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Inflow–outflow tests: principle,
method
Pages in this section include:
This page provides a detailed description of the principle and method for
inflow-outflow channel seepage identification and measurement
technique.
| Principle |
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The inflow-outflow method enables direct measurement
of losses. It is based on a water balance approach,
measuring water flow at both ends of a channel section
and taking into account additional inflows and losses
along the channel length being investigated.
This method is the only one that reflects actual operating
conditions and permits measurement without interruption
to system operations.
| Method |
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The inflow-outflow method is based on measuring the
rates of water flowing into and out of a selected section
of channel. The difference between inflow and outflow
is attributed to seepage, after accounting for inflows
(e.g. rainfall) and known losses (e.g. evaporation).
Accuracy in the results depends on accuracy of inflow
and outflow measurements, including the flow, rainfall,
evaporation and diversions.
The level of the channel should be kept approximately
constant during test periods in order to minimise the
effect of bank and channel storage and changes to the
wetted perimeter of the channel. Diversions must be
measured accurately, and any inflow (e.g. overland
flow or rainfall) into the channel must be taken into
account (ideally there will be no diversions). Measures
should be taken to eliminate as many parameters in
the equation as possible, such as drainage inflows
or diversion outflows. When tests are of long duration,
or are to be repeated in the future, rating curves
and tables can be prepared with water-level recorders
installed at the inflow and outflow gauging sites (to
enable calculation of flow from channel water level).
The basic equation for calculating seepage losses using
the inflow-outflow method is presented below. The figure
below graphically displays these components.
Components of inflow-outflow water balance [insert figure]
Flow rate measurement can be conducted using a number of techniques. The two
most common include:
- Use of a current meter to determine average velocity
(area-velocity method). Discharge equals average
velocity multiplied by the cross-section area.
This
method is usually the most practical.
- Regulating structures such as flumes
or weirs with automatic recording gauges. Without automatic gauges, observations
of water volumes passing through regulating
structures spaced at suitable intervals, with proper time lags between
observations, are reasonably accurate. The best accuracy over a wide range
of discharge
can be obtained with a V-notch weir. Where regulating structures suitable
for incorporating
accurate measuring facilities are absent, temporary weirs or gauging sites
can be established.
Climatic data can be obtained from the nearest weather
station. For potentially more accurate results,
a rain gauge and an evaporation pan can be established
near the section of interest. Corrections from pan evaporation to evaporation
from a shallow water body need to be applied, due to the extra heat taken in
through the pan's sides. The appropriate correction factor to use varies (from
0.6 – 0.8) depending on the rate of evaporation and relatively humidity,
however a value of 0.75 is recommended.
Other factors to consider in conducting tests:
- It is important that the flow rate into the test section is held approximately
constant for the duration of the survey. The channel should be operated
at or close to full supply level during the survey.
- All checks, wheels,
gated structures and doors must remain unadjusted for the duration of the
survey.
- Leaks through Dethridge outlets, etc should be identified and sealed
before commencement of discharge measurements. Structures should be checked
throughout
the survey for leakage.
- Accurate cross-section data must be available
or obtained at the locations where discharge is calculated using the area-velocity
method
described
above.
- Measurements should be conducted within a short survey period
(i.e. several hours) to ensure constant conditions during the period.
- The
water surface area along the section must be known in order to calculate
evaporation.
- If a long section of channel is being investigated, installation
of several rain gauges should be considered.
- The choice between
use of on-site pan evaporation and local evaporation data from a weather
station depends on the proximity
of the station
to the site
and the degree of accuracy required.
More detail of the method as well as specific references
can be found in the Literature review by visiting the Publications section of the IAL website.
| Related
pages |
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For a more detailed description
of the inflow-outflow technique see:
Inflow-outflow tests: summary
Inflow-outflow tests: applicability,
practical implementation, experience from the trials, indicative
costs |
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