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| 3 Collect and evaluate site information |
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| Evaluation
of site conditions |
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Evaluation of site conditions allows identification of key features that influence
the selection of the appropriate techniques for seepage remediation. This may
be part of the process of collecting site information, and should be undertaken
as a distinct and deliberate step in the process, firstly to consolidate knowledge
of the site and secondly to provide focus and direction for the selection of
potential techniques.
Site-specific details are required to assess the suitability of
particular remediation techniques. These include the issues discussed
below and documented in the Site Information Checklist.
Download the Site
Information Checklist: Site_info_checklist_Feb2004.doc (28Kb)
Supply issues and requirements
The ability of a channel to meet existing and future supply requirements
should be considered when remediation options are evaluated. For
example, irrigation practices may have changed, so the water supply
infrastructure may no longer be appropriate. Similarly, future
water needs, whether reduced or increased, due to changes in land
usage, irrigation schedule or demand, need to be taken into account.
When considering the overall viability of a supply system, channel
seepage performance needs to be considered along with other factors
used in evaluating land suitability and capability for irrigation.
Channel capacity and operation schedule
Channel capacity is a factor influencing the selection of remediation
techniques. For example, pipelines can be an effective technique
for smaller capacity channels, but less practical for larger capacity
channels. Smaller channels may not be suitable for compacted earth
lining due to space limitations and higher unit costs for construction
equipment. Flexible membrane and concrete liners are suitable for
both small and large channels, although the use of covered flexible
membrane liners may be limited in smaller channels if compaction
of backfill by earthmoving equipment is required.
Most irrigation channels in Australia are shut down for a relatively
short time each year. This provides limited opportunity for construction
works, but also often coincides with unfavourable construction
conditions. For extensive lengths of seepage remediation, techniques
that involve fast installation and that are less affected by cold
conditions (such as flexible membranes or compacted earth) have
an advantage.
| Impact
of Seepage |
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The type and extent of the impact of seepage
on the surrounding area may affect the choice
of remediation options. For instance, the broader
economic, social or environmental impacts of
seepage may influence the choice of technique
beyond a cost-benefit or technical basis.
Alternatively, it may be possible that channel
seepage produces some positive impacts, whether
it be providing water to a wetland of significance,
or replenishing groundwater supplies. Consideration
of the importance of these benefits, and the
costs of maintaining their supply to these benefits
(if required), may influence whether any seepage
mitigation is undertaken.
| Channel dimensions
and site conditions |
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A survey of the channel cross-section should be undertaken to record
the dimensions and profile of the channel and discover any factors
near the channel corridor that may affect remediation efforts.
The survey could also be used to determine and quantify materials
required. Quantities of some lining materials are likely to be
greater than indicated by the dimensions of the channel due to
overlaps at joins in the material and keying of the material into
anchor trenches in the banks.
| Soil
types and subgrade quality |
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Soil parameters such as permeability, dispersion and shrinkage
characteristics, influence the choice of potential remediation
techniques. In-situ compaction of existing bed and batter material
might achieve the required seepage reduction if its permeability
could be reduced by compaction. Flexible liners or compacted earth
are suitable where soils have a high shrink/swell coefficient,
are prone to settlement, or suffer from piping due to high dispersivity.
In these cases, hard linings may lose support and crack, causing
the lining to be breached.
Where it is not possible to prepare a smooth and uniform subgrade
due to the presence of rocks or boulders, flexible membrane liners
are not appropriate unless a layer of foundation material is laid
to prevent the puncturing.
| Topography
and flow velocity |
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Flow velocities of a channel are influenced by the topography of
the site. Earth linings and covered membrane liners are susceptible
to scouring in high-velocity channels, so are best suited for channels
with flat grades and low velocities. Concrete or other hard-surface
linings are preferable in high-velocity channels.
| Climatic
conditions |
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Climatic conditions affect the timing of remediation works. Irrigation
systems generally shut down when irrigation water is not required
(winter months in southern states and higher-rainfall months in
northern states). Cold and wet conditions hinder the installation
of some lining materials.
