Research projects
- Effects of Macro-Roughness Elements (MREs) on local hydraulics and riverbed morphology
- Hybrid modeling of the Alpine Rhine flood protection project
- Investigations of guide groynes and bank protection measures at the Rheinhalde in Basel
- Large wood management on Lakes Biel, Thun, and Brienz
- Natural and artificial refugia in dynamic river widenings
- Scour protection investigations on pier foundations in a fine sandy riverbed
The implementation of wood in rivers has been recently reevaluated as a useful tool for river restoration projects. Macro-roughness elements (MREs) like rootwads or engineered logjams can enhance flow heterogeneity and riverbed morphologies, increasing diversity of habitats for aquatic species. On the other hand, such structures alter flow dynamics and hence sediment transport, making it difficult to predict their development under flood conditions. As Switzerland aims to restore 4,000 km of river reaches by 2090 (Swiss Waters Protection Act, amended 2011), this study investigated the hydraulic behaviour of scaled macro-roughness elements (MREs) in a laboratory flume under both fixed- and mobile-bed conditions.
The experiments showed that higher Froude numbers and larger relative widths of single, side-placed MREs produced the greatest backwater rise. The local flow field was strongly influenced by the size and distribution of openings within the structures (i.e., porosity), which affected wake length and flow acceleration. The coefficient of variation of flow velocity was highest for emergent, narrow structures, indicating enhanced flow heterogeneity.
Differences in construction materials (e.g., stones versus wooden logs)
generated distinct jet patterns and peaks of turbulent kinetic energy in the
wake. These factors that should be carefully considered when designing suchelements to support aquatic habitats.
Experiments on mobile bed studied how porous logjam influence the scour magnitude. In particular, the effect of placement, relative submergence, porosity, discharge and grain size was tested. Further experiments will implement multiple structures to assess interaction effects.
Natural river systems are shaped by dynamic processes such as water flow, sediment, and wood transport and characterized by high hydro-geomorphological variability. Riverine ecosystems are small-scale, intricate mosaics of aquatic and terrestrial habitats and support high biodiversity. However, numerous rivers in Switzerland and worldwide are heavily impacted by human interference, for example, by channelization (i.e., artificial confinement to a narrow riverbed designed for efficient water and sediment conveyance). In addition, the sediment continuum of rivers is interrupted by transversal structures such as weirs or sediment retention basins. The combined effect of sediment deficit and channelization transformed dynamic alluvial river systems with extensive floodplains into incised, straight rivers with a flat riverbed and little morphodynamic activity. The resulting uniform and static river systems cannot adequately sustain riverine flora and fauna adapted to the high spatiotemporal dynamics of natural river systems. Instead, the flow field is homogeneous, the riverbed is coarse and armored, and connectivity between aquatic and terrestrial habitats is interrupted.
Modern river restoration works towards reestablishing dynamic riverine processes such as sediment transport and the associated morphodynamic activity. Dynamic river widenings are river reaches of a few hundred meters to several kilometers in length designated to increase local river width and morphological variability (Figure 1). After removing bank protection and implementing further initiation measures, bank erosion may increase channel width and sediment input to the river, and a more near-natural morphology may reappear.
The development of dynamic river widenings may be determined by many factors, for example, floodplain erodibility, vegetation growth, or longitudinal slope. Previous studies conducted experiments with a longitudinal slope of approximately one percent to investigate the morphodynamic adjustments of reach scale river widenings and the influence of sediment supply from upstream. In this study, we continue this research and investigate how the widening process is developing for a longitudinal slope of approximately 0.3 percent. The main questions are:
- Does sediment supply influence the morphological development of one-sided dynamic river widenings with low channel slope?
- What are different effects for the morphology investigating implementations of local flow deceleration (e.g. ELJ, initial widening, …) and flow acceleration measures (e.g. flow deflector, …)?
- How does dynamic river widening effect the availability of flood and drought refugia?
These questions will be answered based on a large-scale mobile-bed laboratory experiments representing a one-sided dynamic river widening in a gravel-bed river at VAW. The laboratory results are complemented with numerical modeling using BASEMENT.
This study is part of the transdisciplinary research project Resilient Rivers: Refugia – Connectivity – Stepping stones in the research program Hydraulic structures and ecology (2022-2026) and is co-financed by the Swiss Federal Office for the Environment (FOEN).
