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Inflow–outflow tests: applicability,
practical implementation, experience from the trials, indicative
costs
Pages in this section also include:
This page provides a detailed description of the applicability,
practical implementation, experience from the trials,
and indicative costs for
the inflow-outflow channel seepage identification and
measurement technique.
| Applicability |
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The inflow-outflow method is often used to estimate
approximate seepage losses in a system, but can produce
accurate results if accurate flow measurements are
obtained and losses such as outflow diversions are
measured. Accuracy decreases as the percentage of flow
that is lost to seepage decreases. The inflow-outflow
method is therefore best suited to relatively high-seepage
sections of channel.
The inflow-outflow method can be used at various scales,
from an entire irrigation system to an isolated section
of channel. Measurements are best suited over long
sections of a channel, with appreciable seepage, without
diversions, and with suitable structures to incorporate
measuring devices.
It is a method more suited to high-flow channels where
losses are likely to be much higher than measurement
error. It is not suited to small, low-flow channels
where measurement errors swamp the calculations.
It is perhaps best suited at a system or regional level
for initial identification of long sections of channel
with high losses. It can assist in setting priorities
for detailed seepage assessment of, say, one channel
over another, but not for isolating problem sections
of channel (down to, say, the kilometre scale).
In practice this appears to be how most Rural Water
Authorities presently use inflow-outflow tests.
Inflow-outflow tests were initially included on the
list of techniques to be trialed in the IAL study.
However, investigation into the accuracy of this method
led to its exclusion from the program.
Measurements
using a current meter are, at best, accurate to within
2% and 4% for measurements at both ends of a section
of channel (Theis, pers. comm., 2000). Typically seepage
is only 3-4% of flow in a given section, and therefore
cannot be distinguished from the error bounds of the
measurement. The emphasis in the trials was on relatively
short (1-2km) sections of channel and the isolation
(including identification and measurement) of seepage
within those sections, which is not suited to the inflow-outflow
test method.
Over a relatively long length of channel the inflow-outflow
technique is more appropriate due to the greater volume
of water lost to seepage. It is suitable for identifying
and prioritising, at an authority-wide level, channels
that have higher losses than others. It cannot, however,
identify where within the section the channel is seeping.
Inflow-outflow tests are suitable for remediation assessment
involving a long section of channel, or for a short
section if very accurate gauges are installed immediately
upstream and downstream of the section.
| Practical
Implementation |
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Key issues to consider in determining whether an inflow-outflow
test is practical at a particular site are:
- It is difficult to obtain sufficiently accurate
flow measurements, particularly for short sections
of channel, and channels with low flows or seepage
rates
- All inflows and outflows need to be metered
or measured accurately, as the overall accuracy
depends
on accuracy of inflow and outflow measurements.
- Determining
inflows and outflows between gauged sites is
difficult.
- The test section should ideally contain
suitable structures for incorporating measuring
devices.
- The feasibility of keeping levels in
the channel constant for the duration of the
test needs to
be assessed
- Tests conducted over relatively
long sections do not provide an indication of
spatial variation
of
losses (it provides an average or overall
rate).
| Indicative
costs |
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If structures already exist for measuring flow to a suitable level of accuracy,
costs will be minimal. Costs will be restricted to monitoring of flow in the
gauges over the period of the test. However, if flow is required to be measured
using the velocity-area method of assessment, contractors are likely to be required.
Costs increase with increasing number of outflows (or inflows) along the investigated
reach.
The cost of a recent inflow-outflow test conducted by a contractor for Murrumbidgee
Irrigation (2002) was $7,000 (including analysis and reporting, but not including
cross-section surveying costs). The test was conducted on a 5|km section of channel
over a period of two days. The actual discharge measurements were conducted over
a period of six hours. This included discharge measurement from gauges, various
off-takes and wheels, as well as velocity-area discharge measurements. The cost
included sealing identified leakages.
| Related
pages |
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For a more detailed description
of the inflow-outflow technique see:
Inflow-outflow tests: summary
Inflow-outflow tests: principle,
method |
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