Marine Outfalls 32.3 Predicting Near-Field Dilutions 721
Part C | 32.3
curs in the wastewater plume. One solution is to take
many tracer readings closely separated in space and
time to identify the plume boundaries and the region
of highest tracer concentration.
The fluorescence of rhodamine WT is highly temperature dependent. It loses about 3% of its fluorescence for every one degree Celsius drop in water
temperature. In very cold environments, it may not be
possible to detect a signal at all – as happened to the
authors when first using rhodamine WT in Antarctica.
32.3.5 Far-Field Numerical Modeling
The emphasis in this chapter is on near-field modeling
rather than far-field modeling. The reason for this is because most of the dilution of the discharged wastewater
occurs in the near-field, and environmental guidelines
and licence conditions are usually applied at the end
of the near-field. However, far-field modeling is important when assessing discharges into relatively shallow
waters when mixing in the near-field is incomplete or
when examining potential impacts at sites remote from
the outfall, e.g., beach bathing waters or sensitive marine habitats or communities.
Far-field modeling usually includes hydrodynamic
and water quality components. The hydrodynamic
models are based on the principles of mass and momentum conservation of the marine waters; water quality
models are based on mass considerations of the contaminant(s) or tracer(s) being discharged. Hydrodynamic
models are usually based on a fixed mesh in space (i. e.,
an Eulerian formulation) and produce the depth and velocity fields as output. The water quality models require
the velocity field as input; their formulation may be
based on the same mesh as the hydrodynamic model. In
another formulation (i. e., the Lagrangian formulation),
many parcels of contaminant or tracer may be tracked
as the velocity field transports and disperses them. The
results from an Eulerian formulation yields the contaminant concentrations on the fixed mesh, while the
Lagrangian formulation yields the number of contaminant parcels contained in each volume of fluid bounded
by mesh points or nodes; these numbers can then be
converted into contaminant concentrations.
The most common types of Eulerian models used
are finite difference (i. e., point-wise approximations of
the variables), finite element (piecewise approximations
of the variables), or finite volume (based on fluxes of
mass or momentum within each mesh cell). The meshes
can be regular (i. e., structured) or irregular (i. e., nonstructured); they can be 2-D (2-dimensional; i. e., depth
averaged) or 3-D (three-dimensional).
In a 2-D, hydrodynamic model the mesh is in
the horizontal plane. At each mesh point or node,
the unknowns consist of two velocity components and
a depth. In a 3-D model, the mesh includes the 2-D horizontal plane and the mesh points or nodes in the vertical
dimension. The unknowns at each 3-D mesh point or
node, typically consist of two horizontal velocity components and pressure.
Usually, 2-D models require substantially less computational time and less data for calibration and running
than 3-D models. In water depths exceeding about 20 m,
the velocity vector at a single location in plan, may
vary in magnitude and direction throughout the water
column. If resolution of this variability is considered
significant from the point of view of pollutant movement, a 3-D model may be preferred to a 2-D model.
The horizontal spacing between mesh points or nodes
will depend on the bathymetry and the presence of
islands, headlands, and submarine canyons. The nearfield model results need to be incorporated into the
far-field model. To achieve this effectively, it may be
desirable to refine the far-field mesh in the vicinity of
the near-field.
Far-field models run under various flow scenarios
can be used in the early stages of an investigation to
guide data collection programs before, during, or after
commissioning of an outfall. Such model studies may
be conducted using a coarse mesh for quick turnaround
of results.
32.3.6 Data for Running the Models
A range of information is required to run the numerical models. This includes: the outfall configuration,
wastewater flow, and oceanographic data (currents and
stratification of the water column). Over the long term,
the model results can be used to examine changes in
outfall performance. Below is a summary of the information required to run the models and how that
information may be obtained.
Outfall Configuration
The concept outlined in the following section can provide a starting point for the outfall design. In the
design phase, the outfall configuration can be changed
and refined until the relevant environmental guidelines
are met and engineering feasibility assessed. Once the
outfall has been constructed, its configuration is essentially fixed. However, some flexibility may be enabled.
For example, twin pipelines may be built and only
one pipeline used for present wastewater flows (the
second pipeline being saved for use when wastewater flow increases with future growth in population).
Similarly, a multiport diffuser may have one or more
outlet ports blanked, again in anticipation of future
growth.
