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Retrofitting the Sylvenstein Dam

Protecting Munich from Devastating Floods

Stretching across the beautiful Isar River valley in Bavaria, Germany, the Sylvenstein Dam protects the capital city of Munich and the town of Bad Tölz from devastating floods.

To safeguard the large population downstream of the reservoir, great efforts have been taken to ensure the dam’s stability and prepare for the future, when climate change is expected to increase operational demands. In addition to flood protection, the dam supports two power plants and maintains minimum water levels in the Isar River.

Analysis of existing dam conditions—using information collected from within the dam’s core and at the base of the dam—has prompted the Weilhiem State Office for Water Management to perform precautionary dam strengthening, by adding a slurry cutoff wall and new monitoring system for leakage water. CDM Smith is assisting with the retrofit by providing general civil and structural engineering design, dam stability analyses, landscape conservation planning and onsite supervision services.

Overcoming Challenges
To make sure that the 180-meter-long, 44-meter-high dam is watertight, a cutoff wall will be installed into the existing clay core and attached to the rock abutments. The cutoff wall will reach a depth of at least 70 meters below the water surface—a wall depth that has not been reached in Germany prior to this application.

In addition, a 2.5-meter-wide gallery will be installed at the base of the dam to collect, control and monitor seepage. The water will first be collected in new drainage wells, which will extend from the top of the dam to 40 meters deep, before being discharged to the gallery. A vertical shaft and an access tunnel driven through the rock embankment will provide access to the gallery from both sides.

Assuring Stability
Dr. Knut Langhagen, CDM Smith design team leader, explains that specialized methods needed to be considered because the dam is underlain by deep alluvial soils. “The dam’s foundation is not rock, but a compressible, clayey gravel and sand that is saturated by groundwater. The river previously eroded a 100-meter-deep canyon beneath the river bed, which was then filled with alluvium.”

To design dam improvements in this environment, as well as analyze the interactions between the subsurface conditions and the construction of the tunnel, gallery, and slurry cutoff wall, the finite element calculations method is being used. This sophisticated stability analysis will verify that the team’s design approach is the optimal solution. Langhagen notes, “It is our responsibility to secure the dam and prevent future leakage, so we are taking extra care to understand how the new construction and ground materials will work together.”

Weilheim State Office for Water Management