Research projects

Alpine rivers are ecologically valuable and provide energy as well as recreational benefits to the public but also pose risks in case of floodings and bed failures. In the past, large artificial (concrete) structures were used to reduce the damage caused during such events. Nowadays such structures are not in agreement with the Swiss Waters Protection Act and more natural-like solutions have to be developed. One promising approach is the systematic placement of large roughness elements such as boulders in the rivers to strengthen the river bed and increase the flow resistance (e.g. unstructured block ramps). Although such structures already exist there is little knowledge about the exact flow field and forces acting around and on the roughness elements.

The workflow is separated into three main parts. In a first step, different numerical models will be evaluated using simple cases well-described in literature. In the second step, more complex scenarios will be investigated using multiple roughness elements and nature-like river topography. The validation data comes mainly from laboratory experiments. In the third step, the topography of Swiss rivers will be used to estimate the stability.

During a major riverine or coastal flood event, the water level at a dam or dike can exceed the top elevation of the structure. This causes “overtopping” flow, which sends water flowing along the downstream side of the structure. In the case of earthen embankments, this flow can cause erosion of the embankment material (often sand, gravel, or rock). Given enough time, the erosion can carry away a significant portion of the structure, thereby creating a “breach” that allows uncontrolled outflow through the opening.

In order to prevent the aggradation of the shallower bays around the new Rhine delta in the east-ern part of Lake Constance, dams were built successively to prolong the estuary into the lake and thus convey the transported sediment to deeper parts of the lake to settle. The estuary dams have meanwhile reached their planned extent of about 4 km length.

Riverine environments are amongst the most complex ecosystems on the planet. With several anthropogenic factors increasingly disrupting natural fluvial dynamics, particularly through regularization, the need for re-establishing the ecological properties of these systems has gained relevance in the last decade.

Of particular interest are the floodplains along the river network, primarily for safety against floods, which comprise an extensive realm for restoring ecological functions and establishing new habitats for various species.

However, floodplain vegetation may increase the deposition of suspended sediments and reduce flow conveyance during floods. Considering the central role of rivers in landlocked territories with high development, like Switzerland, the success of river restoration efforts depends greatly on state-of-the-art knowledge, stakeholder cooperation and reliable operational tools for river management and restoration.

The main goal of this study is to contribute to the understanding of the involved processes and answer the following questions:

• How much fine sediment is deposited during floods?
• Which vegetation properties have greater effects on suspended sediment transport and deposition?
• To which extent can 2D numerical models successfully simulate deposition-erosion patterns?

A numerical hydro-morphodynamic 2D model based at VAW (BASEMENT) is herein extended with turbulence and suspended sediment solving capabilities. The implementation follows a High-Performance Computing paradigm, to deliver on the workloads imposed by the spatial and temporal scales required in modelling sediment transport and deposition. Laboratory measurements will provide the support for physical validation while applications to Swiss rivers are planned as case-studies for showcasing the benefits of the developed features.

During the flood event of August 2005, the Inn river received an estimated sediment volume of 300’000 m³ from its tributary Sanna. A considerable part of the sediment was deposited in the vicinity of the confluence, resulting in significant aggradation of several meters and increased sediment transport further downstream.

The numerical modelling of lateral sediment input of this magnitude and the related aggradation and erosion processes is not trivial. By means of a feasibility study, it was investigated whether and how the morphological processes at the Inn, related to the sediment input from the Sanna, can be simulated. Therefore, one-dimensional hydro- and morphodynamic simulations were carried out with the software BASEMENT.

Many rivers have been adapted for flood protection and navigability in the past. In many cases these adaptations have had detrimental effects on the biota living in and around the river by destroying or damaging their habitat.

In recent years increasingly more attention is given to the harmful effects of climate change on freshwater habitat and the focus of river project design has shifted to also include ecological aspects. A fast and objective method to classify areas of ecological fitness on the right spatial scales could aid in these projects and evaluate the effects climate change or management decisions could have on lotic species.

This project aims to develop such a method. The spatial scale will be the mesoscale, which assumes the habitat use of individuals is not only dependent on local parameters but also those of its intermediate surroundings. This scale is easiest to interpret and use in river restoration projects. Several of these methods, like for instance external page http://www.mesohabsim.org already exist. We will aim to expand these methods.

In this project the use of a numerical hydromorphodynamic model (BASEMENT) will be used to obtain spatially and temporally varying data. Based on this data, an unsupervised algorithm will be used to identify mesohabitats (zones of equal habitat suitability) for various species. The primary focus will be on fish species, but invertebrates and other lotic species will also be considered. This unsupervised approach should ensure that the method will be fast and objective, an additional benefit will be that longer reaches can be analyzed.

Field measurements in Swiss rivers are planned. The results from these measurements will be compared to the model outcomes.

River managers and a society that aims at a sustainable interaction with the fluvial system are challenged by the need to ensure flood protection, water resources availability, and ecosystem health in a changing environment.