Earth lining is not impeded by the cold but may be restricted in
wet conditions. Reduced curing times in colder conditions may
limit the application of hard liners.
Flexible membrane liners can be prefabricated to some extent to
make installation easier. Extra care is required with welding in
damp or wet conditions. When installed in hot conditions, exposed
flexible membranes can expand, placing stress on the material and
the joins when lower temperatures cause shrinkage.
Concrete lining is susceptible to low temperatures, particularly
if subgrade material freezes and causes uplift pressures.
| Groundwater
conditions |
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Consideration needs to be given to the groundwater conditions at
the remediation site. High groundwater levels can exert upward
pressure on a liner and dislodge it if the downward pressure is
not sufficient, such as when a channel is drained or if the weight
of the liner is not sufficient. Rigid liners may not accommodate
changes in hydrostatic pressure. Underdrainage of the liner can
alleviate this problem but can be expensive. Compacted earth liners
perform well under these conditions.
| Adjacent
land use |
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The value and productivity of land through which the channel passes
may influence the width of construction activities and the final
width of the channel, whether new or refurbished. Techniques that
can achieve steeper batter slopes may be necessary. The economic
impact of varying the width of a channel may need to be analysed.
| Operation
and maintenance |
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The operation and maintenance profile of the existing channel can
affect the type of remediation technique adopted. A hard channel
lining would be more appropriate in a channel that is subject to
frequent changes in water level or is completely drained and refilled
on a regular basis. Compacted earth and covered flexible membrane
liners can withstand changes in water levels but require more maintenance.
Desilting operations should be considered in the choice of a liner.
Mechanical desilting is not suitable for exposed flexible membrane
liners. All types of liners require considerable care in desilting
operations, although compacted earth and covered flexible membrane
liners are protected to some extent by their earth cover.
Costs of maintenance (for example care required for desilting and
weed removal), should be included in the economic analysis of life-cycle
costs. These costs vary depending on the type of liner adopted
and channel conditions.
| Potential
for damage and exposure |
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When evaluating seepage remediation techniques, an assessment of
the risks and causes of damage to the lining at the particular
site should be undertaken. The choice of lining should be take
into account its ability to resist or prevent damage, thus ensuring
it is able to perform for its expected life.
Remoteness of a site can reduce the potential for damage by humans.
Fencing should be considered to protect the lining if there is
excessive human or animal traffic expected.
Potential causes of damage include:
- Vehicle traffic
- Maintenance works
- Vandalism
- Fire
- Animal traffic (stock and wildlife)
- Burrowing animals
- Groundwater uplift
- High-velocity flow
- Temperature fluctuations
- Ultraviolet light
- Wind.
The remoteness of the site can reduce the potential for damage
from humans. Fencing should be considered to protect the
lining if there is excessive human or animal
traffic expected.
| Structural
integrity of channel profile |
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The structural condition of the channel profile can influence the selection
of remediation techniques. The profiles and banks of old channels can be
undermined
and damaged by:
- Over-excavation
- Animal activity (e.g. cattle, carp, water rats, yabbies)
- Human activity
(e.g. vehicle damage)
- Vegetation (e.g. willows, poplars)
- Erosion of the bank.
If the structural integrity of the channel is not sufficient to support
the application of a liner, some rebuilding of the channel profile
or bank may
be necessary.
Where complete rebuilding is required, this may in itself be sufficient
to reduce seepage losses, depending on the quality of soil used, without
the
need for any
other remediation works.
However, if the bank material provides only
structural support without reducing permeability, further seepage reduction
methods
are required.
| Occupational
health and safety requirements |
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Safety of the public and staff should be considered during the assessment
of remediation techniques. For example, if the channel is located in
a populated area, pipelining could be considered if the channel capacity
is not excessive.
Exposed flexible membrane liners present a smooth and slippery surface
when wet
and so are difficult to negotiate, particularly on steep batters. Exit
points, consisting of alternative materials placed over the batters
(e.g. rubber
mats and ladders) should be provided in high-risk areas. Fencing reduces
the risk
of humans and animals falling into the channel.
| Related
pages |
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Previous: 2 Determine remediation objectives
Next: 4 Review remediation objectives |
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