The
viability of a proposed site must be confirmed before
any detailed design is undertaken. This can be a staged
process that may include preliminary and feasibility
studies.
Preliminary
Study
The preliminary study can mainly be a ‘desk-top’
exercise but should include the sampling and analysis
of the beach sand. Information to be gathered may include,
- representative sand
samples and photographic survey
- geographical maps
and aerial photos (historical & current)
- geological description
and maps (local & regional)
- any available historical
survey data (ie, beach profiles, drilling logs,
beach water table records)
- any available wind,
wave and current information
- tidal variation
- existing reports
and studies
- general information
on the historical development of the site (natural
& human)
- questionnaire survey
of local residence
- any information
on littoral drift (directions, net & total volumes).
Feasibility
Study
The feasibility study should include a comprehensive
report and may require data from the following activities,
- drilling and sampling
of the beach face
- laboratory analysis
of samples (description, grain size distribution)
- filter screening
on at least one bore hole
- analysis of hydraulic
properties of beach deposits - simple test pumping
of filter screened bore holes (including falling
head tests)
- monitoring of ground-water
and sea level changes
- water sampling and
chemical analysis
- evaluation of the
prevailing hyrogeological conditions
- beach profile survey
to 1m below MSL.
Some additional survey
data may be required to facilitate detailed design of
the pumping station and discharge pipeline.
LOCATION AND DIMENSIONS
The design and installation of a beach drainage
system is site specific. The engineering design requires
evaluation of the interaction between many parameters.
The efficiency of the method depends upon the beach
sand having a permeability that falls within a specific
range to allow drainage at a sufficient rate. Wave climate,
tidal range, slope of beach face, shape of near-shore
bottom profile and quantity of gross littoral drift
also affects system efficiency.
The critical elements for the successful design of a
beach drainage system are,
-
the correct evaluation of the beach material with
regard to the cone of depression for water table
draw-down
-
design, position and depth of the filter drains
-
required flow calculations and subsequent pump and
pipeline sizing.
All
of the above elements are site specific. Common elements
to all beach drainage systems are,
Filter drains. A number of designs
and materials have been successfully employed. Existing
system lengths ranging from 180m to 600m connected to
a single pump station. Flow rates have ranged from 100
to 1400 m3/hr. Pipe diameters have ranged from 50mm
to 450mm at depths of 0.8m to 2.5m below MSL.
Pumping station. These can be very
similar in design to common sewer pump stations. The
required chamber depth will vary depending on total
distance of gravity flow and ground elevation at the
site of the pump station (commonly, 4 to 8m deep). The
pumping arrangement is normally gravity wet well with
pressure discharge piping and submersible electric pumps.
Piping materials can be stainless steel or plastic.
The control systems normally associated with the pump
station are incorporated in a small steel cabinet and
generally include a SCADA system.
Discharge pipeline. Design is dependant
on discharge location (back to sea or to a sheltered
location behind the beach) or whether water utilization
is incorporated in the system.
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