Understanding and predicting the interaction between riparian vegetation and river morphology is crucial to assess river evolutionary trajectories under different climate, flow regulation, and river restoration scenarios. In this context, the goal of BASEveg is to quantify important geomorphological processes that so far have been described only in qualitative way. This will be achieved developing a physically-based, quantitative tool integrating both eco-morphological and numerical expertise. Therefore, BASEveg aims to:

In order to increase the hydropower utilization, the hydropower company TIWAG is planning to access new catchment areas to contribute to the existing reservoir “Gepatsch” which belongs to the hydropower plant “Kaunertal” in Tirol. For this purpose, two new water intake facilities shall be built at the main tributaries of the “Ötztaler Ache”. To counteract reservoir sedimentation, an annual flushing of trapped sediments is being planned.

The long-term effects of sediment flushing are investigated by means of 1-D hydro- and morphodynamic simulations using the software BASEMENT. To meet the conditions of the study area, bedload transport was modelled using a multi-grain extension of the Smart & Jäggi transport formula.

Firstly, the actual state was modelled for the period 1997-2012. Sediments are being excavated along the “Ötztaler Ache” to some extend in the scope of flood protection. These removals play an important role and were implemented by different approaches in the numerical model.

Secondly, the sediment flushing during the operational phase of the intakes (planned state) was simulated for the same period of time. A comparison of the results of the actual and planned states was performed in order to evaluate the effect within the diverted river section downstream of the intake facilities.

This computational study aims at describing and quantifying aggradation, degradation and channel rearrangement during floods by means of numerical modelling of braided rivers. For the validation of the numerical model, sophisticatedly monitored flume experiments provide topographical data as well as information on sediment transport with a high resolution in time and space.

The numerical model used to simulate temporal extracts of the flume experiments is based on the two-dimensional shallow-water equations solved with a finite-volume technique and explicit time discretization. Regarding sediment transport the  numerical modelling approach includes diverse treatment of suspended load and bed load transport of sediment mixtures based on an active layer approach.

As far as sediment transport and according empirical relations are concerned, the focus is on bed load transport and the effect of bed armouring, which often occurs in alpine gravel-bed rivers. The numerical simulations are carried out with an initially braided topography based on an extract of the flume experiments and a constant hydrograph as steady inflow boundary condition. The results are verified with the data of temporally corresponding flume experiments. The impact of different bed load formulae and a varying resolution of the computational grid are studied. Furthermore the aspect of the different time scales of water and sediment transport is of interest.

Reservoir dams play a crucial role for society and economy. Large outlet structures, such as spillways or low-level outlets typically ensure the dam safety in case of floods and convey extreme discharges. In the near future, spillways of many existing dams may need refurbishments due to aging infrastructure or to cope with larger flood magnitudes resulting from climate change. Therefore, there is a strong need for safe and reliable design guidelines.
High velocities and turbulent flow conditions typically result in self-aeration of the flow (Fig. 1). The accurate description of the resulting air concentrations along spillways are essential, e.g., to quantify flow bulking and the risk of overtopping of side walls, to quantify risk of cavitation along the spillway, or to design of downstream energy dissipation measures.

The measured and calculated flow properties include flow depths, flow velocities, turbulent stresses, air concentrations, as well as bubble/droplet frequencies and chord lengths. Further, inception point locations are evaluated based on visual observations (Fig. 3).
At the second stage of the project, a simplified numerical model for self-aerated flows on spillways will be developed and validated based on the dataset collected in the first part. This tool will be incorporated into the freely available BASEMENT software, thereby allow for safer and cost-efficient design of outlet structures.

A Description of SAFAIR – Part A is available here.

Man-made barriers like hydropower plants (HPP), weirs, and dams interrupt the longitudinal connectivity of rivers, which hinders migration of fishes and delimitates their natural habitats. The negative impacts on fish communities and consequently on the whole ecosystem underline the need to restore river connectivity allowing for up- and downstream migration of fishes again. While upstream fishways have been implemented successfully and are still being optimized, the focus of current research is more and more on fish guidance structures (FGS) for downstream migration. The proper positioning of FGS, as well as the optimization of upstream fishways, requires in depth knowledge of the site specific hydraulic conditions. Computational fluid dynamics (CFD) has evolved as a powerful tool for investigating the flow under varying operational conditions and therefore has a high potential for fishway designs, for both up- and downstream migration.

In the scope of this project the hydraulic conditions at three different power plants along the Aare River are investigated by means of numerical 3-D simulations using the software packages FLOW-3D and OpenFOAM. The 3-D geometries are constructed from bathymetry measurements of the adjacent river bed and construction plans of the HPP facilities.

The facility Wynau/Schwarzhäusern, located about 5.5 km downstream of Bannwil, consists of a weir and two power houses (Fig. 3). First, simulations of the power plant approach flow conditions will be conducted to identify possible positions for future FGS. In order to validate the CFD results, ADCP measurements are conducted at all three locations.

