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Donald Main Channel
Case Study |
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Date of investigation
1998
Investigation conducted by
Wimmera Mallee Water (WMW) and Sinclair Knight Merz (SKM)
Contact details
Name: Peter Jackson, Wimmera Mallee Water
Phone: (03) 5362 0217
Email: peterjac@wmwater.org.au
| Background
Information |
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The Donald Main Channel is in Wimmera Mallee Water‘s Birchip
district. The channel, fed by Lake Batyo Catyo, supplies about 30,000ML
per year. The channel extends north for about 50km, then east towards
Wycheproof township. The channel, originally constructed about 1912,
was remodelled in 1960 to increase flow to 245ML/d. The channel runs
for about nine months of the year from April to about December.
The study area was defined as the two lengths of the channel shown
in Figure 1, which appear to be worst affected by seepage. The two
sections are described as ‘Sheridans’, a 2.5km section
that runs south-west from the Borung Highway, and ‘Ralstons’,
a 3.5km section between Bishs and Pit Roads.
Figure 1 Study areas on Donald Main Channel
| Channel
characteristics |
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Channel operation
Varies - generally runs April to December.
Capacity
Runs on two-year cycle. Typical flows: 200-240ML/d in first year
and 80-100ML/d in second year.
Operating water level depth/width
Depth approx 1.1-1.3m
Width approx 6-8m.
Soil type
Channel base is underlain by sandy silt in part and silty clay in
part.
Geology
The watertable is within the Shepparton Formation (clay, sandy clay)
overlying Parilla Sand Aquifer (sandy clay and fine to coarse sand).
The channel abuts and sometimes traverses lunettes of Lake Buloke
(clay, sandy clay, fine sand).
Groundwater salinity (ambient)
10,000 mg/L - 40,000 mg/L (approximately 15,000-60,000EC) . Depth
to groundwater
Depth to groundwater away from channel influence (on flats, away
from lunettes) is approximately 2m below surface.
| Seepage
impacts and history |
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The channel has a long history of seepage problems and seepage
trials date back to 1957. Investigations began as a result of
complaints from landholders regarding the salinity of land adjacent
to the channel.
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Investigation
drivers |
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The three main drivers behind the investigation were as follows:
- Land degradation immediately adjacent to the channel,
and landholder concerns regarding salinity impacts.
- Financial
cost of lost water. Economic justification for seepage
control works was based on potential for water savings.
- Regional
contribution to high watertables and salinity, which is
a key environmental issue in the area.
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Investigation
objectives
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The main objectives of the study were as follows:
- To quantify seepage loss from the two sections of channel.
- To
evaluate techniques for determining seepage loss from other
sections of the channel.
- To determine the effect of channel seepage
on land salinity.
- To identify and evaluate of options to reduce
seepage from the channel.
| Summary
of investigations |
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Quantification of leakage
Seepage loss from Sheridans and Ralstons was measured using
pondage tests, which involved:
- Using an excavator to construct temporary earthen banks
across the channel to isolate six separate channel
sections (three
each in Sheridans and Ralstons).
- Lining the banks
with polyethylene sheeting to ensure they were watertight.
- Filling
the six ponds with water.
- Measuring the water level drop in
each of the six ponds over 11 days to determine the seepage
rate.
Based on the pondage test results and normal operational
pattern of the channel, average annual seepage rates
were calculated.
The average annual seepage rates ranged between 52
and 144ML/year per
kilometre length of channel (or between 25 and 67mm/d).
The highest seepage loss was at the southern end
of Ralstons. The seepage
rate for Ralstons was generally higher than for Sheridans.
Figure 2 Cross-section and plan view of channel showing
temporary banks
Evaluation of indirect techniques for measuring channel seepage
Pondage tests are accurate, but
expensive and difficult to coordinate
with channel operations. Pondage tests along the whole 32km length
of the Donald Main Channel would have been impractical and too expensive.
A number of indirect techniques were considered for measuring channel
seepage from the rest of the Donald Main Channel and other channels
in the Wimmera Mallee Water system.
Trial methods included:
- Use of a constant-head permeameter to measure
the seepage rate from shallow holes augered into the base of
the channel.
