Part C | 32.5
734 Part C Coastal Design
quired. As such, this option may only be feasible for
removing larger solids. To decrease the basin/tank footprint and increase its efficiency, internal baffle walls can
be installed to lengthen the flow path. However, the resulting effluent velocity should remain sufficiently low
to allow sediments to settle out.
Design for Self-Cleansing Velocities
If upstream sedimentation control is not feasible, the
outfall conduit diameter can be selected to yield velocities that are high enough to prevent deposition of
solid materials. Given the wide flow variability associated with many types of treatment plants, the outfall
conduit diameter should be based on the lowest-flow
scenario. If, however, the chosen diameter leads to excessively high velocities and head losses during peak
flow, it may not be possible to achieve self-cleaning
velocities for all flow conditions. In this case, the design objective should aim for self-cleaning velocities on
a frequent basis (i. e., at least once per day) and for sustained periods of time (i. e., hours) in order to prevent
build-up of sediments and organic matter over time. The
research literature contains a great deal of information
on the velocities and shear stresses required to move
solids through the conduit and to re-suspend solids that
may have already settled. Velocities to keep solids in
suspension range from approximately 0:52:0 m=s depending on conduit diameter, particle size and specific
gravity, and particle concentration [32.33].
32.5 Outfall Construction
An effective design is not possible without input from
those engaged to construct the outfall. The design must
be detailed in conjunction with how the marine outfall
will be built. Grace [32.34, 35] and Wood et al. [32.4]
provide good information on outfall construction approaches and techniques.
There are two fundamental types of wastewater delivery: via tunnels or pipes (either on trestles or in
trenches). They can be used in combination, a tunnel
for the bulk of the outfall length and a pipeline on the
sea bed comprising the outlet ports. The decision is
often governed by the geography of the region – tunnels may be preferred in areas with rocky coastlines
or where beach access is difficult, while pipelines may
be preferred in areas with easy access such as a sandy
shoreline.
32.5.1 Construction Materials
The materials used for outfall construction include steel,
reinforced concrete, and high density polyethylene. The
marine environment is corrosive and steel structures
must be protected by coating the steel to prevent direct
contact with sea water (often concrete is used) and/or using sacrificial anodes. Ongoing inspections are required
to monitor the integrity of the protective coating or the
anodes and replace them as necessary.
Steel reinforcement in concrete is susceptible to corrosion by chloride salts in marine waters. To help in
preventing such corrosion, low permeability concrete
is used, often in conjunction with additives that inhibit
corrosion. Hydrogen sulphide in wastewater is corrosive to concrete and concrete pipes are lined (usually
with plastic) for protection. Regular inspections are undertaken to ensure the lining has not peeled away from
the concrete. The density of concrete is about one-third
that of steel (although still about double that of seawater) and the concrete pipeline may need to be anchored
to the sea floor.
High-density polyethylene is relatively light, with
a density slightly less than that of seawater. Its big advantage is that it is flexible and can be relatively easy
to deploy. Long sections are welded together on shore,
the pipeline then towed into position and anchored to
the sea floor. Purging of air from the pipe is critical to
prevent the pipe from floating back to the surface.
32.5.2 Construction Methods
Pipelines are often manufactured in sections and assembled at an access point close to the proposed line of the
outfall. When in the water, each section is buoyed to facilitate easy movement. Offshore, a vessel is anchored
and pulls a section of the pipe into a trench along the
line of the outfall. The next section is welded on and
the pull continues. This is a popular technique for steel
pipelines, which are very strong in tension. Concrete
blocks or ballast rock may be used to anchor the pipe to
the sea floor.
The trench is then backfilled either mechanically
or by natural means. Techniques have been devised
whereby trenching, pipe-laying and backfilling are all
done in a single operation. The end of the pipeline terminates in a diffuser and the wastewater is discharged
through outlet nozzles.
The active wave climate of the surf zone makes it
the most critical region. Often, a temporary trestle and
sheet piling is used to protect the pipeline through this
region. Sometimes, wave action can damage the trestle
and sheet piling (Fig. 32.11).
734 Part C Coastal Design
quired. As such, this option may only be feasible for
removing larger solids. To decrease the basin/tank footprint and increase its efficiency, internal baffle walls can
be installed to lengthen the flow path. However, the resulting effluent velocity should remain sufficiently low
to allow sediments to settle out.
Design for Self-Cleansing Velocities
If upstream sedimentation control is not feasible, the
outfall conduit diameter can be selected to yield velocities that are high enough to prevent deposition of
solid materials. Given the wide flow variability associated with many types of treatment plants, the outfall
conduit diameter should be based on the lowest-flow
scenario. If, however, the chosen diameter leads to excessively high velocities and head losses during peak
flow, it may not be possible to achieve self-cleaning
velocities for all flow conditions. In this case, the design objective should aim for self-cleaning velocities on
a frequent basis (i. e., at least once per day) and for sustained periods of time (i. e., hours) in order to prevent
build-up of sediments and organic matter over time. The
research literature contains a great deal of information
on the velocities and shear stresses required to move
solids through the conduit and to re-suspend solids that
may have already settled. Velocities to keep solids in
suspension range from approximately 0:52:0 m=s depending on conduit diameter, particle size and specific
gravity, and particle concentration [32.33].
32.5 Outfall Construction
An effective design is not possible without input from
those engaged to construct the outfall. The design must
be detailed in conjunction with how the marine outfall
will be built. Grace [32.34, 35] and Wood et al. [32.4]
provide good information on outfall construction approaches and techniques.
There are two fundamental types of wastewater delivery: via tunnels or pipes (either on trestles or in
trenches). They can be used in combination, a tunnel
for the bulk of the outfall length and a pipeline on the
sea bed comprising the outlet ports. The decision is
often governed by the geography of the region – tunnels may be preferred in areas with rocky coastlines
or where beach access is difficult, while pipelines may
be preferred in areas with easy access such as a sandy
shoreline.
32.5.1 Construction Materials
The materials used for outfall construction include steel,
reinforced concrete, and high density polyethylene. The
marine environment is corrosive and steel structures
must be protected by coating the steel to prevent direct
contact with sea water (often concrete is used) and/or using sacrificial anodes. Ongoing inspections are required
to monitor the integrity of the protective coating or the
anodes and replace them as necessary.
Steel reinforcement in concrete is susceptible to corrosion by chloride salts in marine waters. To help in
preventing such corrosion, low permeability concrete
is used, often in conjunction with additives that inhibit
corrosion. Hydrogen sulphide in wastewater is corrosive to concrete and concrete pipes are lined (usually
with plastic) for protection. Regular inspections are undertaken to ensure the lining has not peeled away from
the concrete. The density of concrete is about one-third
that of steel (although still about double that of seawater) and the concrete pipeline may need to be anchored
to the sea floor.
High-density polyethylene is relatively light, with
a density slightly less than that of seawater. Its big advantage is that it is flexible and can be relatively easy
to deploy. Long sections are welded together on shore,
the pipeline then towed into position and anchored to
the sea floor. Purging of air from the pipe is critical to
prevent the pipe from floating back to the surface.
32.5.2 Construction Methods
Pipelines are often manufactured in sections and assembled at an access point close to the proposed line of the
outfall. When in the water, each section is buoyed to facilitate easy movement. Offshore, a vessel is anchored
and pulls a section of the pipe into a trench along the
line of the outfall. The next section is welded on and
the pull continues. This is a popular technique for steel
pipelines, which are very strong in tension. Concrete
blocks or ballast rock may be used to anchor the pipe to
the sea floor.
The trench is then backfilled either mechanically
or by natural means. Techniques have been devised
whereby trenching, pipe-laying and backfilling are all
done in a single operation. The end of the pipeline terminates in a diffuser and the wastewater is discharged
through outlet nozzles.
The active wave climate of the surf zone makes it
the most critical region. Often, a temporary trestle and
sheet piling is used to protect the pipeline through this
region. Sometimes, wave action can damage the trestle
and sheet piling (Fig. 32.11).