2 Design Considerations for Tunnelled Seawater Intakes 25
works to suit an ultimate capacity. Due to the size and cost of tunnels, and in
particular their construction risk, a single intake tunnel (and outfall) is typical. Thus
future operation and maintenance requirements associated with a single intake in
deep water needs to be addressed in the design development. Consideration needs
to be made for the:
• inspection (and maintenance) provisions for the tunnels which are typically
20 m below sea bed and 40 m below sea level
• potential dewatering and safely carrying out inspection and maintenance
• limitations on diver access and ROV technology.
Of note is that since the long seawater intake tunnels in Australia have entered
into service (i.e. from 2008 onwards) there have been advances in monitoring
technology. ROV video cameras are now employed for regular monitoring of
tunnel condition, in particular, for signs of sediment ingress and marine growth.
2.3.3 Intake Design
Intakes need to operate in a way that minimizes marine impacts, particularly
impingement and entrainment of marine life. Key environmental performance cri-
teria for intakes include limitations on maximum intake screen aperture and asso-
ciated screen velocities. Similarly, outfall systems are now required to achieve
stringent environmental performance objectives even when running at reduced
capacity.
The upper limit to velocity through the intake screen bars is typically specified as
an average of 0.10 or 0.15 m/s. While this limit on velocity may have the objective
of protecting marine life or to avoid the intake of sediment, it is a major factor
impacting on the size and weight of the seabed intake structure.
Typical screen bar/aperture spacings are in the range of 50–300 mm and, in
combination with an allowance for marine growth on the bars, can have a signif-
icant impact on the overall size of the intake structure. Thus design needs to balance
the size, constructability (in hostile marine environments) and cost factors versus
long-term operation and maintenance. Considerations include:
• providing an allowance for marine growth, and interpreting any flow-on effects
in the context of meeting project performance specifications, ambient ocean
currents, the specified bar spacing and determining resultant impacts on the
structure size
• use of anti-fouling copper-nickel alloys to suppress marine growth
• planning in situ maintenance regimes and/or design of removable screens
• the prevention or control of marine growth inside the riser and tunnels.
26 P. Baudish
2.3.4 Hydraulic Design
Typically the tunnelled intakes and outfalls in Australia have been designed and
constructed as segmented precast concrete lined tunnels. Minimum practical
diameters for tunneling by TBM are about 2.8 m internal diameter. Diameters larger
than 2.8 m are only constructed if required to suit the hydraulic requirements of an
intake.
Large diameter HDPE pipeline style intakes are typically installed by floating,
sinking and anchoring on the ocean floor. These HDPE pipelines are often designed
to be cleaned by pigging, so that their internal condition can be restored. Piping
provided for biofouling control or for other types of maintenance can be located
external to the pipe. For tunnels small diameter piping is usually installed internal to
the tunnel. Thus, the tunnel is not suited to mechanical cleaning or pigging. Due to
their length and depth and internal fixings, tunnels are likely to be difficult to clean
once in service.
Whilst there is published information on friction co-efficients for segmented
tunnels, these co-efficients are not reflective of long-term friction over the 50 or
100 year tunnel design life of a seawater intake. Even if seawater tunnels have been
designed to enable their dewatering and cleaning, the intention is not to do so. Thus,
the designer must take a view as to the potential long-term changes in hydraulic
characteristics due to marine growth and related performance alteration including:
• potential long term loss of diameter in some or all of the intake conduit
components
• long term roughness factors to be adopted.
These are significant issues as they affect the choice of tunnel diameter, long
term pumping heads, intake pump selection and screen design. Long-term data on
marine growth in long conduits where biocide control is practiced is lacking, but it
is clear that some growth will still occur. The highest rates of growth will occur at
the intake screen, riser and tunnel entry, and then decreasing along the tunnel length
where there is less light.
Intake screens require particular attention for the design of cleaning provisions.
Ideally screens should be removable so that they can be mechanically cleaned above
water. Biocides cannot be used for biofouling control on the screen proper due to risk
of escape into the marine environment. They should be applied at the entry to the
intake riser, with particular attention given to achieving effective mixing.
Other factors that need to be considered in design include:
• effect of tidal variations, swells and storm surges, as well as the potential long
term impacts of climate change
• hydraulic limitations caused by fixed brine nozzles which are quite sensitive to
flow rate and with some flow combinations potentially resulting in overflows of
seawater within the site proper. This may require design mitigation measures
such as spill provision from the brine outlet to the seawater intake
• temperature and salinity.
