construction and refurbishment of earthen irrigation channel banks

Lining of channels will not eliminate all conveyance losses, and it is necessary tocarefully distinguished the losses that can be realistically saved by lining. Even,lining of a channel will not reduce the seepage loss to zero, because the liningmaterial used will not be completely impervious.Due to the lack of supervision at the time of construction, the use of poor qualitymaterial and many other reasons, imperfections can easily occur in linings. If thelining is not installed correctly or not properly maintained, the benefits of lining willnot be realised.Seepage losses from properly constructed concrete lined channels should normally berelatively small. However, subsequent cracking due to expansion or settlement orpoor construction or maintenance can result in large losses. Other types of lining arealso susceptible to varying amounts of seepage losses, depending on type, quality ofconstruction and local conditions. The possibility of appreciable losses from linedchannels, particularly after a period of years, should be kept in mind when preparingwater saving estimates.There will be situations where channel lining is not necessary or cannot beeconomically justified. In the foreseeable future, it will not be cost effective to lineall channels and economics dictate that lining efforts need to be concentrated onthose sections of channels that have the highest seepage rates and hence wouldbenefit the most from lining.Care also needs to be taken when talking about water savings. It is commonlyinferred that channel seepage is lost water. In some situations this is valid, but inothers it may not be, for example where the seepage water recharges an aquifer.If the aquifer is fully committed to productive use and seepage losses were reduced,then recharge would decrease and depletion of the aquifer would occur. It thereforemay not be possible in all cases to consider the water savings from channel lining inisolation, but rather a broader river basin perspective may be required to identify ifthe reduction in seepage represents a real water saving.Taking refurbishment, or periodic restoration, as an essential part of the long-termmaintenance of earth channels, there will be periods in the life of a particular channelwhen the water losses are high (before refurbishment) or low (after refurbishment)and its efficiency is respectively below or above average.An earth channel system will inevitably contain channels which are about to berefurbished, channels just refurbished, and those in between. The overall systemefficiency is thus the resultant of cyclically varying individual channel efficiencies.The average efficiencies of individual channels, and probably their refurbishmentcycles, will also vary in relation to soil types. Thus, at any point in time, channelsthat are then in good condition subsidise those then in poor condition and so thesystem efficiency is maintained.The question of lost water becoming available for delivery can only arise when theoverall system efficiency is increased on a permanent basis. This could follow fromimproved operational techniques or conversion from earth channel to pipeline. Therefurbishment of a particular channel does not, however, have this result andtherefore does not, in fact, make recovered losses available.Construction and Refurbishment of Earthen Channel Banks August 2002 - Edition 1.0 22-9