geotechnical engineering perspective so that a deeper understanding of this large landslide event can be gained. We analyzed different remote-sensing datasets to characterize the evolution of the coastal landslide process. Based on the acquired remote-sensing data, Digital Elevation Models were derived, on which we employed a 2D limit equilibrium method in this study to calculate the safety factor and compare the location of the associated sliding surface with the most probable actual location at which this landslide occurred. The calculation results reflect the development process of this slope collapse. In this case study, past earthquakes, rainfall before this landslide event, and glacial melting at the toe may have influenced the stability of this slope. The glacial retreat is likely to be the most significant direct triggering factor for this slope failure. This research work illustrates the applicability of multi-temporal remote sensing data of slope morphology to constrain preliminary slope stability analyses, aiming to investigate large-scale landslide processes. This interdisciplinary approach confirms the effectiveness of the combination of aerial data acquisition and traditional slope stability analyses. This case study also demonstrates the significance of a climate change for landslide hazard assessment, and that the interaction of natural hazards in terms of multi-hazards cannot be ignored

3D-numerische Modellansätze für die Berechnung von Lawineneinstößen in Speicher

Speicherbecken im Pumpbetrieb oder Beschneiungsspeicher können in Zeiträumen großer Lawinengefahr einen hohen Wasserstand aufweisen. In Hinblick auf eine Gefahrenbetrachtung und Risikoabschätzung ist deshalb auch ein möglicher Einstoß einer Lawine in einen gefüllten Speicher zu untersuchen. Der Einstoß einer großen Lawine kann dabei eine Impulswelle erzeugen, welche eine Gefährdung für den Uferbereich, das Absperrbauwerk und in weiterer Folge auch für die Unterlieger darstellt. Um den Effekt abschätzen und mögliche Gegenmaßnahmen (Speicherabsenkung oder bauliche Maßnahmen) überprüfen zu können, haben sich Modellversuche bewährt. Vermehrt kommen aber auch numerische Methoden zum Einsatz, die von allgemein gültigen Berechnungstabellen bis hin zu projektspezifischen 3D-numerischen Berechnungen reichen können. Der Beitrag fasst die Entwicklungen bei der Modellbildung für die Randbedingung und Erzeugung dieser Impulswelle für die 3D-numerische Software FLOW-3D zusammen. Diese umfassen die Definition von (i) Partikelgemischen, (ii) dynamischen Festkörpern und (iii) ausschließlich aus Wasser bestehenden Fluidkörpern in der Sturzbahn der Lawine. Mögliche Konzepte sowie deren Vor- und Nachteile werden aufgezeigt. Zudem wird die Möglichkeit der Modifikation der „Wasserlawine“ durch Modifikation der Startdichte vorgestellt.Avalanche impacts into reservoirs can cause an impulse wave which endanger the area around the reservoir, the dam itself and as well the area located downstream of the reservoir. Especially reservoirs for artificial snow production or pump storage can be filled up to high water levels in periods with significant avalanche hazard potential. The investigation of possible impulse wave scenarios is part of the global risk analysis and can lead to a limitation of the maximum permitted water level in the reservoir under specific weather condition or structural measures. To quantify such an impulse wave, general equations, scale model tests or numerical simulations can be applied. Within the present paper focus is put on different approaches to model an avalanche impact into a reservoir and the corresponding impulse wave generation by use of the 3D-numerical simulation tool FLOW-3D. Different modelling concepts are presented and discussed, consisting of (i) particle-water-mixtures, (ii) moving objects and (iii) pure water avalanches. In addition to this review on the development, preliminary results of the advanced water avalanche are compared to the equations provided by the ETH Zürich. Therefore, the possibility to define a different density for each initial fluid region is used and two parameters (initial velocity of the avalanche at the start and the adapted density of the model avalanche) are varied.(VLID)456579

Experimental and numerical analyses on the capacity and the control management of a large flood retention basin situated at the Inn River in Tyrol

