Open Stone Asphalt Spillways

When reservoir water levels exceed the designed top water level for a dam, water will flow over/in a designed structure and discharged into the watercourse downstream.  One example of this type of structure is a spillweir at the top water level with a spillway on the downstream face of the dam.

The spillway requires erosion protection which is robust enough to withstand the high hydraulic loads but which is also aesthetically acceptable as for the majority of the time the spillway will be dry.  Open Stone Asphalt (OSA) is a suitable material but there was a lack of information on the design of OSA spillways.

With several flood alleviation schemes planned in Germany Karlsruhe University was commissioned to undertake studies on OSA spillways and Hesselberg Hydro was asked to give advice on the mix design, installation routines and material knowledge.

After laboratory studies and tests on scale models the first scheme was installed in 2003. Since then several schemes have been installed. In 2008 Hesselberg Hydro was involved with seawall overtopping tests in the Netherlands. Here, OSA was shown to withstand water velocities of approximately 8.5m/s. The next stage of the research is to commission testing at a facility capable of testing OSA to 10m/s for longer durations.