Seepage measurement techniques include direct and
point seepage measurement, subsurface characterisation, and remote
non-invasive techniques.
| Seepage identification & measurement techniques |
Scale [1]
|
Operational constraints |
Accuracy |
Cost |
Best suited
to... |
| Local |
Inter-
mediate
to large |
Macro |
| Direct & point measurement |
The techniques most likely to provide accurate
measurements of the rate of seepage are pondage tests and
inflow-outflow tests. In addition, point measurements provide
a way of quantifying seepage rates at specific locations
in
a channel. |
| Inflow-outflow
tests |
|
 |
|
Very minimal effect on operation |
Fairly accurate over long sections (depending
on method of measuring flows) |
Depends on whether existing structures in place
for measuring flow - if not, contractors required & will
be moderately expensive |
Regional to large scale investigations for quantifying
seepage at a broad scale |
| Pondage tests |
|
[2] |
|
Must be conducted at start or end of channel
run |
Most accurate method |
Relatively expensive, largely due to cost of
pond bank construction. |
Investigations requiring seepage rates to a
reasonable level of accuracy. Useful for calibrating techniques
with greater density of spatial data (eg, soil and geological
profile classification, geophysics) |
| Point measurement |
|
|
|
Some types can be conducted while channel running |
Not reliable for absolute quantification. |
Depends on density of tests - will be relatively
expensive if reasonable level of accuracy required |
Local scale investigations and assessment of
relative seepage [3] |
| Subsurface character-isation |
Subsurface characterisation of the area around
a channel provides information on which assessments of the
sites of seepage can be made |
| Soil and geological profile classification |
|
|
|
No effect on operation |
Provides indication of broad zones of relative
seepage [3]. Quantification: provides first cut estimate
of seepage only |
Relatively inexpensive if using existing data
/ maps. Moderately expensive when drilling & on-site
classification required |
Site characterisation and assistance in interpreting
other seepage results. |
Groundwater assessment
Including water-level monitoring, mathematical modelling and
hydrochemical investigations |
|
|
|
No effect on operation |
Accurate if aquifer parameters properly characterised,
which can be expensive. However only relevant for slice across
a channel |
Moderately expensive, depending on groundwater
depth. Aquifer characterisation for quantifying rates likely
to be expensive. Most chemical techniques highly expensive. |
Best at local scale due to intensive data requirements.
Provides permanent assessment tool, therefore useful for
post-remediation assessment. |
| Remote non-invasive techniques |
‘Direct and point measurement’ and ‘subsurface
characterisation’ techniques assist in identification
of seepage distribution and rate by directly measuring a
physical property at a single location. For example, groundwater
monitoring of water levels in a bore allows a direct measure
of the watertable, and infiltration tests are direct measures
of the soil properties at a point.
In contrast, Geophysical surveys and remote sensing techniques
use high-density sampling of subsurface and near-surface
properties to provide continuous data along the channel. |
| Geophysical
surveys |
|
[2] |
|
No effect on operation |
Most accurate method for assessing relative seepage2. Most
efficient method for quantification of seepage if calibrated,
eg against pondage tests |
Relatively inexpensive in terms of km channel covered but
proper interpretation and calibration (eg pondage tests)
will increase costs |
Recommended technique for large scale investigations -
when 'calibrated' against pondage tests provides a good indication
of relative seepage AND actual seepage rates |
| Remote
sensing |
|
|
|
No effect on operation |
Can accurately detect surface / near surface seepage over
large areas, but not useful for seepage quantification |
Relatively inexpensive (depending on source data used)
in terms of km channel covered but proper interpretation
will increase costs |
Large scale to regional investigations where the primary
focus is on land degradation (eg, salinity) induced by channel
seepage |
Summary
|
Summary of principle, method, applicability,
practical implementation, experience from the trials, indicative
costs
|
Principle |
Outlines how the technique is used to indicate and measure
seepage. |
Method |
How to undertake the technique. Describes the set-up,
planning, operation, collection and analysis of data and
descriptions
of the way in which results are presented and used, based
largely on examples from the IAL trials. Worked examples
of calculations
are provided.
Descriptions include some of the practical and
contractual issues to be considered in implementing the
measurement techniques |
Applicability |
Reason for using the technique (advantages and disadvantages).
Refers to the outcomes likely to be obtained and how these
meet project objectives. It embraces factors such as scale
and physical conditions (soil, groundwater, etc.). It also
applies to techniques for post-remediation measurement and
monitoring. |
Practical implementation |
Factors to be considered before implementation. Identifies
factors such as scale, timing of testing, and operational and
physical constraints on undertaking the technique at a particular
site. |
Experience from the Trials |
Experience implementing the technique including case studies. |
Indicative costs |
Costs based on various situations. Summarises indicative
costs to set up, implement, monitor and analyse the information.
Costs of interpretation and input to channel management are
not included as these are pertinent to the individual channel
operator.
Indicative costs for each of the techniques are based on
costs from the IAL trials. The costs for a given technique
vary between authorities due to the use
of different subcontractors, use of internal resources rather than subcontractors,
different internal accounting procedures (e.g. different charge rates for
staff) and differences in channel and site characteristics
(e.g. access, channel width).
Where the work can be conducted in-house, the anticipated personnel time
required is provided rather than a dollar cost, as this
varies from authority to authority,
depending on internal charge rates. |
