Marine Outfalls 32.4 Hydraulic Analysis and Design 729
Part C | 32.4
One option is to use variable-orifice nozzles such as the
duckbill valves described in Sect. 32.4.3.
Another option is to use rosette style outfall structures. A similar analysis of flow conditions through
a manifold configured with rosettes shows that port discharges can be more evenly distributed. This better flow
balance occurs because head loss coefficients along the
manifold are nearly uniform along its length, provided
the friction losses and local losses between the conduit
and risers are kept low. In this case, flow rates into each
rosette will be nearly equal. Furthermore, the flow rate
through each port on a given rosette structure will be
the same for axisymmetric rosette designs because the
total head loss coefficient will be the same regardless
of which port the effluent flows through. Rosettes can
have any number of nozzles, however, there tends to be
an upper limit beyond which the additional nozzles will
impede individual jet mixing processes. This behavior
occurs because neighboring jets tend to coalesce due to
reduced pressures caused by entrainment of the ambient water between them. The result is to reduce overall
dilution performance. Near-field modeling can be used
to determine the maximum number of nozzles for a specific rosette structure configuration.
32.4.3 Flow Variability
The wide range of flows that may be experienced at
treatment plants is one of the greatest challenges in
designing an outfall system. The preceding discussion
assumes that the nozzle diameters have been sized to
provide the required dilution for the maximum design
flow rate, while at the same time maintaining sufficient
velocity to prevent salt water intrusion at the ports and
avoiding problems arising from sedimentation. In reality, treatment plants operate over a wide range of flows,
and it is quite possible that the design flow might not
be achieved until many years after the plant is initially
constructed. In addition, plant start-up and ramp-down
operations as well as any major changes to process
flow requirements could create situations in which the
plant will operate well below the design flow rate. The
frequency and duration of any such events that result in low dilutions should be considered. If low flow
scenarios are likely to result in sustained periods of insufficient dilution, the following mitigation schemes for
managing low-flow scenarios can be used. Each of these
options has advantages and disadvantages, and a hybrid
solution may turn out to be the most suitable approach.
Supplemental Flows
For desalination plants, a connection between the intake and outfall systems can be incorporated in the plant
design to provide a way for seawater to be bypassed
directly into the outfall system during periods of low
outlet flow. The bypassed seawater effectively increases
the outlet flow rate and nozzle exit velocity, thereby
providing additional momentum for dilution by mixing.
The bypassed seawater also acts as an initial diluting
agent before effluent is discharged into the sea; as such,
adequate levels of dilution may be obtained with lower
nozzle exit velocities for a given brine flow. The amount
of bypassed seawater required for any particular combination of outlet flow rate and salinity can be determined
using results of the near-field modeling and basic conservation principles.
Although this option provides a near instantaneous
level of control for managing the outfall system (when
the appropriate flow control devices are included in the
design), the addition of bypassed seawater will lead to
increased energy use and operating costs due to additional pumping of seawater. These costs should be
considered when deciding whether this option is an appropriate strategy for managing low flows. Depending
on the frequency and amount of bypassing required, this
solution may, in fact, be found to be the most practical
or cost effective when compared with other alternatives.
The incorporation of a seawater bypass system also provides a way to commission both the marine intake and
outfall systems independently from the rest of the plant.
Staged Construction of Marine Works
Many treatment plants are designed to be initially operated at a low flow rate, then periodically upgraded until
the maximum design capacity is attained. To manage
the low flows associated with early stage plant operation, the marine construction works can be staged such
that only some of the nozzles or rosette structures required for the ultimate flow case are initially installed.
Additional nozzles, diffuser structures, or sections of
a line diffuser can be installed each time the plant is
upgraded; near-field modeling will inform the number
of nozzles to be installed at each stage.
The main disadvantage with this solution is that additional marine construction works are required whenever the plant is upgraded. Moreover, it assumes that
once the plant is upgraded to ultimate capacity, it will
not operate at the lower early stage flows. This assumption may not be valid if demand can fall below the
plant’s maximum capacity after it is upgraded (for example, lower demand from a desalination plant due to
a temporary period of increased rainfall). This solution
also does not address periods of low flow that will occur
even for ultimate capacity when the plant ramps up or
shuts down due to operations or maintenance requirements. These periods of low flows may not be an issue
if they occur infrequently and for short durations, and if
environmental licences allow for temporary periods of
Part C | 32.4
One option is to use variable-orifice nozzles such as the
duckbill valves described in Sect. 32.4.3.
