construction and refurbishment of earthen irrigation channel banks

In high watertable areas even a small quantity of seepage may be detected by visualobservations. On the other hand, in deep watertable areas it is possible that nosurface effects are evident on land adjacent to channels having high seepage losses.There will be many locations where the origin and magnitude of seepage losses arenot obvious and a seepage study will be required to detect and measure the losses.Seepage losses can vary widely along different sections of channel depending onsoil and geological conditions. In some irrigation schemes a high proportion oftotal seepage can be caused by relatively short sections of high seepage ratechannels. If the sections of channel where the seepage is the greatest can be locatedand treated, this can present the most cost-effective means of reducing overallseepage losses.However it should be noted that inaccuracies in channel flow measurement can attimes create the perception of severe seepage losses, and some channels suspectedof having high seepage losses may have no such problem. The suspected seepagelosses may simply be due to inaccurate flow measurement.Generally more than one method of detecting and measuring seepage is used in achannel seepage study:• Visual observations for signs of seepage• Pondage tests• Inflow-outflow studies• Seepage meter• Aerial photography• Piezometer survey• Mapping of salinised areas along the channel• Soil maps and surveys• Water balance estimates• Water chemistry analysis• ElectroMagnetic (EM) survey• PermeameterAny assessment of methods needs to consider the differences between seepagequantification and seepage identification, the time available, the likely magnitude ofseepage losses, the availability of equipment and skilled technical personnel, andthe trade offs between cost and accuracy.A staged investigation of seepage sites may involve:(i)Initial IdentificationIdentification of suspected high seepage sites by visual inspection of localisedproblems, landholder reports and knowledge of soil and geological conditions.Visual indicators of seepage are weed growth or bare ground beside a channel,the presence of spongy ground underfoot, unhealthy crops or pasture near achannel and the presence of water on the ground surface next to a channel.However seepage may occur through the bed of a channel and so directly into anunderlying aquifer, thus the evidence of the seepage may not be apparent on theground surface. In these situations, slow filling and fast emptying channels maybe tell-tale signs of high seepage losses.Construction and Refurbishment of Earthen Channel Banks August 2002 – Edition 1.0 21-23