Understanding and controlling the hydraulic behaviour of a surge shaft during a fast unsteady flow event: a major safety issue, to prevent the risk of significant damage to the equipment such as: penstocks, turbines and gates.
A surge shaft plays an essential protective role in the headrace structures of a hydropower scheme on which the upstream underground waterway, excavated through the mountain, is relatively long. This very high vertical shaft absorbs the strong water hammer that occurs when a turbine unit is started up or stopped, and it can also assist during the start of turbining, by reducing the inertia and the hydraulic start-up time on the upstream waterway. .
In particular, during an unsteady flow phenomenon related to a sudden variation in turbine flow rate, water level oscillations in the shaft allow the strong pressure variations that propagate through the tunnel or the penstock to ba absorbed. However, the unsteady hydraulic behaviour of surge shafts can be difficult to comprehend through analytical calculations or numerical modelling if the geometry is especially complex or the structures are operated in a specific manner (overflow or interconnected shafts, for example).
Contribution of the physical model
In this instance, physical modelling is a powerful tool which allows us to replicate hydraulic phenomena accurately and very tangibly using the laws of similitude, by reproducing the extremely rapid variations in flow rate at a reduced scale.
Models of surge shafts are hence rather specific, due for one thing to their exceptional size (a shaft with a true height of 300m can thus be represented by a model around 6m in height!), and also to their highly sophisticated equipment and instrumentation: a system of automated gates, computer-controlled, cuts off the flow by implementing a standard sequence of actions that generally take a few tens of milliseconds.
The amplitude of changes in water level and the speed of rise and fall are measured as a function of the flow cutoff time. The internal hydraulic behaviour is observed and analysed: loss of head at the base of the shaft, overflow capacity, ease of drainage of any horizontal sections, etc.
The geometry of the structure can thus be optimised as a function of the results obtained, in particular near the base of the shaft, where the head losses play an essential role in the overall behaviour, or at the top of the shaft if it is an overflow structure.