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This page provides a detailed description of the principle and method for the pondage tests channel seepage identification and measurement technique. Principle The pondage test method uses a water balance approach to determine seepage losses in an isolated section of channel. Seepage losses constitute the drop in water level over time in the pond (or volume added to maintain a constant level) after accounting for evaporation, rainfall and other inflows and outflows. This method is a direct way of recording the losses through a section of channel. As such it is considered to be a datum against which other methods can be compared and calibrated. Method The pondage test method uses barriers or constructed pond banks to form a pond within a section of channel. The location of the barriers depends on project objectives, and might be determined by geophysical surveys or perhaps anecdotal information. Existing structures suitable for forming a sealed barrier can be used to minimise the number of new barriers to be constructed. A section of channel is blocked off with barriers at each end and filled with water up to, or slightly higher than, the level at which it usually flows during operation. As the water level in the channel section falls, the level is measured by a staff, hook gauge, or a water-level recorder. The time between measurements is also recorded, necessary corrections for evaporation and rainfall made, and the resulting seepage loss rate computed. A variation of the normal pondage test involves adding water to maintain a constant surface level. The volume of water added is measured and is considered the seepage loss (allowing for evaporation and rainfall). This method has the advantage of more accurately representing normal flow conditions than the falling level test but depends on accurate measurement of the pond depth. The equation for calculating seepage losses using the pondage test method is presented below (modified after Frevert and Ribbens (1988) to allow for rainfall and evaporation). See also the graph below.
If the only inflow into the reach is rainfall (which is often assumed to be the case), the length of the reach drops out and the equation reduces to:  Where (for above 2 equations):
These parameters are illustrated below:  Figure 1 – Channel cross section indicating parameter values to calculate the seepage rate
Estimates of evaporation and rainfall can be obtained from the nearest weather station. However, more accurate results can be achieved using an evaporation pan and rainfall gauge adjacent to the pond. As well as soil and hydraulic data for the channel, information about silt layers lining the channel is important in interpretation of results. Site survey The site should be surveyed before channel filling [while the channel is empty] to collect information on channel bed levels, channel cross-sections and structure or bank details. The survey should take account of the following requirements.
Survey definitions Left-hand side (LHS) – Left-hand side based facing downstream Right-hand side (RHS) – Right-hand side based facing downstream  Figure 2 –Survey points to be included in channel cross-section Elevations should be surveyed to Australian Height data (AHD). It is beneficial to survey to Australian map grid (AMG) coordinates to allow consistency with other spatial techniques such as geophysics and groundwater bore locations for input to GIS mapping and analysis systems. A site plan showing all survey channel data (preferably an electronic version), should be prepared to help interpret pondage test data and other related information. Pondage test soil barrier construction
Individual ponds can be tested for a site-specific investigation. Multiple ponds may be used if data are required to extrapolate to other parts of the channel in conjunction with other data such as geophysical response for calibration. Barriers should be located so that individual ponds coincide with sections showing similar geophysical response. A minimum length of 50m is recommended. Back-to-back construction minimises the number of barriers required to be constructed. Using an excavator and plastic or rubber membrane liner, barriers are compacted, lined and covered with soil to minimise or prevent seepage. An excavator will be required for construction of the banks. See figures 3 and 4 above showing two stages of a barrier under construction in a full channel. Landowner permission to use soil adjacent to temporary bank locations saves on transportation. Holes are filled in when temporary barriers are dismantled after the tests. Soil used for barriers should have sufficient clay content.
Hook gauges Hook gauges are to be installed at both ends of each pondage section to eliminate potential wind effects. Gauges should be calibrated to millimetre accuracy. Data loggers can produce more accurate results and eliminate the need for gauge monitoring during the test. Rainfall gauge and evaporation pan Rainfall and evaporation must be measured to develop the water balance. A rainfall gauge and class A evaporation pan should installed in middle of the pond (or at the middle pondage for a series of ponds). More accurate evaporation rates are obtained if the evaporation pan is allowed to float in the pond. Specifications can be obtained from the Bureau of Meteorology. Filling methodology Pondage tests within each section should start as close as possible to simultaneously, with water levels at full supply level (FSL). To account for losses during the initial wetting period of the soil, the methodology should include one of the following:
Set-up methods differ slightly vary depending on whether tests are being conducted at the start or end of the irrigation season. Filling methodology at end of channel operation The furthest downstream bank should be constructed first, working backwards towards the furthest upstream bank. The water level in the most downstream pondage should be allowed to back up to the highest degree, as this will experience the highest losses by the time the final pondage is constructed. A pump might be required to top up levels in the lower ponds before starting the test. Filling methodology at beginning of channel operationIn dry conditions, the downstream bank can be fully constructed before the release of the water. The remaining barriers can also be constructed, taking advantage of the dry conditions.To allow water to be transferred through the barriers, a suitably sized pipe should be installed towards the top of each bank, and each pipe adequately sealed when the filling of the ponds is completed. The use of these pipes will enable equilibrium to be maintained across all the ponds. A portable pump may be required to top up ponds. Water being released to fill the ponds needs careful control. Once the test has commenced the flow coming down the channel needs to be virtually stopped. Overflow of the upstream bank cannot occur during the test. This makes the timing of the test crucial and operations personnel need to be prepared to shutdown the system for the test.
During and after filling of the ponds, an inspection should be carried out of all sealed structures and along both barriers of the test reach, to check for leakage and other signs of water loss (e.g. stock usage, pumps). If inflow or outflow sites are found, attempts should be made to measure or eliminate them. Pondage test operation Duration of test
The field-recorded data is used in the equations to determine the estimated seepage rate. Download the Microsoft Excel spreadsheets below for an example of, or to record your own pondage test calculation output.
Pondage
tests: summary |
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5m
from toe of bank)
Pondage_example_Sep2003.xls