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| 4 Select seepage measurement techniques |
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Purpose
Identification
of the best technique for obtaining seepage data to meet management
objectives and minimise bias based on personal preferences. This
provides the basis for developing a work plan for the project, providing
advice to management on costs and expected outcomes.
There are two basic approaches to selection
of techniques based on whether the investigations are local or larger-scale
programs.
Inputs
- Management decisions on project scope
- Identification of the scale of the project (e.g. local site
or larger scale)
- Site condition data from previous task
- Operational factors related to conducting channel works including:
- Cost
- Channel operating schedule and operational constraints
- Personnel and resource constraints
- Information about particular techniques, including the principle,
method, practical implementation, applicability, experience and
costs)
| Description of activity |
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Information obtained from the site condition assessment is used to
determine which techniques can provide meaningful data on seepage
within the operational conditions of the channel. Whether the assessment
involves local or larger scale approaches need to be identified at
an early stage.
Evaluation of the management issues should include the practical
implementation issues for each technique. General considerations
include:
Operational constraints
Operational constraints may reduce the number of approaches available.
Key questions to consider include:
- Does the channel require continuous operation?
- Over what period is the channel shut down?
- What time is available for the investigation work?
Resource constraints
Resource constraints may impose further restrictions
on options available. Important questions to ask include:
- Are the personnel and resources required to plan, conduct and
analyse tests available in-house?
- Are equipment and skilled technical personnel available for
making measurements?
- If not can these be readily obtained outside of the organisation?
Accuracy/cost trade-off
Investigations are likely to benefit from adjustment, with consideration
given to accuracy requirements and related cost constraints. Questions
to ask are:
- Can the accuracy requirements be met within the available budget?
- Is allocation of additional funds warranted?
- Can the area to be assessed be fine-tuned (i.e. representative
sub-areas studied)
- Can an extrapolative technique be employed to lower costs?
- Does the accuracy requirement need review?
| Responsibility/decision
makers |
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This is generally the responsibility of a project manager or project
officer, who makes a recommendation to management.
Output
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A decision on the most suitable techniques that can be practically
implemented for the investigations needed for the particular site.
A recommendation to management on expected outcomes of the data to
be obtained, how the data can be used, and costs and benefits.
This may be a staged recommendation, which may identify value in
undertaking certain tasks, reviewing the results and making decisions
to draw conclusions or to do further work.
Local-scale
recommendations
[Up20/10/07 | |
The types of techniques appropriate for local investigations depend
on whether there is a need for:
- Measurement of seepage rates at specific
locations; or
- Identification of higher permeability zones within a section,
and mapping of seepage variability
Specific locations
For measurement of seepage rates at specific locations the techniques
that should be considered are:
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| Point
measurement |
Point measurement refers to
techniques (such as Idaho seepage meter, infiltrometer) that
measure infiltration/hydraulic conductivity at a given point,
usually involving adding water to the channel and measuring
the rate at which it drains away.
Provide an indication of the distribution of losses along the
channel.
Often used in conjunction with soil surveys to assign a seepage
rate to a particular soil type. More on point
measurement
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| Groundwater
assessment |
Groundwater levels, measured using a series
of piezometers located at right angles to a channel, can be
used to estimate seepage by subtracting groundwater flow before
channel influence from groundwater flow after channel influence.
The seepage rate can be estimated from groundwater flow equations,
provided the hydraulic conductivity (K) of the aquifer is determined
with sufficient accuracy.
Useful for post-remediation assessment of watertable/salinisation
impacts.
Hydrochemical or isotopic concentration of seepage water can
be used to define a plume and also the volume of water that
has escaped from the channel. More on groundwater
assessment
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| Pondage
tests |
Seepage losses are estimated by measuring
the drop in water level over time in an isolated reach of channel
(or volume added to maintain a constant level) after accounting
for evaporation and rainfall.
The most accurate means of measuring channel seepage. Can be
used on both lined and unlined channels. More on pondage
tests
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Mapping seepage variability
If mapping of zones of different seepage or potential seepage is
required, there is a need to use tools (such as geophysical
surveys), subsurface methods (such as soil
and geological profile classification), and both groundwater and
surface observations. Estimates of the rate for different mapped
zones can be undertaken using the techniques listed above, once the
mapping is complete.
