construction and refurbishment of earthen irrigation channel banks

Concrete linings are suitable for both large and small channels, for both high and lowvelocities.Compacted earth and covered geomembrane linings can be used on comparativelysmall lengths of channel, say 300 metres, without any elaborate arrangements or extracosts.22.5.10 Acceptable Seepage RateThe objective of channel lining is rarely to completely stop all seepage, but to reducethe seepage to an acceptably low rate. Some irrigation authorities are using a targetseepage rate of 0.002 to 0.003 m 3 /m 2 /day for channel lining projects. However thebasis of this target was not always apparent and its adoption may have beensomewhat arbitrary.Seepage from channels can be reduced to acceptable limits by lining, however thecost of lining all channels in an irrigation scheme may be prohibitive. Seepage canvary widely among different reaches of a channel, and overall seepage losses can bereduced to acceptable limits at reasonable cost by lining reaches of channel whereseepage is greatest, if these reaches can be definitely located. The challenge is toidentify these sites when there is no easy method of monitoring seepage losses on anextensive and continuous basis.Acceptable seepage rates will depend on the individual circumstances of each projectand will influence the choice of lining. No channel lining is completely impervious.The overall permeability of a lining depends partly on the intrinsic properties of thematerial, and partly on the existence of flaws. Probably the most impermeable andlong-lasting is a concrete lining placed over the top of a geomembrane, while theleast durable are some of soil sealant treatments. Indicative permeabilities fordifferent channel lining materials are given in Table 22-4.Type of LiningExpectedSeepage RateReductionIndicative Permeabilitym 3 /m 2 /dayCompacted earth 70-90% 0.0005 – 0.002 but varies widelyBentonite lining 60-70% 0.0005 – 0.001Chemically stabilised soil 60-90% 0.0005 – 0.001 but varies widelywith material usedConcrete 70-95% Below 0.0005 if well constructed& maintainedGeomembrane 85-95% 0 - 0.0005 but variesUnlined – ordinary clay loam 0.005 – 0.025Table 22-4Indicative Channel Lining Permeability’sReported seepage rates in Australia and overseas for the same type of channel liningcan vary significantly depending on such factors as depth of water, level ofwatertable, lining thickness, local conditions and construction standards. Directcomparisons can therefore not always be made.If the value of water lost through seepage is high or the secondary effects areparticularly adverse, the aim should be to adopt a relatively watertight lining.Construction and Refurbishment of Earthen Channel Banks August 2002 - Edition 1.0 22-17