14.1  Seepage control works

Seepage investigations in the CIA during 2000/2001 consisted of the examination of two km of EM-31 data for the Boona Main Supply Channel at the Eastern end of Bull Road.  Seepage investigation techniques are now being applied on-farm to assist shareholders in identifying farm channel seepage problems.

Seepage treatment works carried out in the CIA (see below) include clay re-lining of banks, bank reconstruction and bank and bed reconstruction.

• A 400m section of the left bank of Yamma 2 Supply Channel upstream of Kidman Way was re-lined with clay.  Due to high accession to the groundwater through the bed of the channel at this location, 325m of bed re-lining was necessary.
• A 400m section of the left bank of Bundure 3 Supply Channel was reconstructed at Farm 606. The presence of soil with a high content of gypsum shot necessitated the re-lining of 400m of the bed of this channel as well. 
• A 50m section of both banks and the bed of the Argoon 3 Supply Channel were re-lined with clay.  The loamy texture of the soil and the infiltration of groundwater at this location meant that the bed had to be underlined with bentonite-impregnated geotextile material. 
• A 150m section of the right bank of the Boona Main Supply Channel near the town industrial area was re-lined with clay.

Table 14.1 shows a summary of seepage investigations and treatments since 1993/94.  Treatments carried out in 2000/2001 include works carried out during the 2001 winter maintenance program.

Table 14.1  Summary of seepage investigations and treatments since 1993/1994

It is possible to make an estimation of the water losses saved by clay lining works undertaken this year by way of making several assumptions:

• The Idaho testing locations along the channel can be averaged over the whole section of channel being clay-lined
• Prior to clay lining, an average infiltration rate of 25mm/day is observed
• After clay lining, an average infiltration rate of one mm/day is observed
• The area over which infiltration occurs is the wetted perimeter of the channel section multiplied by the length of channel
• The average duration for which channels are full of water is 270 days/year

Using these assumptions it is estimated that the 2001 clay lining and bank reconstruction of 4,975m² wetted perimeter would equate to one kilometre of channel and would save approximately 20 ML/year.

The results of the clay lining are consistent with the projections of SKM seepage investigations.  These indicated that the cost of seepage reduction can not be justified by the value of the water saved alone.

Most of this year’s remedial seepage work has revealed that the identified seepage area is due to the bed of the channel intercepting a “gypsum shot” soil layer. By preventing water entering this layer there should be a change in the visible symptoms of the seepage. This may be a drying-up of an area of swampy ground beside the channel, reduction in damage to roads beside channels or the containment of the spread of salt-affected patches beside channels.  These identifiable improvements are in addition to the area-wide reduced accessions to the watertable that are achieved by seepage reductions.