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Pondage tests: applicability, practical implementation,
experience from the trials, indicative costs |
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Pages in this section include:
This page provides a detailed description of the
applicability, practical implementation, experience from the trials
and indicative costs for the pondage tests channel seepage identification
and measurement technique.
Applicability
Pondage
tests are considered the standard for channel seepage quantification.
The most accurate means of measuring channel
seepage, they are suitable for benchmarking
and assessing other methods.
Pondage tests can be undertaken at most locations
to provide the channel seepage base data. At locations where there
is
hard rock, care must be taken to secure
the barriers. In some very large channels, the cost of earth works may be
prohibitive.
Inconvenience of conducting tests outside normal channel operation
times and the cost of bank installation are factors to be considered
for pondage
tests.
In the trials RWAs found it convenient to undertake tests at the end of
the season.
The extent of pondage test work to be undertaken at a site depends
on the objectives of the investigation.
- If the investigation is aimed solely at determining the seepage
rate along a specific length of channel, a single pond would
be appropriate.
- While a long section can be tested at minimal
cost (i.e. only two barriers), if
pondage tests are to be used to identify seepage within short-to-medium-length sections,
multiple barriers will be required, which (especially for large channels)
can be very expensive.
- A minimum of four ponds is recommended for
a section if results are to be correlated against geophysical data,
point test data or groundwater analyses and used for extrapolation
to larger lengths
of channel.
Pondage tests are probably the most accurate post-implementation
measurement technique. A difficulty may be obtaining a suitable
seal between barriers and
remediated sections.
| Practical implementation |
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Pondage tests must be conducted outside normal channel operation,
and non-flow conditions introduce some inaccuracies. This means
that:
- The channel must remain out of use during tests.
- Cost of barriers
can be high, and sometimes prohibitive for very large channels.
- Conditions
do not reflect velocities and sediment loads carried during
normal channel flow.
- Pondage test results do not provide an indication
of the spatial variation of losses within the reach isolated
and only provide a bulk figure for seepage.
The best times for conducting pondage tests are at the end
or beginning of the
water distribution season, so as to minimise disruption to
the system. Of these
two options, the ideal time is at the end of the water distribution
season, immediately before the shutdown of the system, as
subsurface conditions are closest to those
encountered during operation. Pondage tests conducted at
the beginning of the season can be of advantage if initial
channel start-up losses are of special interest.
| Experience from the trials |
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The set-up for the trials involved installation of six back-to-back
pondage sections, each 150-300m long. Locations were
determined based on inferred zones of high or low seepage.
The results are an average of the seepage rate over the
pond length and do not necessarily target areas with
the highest seepage zones.
Trial results suggest that under full-scale investigation
and operational programs, ponds could be located according
to geophysical indicators of seepage. Where possible,
barriers should be located so that they coincide with
sections showing similar geophysical response. A minimum
pond length of 100m is recommended.
Pondage tests were conducted successfully at a large
number of sites in the trials. Six
150-300m long ponds were installed and monitored
over a two-week period. A summary of the results
is presented below.
Pondage test summary from the trials
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Channel |
Seepage rate range
(mm/d) |
Average seepage rate
(mm/d) |
Pondage section |
|
Donald |
9-48 |
35 |
6 |
Toolondo |
1-11 |
7 |
6 |
Rocklands |
4-13 |
8 |
6 |
Tabbita |
6-10 |
7 |
6 |
Lake View |
7-9 |
8 |
4 |
Dahwilly |
4-16 |
10 |
6 |
Finley |
4-7 |
4 |
6 |
Waranga Western |
1-13 |
5 |
11 |
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The sites tested were considered by RWAs to
be high-seepage sites. The rates of seepage determined were
variable along each channel. The range of rates
along the channel sections appeared to be consistent
with the geological profile at each site and
the perceived significance of the seepage problem.
However, there were anomalies identified in
several locations.
Although pondage test seepage rates are the
most accurate means of measuring channel
seepage, they may still lead to an underestimation of seepage
compared to channel flowing conditions. For example, at Dahwilly
in SE NSW, where the geological profile is very sandy, it was
expected that the rates of seepage would be high, although
pondage test results from the selected sites suggested low
rates of seepage. This is due to the effects
of clogging caused by siltation when the
water is stationary.
| Indicative costs |
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The most significant cost element for pondage
tests is bank construction. Next is personnel time
for monitoring during the tests. The
cost estimates below assume that the
RWA has the required equipment available
for the tests, including hook gauges,
rainfall gauge and evaporation pan.
Bank construction/removal:
- Earthmoving contractor costs - These
costs vary considerably depending
on the availability
of a suitable local clay source.
Approximate costs for bank construction
and removal in the IAL trials
are listed below. These were for
channels
between 10-20m wide (at FSL) and
1.5-2m deep:
- Wimmera Mallee Water: $1,000
per bank for seven barriers
- Murray
Irrigation: $715 per bank for
10 barriers
- Murrumbidgee Irrigation:
$780 per bank for seven barriers
(including importing clay)
and $560 per bank for
five barriers (using on-site soil)
Differences between authorities
are attributable to different
channel sizes, economies
of scale for different number
of barriers constructed and
different
charge-out rates for internal
personnel.
- Supervision by RWA personnel
- Generally a least half
a day would
need to be
allowed for supervision of
bank construction.
- Plastic
lining - Plastic lining is
recommended for
barriers, and essential
if the bank is not constructed
with a suitable percentage
of clay material. The cost
of this for one pondage bank
is relatively minor (e.g.
$50-$100/bank). Additional
personnel (two recommended)
are required to lay this
plastic. About
one hour per bank should
be allowed.
- Cross-section
and hook gauge surveying – A
least one cross-section per
200m, or
one at the upstream
and downstream ends of the
pond is recommended.
Typical costs in this study
for the cross-section surveying
for
six cells (every 200m) ranged
from $600 (Murray Irrigation),
$1,200 (Wimmera Mallee Water)
to $2,000 (Murrumbidgee Irrigation).
The variation is largely
attributed to internal
charge-out rate differences
and
use of external contractors.
- Set-up
costs – Activities
including installing hook
gauges, rainfall gauge and
evaporation
pan) can be conducted by
RWA staff. This is likely
to take
half to one day.
- Monitoring
costs – Monitoring
can be conducted by RWA staff.
Costs depend on the number
of cells at the site and
bank spacing. Excluding
travel time to
the site, the daily time
required on site for one
cell is about
15 minutes. During trials
the typical
time required for monitoring
six cells at one site was
about half to one hour, including
measurement
of water levels, rainfall
and
evaporation. Monitoring should
be conducted once per day
for at least seven
days.
| Related pages |
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Pondage tests:
summary
Pondage tests: principle,
method
|
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