Part C | 32.3
curs in the wastewater plume. One solution is to take
many tracer readings closely separated in space and
time to identify the plume boundaries and the region
of highest tracer concentration.
The fluorescence of rhodamine WT is highly temperature dependent. It loses about 3% of its fluorescence for every one degree Celsius drop in water
temperature. In very cold environments, it may not be
possible to detect a signal at all – as happened to the
authors when first using rhodamine WT in Antarctica.
32.3.5 Far-Field Numerical Modeling
The emphasis in this chapter is on near-field modeling
rather than far-field modeling. The reason for this is because most of the dilution of the discharged wastewater
occurs in the near-field, and environmental guidelines
and licence conditions are usually applied at the end
of the near-field. However, far-field modeling is important when assessing discharges into relatively shallow
waters when mixing in the near-field is incomplete or
when examining potential impacts at sites remote from
the outfall, e.g., beach bathing waters or sensitive marine habitats or communities.
Far-field modeling usually includes hydrodynamic
and water quality components. The hydrodynamic
models are based on the principles of mass and momentum conservation of the marine waters; water quality
models are based on mass considerations of the contaminant(s) or tracer(s) being discharged. Hydrodynamic
models are usually based on a fixed mesh in space (i. e.,
an Eulerian formulation) and produce the depth and velocity fields as output. The water quality models require
the velocity field as input; their formulation may be
based on the same mesh as the hydrodynamic model. In
another formulation (i. e., the Lagrangian formulation),
many parcels of contaminant or tracer may be tracked
as the velocity field transports and disperses them. The
results from an Eulerian formulation yields the contaminant concentrations on the fixed mesh, while the
Lagrangian formulation yields the number of contaminant parcels contained in each volume of fluid bounded
by mesh points or nodes; these numbers can then be
converted into contaminant concentrations.
The most common types of Eulerian models used
are finite difference (i. e., point-wise approximations of
the variables), finite element (piecewise approximations
of the variables), or finite volume (based on fluxes of
mass or momentum within each mesh cell). The meshes
can be regular (i. e., structured) or irregular (i. e., nonstructured); they can be 2-D (2-dimensional; i. e., depth
averaged) or 3-D (three-dimensional).
In a 2-D, hydrodynamic model the mesh is in
the horizontal plane. At each mesh point or node,
the unknowns consist of two velocity components and
a depth. In a 3-D model, the mesh includes the 2-D horizontal plane and the mesh points or nodes in the vertical
dimension. The unknowns at each 3-D mesh point or
node, typically consist of two horizontal velocity components and pressure.
Usually, 2-D models require substantially less computational time and less data for calibration and running
than 3-D models. In water depths exceeding about 20 m,
the velocity vector at a single location in plan, may
vary in magnitude and direction throughout the water
column. If resolution of this variability is considered
significant from the point of view of pollutant movement, a 3-D model may be preferred to a 2-D model.
The horizontal spacing between mesh points or nodes
will depend on the bathymetry and the presence of
islands, headlands, and submarine canyons. The nearfield model results need to be incorporated into the
far-field model. To achieve this effectively, it may be
desirable to refine the far-field mesh in the vicinity of
the near-field.
Far-field models run under various flow scenarios
can be used in the early stages of an investigation to
guide data collection programs before, during, or after
commissioning of an outfall. Such model studies may
be conducted using a coarse mesh for quick turnaround
of results.
32.3.6 Data for Running the Models
A range of information is required to run the numerical models. This includes: the outfall configuration,
wastewater flow, and oceanographic data (currents and
stratification of the water column). Over the long term,
the model results can be used to examine changes in
outfall performance. Below is a summary of the information required to run the models and how that
information may be obtained.
Outfall Configuration
The concept outlined in the following section can provide a starting point for the outfall design. In the
design phase, the outfall configuration can be changed
and refined until the relevant environmental guidelines
are met and engineering feasibility assessed. Once the
outfall has been constructed, its configuration is essentially fixed. However, some flexibility may be enabled.
For example, twin pipelines may be built and only
one pipeline used for present wastewater flows (the
second pipeline being saved for use when wastewater flow increases with future growth in population).
Similarly, a multiport diffuser may have one or more
outlet ports blanked, again in anticipation of future
growth.