Keynote Speaker

In today’s spillway design of dams there is a tendency of increasing the unit discharge of high-velocity jets leaving the appurtenant water release structures. For gated chute flip bucket (ski jump) spillways, unit discharges between 200 and 300 m 3 /s/m are not rare anymore, since the cavitation risk is mitigated by chute bottom aerators. Crest spillways for arch dams are currently designed for unit discharges from 70 m 3 /sm to 120 m 3 /sm by installing crest gates. With the latest high-pressure gate technology, mid- and low-level orifice spillways evacuate unit discharges up to 400 m ³ /s.m. Scour of rock is of concern when assessing the effect of jets released from spillways plunging onto rock foundations downstream of dams or designing plunge pools. Rock scour is a complex problem studied extensively experimentally but often in a simplified way by using granular material. Nevertheless, reliable scour prediction in rock must consider the relevant physical processes when fluctuating turbulent pressures of highly aerated flow in the plunge pool are propagating into fractured rock masses resulting in a strong fluid-air-rock interaction. In modern scour evaluation methods, the potential and extent of rock scour is determined by employing the principles of fracture mechanics and Newton’s second law. Continuing research into various aspects of turbulent flow and its interaction with rock formations refine this procedure. The main difficulties encountered when estimating scour depths are discussed, including the choice of the appropriate theory for rock scour assessment, interpretation of hydraulic model tests and prototype observations, the scour rate, and the prevailing discharge. The selection of the flood return period is addressed, for which the scour formation and the control measures have to be evaluated, as also options of measures for scour control. Finally, the challenges for dam designers are presented when answering relevant questions reading dam safety and powerhouse operation.