Morning Glory Spillway of the Val Noci Dam - Physical Model Investigation (2008)

Morning Glory Spillway of the Val Noci Dam - Physical Model Investigation

Mediterranea delle Acque operates the Val Noci reservoir in Montoggio (Italy) for the water supply of the city of Genoa. The Val Noci reservoir is stored by a 56 m high dam which impounds a volume of 3.3 Mio m³ usable storage. Three existing spillway structures, i.e. a gated chute spillway, a middle and a bottom outlet serve to discharge floods. A recent estimation of the flood hydrograph shows, that it exceeds the installed discharge capacity. To counter this shortage Mediterranea delle Acque plans to erect an additional morning glory spillway. It will use the existing middle outlet tunnel for a cost saving design.

The performance of the morning glory spillway had to be verified. In the physical model with a scale factor of 25 the project design as handed in by the client was investigated first. These model tests on the original design showed that, by the transition from free to pressurized flow condition in the vertical shaft, the up scaled minimum pressures drop below the vapour pressure, resulting in a change of phase from water to vapour, i.e. water column separation. This phenomenon would cause strong vibrations at prototype site and consequently structural damage and limited spillway capacity.

VAW optimised the original design of the morning glory spillway by widening the vertical shaft and installing a throttled bottom bend. Furthermore, VAW added overflow piers at the morning glory crest to ensure radial flow in the overflow structure avoiding air-entraining vortex formation. Thereby, the required discharge capacity of 75.1 m³/s is ensured and extreme pressure fluctuations in the vertical shaft are limited. An air supply system is installed behind the throttle to ensure free aeration of the spillway tunnel.

The model tests were completed in the autumn 2007.