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Direct and point seepage measurement techniques
The techniques considered most likely to provide accurate
measurements of the rate of seepage are direct measurement
techniques, which include inflow-outflow tests and
pondage tests. In addition, point
measurements provide a way of
quantifying seepage rates at specific locations in a channel.
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Pondage tests: summary
Pages in this section include:
This page provides a summary of the pondage tests
channel seepage identification and measurement technique.
Principle
The pondage method uses a water balance approach to determine
seepage losses in an isolated section of channel. Seepage losses
constitute
the drop in water level over time in the pond (or volume added
to maintain a constant level) after accounting for evaporation,
rainfall
and other inflows and outflows.
More information
| Method |
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The pondage test method uses barriers or constructed pond banks
to form a pond within a section of channel. The location
of the banks
depends on project objectives, and might be determined
by geophysical surveys or perhaps anecdotal information.
Existing structures suitable for forming a sealed barrier
should be used where possible to minimise the number
of barriers required
to be constructed.
A section of channel is blocked off with embankments
at each end and filled with water up to, or slightly
higher
than,
the level
at which it usually flows during operation. As the
water level in the
channel section falls, it is measured by a staff, hook
gauge, or a water-level recorder. The time between
measurements is also recorded,
necessary corrections for evaporation and rainfall
made, and the resulting seepage loss rate computed.
More information
| Applicability |
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Pondage tests are considered the standard for channel
seepage quantification. The most accurate means
of measuring channel
seepage, they are
suitable for benchmarking and assessing other methods.
Inconvenience of conducting tests outside normal
channel operation times and the cost of bank
installation are
factors to be considered
for pondage tests. In the trials RWAs found it
convenient to undertake tests at the end of the
season. Pondage tests are probably the most accurate
post-implementation measurement technique.
More information
| Practical
implementation summary |
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Pondage tests must be conducted outside normal
channel operation times and the channel must
remain out of
use during tests
- Installation cost of embankments to isolate reaches
of the channel can be high
- Non-flow conditions introduce some
inaccuracies as they do not reflect velocities and sediment
loads carried
during normal channel flow conditions
- The result does not provide an indication
of the spatial variation of losses within the reach
More information
| Indicative
costs |
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The most significant cost of pondage tests
is bank construction, which varies considerably
depending
on proximity of a
suitable clay source. During the IAL
trials (for channels between
10-20m
width and 1.5-2m
deep), bank construction and removal
costs ranged from $560 to $1,000
per bank. Other costs include cross-section
and hook gauge surveying. Surveying costs
in the
IAL trials
for six cells (every
200m)
ranged from $600 to $2,000.Daily monitoring
is required and can
be conducted by RWA staff.
More information
| Related
pages |
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For a more detailed description
of the inflow-outflow technique see:
Pondage tests: principle, method
Pondage tests: applicability,
practical implementation, experience from the trials, indicative
costs |
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