Flood Protection at River Würzenbach, Luzern: Physical experiments to optimize bed load and large wood retention at a spillway tunnel intake
Röber, Isabel Christin
Heavy rainfall events in summer 2015 caused spacious inundations in the quarters of Würzenbach in Luzern. An operating spillway tunnel upstream of a dam in the Würzenbach is prone to lose its discharge capacity during flood events due to clogging with large wood. Furthermore, the sediment retention basin is not working properly which results in large sediment inputs into the tunnel.
The lower reach of the Würzenbach downstream of the dam has a discharge capacity of 5 m3/s, whereas the estimated discharge of a 300-year flood amounts to 30 m3/s. The spillway tunnel can discharge up to 27 m3/s. Therefore, it needs to be fully operational in case of bigger flood events.
Short-term safety measures were implemented after the flooding in 2015 including two large drift wood racks upstream of the intake. Further improvements are necessary to ensure permanent effectiveness including a fully operational intake during floods. Because of the hazard and damage potential, canton Luzern commissioned Hunziker, Zarn & Partner AG to find the best solution for this flood protection project.
The VAW was initially involved in the planning stage due to the comprehensive experiences with impacts of driftwood, clogging of hydraulic structures and driftwood retention. However, small system changes in cases of combined retention of sediment and driftwood can entail major and unexpected impacts on the total system. Since the relief capacity must be preserved for a multitude of load cases, physical experiments are inevitable.
The physical model is built at a scale of 1:15 with dimensions of 14.5 m x 3 m, reproducing around 170 m of the River Würzenbach. The aims of the model are surveying the functionality of suggested reconstructions (a lateral weir and an upstream located rack) in the spillway tunnel´s intake area to achieve a sufficient retention of bed load and driftwood. To guarantee the connection of longitudinal river passability for fish, a lateral fish ladder will be implemented.
Physical model experiments on the Sihl River culverts at Zurich main station
Essential parts of downtown Zurich are located on the alluvial fan of the Sihl River. In the 20th century, the Sihl River was strongly integrated into the urban infrastructure.
At Zurich main station, the Sihl River crosses the station building in an intermediate floor through 5 culverts with a length of 190 m and a clear opening of 12 m x 3 m each. It passes underneath the station’s main tracks while two underground railway stations are situated under the river bed.
Beside the limitation of the river’s flow capacity by the culvert dimensions, this specific situation enhances driftwood and floating debris accumulations and induces a prompt discharge capacity reduction in case of deposits in the culverts.
The estimated potential damage for extreme flood events is remarkable as Zurich main station is an important hub for the national and international railway network. Furthermore, the inundation of large parts of the city center and of the surrounding railway infrastructure contributes to an estimated potential damage of 5.5 billion CHF. During the 30-year flood event of August 2005, the city narrowly escaped high damages, an event that pointed out existing deficits in flood protection and the need for action.
In order to plan effective control measures for future flood events, basic data on the discharge capacity of the culverts at Zurich main station are essential. Additionally, the risk of driftwood accumulations at the culverts and possible erosion or sedimentation processes of the river bed have to be assessed. To that end, the VAW was commissioned by the AWEL (Office of Waste, Water, Energy and Air of the Building Department, Canton of Zurich) and the SBB (Swiss Federal Railways) with physical model investigations at a scale of 1:30.
The physical model reproduces nearly 1 km of the Sihl River, leading to model dimensions of 30 m x 6.7 m. The main river bed of the Sihl is represented on both sides of the main station, including several bridges and the river confluence with ‘Schanzengraben’, a moat which originates in Lake Zurich. The 5 culverts as well as adjacent parts of the railway station are represented in acrylic glass for better visualization of the main processes. The model is equipped with a movable bed to accurately reproduce sediment transport processes during flood events. Flow and sediment hydrographs of different flood scenarios can be reproduced.
The main goal of the investigations is the exact determination of the culverts’ discharge capacity for different flood scenarios. Furthermore, the physical model serves to investigate optimization possibilities of the culvert design to increase the discharge capacity.