DESIGN OF ROCKFILL SPILLWAYS ON OVERTOPPABLE EARTHDAMS

An overtoppable section with a rockfill protection layer is for dams and levees a reasonable alternative to conventional spillways made of concrete. Failure of the spillway and hence of the dam occurs when the rocks of the spillway start to be eroded or the whole protection layer is sliding. To evaluate the risk of erosion, the complex flow over and through the rockfill spillway associated with air entrainment as well as the hydrodynamic pressures must be investigated, influenced by a lot of parameters such as size and form of the rocks as well as their structural performance. Especially in case of supercritical flow conditions, high positive and negative hydrodynamic pressures as well as the forces show a large spatial and temporal variability. Hence the design of those spillways is subject to a large uncertainty. At the Institute of Hydraulic Engeneering of Stuttgart University large scale experiments on rockfill spillway sections with different slopes from 1:25 to 1:3 have been realised to evaluate the stability and the hydraulic loading of rockfill spillways with the aim to find new design criteria. Therefore, regular placed stones as well as loose stone layers with a different thickness have been tested with specific discharges up to 0,350 m³/sm. Measurements of the hydrodynamic pressures and the flow velocities give an idea about the magnitude of the pressure fluctuations. Objective is to propose design standards which help to manage the uncertainty by specifying the safety as the ratio of failure load/actual load and which enable to dimension rockfill spillways by using a certain safety coefficient.