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Waranga Western Channel
Case Study |
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Date of investigation
2001-2003
Investigation conducted by
Goulburn Murray Water, Sinclair Knight Merz (SKM), and various contractors
Contact details
Name: Nick Kelleher
Phone: 03 5833 5404
Email: NKelleher@skm.com.au
Background Information
It was proposed that the WWC, an open irrigation channel maintained
by Goulburn Murray Water, be upgraded in capacity. The proposed upgrade
area was from the Loddon Weir to west of Boort, approximately 50km
in total length. The increase in capacity was to supply additional
irrigation water to a new horticultural development near Boort, as
well as to meet existing needs.
Seepage problems along the channel have been well documented. In
addition to existing problems, there was concern that new seepage
paths may be opened up during the upgrading works program.
Geotechnical and geophysical investigations were carried out along
the channel to identify and sections with existing seepage problems
and quantify seepage.
| Channel
characteristics |
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Channel operation
Annually, August-May
Capacity
Before upgrade 270ML/d (at 8-mile regulator). After upgrade 570ML/d.
Operating water level depth/width
Depth approx 2.3-2.5m
Width approx 18.5-21.5m
Soil type and geology
The channel traverses through Shepparton Formation surface sediments
(predominantly clays and sandy clays, with shoestring sand deposits
- fine to medium-grained) at various locations along the study length.
Underlying the Shepparton Formation along most of the channel length
is extremely weathered to moderately weathered sandstone. The sandstone
represents the indurated (ie, hardened) zone of the top of the Parilla
Sands aquifer. In sections along the channel this indurated zone
rises very close to the surface and in part intersects the channel.
Figure 1 Waranga Western Channel works
Groundwater salinity (ambient)
3,000-35,000 mg/L TDS.
Depth to groundwater
Depth to groundwater varies along the channel and depth of the watertable
is generally between 6-10 metres. There are some locations where
the watertable has been recorded close to the surface immediately
adjacent to the channel.
Adjacent land use
The land adjacent to the site has been used for irrigated grazing
and cropping, with expansion of grape and olive production in recent
years.
Channel maintenance
Over the last 25 years no desilting has been conducted and a significant
silt deposit has accumulated on the channel bed. It is likely that
this layer is the controlling influence on seepage rates, rather
than underlying soil types and geology.
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Seepage impacts and
history |
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The channel has been operating for more than 70 years and has a well-documented
record of existing seepage problems. The extent of channel seepage
in the Boort West of Loddon Salinity Management Plan is of concern
to landholders. A channel seepage program was initiated in 1993.
However, earlier investigations did not provide satisfactory information
to prioritise sites for remedial works.
| Investigation
drivers |
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The main driver behind the investigation was risk of high seepage
losses from new seepage paths being opened up due to the channel
upgrading, and the resulting impacts on adjacent land. A secondary
concern was the water loss resulting from the existing seepage.
| Investigation
objectives |
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The main objectives of the study were to:
- Quantify sections of the channel with existing seepage problems.
- Identify
sections and quantify potential seepage from opening up of
new seepage paths.
| Summary
of investigations |
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The following investigations were undertaken:
- EM31 survey - November 2001: A 46km EM31 survey was conducted
on-channel and on-land beside each bank. This was coupled
with drilling of 128 bores adjacent to the channel (to 4m depth)
to ground truth the survey.
- Additional geotechnical drilling
- March 2002: An additional 107 bores were drilled and 34 piezometers
installed. Bores
were generally drilled to a depth of at least 6m, and some
up to 10m.
- Pondage tests - May-June 2002: 12 pondage tests were
conducted at various locations along the length of channel
under investigation.
- Construction review panel field investigations
- June-August 2003.
EM31 survey
Due to the depth of the watertable generally being beyond the penetration
depth of EM31, the EM31 survey inferred likely seepage areas based
on unsaturated zone soil properties (rather than direct seepage detection
of the impact of seeped water in the watertable). The high-risk section
of the channel, as categorised by the EM31 contractor based on EM31
magnitude, totalled approximately one-third (15km) of the length
of the investigated section of the channel. Figure 2 illustrates
the classification of the channel into high, medium and low seepage
risk categories (red being high and green being low).
