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| General issues in channel seepage identification and measurement |
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General issues that affect channel seepage identification and
measurement, the location and rate of channel seepage, how it impacts
local amenities
and how it can be remediated include:
- Physical conditions: soil and water conditions near and within
the channel
- Seepage mechanisms: shallow surface and deep vertical seepage
- Scale: geographic extent of seepage – local, intermediate
to large or macro
Physical conditions
Factors affecting seepage from channels include:
- soil;
- hydraulic;
- channel water characteristics.
Soil characteristics
An important factor influencing channel seepage is the permeability
of soil layers forming, or lying immediately below, the
wetted perimeter of the channel. Another important factor is
formation
of silt sediment
on the channel base.
Hydraulic characteristics
Hydraulic characteristics of a channel include depth
of water in the channel, wetted perimeter of the
channel and
depth
to groundwater.
Seepage losses generally increase with greater water
depth in the channel and as the difference between
water level
in the
channel
and watertable increases (until an equilibrium
is reached). The depth below the channel bed within
which the nature
of the soil
affects
seepage losses is approximately five times the
bed width of the channel. Laterally, at a distance of
approximately ten
times
the bed width
of the channel, the effect of seepage losses on
the original
watertable elevation is minimal, although this
varies depending on channel
dimensions and local hydrogeology.
Channel water characteristics
Material suspended in channel water is carried
by seepage water into the pores in the soil
in which
the channel
is constructed.
If the
water contains considerable amounts of suspended
material, the seepage rate may be reduced in
a relatively short
time. Even
small amounts
of sediment have a sealing effect over a long
period of time.
Salinity and Sodium Adsorption Ratio
The Sodium Adsorption Ration (SAR) and salinity of water will
affect infiltration rates, however with respect to channel
seepage, the range of SAR and salinity of water within channels
is unlikely to vary significantly. While waters with high SARs
and low salinity are likely to decrease the permeability of
the channels, water with a high SAR would not be desirable
for irrigation. Therefore water within an irrigation supply
system is unlikely to have high a SAR.
In particular for older
channels, the water quality will have minimal impact on the
permeability of the soil layers due to the equilibrium previously
established between the channel water and the soils immediately
beneath the channel. Other factors such as the suspended sediment
load are likely to have a more significant effect on seepage
rates.
| Seepage mechanisms |
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Seepage from channels can be mainly horizontal
or vertical, or a combination of the two.
The dominant mechanism
at a site affects
the rate of seepage, nature of impact and
approach to remediation.
Site conditions can be used to determine
appropriate investigation techniques.
The mechanisms of
seepage from channels are
illustrated below.
Figure 1.Channel seepage mechanism
1. Shallow surface seepage
Lateral seepage through horizontal pathways is a major reason
for channel remediation in Australia. It can lead to perched
watertables,
soil waterlogging, degradation, and bank instability due to saturation.
Even if water loss is not necessarily high, there can be a
major impact on channel operation and maintenance and the
local environment.
Estimation of seepage rates is not always possible, and may
be of significance mainly because of its impact on soil conditions,
especially waterlogging and salinity. Locations can be detected
by surface mapping and remote sensing. Remediation using cut-off
walls, trenches and bank lining may be all that is needed and this
is likely to be less expensive than
lining of the entire wetted perimeter of the channel.
2. Vertical seepage
Where there is no surface expression the geographic
extent of seepage is more likely to be governed by
vertical seepage
processes and
seepage is more difficult to identify. However, there are local
and perhaps regional groundwater impacts from seepage, including
increased recharge and rising watertables. The assessment of impact
of seepage on watertables needs to take into consideration external
groundwater flow systems before extensive remediation is undertaken.
For example, in some irrigation areas, existing high watertables
from regional irrigation cause land degradation. Remediation might
have no effect on adjacent land systems if high watertables are
due to external factors that are not altered by channel works.
Under conditions of deep (vertical) drainage, channel works
can increase seepage. Examples include deepening of channels
and exposure
of potentially high-seepage pathways, or de-silting of the existing
channel and reopening blocked seepage pathways.
| Issues
of scale |
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The IAL trials, RWA survey, the Task Force, and others involved
in seepage evaluation, suggest that the geographic scale of the
investigation is an important factor for most investigations. Scale
should be
considered when management needs for doing the project are being
evaluated (e.g. extent of the project, expenditure, reason for
the project, due diligence), and they should be reconsidered in
finalising the selection of the technique.
Three scales of investigation are considered appropriate in
the selection of a technique:
a) Local scale
Short (up to approximately 400m
in length) where anecdotal or other evidence exists for some leakage
and a quick assessment
is
needed. This leads to the possibility of undertaking site specific
measurements at a very local scale (e.g. point
measurement, groundwater
monitoring, pondage tests).
b) Intermediate to large scale
Hundreds of metres to tens of kilometres. For investigations
of this length, it is considered best to apply a combination
of mapping
channel characteristics (e.g. geophysical surveys, remote
sensing),
identifying seepage rates at test sections (pondage tests),
and interpolating
between the test sections.
c) Macro scale
Tens of kilometres to entire systems. A similar approach to
(b) when testing is undertaken, but for some investigations,
water
balance estimates (e.g. Inflow – outflow)
may be sufficient. Remote
sensing techniques may also be useful at this regional level
of investigation.
The basis for consideration of these scales is twofold:
- A key operational need is often to identify the location
of high-seepage zones to set priorities in remediation expenditure.
To be cost-effective
in remediation programs, the measurement technique needs to be
able to target channel sections in greatest need of works. Therefore
the measurement technique may need to look at the distribution
and rate of seepage across lengths of channel, which may be (a)
local or (b) intermediate to large scale.
- In contrast there are
broad scale issues of quantifying losses within a system
and estimating the cost of the lost resource.
This could effectively be done at (c) macro scale based on relatively
simple water balance estimates, using inflow – outflow
measurements. Knowledge of the system configuration and operation,
including
diversions, are needed. For this type of water resource analysis
the detail of where, how and at what rate seepage is occurring
may not necessarily be as important as the overall water balance.
The question may then remain, if losses can be detected and estimated
at macro scale, is there a need for more detailed quantification
at smaller scales (a,b).
Once the scale has been identified, the selected technique
needs to take into consideration the cost and the ability
to extrapolate
or interpolate the results. It is important that when test
information is extrapolated to the entire section of
interest in the channel
in a large-scale investigation, the test results are representative
of the channel conditions over the total length for which the
extrapolation has been made.
For the different scales and management requirements, there
are generic approaches to conducting a seepage investigation.
These
have been developed largely form the experience gained in
the field trials and are described in Recommended
approach and
techniques. In some circumstances there are constraints
on performing the
techniques
and these need to be understood by examining the detailed
description of each technique (see Techniques:
identification and measurement).
Example: Retreat Channel, Murray Irrigation
In one case a planned pondage test on the Retreat Channel
operated by Murray Irrigation was not conducted because
of the size
of the channel and the estimated cost of earthworks.
In that case
the
preferred approach for this investigation was not adopted
and alternative means were required to assess the seepage
rate.
| Related
pages |
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Recommended approach and
techniques |
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