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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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Inflow–outflow tests: summary
Pages in this section include:
This page provides a summary of the inflow-outflow channel
seepage identification and measurement technique.
Principle
The inflow-outflow method enables direct measurement
of losses. It is based on a water balance approach, measuring
water flow at both ends of a channel section and taking
into account additional inflows and losses along the
channel length being investigated.
More information
| Method |
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The inflow-outflow method is based on measuring the rates
of water flowing into and out of a selected section of
channel. The difference between inflow and outflow is
attributed to seepage, after accounting for inflows (e.g.
rainfall) and known losses (e.g. evaporation). Accuracy
in the results depends on accuracy of inflow and outflow
measurements, including the flow, rainfall, evaporation
and diversions.
Discharge measurement can be conducted using a number
of techniques. The two most common include:
- Use of a current meter to determine average
velocity. Discharge equals average velocity multiplied
by the cross-section
area. This method is usually the most practical.
- Regulating
structures such as flumes or weirs with automatic recording
gauges.
More information
| Applicability |
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The inflow-outflow method is often used to estimate
approximate seepage losses in a system. Accuracy decreases
as the
percentage of flow that is lost to seepage decreases.
The inflow-outflow method can be used at various scales,
from an entire irrigation system to an isolated section
of channel. Measurements are best over long sections
of a channel, with appreciable seepage, without diversions,
and with suitable structures to incorporate measuring
devices.
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, kilometres).
More information
| Practical implementation |
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It is difficult to obtain sufficiently accurate flow
measurements, particularly for short sections of channel,
and channels with low flows or seepage rates. The feasibility
of keeping levels in the channel constant for the duration
of the test needs to be assessed.
There is the need to determine potential inflows and
outflows between gauged sites, which may be difficult.
More information
20/10/07ame="description">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.
As an indication of costs, a recent inflow-outflow
test conducted by a contractor for an RWA in the Murray
Basin
was $7,000. This was conducted on a 5km section of
channel over a period of two days.
More information
| Related
pages |
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For a more detailed description
of the inflow-outflow technique see:
Inflow-outflow tests: principle,
method
Inflow-outflow tests: applicability,
practical implementation, experience from the trials, indicative
costs |
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