- Use of an electromagnetic technique (EM34) to measure
the electrical conductivity of the soil to a maximum depth of
20m. In high seepage
areas the conductivity adjacent to the channel is lower due
to the presence of fresher water from the channel. In this way,
EM34 was
used to map the areas of channel seepage.
Both the constant-head permeameter and EM34 surveys were conducted
along the same lengths of channel as the pondage tests. Results
were compared with seepage rates from the pondage tests.
The correlation between constant-head permeameter results and
seepage rates and was very weak, but strong for EM34 results.
The EM34 results were therefore used to calculate channel seepage
rates from a length of channel at Ralstons not subject to pondage
tests.
In addition, other sections of the channel outside of these two
sections were assessed using the same EM34 – pondage test
seepage relationship. This provided an indication of the variation
of seepage along the channel
and helped to identify and prioritise sites for seepage control
works (refer Figure 3).
The methodology used in this study could be applied to other channel
systems.
Figure 3 Sheridans partitioned yearly seepage rates
Land salinity
Salinity mapping showed the distribution of salt-affected
land. Both Sheridans and Ralstons have severe land
salinity immediately
down-slope
of the channel. This is a result of a near-surface watertable that
has brought saline groundwater to the plant root zone.
Possible causes for the high watertable near the channel include:
- Channel seepage.
- Land clearing in the region and in the highlands
to the south, resulting in increased groundwater recharge
(ie increase in
rainfall infiltration)
and a rise in watertable.
- Elevated water level in nearby Lake
Buloke, causing leakage from the lake and creating a barrier
to natural groundwater discharge.
Investigation to determine the relative contribution of these
processes to salinity included:
- Comparing groundwater levels to channel water levels, using
monitoring results from four lines of bores intersecting the
channel at right
angles.
- Comparing the groundwater chemistry to channel water
chemistry (salinity, oxygen-18 and deuterium).
- Comparing the
pattern of land salinity with the position of the channel.
- Using
past investigations into the effect of Lake Buloke on local
groundwater levels.
The results showed that the channel influence on groundwater
levels, groundwater chemistry and land salinity decreases with
increasing
distance from the channel.
The relative influence of channel seepage, regional groundwater
processes and lake’s effect on land salinity was difficult
to determine. The study concluded that the salinised area at
Sheridans and Ralstons,
within about 50-100m west of the channel, is predominantly channel
influenced, with regional groundwater processes and Lake Buloke
having a less dominant effect. Outside this near-channel zone,
land salinity
is likely to be predominantly caused by regional groundwater
processes and lake effects. The limit of channel seepage is unlikely
to be
greater than about 200 metres from the channel, as shown by groundwater
chemistry
data.
Seepage control works
The following seepage control options were considered:
- Plastic liners covering the full perimeter of the channel,
e.g. polypropylene or high-density polyethylene
- Core trench
in the banks using either plastic liners or a clay-bentonite
backfill
- Clay lining
- Clay-bentonite lining
- Shotcrete lining
- Seepage interception drains
- Tree planting
- Replacing the channel with a pipeline.
Replacing the channel with
pipeline was not considered feasible because of the high capital
and maintenance costs.
The most cost-effective option was found to be plastic
lining with either high-density polyethylene or polypropylene.
The
cost of
plastic lining was estimated to be $120 per metre (1998
costs).
| Recommendations |
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The main recommendations of the study included:
- The rankings in Figure 4 be used to prioritise
channel segments for seepage control work.
- Channel lining
with plastic liner be considered as the most cost-effective
seepage control option for Sheridans
and Ralstons.
- Further investigation is required to verify
the link between channel seepage and the EM34 survey
results.
If this relationship
is verified,
EM34 should be used to calculate seepage rates along
other stretches of the channel.
Costs of investigation
The costs for the consultant to undertake channel seepage
measurement investigations (including contractor costs,
but excluding earthworks
for pondage tests) was $40,000 (1998 costs).
Figure 4 Channel base in foreground, salinity
in background
Figure 5 Pondage bank in background, hook gauge in foreground
Figure 6 Line of groundwater bores (view from canal towards
salinity area)
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