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5a Implement seepage measurement techniques:
Intermediate to large scale recommendations |
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This page details the recommended approach to implement a seepage
identification and measurement investigation for intermediate to
large-scale sites. The approach includes the following elements:
- Map spatial variability in geophysical response
- Basic seepage
quantification
- Relationship between seepage and geophysical
response
- Interpretation and extrapolation of results beyond
the test sections
- Extrapolation of seepage measurements
- Extrapolation
of geophysical data
- Extrapolation of soil and
geological information
Note: A similar approach can be used for local-scale
investigations where there is a need to map seepage variability.
| Map spatial variability in geophysical
response |
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The recommended procedure
for undertaking an intermediate
to large-scale investigation is as follows:
- Undertake a geophysical survey over the section of
interest, giving due consideration to factors such as appropriate
timing and
important variables
- Plot geophysical survey results along the section
and overlay with known site conditions (soils, geology, hydrogeology
and hydraulic
data). Based on these plots, identify areas of suspected
high,
low and moderate seepage, assuming low conductivity/high resistivity
equates to higher seepage
- Soil bores are drilled at appropriate
intervals along the length of the geophysical survey to assist
with interpretation
of
the geophysical survey. Drilling is conducted across a range of low,
moderate and
high conductivity/resistivity sites:
- Some bores
should be drilled into the watertable, and some should
be constructed as permanent groundwater
observation
bores.
- Generally drilling should be conducted
on the outside toe of the channel.
- Logging and sampling
of the bores should ideally be undertaken by someone trained
in soil/geological
classification
and
a consistent classification system should
be followed.
- Depending on the density of data collected,
presenting the results for a long section
should be considered
so that a picture
of soil
variability is obtained
| Basic
seepage quantification |
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The IAL trials found pondage tests
to be effective for quantifying seepage.
The
measured seepage
rates can be
used as a calibration/correlation
datum on which to assess the significance
of other techniques.
For local-scale mapping of seepage
variability, point
measurement and groundwater
assessment can be used in addition
to pondage
tests
using the approach
described below).
Pondage tests are conducted at
intervals along the entire length
of the channel.
The number
of tests
depends on
the length of
channel surveyed and the variability
of conditions along the channel.
The following basic principles
apply: - Pondage tests should be conducted across a range of
low, moderate and high conductivity/resistivity sites
to establish a regression equation that represents the range of geophysical
response across the area.
- Based on soil drilling results, pondage tests
should be based on a range of different soil
types and/or groundwater conditions.
- Pondage tests must be conducted over areas
of like conductivity/resistivity. They should not straddle
areas of significantly different geophysical response, as this will complicate
interpretation
of the results
and development of a regression equation.
- Due
to the cost of conducting pondage tests, it is recommended
that at least two cells back-to-back
should be conducted at each site for efficiency purposes. Using available
structures should
be considered
to minimise barrier
construction costs.
- Pond length varies, but as a guide one pondage
cell should generally not be more than 400-500|m
and not less than 50|m.
By conducting pondage tests in this manner across
the area of the
geophysical survey, prediction of seepage
rates
outside of
pondage
test areas can
be based on extrapolation between ponds rather than into
entirely untested
environments.
This
improves confidence
in
the predicted seepage.
While pondage tests
are expensive,
they are an
important part
of the investigation
process.
While seepage
rates determined
using pondage
tests are the
most accurate
means of measuring
channel seepage
they can sometimes
lead
to an underestimation
of seepage compared
to channel
flowing conditions.
This is due to
the effects
of clogging caused
by siltation
when the water
is stationary,
which
are lessened
when the channel is
running.
For example,
if the project
objectives
are to determine
likely seepage
losses after
deepening and
widening
of the channel,
pondage tests
may not provide
very
accurate predictive
information
for this scenario,
particularly
if a significant
silt
layer
has built
up in the channel.
Therefore,
it is recommended
that
where possible,
geological
and
soil profiles should
be examined
to help evaluate
the pondage
test results. For
this scenario
the geological
profile
data will
also form an
important
part of the
evaluation
process.
Pondage tests
are usually
necessary
for calibration
of other
techniques and are
used in all
but
the simplest
investigations.
Certainly
for large-scale
investigations,
they are
essential
for quantification.
Other direct
measurements
can be
correlated against geophysical
results.
