Hydraulic Structures: Fourth Edition

SEA OUTFALLS 659

The frictional loss in the straight pipe of the outfall leading to the dif-
fuser is calculated using the Darcy–Weisbach equation (equation (8.4))
with friction factor computed by equation (8.5) or the Colebrook–White
equation (Worked example 13.1). During the operation of the outfall,
slime builds up on the wall of the pipe increasing the roughness height and
reducing the effective diameter of the pipe; this should be taken into
account in headloss calculations.
Figure 15.14 shows typically a diffuser with three ports with vertical
risers. An expression for the discharge through a port is obtained by
applying the energy equation between the exit of the port and the centre-
line of the diffuser at the junction. Consider for example, port 2:

 lp2 (kekstkb) (15.47)

wherep 2 andV 2 are the pressure and velocity respectively just before the
junction at 2. lp2is the vertical distance between the port exit and the
centreline of the diffuser and pa2is the ambient pressure at the port exit.
ke,kstandkbare respectively the coefficients for the headlosses at entry to
the riser, along the straight riser and in the bend (Miller, 1994). The dis-
charge through the second port Qp2is then Qp2ap2Vp2.
Wall ports are simply holes through the wall of the diffuser and may
have a rounded or sharp-edged entrance. Equation (15.47) applies with
lp2kstkb0. Therefore for Vp2

V^2 p2



2 g

V^2 p2



2 g

pa2



g

V^22



2 g

p 2



g

Fig. 15.14 Diffuser with risers