Hydraulic Structures: Fourth Edition

SEA OUTFALLS 661

p 3 andV 3 are respectively the pressure and velocity in the diffuser at the
section immediately downstream of junction 3 (Fig. 15.14). p 2 andV 2 refer
respectively to pressure and velocity occurring upstream of junction 2
(Fig. 15.14). ∆Z 23 is the change in elevation. hf23is the frictional head loss
in the straight pipe and hcis the head loss in the contraction which is
usually negligible. Z 2 andZ 3 are the elevations of junctions 1 and 2 respec-
tively (Fig. 15.14).
The pressure across the junction (Fig. 15.14) is determined using the
energy equation which, as an example, for junction 2 is

k 2. (15.54)

Note that

Q 2 Q 2 Qp2 (15.55)

whereQ 2 ,Q 2 andQp2are respectively the discharges upstream and down-
stream of the junction and through the port.
The analysis of flow in the diffuser is a step-by-step procedure start-
ing from the extreme seaward end. The calculation procedure for the wall
ports (or ports with risers) is as follows.

1. Estimate E 1 relevant to port 1 in the diffuser at the junction with the
   first port for the discharge Qp1through the first port, using equation
   (15.49); determine p 1.


2. Determine the pressure p 2 just downstream of junction 2 from equa-
   tion (15.53).


3. Solve equations (15.54) and (15.55) or (15.50) to find the pressure p 2
   just upstream of the junction and the flow Qp2through the second
   port.


4. Continue the procedure to port 3 and so on.


5. Check whether the sum of the discharges through all the ports is the
   same as the design discharge.

The calculation procedure can be carried out using computer programs.
It is difficult to avoid at all times deposition of sediments in the
outfall pipe and the diffuser (at low flows) as a small pipe diameter
increasing the flow velocity would result in large energy losses and greater
pump power requirements. Usually some sediments are allowed to deposit
at low flows to be flushed out at high flows. Generally velocities in
excess of about 0.9 m s^1 will be required at high flows. For further
information on sediment transport in sewers and outfall see Novak
and Nalluri (1987) and Ackers (1991). In order that the velocities along
the diffuser are sufficiently large to dislodge the sediments from the bed

V 2 ^2



2 g

V 2 ^2



g

p 2



g

V^22



2 g

p 2



g