Calculation of the discharge capacity of the Sihlhölzli Bridge during the construction phase

The Sihlhölzli Bridge in Zurich is being widened to improve the safety of pedestrians and cyclists. For this purpose, a temporary access ramp from Stauffacherquai to the Sihl River, as well as a ford crossing, will be constructed. In addition, sheet pile walls will be installed in the Sihl River for the construction of a pier and an abutment. In accordance with AWEL guidelines, a free-flow area must be ensured in the Sihl River to maintain discharge capacity. This section can remain open or be equipped with permeable ford elements.

The openings and ford elements must together ensure a discharge capacity (including minimum freeboard) of at least 40 m³/s. This corresponds approximately to a 5-year flood and is intended to provide a ford crossing that remains dry and accessible for construction equipment during the works. The corresponding minimum protection level was determined by TK CONSULT AG using numerical flow simulations.

A detailed 2D hydraulic model with a high-resolution computational mesh was created, extending from the Sihlhölzli Weir upstream of the Sihlhölzli Bridge to the Stauffacher Bridge. Bridge piers, ford elements, and sheet pile walls were incorporated into the model based on the site plan geometry.

As part of the modeling, water surface elevations, energy line levels, and local energy losses due to contractions and expansions were determined. Based on these analyses, a protection level (i.e. water level + freeboard) was recommended. In addition, a hydraulic assessment was carried out to estimate potential additional losses caused by the flow impinging on the partition walls of the ford elements.