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Point measurement: principle,
method
Pages in this section include:
This page provides a detailed description of the principle and
method for the point measurement channel seepage identification
and measurement technique.
Principle
A point test measures seepage at a given point. This usually
involves the application of water to the surface or hole within
the channel and measurement of the rate of water loss. The infiltration
rate has a direct relationship to the seepage at that point.
Each point test provides a unique value and when individual values
are collated they can be useful for identifying seepage hotspots
and relative seepage potential.
| Method |
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Point measurements can be undertaken either when the channel is operating
or when it is not running. Techniques (described in detail in
the literature review (IAL, 2000a)) that have been applied
to channels around the world include:
Channel-operating techniques
- Seepage meters
- Barrel tests
- Well permeameters
Channel-empty techniques
- Constant head
permeameters
- Double tube method
- Disc permeameters, infiltrometers
- Ring infiltrometers
- Two-stage borehole permeameters
Point testing methods involve
introducing water into a hole or the surface of the channel
and measuring the rate at which it infiltrates the channel
substrate.
It is presumed that the water used for measurement is similar
in chemical make-up to the water that normally runs in the
channel. The infiltration rate has a direct relationship
to potential
seepage. Results are used to infer the point distribution
of seepage potential at a point. To obtain a broad coverage
of the
infiltration variability many point measurements are usually
required. In Australia, the most commonly used techniques are
those for
which equipment and experienced operators are available.
These
are:
- Idaho seepage meter (operating channel)
- Ring infiltrometers
(channel empty)
- Disc infiltrometers (channel empty)
These techniques are the most appropriate for operations
in Australia and are the recommended point measurements in these
guidelines. Results
of their use in Australian conditions can be obtained from the
project trials (IAL, 2003) with supporting information in the
Literature Review.
Seepage meters (channel-running conditions)
Seepage meters are cylindrical infiltrometers modified for use
under water. The seepage meter method involves the use of a water-tight
bell housing embedded into the channel bed. The water lost per
unit area through the base of the bell is the seepage loss from
the channel. Although many kinds are in use, most involve the
insertion of a relatively small ring structure (covered by a
water-tight bell) into the channel bed.
The structure is typically 30-60cm in diameter and is connected
by a hose or pipe to a water supply reservoir, which allows the
rate of loss of water from the bell to be measured. The meters
generally used are based on either a variable or a constant head.
The figure below presents a simple version of a constant-head
seepage meter. It consists of a watertight bell connected by
a hose to a flexible (plastic) bag floating on the water surface.
By keeping the water bag submerged, it will adapt itself to the
shrinking volume so that heads on the areas within and outside
the bell are equal. This type of meter is described in more detail
below.
Figure 1: Seepage meter with
submerged plastic bag
The seepage meter commonly used in Australia is a constant-head
unit referred to as an Idaho seepage meter (see figure below),
which operates using a reservoir (rather than a floating bag)
from which seepage can be calculated directly from the rate of
fall of water in the reservoir. The work is best conducted by
contractors who have the appropriate equipment.
Figure 2: Installation of Idaho
seepage meter
Ring infiltrometers (dry-channel conditions)
Ring infiltrometers are devices for determining the rate of
infiltration into soil. Single-ring infiltrometers are a
simple device used for gaining an estimate of infiltration
rates (Figure 3). Single-ring infiltration rates
can be affected by ‘edge’ effects. To minimise
the effect of lateral spreading, a double-ring infiltrometer
can be used, which is a ring infiltrometer with a second,
larger ring around it. Both the inner and outer rings are
filled with water, which causes essentially vertical flow
through the inner ring into the soil. The relative accuracy
of both methods is not clearly established.
Ring infiltrometers normally have metal rings with diameters
from 30-100cm and a height of about 20cm. The ring is driven
about 5-10cm into the ground, water is applied inside the
ring with a constant head device, and intake measurements are
recorded until a steady infiltration rate is observed. For
a double-ring infiltrometer, both rings have water applied
to them (to a depth of about 15cm), the head (water level)
is recorded, and measurements are taken inside the inner ring.
If a constant-head device is not available to add water to
the hole, a constant head can be maintained by adding water
and recording the volume of water added, at approximately 10-minute
intervals for a period of one to three hours (depending on
soil type).
The ring infiltrometer can be conducted as a falling head test,
and measurements of the drop in water level can be taken at
regular intervals, rather than maintaining a constant water
level in the rings. The infiltrometer does not provide a seepage
rate but allows the variability in hydraulic conductivity and,
by implication, the potential for seepage to be identified.
Figure 3 Ring infiltrometer
in use in a dry channel
Disc
permeameter (dry-channel conditions)
The disc permeameter is an instrument developed and commonly
used in Australia to measure the hydraulic conductivity of
soil at (or near to) saturation. A disc covered with a semi-permeable
membrane (typically 20cm in diameter) is placed on a surface
and the subsequent infiltration of water allows calculation
of the hydraulic conductivity of that surface.
The test requires a specialist in undertaking this type of
work, including the interpretation of hydraulic conductivity.
This measurement method can only be used in empty channels
or in areas that are not currently inundated. The method can
be used on an undisturbed channel bed. The disc should be mounted
on an approximately level surface and thus the approach is
less useful on the side walls of channel beds.
Soil properties beneath the surface of the channel can be assessed
by excavating to the required depth. Preparation for testing
of depths greater than around 50cm will be difficult.
| Related
pages |
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For a more detailed description of the point measurement technique
see:
Point measurement: summary
Point measurement: applicability, practical implementation, experience
from the trials, indicative costs |
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