Estuary Processes and Managment - River Dynamics and Integrated River Management - page 518

jet configuration is still dominant. By contrast, the prediction using an actual source (AS) approach (with
the volume and pollutant mass sources introduced at the point of discharge) results in significant
over-prediction of upstream buoyant spread.
Fig. 8.60 Computed surface tracer concentration field (in units of 0.01 C 0 ) and observed surface field for a finite
line plume in a perpendicular cross flow ( F = Froude number, C 0 = effluent tracer concentration)
8.5.3 Diffuser Design
Initial dilution is determined by the choice of the outfall location and length. In addition, the impact on the
receiving water is strongly affected by the diffuser configuration (number of jets and risers, jet orientation,
riser geometry details, diffuser orientation and length). The optimal design of an outfall diffuser is an
unavoidable step in the planning of a sewage disposal scheme. Theory and practice have shown that the
multiport diffuser is an efficient device that is able to achieve rapid dilution of the effluent discharge
through a large number of orifices with high flow rates (Jirka, 1982). The diffuser could have a simple
port configuration installed in the wall of the pipe (Fig. 8.61(a)); some additional elements (e.g., duckbill-
shaped elastomer valve, Fig. 8.61(b) could be installed to improve the performance of diffuser. If diffusers
are covered with ballast, laid in a trench or even tunneled in the ocean floor, vertical risers (riser/port
configuration, Fig. 8.61(c) need to be added. For deep-tunneled solutions rosette-like port arrangements
(Fig. 8.61(d)) are often used to save on the number of costly riser installations (Bleninger, 2007).
Sea water intrusion into tunneled outfalls —Ocean sewage outfalls on exposed high energy coastlines
are often most economically constructed by tunneling beneath the sea floor and discharging the sewage
into the ocean through diffusers consisting of a series of vertical risers. Systems of this type have been
constructed at Hastings, Weymouth, and Aberdeen (U.K.), San Francisco, Sydney, Shanghai, and Hong Kong.