SPILLWAYS 225
Although, in some of the older shaft spillways placed well away from
the banks and operating under free-flow conditions, anti-vortex devices
were also provided, it is in fact advantageous in these cases to induce a
vortex at the inflow by placing low curved vanes below the spillway crest.
These vanes do not measurably reduce the discharge, but by inducing
spiral flow along the shaft walls they substantially reduce vibration and
pressure fluctuations induced by an otherwise free-falling jet, particularly
at low (and thus frequent) discharges.
As for free flow, Q"H3/2, whereas for submerged flow even a small
increase in Qresults in a substantial increase of the required head H(Fig.
4.13), for safety shaft spillways are usually designed to operate only as
free-flowing, i.e. with Hs/DsH/Dc0.225. For values of H/Dc0.1 and
close to a bank, a wide-crested spillway may be advantageous.
The tunnel below shaft spillways can be designed for four different
flow régimes shown in Fig. 4.17, depending on the relative position of the
tunnel soffit and the downstream water level. The configuration in Fig.
4.17(d) is the most favourable one from the point of view of stability of
flow in the tunnel and prevention of vibrations. The aerated transition
from the shaft bend into the tunnel is an important feature of the design.
Its purpose is to establish a control section for the shaft and bend and, at
the same time, to stabilize the free or pressure flow (with a stable jump) in
the tunnel. For some further design features and details of shaft spillways,
see Novak and Cˇábelka (1981), Haindl (1984), US Bureau of Reclamation
(1987), Vischer and Hager (1998) and Ervine (1998).
Fig. 4.17 Shaft spillway and tunnel configurations (after Novak and
Cˇábelka, 1981)