However,
because
of the variability
of
the results
of point
measurement
and the
need for
many results
to provide
representative
values,
they are
generally
not
as useful
as pondage
tests.
Furthermore, point
measurements are
labour-intensive
in comparison
with pondage
tests.
| Relationship
between seepage and geophysical response |
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To evaluate
this
relationship,
the
following key steps
should
be
conducted:
- Plot geophysical response against pondage test seepage.
The average geophysical response
(e.g. average conductivity) for the test section and the steady
state pondage test
seepage
rate
are used in establishing
the relationships.
- Outliers should be assessed in light of all
available information,
including the conceptual seepage mechanism, test drilling results, channel
hydraulics, etc.
There are sometimes legitimate
grounds
for excluding
outliers.
- If two or more trends can be observed due to identifiable differences
in subsurface conditions, then different regression equations
should be generated
for each one.
- Regression lines should be fitted to the data.
- Statistical
analysis should be conducted to determine the degree
of confidence that can be placed in the derived relationship
- Using the derived
relationship the channel length should be divided
into seepage categories of various seepage rates based on geophysical response,
with accompanying error
estimates.
| Interpretation and extrapolation
of results beyond the test sections |
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Test results
are evaluated to interpret
what they mean in
terms of seepage
beyond the test sections.
Investigations must
include an
assessment of
how accurately measurements
reflect actual
channel seepage rates
or distributions and how
can they be
used in channel
management, including
after remediation.
Extrapolation
of seepage measurements
Extrapolation
involves taking
the results
of an
accurate method
of assessment
(e.g., direct
measurement) and
applying them
to other
areas based
on results
of a
more-cost effective
but less
accurate method.
The basic
approach to
extrapolation is discussed in
the Literature
review. The
following principles
are relevant:
- The technique upon which the extrapolation is based
is applied locally,
restricting the chance of significant variability in conditions to the area
for which the extrapolation
is applied.
- As many tests as possible
are conducted to clearly establish the
relationship between the primary technique and the extrapolated technique.
- Sufficient
information is gathered about the area over which the
data is extrapolated to ensure the basis for the development of the relationship
is not compromised.
Extrapolation is based on the availability of detailed
spatial knowledge that
can be obtained quickly. The information
of most relevance is extensive soil
and
geophysical mapping and
geophysical
survey mapping.
This is the reason
for evaluating the conditions
along the entire channel length being considered (see
2 Collate
site
physical
condition data). Extrapolation can be
based on:
- Geophysical data
- Soil and geological information
Extrapolation
of geophysical data
In the IAL tests low average conductivity/high
average resistivity
were related to high seepage estimates from pondage tests.
On this basis,
the
mapped distribution of low conductivity/high
resistivity zones can be taken to indicate
the
zones of highest seepage.
This has been verified in field trials.
The relationship of
geophysical
results to measured seepage rates from ponds
can be used
to extrapolate the inferred
seepage rate to other sections of channel. However, estimated
seepage is
not
absolute and
the rate is within error bands. For
a specific channel
a strong relationship may be recognised.
Extrapolation of
seepage
measurements should
be related to zones of similar
geological conditions.
One of the
key questions to address
is whether there
is sufficient
confidence in
the derived relationship.
In addition
to
the particular statistics
of the regression line, this
largely depends
on the
project objectives.
Further pondage tests or
other testing
may be required
to further
improve confidence
in the relationship.
Extrapolation
of soil and geological
information
The
use of
soil types
and geological
profiles
to identify
seepage zones
or potential
seepage zones
is also
based on
extrapolation.
Based
on the
seepage
rates for
a particular type
of soil
(determined
by a
seepage
meter,
pondage
tests, etc.),
projections
are
made for
all the
soil
types along
the channel.
The
cheaper and
more rapid
method
of soil
surveying/mapping,
effectively
replaces the
more
expensive method
of seepage
meter testing
or
pondage test
measurements.
Extrapolation
of soil
and geological
information
is based
on inferring
that,
where
the same
type of
soil
or geological
profile exists, similar
rates
of
seepage
can
be expected.
Groundwater
monitoring
data
can also be extrapolated
to similar
soil and
geological
profiles
to estimate relative seepage rates.
| Related
pages |
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Previous: 5 Implement
seepage measurement techniques
Next: 6 Interpret results |
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