Figure 2 High, medium and low seepage risk categories
A combination of the EM31 results and impermeability grade (a lithological
classification devised for the investigation based on the amount
of clay in the profile) was used to identify sections of channel
considered to represent ‘very high’ risk areas. Four
significant lengths of channel were identified as ‘very high’ risk.
The total length of ‘very high’ risk area was 7.2km.
Further investigation was required before committing to the significant
expense of lining such a length of channel.
Additional geotechnical drilling
A further 107 deeper bores were drilled. Again, a combination of
the EM31 results and impermeability grade (incorporating results
from the new bores) and EM31 response was conducted. Following the
review of the additional drilling, the areas classified as ‘very
high’ seepage risk increased by about 1km (from 7.3 to 8.3km).
This resulted from removing as well as adding some areas to the ‘very
high’ risk category.
Pondage tests
In addition to the drilling program, pondage tests were required
to quantify seepage rates (and potentially identify a relationship
between EM31 conductivity and pondage test seepage rates) and confirm
interpretation of seepage rates based on geology and EM31 data. Twelve
pondage tests were conducted at various intervals along the channel,
covering a range of environments and areas of different geophysical
response. (Seepage rates ranged from 1-14mm/d).
Only a moderately strong relationship (based on the correlation coefficient)
was recorded between EM31 conductivity and pondage test seepage,
as shown in Figure 3. However, this is not surprising given that
these represent seepage sites more than 20km apart (significantly
further than other sites tested in the trials).
Figure 3 Relationship between EM31 conductivity and pondage test
seepage
The watertable along the WWC is generally beyond the penetration
depth of the EM31 and therefore the likelihood of seepage had to
be inferred based on soil properties beneath the channel. This contributes
to misleading results when comparing unsaturated zone lithology to
seepage rates, and is the cause of some of the scatter in the WWC
pondage test/EM31 regression relationship.
Based on the results of the pondage tests and relationship with EM31
and the drilling results, areas recommended for remediation were
proposed. Sites were defined as either priority one or priority two
seepage zones, depending on the degree of perceived seepage risk.
Priority one sites were considered to require remediation as part
of the upgrade, while priority two sites were monitored closely for
seepage following the upgrade. Given the uncertainty in the EM31/seepage
relationship, the EM31 predicted seepage was not used as the sole
means of assigning seepage risk and geological data and visual observations
were also considered.
Detailed site inspection after the remedial works indicated that
the EM31 survey results had a good correlation with the in-situ channel
material and accurately highlighted the delineation between clay
and sandstone areas (with the lower values of electrical conductivity
correlating well with sandstone areas). Many of the areas containing
cemented sandstone were deemed to be relatively impermeable and were
not lined.
Construction review panel
The EM31 provided an indication of where the channel did not have
seepage (e.g. clay areas) and where it might have high seepage (e.g.
sandstone areas). While it did not provide definitive results, it
assisted in selecting the areas requiring field checking. A construction
review panel comprised of experienced professionals was established
to preside over decisions regarding clay lining in sandstone areas.
Following inspection by the panel, it was determined that only 2.25km
of full clay lining and 1.09km of partial clay lining would be required.
This was approximately half that estimated based on the EM31 and
pondage test analysis. The partial clay lining was adopted in marginal
areas where it was considered that full lining would not be warranted.
The rules developed for areas requiring lining were included maximum
allowable seepage of 4mm/d in horticultural areas (priority one zones)
and maximum allowable seepage of 8mm/d in cropping areas (priority
two zones). While these rates may not appear high, similar rates
had caused damage adjacent to other sections of the channel.
More details on the clay lining aspects and remediation costs of
the WWC upgrade can be found under Remediation.
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Conclusion |
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The WWC seepage investigation is a good example of the
integration of geophysical, geological, pondage test data
and detailed field inspections to determine areas of highest
seepage risk. Each stage was used to refine areas of potential
seepage, with EM31 providing a tool for identifying areas
of potential seepage subsequently verified by field inspection.
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Costs of
investigation |
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EM31 survey - $16,363
Drilling - $40,521
Pondage tests - $14,000
Seepage interpretive work - $12,457
Project management costs are not included in these figures.
Figure 4 EM31 being undertaken along channel
Figure 5 Aerial view of clay lining works
Figure 6 Preparation of channel cross-section for lining,
with clay lining being undertaken in the background
Figure 7 Channel cross-section nearing completion, with
crushed rock being placed on the batters
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