Hydraulic Structures: Fourth Edition

SEA OUTFALLS 657

and

Sm0.54FD 0.66

5/3

for '0.5FD. (15.41)

Smis the minimum dilution occurring along the centreline of the plume.
For vertical port discharges, the average plume dilutions Sain the initial
mixing region can be determined by the equations due to Muellnoff et al.
(1985) with Qpbeing the port discharge:

Sa0.13g1/3Qp^ 2/3y 0 5/3

which can be written in non-dimensional form as

Sa0.13

π

4



2/3



5/3

. (15.42)

The average plume dilution Sais approximately 1.8 times the minimum
dilutionSmat the centreline.
Figure 15.13 shows that the same dilution can be obtained for either
low or high exit densimetric Froude numbers. Low velocities mean low
driving heads while large velocities need large heads. The choice of low
velocities can lead to malfunctions of the outfall if deposits of sediments on
the bed occur or if the densimetric Froude number is below the critical limit
of unity when salt water intrusion reducing the effluent flow will develop.

15.7.5 Moving receiving water

When the effluent is discharged into receiving water moving with velocity
Vathe dilution is significantly modified from that in calm water. Field
studies (Agg and Wakeford, 1972) give an equation for an additional dilu-
tion factor
which is a ratio of the dilution in moving water to the dilution
in still water. The empirical equation for
applicable in the range
0.1Va/Vp2.0 is

log
1.1070.938 log(Va/Vp). (15.43)

(The dilution in moving water is determined by multiplying the dilution in
still water of Fig. 15.13 by the factor
.) Lee and Neville-Jones (1987)
present the following equations for dilution in moving water:

Sm for y 0  , (15.44)

5 B



V^3 a

0.31B1/3y 0 5/3



Qp

y 0



Dp

1



FD2/3

y 0



Dp

0.38y 0



DpFD