WORKED EXAMPLES 669
RUN-UP
Peak Irribarren number at the structure p(1/3)(2.17/172.0)^ 0.5
2.7.
The correction factor rfrom Table 15.1 is 0.8.
Ru2.17(2.11 0.092.7)0.83.24 m.Worked Example 15.4
Produce a preliminary design for a sea outfall. The dry weather flow
through the outfall is 0.1 m^3 s^1 and the peak flow is 0.5 m^3 s^1. Site investi-
gations show that a sea outfall extending to 2 km seaward would not cause
pollution of foreshore and recreational waters. The still water depth there
is 15 m. The density of sea water is 1026 kg m^3.
If a minimum dilution of 50 is required, estimate the diameter of the
ports, number of ports and the length of the diffuser for still water.
Solution
Cederwall equations (15.40) and (15.41) are used to obtain the following
table (choose FDand compute y 0 /DpforSm50).
FD 01 02 0510131520050
y 0 /Dp 38 49 67 82 88 91 96 100
Dp(m) 00.39 00.31 00.22 00.18 00.17 00.165 00.156 000.15
Vp(m s^1 ) 00.32 00.56 01.2 02.16 02.7 03.1 04.0 009.8
Qp(m^3 s^1 ) 00.038 00.041 00.047 00.057 00.062 00.066 00.076 000.17
The diameter of the ports is chosen as 175 mm which is large enough to
avoid blockage. The exit velocity is about 2.4 m s^1 which is less than the
highest acceptable velocity of 3 m s^1 to avoid undue headloss at the exit.
The discharge through the port of 175 mm diameter0.06 m^3 s^1.
Number of ports is 0.5/0.0610 (say). The flow through the ports at
DWF0.1/100.01 m^3 s^1. The velocity through the ports at DWF
0.01/(π0.175^2 /4)0.4 m s^1.Discharge through the ports at peak flow0.5/100.05 m^3 s^1. Velocity
through the ports at peak flow