Hydraulic Structures: Fourth Edition

4.7.6 Stepped spillways

Stepped spillways (ordinary overfall spillways followed by steps on the

spillway face) have been used for a very long time – the earliest was built

around 700 BC (Chanson, 1995). In modern times their use was rather

limited but during recent decades they have been attracting increased

attention mainly because of new material technologies (RCC dams and

prefabricated blocks); they are used mainly for auxiliary but also for main

spillways, where their enhanced energy dissipation contributes to the

economy of overall design. For examples of stepped spillways see Chanson

(1995).

For stepped spillways the crucial problems are the flow régime (nappe

or skimming flow) and the associated questions of air entrainment (and thus

cavitation protection) and energy dissipation. In the skimming flowrégime

water flows over the stepped spillway as a stream skimming the steps and

cushioned by recirculating fluid trapped between them, whereas the nappe

flowwith fully developed hydraulic jumps and a succession of free fall jets is

essentially a flow in a series of drop structures – see Section 10.3.

The early systematic investigation of hydraulic conditions on cas-

cades with a slope 1:1 was carried out by Essery and Horner (1978) who

produced useful design curves for nappe flow.

Denoting the step height hand the length lChanson (1995) showed

that full onset of the skimming flow régime is characterized by a value of

critical depth yc((q^2 /g)1/3) given by

yc/h1.057 0.465h/l. (4.63)

Chamani and Rajaratnam (1999) suggest the equation

h/l0.405(yc/h)^ 0.62 (4.64)

for the upper boundary of nappe flow and

h/l (^0) .8 (^9) 

1
^ 

0.3

4
^1 .5^ ^1 (4.65)
as the lower boundary for the skimming flow. Equation (4.65) gives results
agreeing with experiments for h/l1; for h/l0.8,yc/hwas found to be
almost constant at 0.8 Boes and Hager (2003b) suggest for the onset of
skimming flow:
yc/h0.91 0.14 tan  (4.66)
There is a transition zone between the two flow regimes and any differ-
ences between the experimentally derived equations 4.63–4.66 are mainly
yc

h
yc

h

228 DAM OUTLET WORKS