Hydraulic Structures: Fourth Edition

Sm for y 0 ' (15.45)

and

BQpg.

For small values of y 0 /Dp, the results of Agg and Wakeford are comparable

with equations (15.44) and (15.45) but, for y 0 /Dpbetween 50 and 100, they

underpredict the initial dilution (Neville Jones and Dorling, 1986).

For vertical single ports discharging into moving receiving water,

Muellenhoffet al.(1985) present the following equation for average dilu-

tionSa:

Sa. (15.46)

For further information about dilution in moving fluid, either stratified or

unstratified, refer to Wood et al. (1993) and Roberts (1996).

15.7.6 Spacing of ports

The spacing of the ports depends on the geometry of the plume as it

spreads. Under still water conditions the buoyant rising plumes should

not overlap till they reach the surface. The geometry of the buoyant

plume in still water according to Brooks (1970) is shown in Fig. 15.12.

The dimensionless radial extent of the plume W/Dpat the free surface is

presented in Fig. 15.12 as a function of the densimetric Froude number

FDandy 0 /Dp. The spacing of the ports should be greater than W. In

moving receiving water, the size of the plume is increased with the

increased degree of dilution. Neville-Jones and Dorling (1986) suggest

that for Va/Vp0.2,W0.5y 0 fory 0  2 B/V^3 a. The width W0.9y 0 for

Va/Vp0.2.

15.7.7 Diffuser design

The flow through the outfall may be due to gravity depending on the head

available between the headworks and the highest water level at the dis-

charge location in the sea. If enough head is not available, then the waste-

water is pumped through the outfall.

0.49Vay^20



Qp

(^) a (^0)

(^0)

5 B



V^3 a

0.32Vay^20



Qp

658 COASTAL ENGINEERING