Part C | 32.3
722 Part C Coastal Design
Information needed for the outfall configuration includes:
Water depth in which the diffuser section is located.
Length of the diffuser section.
Configuration of the diffuser (e.g., a single or multiport outlet).
Diameter of each outlet port.
Whether the outlet ports are fitted with nonreturn
check valves.
Wastewater Flow
Wastewater flow is usually measured in the outlet pipe
at the end of the treatment processes. A range of flow
measuring devices are available including flows based
on electromagnetic, pressure, ultrasonic, or capacitance
sensors. Also important is the density of the wastewater in relation to the density of the marine waters into
which the wastewater is discharged. Usually it is safe to
assume that the density of the wastewater is close to that
of fresh water [32.2], although large amounts of particulate material in the wastewater may alter the density of
the wastewater.
For numerical modeling purposes, wastewater flow
is usually assumed to be uniform throughout each outlet
port. This may not be necessarily the case. Energy loss
may be significant over long diffuser sections, resulting
in reduced flows through outlets lying further offshore.
Low flows may result in the intrusion of seawater into
the diffuser and a reduction in its performance. To help
establish uniform flow, diffuser sections may be tapered
(Fig. 32.1) and to help prevent the intrusion of seawater,
outlet ports may be fitted with nonreturn check valves.
Currents
Currents determine the movement and dilution of the
wastewater. Often a moored Doppler profiler is used
to measure the current speed and direction throughout
the water column. Doppler profilers can also be ship
mounted, which allows a spatial picture of the currents
to be obtained. Remote sensing and shore-based radar
systems can provide detailed spatial coverage of the
surface currents. However, it is subsurface current data
which is critical for running the near-field models.
The choice of mooring location should be as close
as possible to the diffuser. However, a compromise is
often made, balancing the proximity of the diffuser to
the mooring, with the health of workers who service the
mooring (in waters that may be contaminated with diluted wastewater) and the security of the mooring itself.
Stratification
Stratification is a rapid vertical change in the density
of the marine waters. In coastal waters, changes in the
density are dominated by changes in temperature and
salinity. The height to which a wastewater plume rises
in the water column is largely governed by the strength
of the stratification.
Measurements of temperature and salinity are often
made using a conductivity, temperature, depth (CTD)
probe. (Salinity is calculated from conductivity and
temperature). The CTD probe can be lowered from
a boat providing a continuous profile through the water column. CTD probes can also be moored, thereby
providing a time series of density data at a fixed point.
Historically, conductivity data from moored CTDs drift
with time due to the gradual build-up of biofilm on
the sensors. While there have been substantial improvements in the reliability of moored conductivity sensors
in recent years, the quality of the data may still be
highly variable. Temperature sensors (unless heavily
fouled with marine growth) do not suffer the same problem. Hence changes in stratification using data from
long-term moored systems are usually estimated from
temperature sensors alone.
32.3.7 Conceptual Design
for Positively Buoyant Discharges
Wilkinson [32.23] described a method by which the
minimum length of a simple outfall could be determined. In his concluding remarks, Wilkinson [32.23]
was careful to point out that this provides a preliminary
estimate only, and he provided some suggestions on
ways in which the outfall configuration could be further
refined. This analysis is only intended as a starting point
for outfall design. Detailed analyses are site specific and
must be undertaken for final design. Some site specific
factors include: the bathymetry, environmental guidelines, level of wastewater treatment, and the likelihood
of plumes reaching the surface or sensitive ecological
areas. Wilkinson’s [32.23] approach is modified here, by
using the single set of equations (Table 32.2) and introducing construction cost as criteria for outfall design.
The following analysis is applicable to nonzero ambient currents, which is usually applicable to marine
waters (e.g., currents near the Sydney, Australia deep
water ocean outfalls exceed 0:05 m=s more than 90%
of the time).
The total cost (T c ) of a marine outfall can be expressed as
T c D lL p C mL D C nn ports
(32.1)
where l is cost per meter of the outfall pipeline or tunnel [$=m], L p D length of the outfall pipeline or tunnel
[m], m is the cost per meter of the diffuser [$=m], L D
722 Part C Coastal Design
Information needed for the outfall configuration includes:
Water depth in which the diffuser section is located.
