Estuary Processes and Managment - River Dynamics and Integrated River Management - page 519

Fig. 8.61 Examples of diffuser configurations: (a) simple port; (b) variable-area duckbill valve; (c) riser/port configuration;
(d) rosette like riser/port arrangement (after Lee et al., 2001b; Bleninger, 2007)
Outfall tunnels are typically 2 km-5 km in length and are located 30 to 100 m beneath the sea floor. The
risers are often capped with multi-port heads to increase the initial dilution of the sewage plume. A schematic
diagram of a tunneled ocean sewage outfall is shown in Fig. 8.62 (Wilkinson, 1984). Under certain
conditions, it is observed that sea water from the ocean can intrude into the outfall tunnel, with the lighter
sewage discharging through only some of the risers. The problem arises because the seawater has a greater
density than the sewage and a pressure surcharge is required to purge the seawater from the tunnel.
Figure 8.63 shows a tunneled outfall discharging through the landward risers, while sea water enters
through the seaward risers, setting up a circulation.
Fig. 8.62 Schematic diagram of tunneled sewage outfall (after Wilkinson, 1984)
This sea water intrusion is clearly hydraulically inefficient, and may lead to poorly diluted effluent and
violation of water quality standards. In addition, silt and sand may settle out of the sea water and ultimately
constrict the tunnel. The presence of circulating sea water will also promote marine growth on the tunnel
walls which may also lead to tunnel blockage. The reason for the failure of some ocean outfalls to purge
intruded sea water lies in the failure to account for the possible development of a salt water wedge within
the outfall tunnel. In general, intrusion into a diffuser port will not occur if the jet densimetric Froude