The consideration of recent extreme events in flood statistics implies an increase of design flood peaks and discharge loads. With the focus on the 75 km long Tyrolean Inn River reach downstream the regional capital city Innsbruck, the harmonization of the 100-year flood peak and comprehensive 2d-hydrodynamic modelling simulations indicate the need for an extension of the existing flood protection measures. Lateral protection and object protection measures represent the only feasible option due to the confined areal conditions. However, an increase of the channel capacities would worsen the situation for downstream areas demonstrably. In order to counter this impact, it is planned to build several large controlled flood retention basins situated along the Inn River at the valley floor between Innsbruck and the border to Germany. The retention basin “Voldöpp” as one of these flood polders features a maximum capacity of 1.7 million m³ and a maximum design water depth of 3.6 m. According to current planning the inlet structure consists of four uniform weir fields with two gates each. Aims of the presented experimental and numerical analyses are the investigation of the flow characteristics in close range of the inlet structure, the weir capacity and a possible weir control management. Hydraulic model tests are accomplished at the scale 1:35 according Froude similarity and numerical modelling is done with the software FLOW-3D. Preliminary modelling results confirmed the functionality of the inlet structure and pointed out the need of further tests concerning the potential impacts of intense sediment transport and woody debris

Discharge Calculation of Side Weirs with Several Weir Fields Considering the Undisturbed Normal Flow Depth in the Channel

Discharge behavior at side weirs is significantly influenced by the water surface profile along the weir crest. In the past century, different approaches were developed to describe this profile and the associated discharge coefficients. However, the application of these methods to practical problems poses a particular challenge, as a complex three-dimensional funnel is formed due to the discharge reduction, leading to significant uncertainties in determining the relevant flow depth. For this reason, a new approach for the determination of the discharge coefficient of side weirs was developed that refers to the undisturbed normal flow depth in the main channel. Based on a comprehensive parametric study utilizing 3D-numerical simulations, the influence of the weir and channel characteristics on the discharge behavior at the side weir was analyzed. A revised formula for estimating the discharge coefficient for side weirs with multiple weir fields was derived using multiple regression analyses. Validation of the numerical simulations was carried out by applying a physical scale model, showing good agreement between the results

Geometry-Based Preliminary Quantification of Landslide-Induced Impulse Wave Attenuation in Mountain Lakes

In this work, a simple methodology for preliminarily assessing the magnitude of potential landslide-induced impulse waves’ attenuation in mountain lakes is presented. A set of metrics is used to define the geometries of theoretical mountain lakes of different sizes and shapes and to simulate impulse waves in them using the hydrodynamic software Flow-3D. The modeling results provide the ‘wave decay potential’, a ratio between the maximum wave amplitude and the flow depth at the shoreline. Wave decay potential is highly correlated with what is defined as the ‘shape product’, a metric that represents lake geometry. The relation between these two parameters can be used to evaluate wave dissipation in a natural lake given its geometric properties, and thus estimate expected flow depth at the shoreline. This novel approach is tested by applying it to a real-world event, the 2007 landslide-generated wave in Chehalis Lake (Canada), where the results match well with those obtained using the empirical equation provided by ETH Zurich (2019 Edition). This work represents the initial stage in the development of this method, and it encourages additional research and modeling in which the influence of the impacting characteristics on the resulting waves and flow depths is investigated

Bedeutung des Geschiebetransportes für die Planung von Hochwasserschutz- und Retentionsmaßnahmen in Talflüssen - Numerische Modellierung des Inns im Tiroler Unterinntal