Length of the diffuser section.
Configuration of the diffuser (e.g., a single or multiport outlet).
Diameter of each outlet port.
Whether the outlet ports are fitted with nonreturn
check valves.
Wastewater Flow
Wastewater flow is usually measured in the outlet pipe
at the end of the treatment processes. A range of flow
measuring devices are available including flows based
on electromagnetic, pressure, ultrasonic, or capacitance
sensors. Also important is the density of the wastewater in relation to the density of the marine waters into
which the wastewater is discharged. Usually it is safe to
assume that the density of the wastewater is close to that
of fresh water [32.2], although large amounts of particulate material in the wastewater may alter the density of
the wastewater.
For numerical modeling purposes, wastewater flow
is usually assumed to be uniform throughout each outlet
port. This may not be necessarily the case. Energy loss
may be significant over long diffuser sections, resulting
in reduced flows through outlets lying further offshore.
Low flows may result in the intrusion of seawater into
the diffuser and a reduction in its performance. To help
establish uniform flow, diffuser sections may be tapered
(Fig. 32.1) and to help prevent the intrusion of seawater,
outlet ports may be fitted with nonreturn check valves.
Currents
Currents determine the movement and dilution of the
wastewater. Often a moored Doppler profiler is used
to measure the current speed and direction throughout
the water column. Doppler profilers can also be ship
mounted, which allows a spatial picture of the currents
to be obtained. Remote sensing and shore-based radar
systems can provide detailed spatial coverage of the
surface currents. However, it is subsurface current data
which is critical for running the near-field models.
The choice of mooring location should be as close
as possible to the diffuser. However, a compromise is
often made, balancing the proximity of the diffuser to
the mooring, with the health of workers who service the
mooring (in waters that may be contaminated with diluted wastewater) and the security of the mooring itself.
Stratification
Stratification is a rapid vertical change in the density
of the marine waters. In coastal waters, changes in the
density are dominated by changes in temperature and
salinity. The height to which a wastewater plume rises
in the water column is largely governed by the strength
of the stratification.
Measurements of temperature and salinity are often
made using a conductivity, temperature, depth (CTD)
probe. (Salinity is calculated from conductivity and
temperature). The CTD probe can be lowered from
a boat providing a continuous profile through the water column. CTD probes can also be moored, thereby
providing a time series of density data at a fixed point.
Historically, conductivity data from moored CTDs drift
with time due to the gradual build-up of biofilm on
the sensors. While there have been substantial improvements in the reliability of moored conductivity sensors
in recent years, the quality of the data may still be
highly variable. Temperature sensors (unless heavily
fouled with marine growth) do not suffer the same problem. Hence changes in stratification using data from
long-term moored systems are usually estimated from
temperature sensors alone.
32.3.7 Conceptual Design
for Positively Buoyant Discharges
Wilkinson [32.23] described a method by which the
minimum length of a simple outfall could be determined. In his concluding remarks, Wilkinson [32.23]
was careful to point out that this provides a preliminary
estimate only, and he provided some suggestions on
ways in which the outfall configuration could be further
refined. This analysis is only intended as a starting point
for outfall design. Detailed analyses are site specific and
must be undertaken for final design. Some site specific
factors include: the bathymetry, environmental guidelines, level of wastewater treatment, and the likelihood
of plumes reaching the surface or sensitive ecological
areas. Wilkinson’s [32.23] approach is modified here, by
using the single set of equations (Table 32.2) and introducing construction cost as criteria for outfall design.
The following analysis is applicable to nonzero ambient currents, which is usually applicable to marine
waters (e.g., currents near the Sydney, Australia deep
water ocean outfalls exceed 0:05 m=s more than 90%
of the time).
The total cost (T c ) of a marine outfall can be expressed as
T c D lL p C mL D C nn ports
(32.1)
where l is cost per meter of the outfall pipeline or tunnel [$=m], L p D length of the outfall pipeline or tunnel
[m], m is the cost per meter of the diffuser [$=m], L D