4.7.5 Siphon spillways
Siphon spillways are closed conduits in the form of an inverted U with an
inlet, short upper leg, throat (control section), lower leg, and outlet.
For very low flows a siphon spillway operates as a weir; as the flow
increases, the upstream water level rises, the velocity in the siphon
increases, and the flow in the lower leg begins to exhaust air from the top
of the siphon until this primes and begins to flow full as a pipe, with the
discharge given by
QCdA(2gH)1/2 (4.62)
whereAis the (throat) cross-section of the siphon, His the difference
between the upstream water level and siphon outlet or downstream water
level if the outlet is submerged and
Cd
wherek 1 ,k 2 ,k 3 andk 4 are head loss coefficients for the entry, bend, exit,
and friction losses in the siphon.
Once the siphon is primed the upstream water level falls but the siphon
continues to operate, even though this level may have fallen below the crest
of the siphon; a further increase in the upstream water level produces only a
small increase in the discharge. Priming normally occurs when the upstream
level has risen to about one-third of the throat height. Well-designed siphons
can thus control the upstream water level within fairly close limits.
When the discharge decreases this process is reversed, with a hys-
teresis effect which may cause instabilities; these are reduced by using
multiple siphons with differential crest heights or air-regulated siphons, of
which the design in Fig. 4.18 is an example. This siphon spillway at Spelga
dam (Poskitt and Elsawy, 1976) was built to augment the flow over an
older overfall spillway; there are four batteries of three siphons, each with
a special ‘duck’s bill’ inlet for air regulation.
Another type of air-regulated siphon spillway developed from a
model study has a downward projecting hood with an upstream baffle and
a series of holes which can be selectively sealed to compensate, if neces-
sary, for any scale effects and to ensure the required head-discharge rela-
tionship (Hardwick and Grant, 1997).
At the apex of the siphon the pressure falls below atmospheric, with
the lowest pressure (and greatest danger of cavitation) on the inside of the
bend.
Apart from negative pressures, and stability and regulation dif-
ficulties, siphons have to be protected against blocking by floating debris
and freezing; their main advantage over other types of spillways is the
1
(k 1 k 2 k 3 k 4 )1/2
226 DAM OUTLET WORKS