Intermediate
to large-scale recommendations
[Hundreds of metres to tens of kilometres
in length] |
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The trials conducted in the IAL study and the case study of the
Waranga Western Channel have indicated that for most channel seepage
projects at intermediate to large-scale (i.e. greater than approximately
400m in length), the most appropriate approach is to:
- Map seepage variability
- Quantify seepage rates
- Extrapolate results
- Verify results
Map seepage variability
Zones of highest seepage can be rapidly and cost effectively identified
using a mapping process based on geophysical
surveys or remote sensing.
Geophysics is often the preferred technique as it is applicable in
a wider range of situations. Remote sensing also has the potential
to rapidly and cost effectively cover large areas, but was not investigated
in the IAL trials. It is applicable at the large to macro scale
level of investigation).
Geophysical techniques are usually effective in rapidly mapping sections
of seepage along extensive channel systems. Coupled with appropriate
soil and groundwater investigations as well as local knowledge, geophysical
techniques offer the potential to effectively map spatial variability
of subsurface conditions along channels. This variability is the
basis on which zones of significant seepage can be inferred.
One important conclusion from the IAL trials is that the geophysical
surveys work best when the conductivity measurement is focused at
the depth where seepage has the greatest influence on groundwater
salinity. Thus, depending upon local conditions, different systems
have performed better at different sites and at different times.
See Geophysical surveys for more
information about selecting the most appropriate geophysical technique.
The appropriate geophysical method depends on local conditions. In
general, for a shallow watertable (surface to approximately 4m) EM31
(vertical dipole) is suitable for detection of seepage impacts at
the top of the watertable. For deeper watertables, EM31 could be
used to map inferred seepage based on soil properties in the unsaturated
zone. However, it must be ensured that seepage is controlled by the
unsaturated zone and not surface-clogging processes. Otherwise an
instrument with deeper capability targeted to the top of the watertable
should be selected.
The optimum geophysical seepage measurement technique has a depth
focus on and immediately below the watertable. Whether this is achieved
using EM or resistivity is not important. However, generally it is
easier to focus on a given depth with resistivity (EM provides an
average across a range), which can be done without knowledge of groundwater
depth. Techniques are discussed in more detail in Geophysical
surveys and should be assessed in relation to project objectives
and to site conditions.
Remote sensing mapping techniques
aim at providing cost-effective means of assessing long sections of
channel. It may be useful for identifying saturation at the ground
surface from lateral seepage through channel walls and banks, but
not where seepage is relatively deep or with no surface expression.
Therefore understanding the seepage mechanism is important. Results
are limited to date, although there is potential if suitable frequencies
are adopted.
At this stage there is little evidence that spatial data analysis
is useful for quantifying seepage. However, it may be worthwhile
considering combining the results from seepage measurement such as
pondage test results, and using GIS to compare actual seepage with
areas mapped by remote sensing.
Quantify seepage rates
Seepage rates can be measured using pondage tests,
although for particular purposes point
measurement or groundwater investigations
may be undertaken.
Correlated against direct measurements of seepage rates (usually
from pondage tests), a general relationship between the geophysical
response and seepage rate can be established. This approach is superior
both technically and economically to other techniques assessed for
most projects, particularly those in which there are significant
lengths of channel being investigated.
Therefore, following the recommended approach, the techniques to
be applied are:
- Map the distribution of lengths of the channel considered likely
to be sites of seepage by using ground based geophysics
- Quantify the seepage rate using direct testing in sections
of the channel using pondage tests
Extrapolate results
Results may be extrapolated beyond the test sections
reaches of channel of interest. This involves comparison of conditions
at the test sections with those of the broader area of interest.
Verify results
The inflow-outflow method can
be used to verify observations along the section of the channel of
interest.
This cost-effective procedure can provide an indication of the extent
and magnitude of seepage along a channel. It can be applied at any
scale and is more cost-effective for larger lengths of channel where
there is also more opportunity for meaningful verification.
Some knowledge is necessary for targeting parts of a system for investigation.
This information is often held within the organisation, based on
flow records or visual observations of historical seepage. However,
techniques such as remote sensing might
be considered for preliminary identification of areas susceptible
to channel seepage at a regional level.
| Related
pages |
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Previous: 3
Evaluate site condition data
Next: 5 Implement seepage measurement
techniques
Techniques: identification and measurement |
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