Another option is to use rosette style outfall structures. A similar analysis of flow conditions through
a manifold configured with rosettes shows that port discharges can be more evenly distributed. This better flow
balance occurs because head loss coefficients along the
manifold are nearly uniform along its length, provided
the friction losses and local losses between the conduit
and risers are kept low. In this case, flow rates into each
rosette will be nearly equal. Furthermore, the flow rate
through each port on a given rosette structure will be
the same for axisymmetric rosette designs because the
total head loss coefficient will be the same regardless
of which port the effluent flows through. Rosettes can
have any number of nozzles, however, there tends to be
an upper limit beyond which the additional nozzles will
impede individual jet mixing processes. This behavior
occurs because neighboring jets tend to coalesce due to
reduced pressures caused by entrainment of the ambient water between them. The result is to reduce overall
dilution performance. Near-field modeling can be used
to determine the maximum number of nozzles for a specific rosette structure configuration.
32.4.3 Flow Variability
The wide range of flows that may be experienced at
treatment plants is one of the greatest challenges in
designing an outfall system. The preceding discussion
assumes that the nozzle diameters have been sized to
provide the required dilution for the maximum design
flow rate, while at the same time maintaining sufficient
velocity to prevent salt water intrusion at the ports and
avoiding problems arising from sedimentation. In reality, treatment plants operate over a wide range of flows,
and it is quite possible that the design flow might not
be achieved until many years after the plant is initially
constructed. In addition, plant start-up and ramp-down
operations as well as any major changes to process
flow requirements could create situations in which the
plant will operate well below the design flow rate. The
frequency and duration of any such events that result in low dilutions should be considered. If low flow
scenarios are likely to result in sustained periods of insufficient dilution, the following mitigation schemes for
managing low-flow scenarios can be used. Each of these
options has advantages and disadvantages, and a hybrid
solution may turn out to be the most suitable approach.
Supplemental Flows
For desalination plants, a connection between the intake and outfall systems can be incorporated in the plant
design to provide a way for seawater to be bypassed
directly into the outfall system during periods of low
outlet flow. The bypassed seawater effectively increases
the outlet flow rate and nozzle exit velocity, thereby
providing additional momentum for dilution by mixing.
The bypassed seawater also acts as an initial diluting
agent before effluent is discharged into the sea; as such,
adequate levels of dilution may be obtained with lower
nozzle exit velocities for a given brine flow. The amount
of bypassed seawater required for any particular combination of outlet flow rate and salinity can be determined
using results of the near-field modeling and basic conservation principles.
Although this option provides a near instantaneous
level of control for managing the outfall system (when
the appropriate flow control devices are included in the
design), the addition of bypassed seawater will lead to
increased energy use and operating costs due to additional pumping of seawater. These costs should be
considered when deciding whether this option is an appropriate strategy for managing low flows. Depending
on the frequency and amount of bypassing required, this
solution may, in fact, be found to be the most practical
or cost effective when compared with other alternatives.
The incorporation of a seawater bypass system also provides a way to commission both the marine intake and
outfall systems independently from the rest of the plant.
Staged Construction of Marine Works
Many treatment plants are designed to be initially operated at a low flow rate, then periodically upgraded until
the maximum design capacity is attained. To manage
the low flows associated with early stage plant operation, the marine construction works can be staged such
that only some of the nozzles or rosette structures required for the ultimate flow case are initially installed.
Additional nozzles, diffuser structures, or sections of
a line diffuser can be installed each time the plant is
upgraded; near-field modeling will inform the number
of nozzles to be installed at each stage.
The main disadvantage with this solution is that additional marine construction works are required whenever the plant is upgraded. Moreover, it assumes that
once the plant is upgraded to ultimate capacity, it will
not operate at the lower early stage flows. This assumption may not be valid if demand can fall below the
plant’s maximum capacity after it is upgraded (for example, lower demand from a desalination plant due to
a temporary period of increased rainfall). This solution
also does not address periods of low flow that will occur
even for ultimate capacity when the plant ramps up or
shuts down due to operations or maintenance requirements. These periods of low flows may not be an issue
if they occur infrequently and for short durations, and if
environmental licences allow for temporary periods of