Run-off-river hydropower plants may be barriers to up- or downstream migrating fish. For this reason, efforts have been made to make hydropower plants passable for fish. While there are common measures for upstream migration such as fish ladders or bypasses, downstream migration of fish remains a challenge.

The fish downstream migration is hindered at many hydropower plants in Switzerland. The operator St.Gallisch-Appenzellische Kraftwerke AG (SAK) is developping a pilot project at the river Thur in the Canton St.Gallen in Switzerland to improve fish migration. The design discharge at HPP Herrentöbeli amounts to 11 m3/s and the HPP is equipped with two Kaplan turbines. In the current state, there is no residual flow unless the discharge of the Thur is above the design discharge (weir overflow). Therefore, fish downstream migration is only possible through the turbines with a high risk of injuries or mortality due to blade strike and strong pressure gradients.

Tsunamis in lakes represent one of the natural hazards in Switzerland. On a historical basis, inland lakes have been subject to severe wave inundations in the past, with major events as Lake Geneva in 563 AD and Lake Lucerne in 1601 AD, with reported wave heights of several meters and high destructive impact on adjacent settlements on the shoreline. The trigger mechanisms, preconditions, processes and impacts of lake tsunamis are currently still under-explored and therefore no warning mechanisms are yet available. This project is within the interdisciplinary SNSF Sinergia research project.

The objective of the numerical modeling is to provide a workflow for the probabilistic hazard assessment of tsunami events on Swiss lakes. In particular, the wave generation, propagation and run-up of tsunami waves will be modeled, to allow a future estimation of the inundated area and flood propagation velocities. There are two main investigation strategies: i) numerical simulations carried out via BASEMENT v3 will be integrated actively within the workflow of the Sinergia project; ii) laboratory experiments will be used to verify numerical evaluations.

The outcome of this challenging project will help to enhance the understanding of the process and will substantially contribute for assessing the risks for people, lifelines and structures along lake-shores and coasts. This study will contribute in defining the needs and benefits of potential mitigation strategies.

Flood discharges may lead to inundation that threatens people and buildings as well as damages agricultural land. Flood polders are a measure, particularly common in Germany, to reduce flood peaks and thus mitigate flood disasters. A flood polder is a retention area arranged laterally to the river, where a large volume of water can be stored during a flood event.

The inlet structure of a flood polder is an important component of a well-functioning flood retention system. Inlet structures are used to divert the discharge when the flood level rises and thus to reduce the flood peak discharge in the river. Inlet structures can be distinguished according to their controllability. Controlled inlet structures are classically equipped with flaps or gates. Uncontrolled structures may be designed as overflow dams or are equipped with tilting elements, for instance.

In order to support the design of flood polder inlet structures, this project compiles various possibilities of types and arrangements of inlet structures. For this, a literature study is carried out. In addition, a survey addressed to operators of flood polders is launched to collect data and experience about the construction, operation and maintenance of inlet structures.

The different types of inlet structures are compared and evaluated using criteria such as costs for construction and maintenance, robustness, flexibility with regard to flood management, impact on the landscape, etc. For the implementation of the inlet structure, the environment and boundary conditions (e.g. risk of drift wood clogging, accessibility of the structure, etc.) are essential and must be taken into account for the evaluation.

The construction of a run-of-river power plant may involve the separation of floodplains from the river. Consequently, the flood peak discharge will not be reduced due to inundation of the floodplains anymore and thus the flood wave will not be damped by retention. This may lead to more acute flooding problems downstream of the power plant, especially for people and buildings in densely populated regions.

The run-of-river power plant Langkampfen on the Tyrolean Inn, operated by TIWAG – Tiroler Wasserkraft AG, was built in 1998. Due to the construction of the weir and the embankments along the river, a reservoir has been formed. This artificial reservoir shall be used to reduce the flood peak discharge. The objective is to control the weir during the passage of the peak discharge to refill the storage and to facilitate an optimal capping.
The volume of the instream reservoir is small relative to the flood volume of the Inn river, so that only peak discharge capping becomes possible. The potential retention volume depends on the discharge as well as the corresponding water level. The higher the discharge, the lower is the retention volume. Furthermore, a forecast of the flood hydrograph to be capped must be provided for optimal retention.

The flood routing and the control of the weir are simulated with a hydrodynamic 1-D model developed at VAW, namely BASEMENT. The model includes a PID (proportional integral derivative) controller that can be used for controlling the water level in the reservoir. The objective is to adapt the PID controller automatically to the situation before, during and after the flood event. Therefore, requirements for the controller regarding the gate operation during the passage of the flood wave must be predefined. For the capping operation, the main constraints are not to exceed the normal operation water level and to comply with the freeboard regulations. The model is tested by investigating past flood events and possible scenarios. Finally, the model will be implemented for real time operation to provide recommendation to the operator for weir control during a flood event.