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Point measurement: applicability,
practical implementation, experience from the trials, indicative
costs
Pages in this section include:
This page provides a detailed description of the
applicability, practical implementation,
experience from the trials, and indicative
costs for the point measurements channel seepage
identification and measurement technique.
Applicability
Direct seepage measurements are obtained using point measurements under
channel-full conditions using Idaho or other types of seepage
meters. Channel-empty conditions operate under different hydraulic
and soil saturation conditions and provide a measure of soil
hydraulic conductivity from which seepage rates can be calculated.
Channel-empty techniques measure the hydraulic conductivity of
soil, which can be used in groundwater equations to determine
the seepage rate. Silt layers at the channel base can have a
strong influence on the interpretation of measurements.
Point measurements are best suited for determining the distribution
of seepage losses or potential losses (i.e. relative seepage).
However, the frequency distribution of seepage rates is usually
positively skewed. That is, a high percentage of the seepage
occurs through a relatively small percentage of the channel.
Therefore, multiple measurements are required to obtain a reliable
estimate of the mean. Taking too few samples tends to produce
biased estimates of the total seepage rate, and the number of
measurements required to accurately estimate the actual seepage
is generally prohibitive.
Due to variable and sometimes erratic values obtained and the
large number of tests required to accurately determine the average
seepage rate, point measurements are not considered reliable
for absolute quantitative purposes. However, they do provide information
on
seepage distributions.
Point measurement techniques are not a reliable stand-alone method,
but a soil survey can help determine where to take measurements,
how many to take, identification of likely hotspots and extrapolation
of results. Point measurement techniques can be used in conjunction
with soil surveys to assign a seepage rate to a particular soil
type. With this approach there needs to be adequate understanding
of surface and sub-surface soil profiles.
Point measurements are not sufficiently reliability for absolute quantification
of channel losses and are best suited for determining the distribution
of seepage losses (relative seepage), and then generally over
short lengths of channel (e.g. defining a hotspot).
- They are generally not reliable for direct (absolute)
quantification of channel seepage losses due to the variable
nature of soil
and channel beds.
- Typically a high percentage of seepage occurs
through a relatively small percentage of the channel, so
numerous point measurements
are required to obtain a reliable estimate of the mean.
- To
obtain reliable and meaningful results, tests require a
skilled operator or technician, so generally tests will
not be
able to be conducted in-house.
- In principle, they are
suitable for measuring local seepage rates in channels
without artificial lining.
They are not
practical for a post-remediation assessment, other
than for clay-lined
channels.
| Practical implementation |
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Relative to most other seepage measurement techniques, point
measurements are quickly and easily conducted and are most useful for
locating short sections of a channel where seepage is excessive.
Seepage meters should be installed with the least possible disturbance
of the bed material. Disturbance of the soil during insertion
of the meter can cause indicated seepage rates to be higher or
lower than actual due to potential leakage around the bell or
changes in the soil bed structure. Infiltrometers and permeameters
can also disturb the channel base during installation.
Multiple measurements are required to obtain a reliable estimate
of the mean, so that the point test method requires a large number
of tests to obtain a representative seepage rate over a given
length of channel.
Dry-channel measurement techniques can be time-consuming. A single
measurement might take several hours depending on the hydraulic
conductivity and the moisture condition of the channel bed.
| Experience from the trials |
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Comparison of representative seepage rates for each pond derived from
channel-empty point measurements (disc permeameter and ring infiltrometer)
and channel-full point measurements (Idaho seepage meter) have either not
compared favourably with the pondage test seepage rates or have been
inconclusive. Point test results do not typically cover large enough
segments of the channel, either laterally or vertically to be representative
of the larger ponded section. Surface sediment layers, which were not
the most restrictive layers, also significantly adversely affected
some trials.
The results do not preclude the use of point measurements for some aspects
of channel seepage assessment (e.g. for qualitative purposes such as
isolating hotspots). However, using point test data is limited by the
variability in the soil, and the results indicated that significantly
higher numbers of tests were required to characterise the channel.
The number of tests required to adequately characterise the seepage
over a defined length of channel was found to be financially prohibitive.
| Indicative cost |
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Point measurement techniques generally need to be conducted by an operator
outside the RWA with expertise in using the equipment. The greatest variable
influencing the cost of point measurement is the density of testing. The
cost of testing also depends on the duration of individual tests, which
is a function of the time required to reach steady-state infiltration (which
in turn is a function of the soil type or clogging layer at the base of
the channel).
Subcontractor costs are provided below for infiltration tests conducted
during the IAL study. However, none of these tests were conducted at
sufficient density to accurately measure seepage within the reaches concerned.
To properly characterise a reach, testing at a greater density than conducted
in these studies is required.
- Idaho seepage meter - For Idaho tests at 22 sites (four individual
tests at each site, over the channel cross-section) along 800m
of channel, the
total subcontractor cost (including reporting) was $6,200. Testing
was conducted over four days (plus travel time).
- Ring infiltrometer
- For 29 individual ring infiltrometer tests, over 600m of channel,
the total subcontractor cost (including
reporting) was
$5,000. Testing was conducted over three days (plus travel time).
- Disc
permeameter - For 24 disc permeameter tests, over 600m of channel,
the total subcontractor cost was $4,000 (including
reporting). Testing
| Related
pages |
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For a more detailed description of the point measurement technique
see:
Point measurement: summary
Point measurement: principle, method |
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