Die Bedeutung des Hochwasserrisikomanagements im Alpenraum nimmt immer stärker zu. Ziel der Planung ist die Verringerung der hochwasserbedingten nachteiligen Folgen auf die menschliche Gesundheit, die Umwelt, das Kulturerbe und wirtschaftliche Tätigkeiten. Der Inn im Tiroler Unterinntal steht gegenwärtig im Mittelpunkt intensiver Untersuchungen zur Verbesserung des Hochwasserschutzes der angrenzenden Siedlungsräume. Die Betrachtung des Feststoffhaushaltes eines Fließgewässers bzw. die sich im Verlauf eines Hochwassers einstellenden Sohllagenänderungen können einen maßgeblichen Einfluss auf die Wirkung von Hochwasserschutzmaßnahmen und die Dotierung von Retentionsräumen haben. Der vorliegende Artikel befasst sich mit der Bedeutung des Geschiebetransportes für die schutzwasserbaulichen Planungen am Inn, welche Maßnahmen zum Schutz von Siedlungs- und Gewerbegebieten, wie Linearmaßnahmen, Erhalt der natürlichen Überflutungsräume und ungesteuerte und über ein Pegelkollektiv gesteuerte Retentionsräume umfasst. Anhand einer fraktionierten Geschiebetransportberechnung für den 73,5 km langen Gewässerabschnitt konnten sensible Bereiche für die Dotierung der Retentionsräume ausgemacht werden. Der Einfluss der morphologischen Änderungen bzw. der sich einstellenden Sohllagenänderungen während eines 100-jährlichen Hochwasserereignisses auf die Planung der Hochwasserschutz- und Retentionsmaßnahmen konnte aufgezeigt werden. Die aus dem durchgeführten Projekt gewonnenen Modellierungsergebnisse bilden eine wichtige Grundlage für die weiteren Planungsschritte, um eine hinreichende und optimale Scheitelreduktion im Hochwasserfall sicher zu stellen.Flood risk management has become increasingly important in Alpine regions. Hence, the Inn River located in the lower Inn valley is focus of intensive research that aims to decrease flood risks for the densely settled valley plain. The movement of sediments within the river system or rather the development of the river bed during floods can have a significant impact on the effectiveness of flood protection measures and the dimensioning of retention areas. The study presented in this paper deals with flood mitigation systems for the Inn River, such as measures for protection of individual objects, lateral protection structures and uncontrolled and controlled retention areas. Through the project, understanding of potential impacts of morphodynamic changes on the efficiency and functionality of flood-protection measures was achieved. In order to identify sensitive areas for retention control, a multi-fractioned bed-load transport simulation was applied for the 73,5 km long reach of the Inn River. The simulation could prove the influence of morphological changes and resulting river bed formations during a 100 yr-flood on flood protection and retention measures. Presented modelling results aim to provide a profound basis for the further planning procedure of flood protection and retention measures in order to reach an efficient and optimized peak reduction in case of floods.(VLID)456930

Effects of catchment characteristics and hydro-meteorological scenarios on sediment connectivity in glacierised catchments

In the past decade, sediment connectivity has become a widely recognized characteristic of a geomorphic system. However, the quantification of functional connectivity (i.e. connectivity which arises due to the actual occurrence of sediment transport processes) and its variation over space and time is still a challenge. In this context, this study assesses the effects of expected future phenomena in the context of climate change (i.e. glacier retreat, permafrost degradation or meteorological extreme events) on sediment transport dynamics in a glacierised Alpine basin. The study area is the Sulden river basin (drainage area 130 km2) in the Italian Alps, which is composed of two geomorphologically diverse sub-basins. Based on graph theory, we evaluated the spatio-temporal variations in functional connectivity in these two sub-basins. The graph-object, obtained by manually mapping sediment transport processes between landforms, was adapted to 6 different hydro-meteorological scenarios, which derive from combining base, heatwave and rainstorm conditions with snowmelt and glacier-melt periods. For each scenario and each sub-basin, the sediment transport network and related catchment characteristics were analysed. To compare the effects of the scenarios on functional connectivity, we introduced a connectivity degree, calculated based on the area of the landforms involved in sediment cascades. Results indicate that the area of the basin connected to its outlet in terms of sediment transport might feature a six-fold increase in case of rainstorm conditions compared to “average” meteorological conditions assumed for the base scenario. Furthermore, markedly different effects of climate change on sediment connectivity are expected between the two sub-catchments due to their contrasting morphological and lithological characteristics, in terms of relative importance of rainfall-triggered colluvial processes vs temperature-driven proglacial fluvial dynamics

Experimental measurements of flood-induced impact forces on exposed elements

Torrential flood hazards are a major threat for inhabited alluvial fans. They have the potential to relocate large amounts of sediment from the upper catchments to settlement areas on the alluvial fans where typically distributary processes take place. The approaching water-sediment-mixture impacting on building walls are part of a set of damage-generating mechanisms and may cause severe damages to buildings and infrastructure. It is difficult to predict the magnitude and temporal forces on buildings due to the complex flow patterns and sediment deposition processes around obstacles on the floodplain. Our work focuses on experimental measurements of impact forces of flood events on buildings at a 1:30 scale model. It covers the alluvial fan of the Schnannerbach torrent (Austria) with a set of building structures which are equipped with force measurement devices. The measured impact forces are correlated to the approaching flow heights. Influencing factors on the impacts forces such as surrounding buildings on the floodplain and the presence of openings in the building envelope are also analysed. The influence of different hydraulic flow patterns on the impact forces and regression analyses for an estimation of impact forces are presented