In July 2021, the discharges in the Rhine near Basel was above HQ2 for almost two weeks, with a peak discharge of > HQ10. This prolonged stress on the banks of the Rheinhalde led to scouring at the foot of the embankment and erosion of the bank over a length of approximately 300 m. Immediate measures were taken during the peak discharge to protect the remaining banks and, in particular, the road behind them. These immediate measures were supplemented after the event with a berm (Figure 1).
Large areas of the banks consist of Nagelfluh and sand layers, which were exposed during the event and are still visible today. In the Rheinhalde nature reserve, these form a significant ecological asset that should be preserved in a long term perspective. Structuring the bank protection measures should also enhance the aquatic habitat, while at the same time ensuring the stability of the bank up to an HQ100. A combination of guide groynes and bank protection measures should meet these requirements (Figure 2).
VAW was commissioned by the Tiefbauamt Basel-Stadt to test the stability of the planned measures in a hydraulic model on a scale of 1:50. The upper boundary condition of the model is difficult to define, as the flow is strongly influenced by the power plant Birsfelden. The simplified modeling of the power plant is intended to provide an understanding of the flow to the riverbank of the Rheinhalde. The riverbed in this section consists mainly solid rock and is therefore reproduced using a solid base. The precise modeling of the rock channels in the base is of central importance for the flow to the bank.
The Bernese lakes – Lake Biel, Lake Thun, and Lake Brienz – repeatedly experience events large wood from their tributaries. In the lakes, this wood poses various risks: for example, it can cause accumulations at the weirs in Thun and Interlaken,destroy shoreline vegetation (reed zones), or hinder navigation.
To prevent these adverse effects, the driftwood is currently secured by the police using floating barriers. However, both the securing process and the subsequent recovery and disposal are very laborious and generate third-party costs of up to half a million Swiss francs per year.
For this reason, the Office for Water and Waste (Amt für Wasser und Abfall, AWA) of the Canton of Bern aims to re-evaluate its approach to large wood management. VAW has therefore been commissioned to examine the current management concept and to assess measures that could improve it. Possible solutions include retention racks in rivers, retention at power plants, fixed barriers at river mouths, or local protective measures for conservation areas. The challenge lies in identifying the most effective measures while considering various, sometimes conflicting, objectives regarding safety, ecology, and economic efficiency.
Ultimately, the project aims to show how large wood management can be designed for the future.
Scour protection investigations on pier foundations in a fine sandy riverbed
The expansion of the highway to six lanes between Luterbach and Härkingen (external page A1 Luterbach–Härkingen 6-Streifen-Ausbau) means that an additional bridge with two pier foundations must be built in the Aare river near Wangen an der Aare (current stat in figure 1).
Due to the special fine sandy riverbed of the Aare (dm ≈ 0.1–0.2 mm) covered only with a thin armor layer and a deep scour hole between the existing bridge piers, it was difficult to estimate the changes in terms of hydraulics and the stress on the riverbed.
The 1:35 hydraulic model was used to investigate the impact of the additional bridge piers in the project status and the influence of a temporary bridge during the construction phase.
This showed that at certain placed with high stress, a scour protection must be implemented during the construction phase in order to prevent any deterioration of the existing conditions. Furthermore, the scour protection in the area of the pier foundations could be optimized so that, despite the fine sandy riverbed, the stability of the piers remains uncompromised (figure 2).
The flood protection project “Alpenrhein Internationale Strecke km 65 bis 91” (www.rhesi.org) aims to increase the discharge capacity of the Alpenrhein from the current 3,100 m3/s (approx. HQ100) to 4,300 m3/s (approx. HQ300) over thecoming decades.
The hydraulic and river engineering measures planned in the project were tested and optimized by VAW on behalf of the external page Internationalen Rheinregulierung using a hybrid modelling approach with a physical model (scale 1:50) and different numerical models. The following issues were investigated:
- Flood safety testing
- Morphological development of the
river bed - Processes involved in changes to the
cross-section (narrowing/widening) - Bank protection and embankment
safety - Bridge structures (freeboard, risk
of log jams, pier scouring) Vegetation on gravel banks (backwater effects in the event of flooding and retention potential of driftwood, influence on morphological development
The most important results of the investigations are explained in more detail in the journal “Wasser Energie Luft, WEL” issue 116 (2) from June 2024.
A virtual tour of the industrial hall, which was specially converted for the model tests, is available at the following link thanks to a digital twin of the Oberriet/Koblach reach:
