Part C | 32.6
736 Part C Coastal Design
increasing the flow may not always be possible). The
addition of salinity sensors on individual risers can confirm whether seawater intrusion has occurred.
Seawater Intrusion and Purging
A rule-of-thumb for preventing seawater from entering the outfall is to keep the port densimetric Froude
number (32.9) well above unity (e.g., on Sydney’s
deepwater ocean outfalls, the port densimetric Froude
number is of the order of 2030). Occasionally, the port
densimetric Froude number may become small and seawater may enter the outfall. One method to help prevent
such an intrusion of seawater is to use nonreturn check
valves. However, not all outfalls are so equipped and
it may be necessary to periodically purge the seawater
from the outfall. This is done by backing up the wastewater in the treatment plant and releasing the wastewater
at high velocity. This also has the advantage of clearing
the outfall of sediments.
Outfall Maintenance
Regular inspections of the marine outfall should be carried out as part of a regular maintenance program to
ensure there is (a) no physical damage to any components and (b) no blockage of any outlet nozzles. This
can be carried out by divers although this is accompanied by substantial health and safety concerns (e.g.,
diving in contaminated waters, wave action, and decompression requirements for deep diving). Our preference
is to carry out inspections using remotely operated vehicles. The frequency of maintenance inspections will
depend on a range of factors such as environmental
conditions and construction materials. Often, annual inspections are adopted.
32.6 Environmental Monitoring
The fundamental objective of environmental monitoring is to quantify impacts that may arise as a result of
the discharge of wastewater to the marine environment.
Some of the questions that an environmental monitoring program should address include:
Is it safe to swim?
Is it safe to eat the seafood?
Will the marine communities be protected into the
future?
Will the beaches be free from contamination?
Two distinct environmental monitoring programs
are often implemented. The first is a pre- and postconstruction monitoring program. This is usually an
intense program of short duration (perhaps five years)
aimed to quantify the initial impacts of the discharges
from the marine outfall. The second is a long-term (ongoing) monitoring program, which usually takes a subset of the information obtained from the pre- and postconstruction monitoring program, and uses it to define
acceptable limits of change in the environmental indicators. Long-term monitoring is designed to determine
whether a subsequent change lies outside these limits.
There is an assumption, often tacit, that the engineering aspects of a sewage treatment system and
marine outfall are operating as designed. If there is
a treatment bypass, breakdown or blockage of some
of the outlet ports, the quality of the effluent (or its
delivery to the marine environment) will be less than
expected. Monitoring the operational performance is an
important element of environmental monitoring.
32.6.1 Change Versus Impact
The marine environment is in a state of continual flux.
The challenge of a monitoring program is to separate
change that occurs via natural processes from change
that is a direct result of the discharge from the marine outfall (i. e., impact). It is critical to define what
constitutes an impact prior to the execution of the postcommissioning monitoring program. This places clear
bounds on the interpretation of change that may be observed in the marine communities.
32.6.2 Pre- and Post-construction
Monitoring
Broadly, the pre- and post-construction monitoring program compares conditions before and after the marine
outfall is commissioned. It is critical that sufficient time
is allowed to carry out the before monitoring. An after
monitoring program can be conducted at any time after commissioning of the marine outfall. However, we
only get one chance at the before phase of the monitoring program.
A monitoring program may be of several years’
duration. Consequently, the cost of an environmental
monitoring program may be large – perhaps 5% of the
construction costs may be needed to properly address
the main environmental issues.
Monitoring Philosophy
Some of the characteristics that constitute a good monitoring program are outlined below. These characteris-
736 Part C Coastal Design
increasing the flow may not always be possible). The
addition of salinity sensors on individual risers can confirm whether seawater intrusion has occurred.
Seawater Intrusion and Purging
A rule-of-thumb for preventing seawater from entering the outfall is to keep the port densimetric Froude
number (32.9) well above unity (e.g., on Sydney’s
deepwater ocean outfalls, the port densimetric Froude
number is of the order of 2030). Occasionally, the port
densimetric Froude number may become small and seawater may enter the outfall. One method to help prevent
such an intrusion of seawater is to use nonreturn check
valves. However, not all outfalls are so equipped and
it may be necessary to periodically purge the seawater
from the outfall. This is done by backing up the wastewater in the treatment plant and releasing the wastewater
at high velocity. This also has the advantage of clearing
the outfall of sediments.
Outfall Maintenance
Regular inspections of the marine outfall should be carried out as part of a regular maintenance program to
ensure there is (a) no physical damage to any components and (b) no blockage of any outlet nozzles. This
can be carried out by divers although this is accompanied by substantial health and safety concerns (e.g.,
diving in contaminated waters, wave action, and decompression requirements for deep diving). Our preference
is to carry out inspections using remotely operated vehicles. The frequency of maintenance inspections will
depend on a range of factors such as environmental
conditions and construction materials. Often, annual inspections are adopted.
32.6 Environmental Monitoring
The fundamental objective of environmental monitoring is to quantify impacts that may arise as a result of
the discharge of wastewater to the marine environment.
Some of the questions that an environmental monitoring program should address include:
Is it safe to swim?
Is it safe to eat the seafood?
Will the marine communities be protected into the
future?
Will the beaches be free from contamination?
Two distinct environmental monitoring programs
are often implemented. The first is a pre- and postconstruction monitoring program. This is usually an
intense program of short duration (perhaps five years)
aimed to quantify the initial impacts of the discharges
from the marine outfall. The second is a long-term (ongoing) monitoring program, which usually takes a subset of the information obtained from the pre- and postconstruction monitoring program, and uses it to define
acceptable limits of change in the environmental indicators. Long-term monitoring is designed to determine
whether a subsequent change lies outside these limits.
There is an assumption, often tacit, that the engineering aspects of a sewage treatment system and
marine outfall are operating as designed. If there is
a treatment bypass, breakdown or blockage of some
of the outlet ports, the quality of the effluent (or its
delivery to the marine environment) will be less than
expected. Monitoring the operational performance is an
important element of environmental monitoring.
32.6.1 Change Versus Impact
The marine environment is in a state of continual flux.
The challenge of a monitoring program is to separate
change that occurs via natural processes from change
that is a direct result of the discharge from the marine outfall (i. e., impact). It is critical to define what
constitutes an impact prior to the execution of the postcommissioning monitoring program. This places clear
bounds on the interpretation of change that may be observed in the marine communities.
32.6.2 Pre- and Post-construction
Monitoring
Broadly, the pre- and post-construction monitoring program compares conditions before and after the marine
outfall is commissioned. It is critical that sufficient time
is allowed to carry out the before monitoring. An after
monitoring program can be conducted at any time after commissioning of the marine outfall. However, we
only get one chance at the before phase of the monitoring program.
A monitoring program may be of several years’
duration. Consequently, the cost of an environmental
monitoring program may be large – perhaps 5% of the
construction costs may be needed to properly address
the main environmental issues.
Monitoring Philosophy
Some of the characteristics that constitute a good monitoring program are outlined below. These characteris-