Hydraulic Structures: Fourth Edition

to reduce and localize the scour (in a position where it can be controlled

and is not harmful to the dam), not to eliminate it. The end sill of the basin

has to be protected against retrogressive erosion and/or designed in such a

way as to encourage sediment accumulation against the sill rather than

erosion (Fig. 5.4).

The extent and depth of the local scour depend on hydraulic para-

meters, geological structure (erodibility index (Annandale, 1995)) and

basin geometry. Several methods are in use for its computation including

model studies, but perhaps the simplest is to estimate the scour depth as a

percentage of the depth which would occur at the foot of a free overfall

without a basin; this in turn can be computed from several equations

(Schoklitsch, Veronese, Jaeger, etc.). Using Jaeger’s form Novak (1955)

expresses the scour depth (downstream of hydraulic jump basins as deter-

mined from model experiments of relatively low head structures – say

S10 m – with coarse sand and limited field observations) as

ys0.55[6H*0.25q0.5(y 0 /d 90 )1/3 y 0 ] (5.15)

whereysis the scour depth below the river bed (m), H*is the difference

between upstream and downstream water levels (m), y 0 is the tailwater

depth (m), qis the specific discharge (m^2 s^1 ), and d 90 is the 90% grain size

of sediment forming the river bed (mm). Equation (5.15) thus indicates

that the stilling basin reduces the potential scour by 45–50%.

For erosion downstream of an apron see e.g. Dey and Sarkar (2006).

258 ENERGY DISSIPATION

Head drop from reservoir level to

tailwater level, H* (m)

Key

simple hydraulic

jump basins

baffle basins

free trajectory jet dissipators

(^0020406080100120)
2000
4000
6000
8000
10 000
12 000
Discharge capacity of dissipator, Q (m
3 s
1 )
Fig. 5.9 Preferred ranges of use for the main types of dissipators (after
Mason, 1982)