construction and refurbishment of earthen irrigation channel banks

In many Australian irrigation schemes the annual window of opportunity for liningexisting channels is at most 3 months and there is a limit to the amount of on-sitework that can be achieved in a year.If long lengths of existing channel have to be lined in relatively short periods of timeand the weather conditions are cold and wet; the low temperatures, wet subgrade anddifficult plant operating conditions do not favour hard-surface linings or earth-typelinings. Where as, a covered geomembrane lining may be a better choice under thesecircumstances.22.5.7 Land UseIn areas of high land value and intensive cultivation, the fullest productive use ofland is made, and land acquisition costs are high. Such situations favour theinstallation of pipelines or hard-surfaced lined channels with steep batter slopes. Itwould be expensive in such cases, to use a buried membrane lining which normallyrequires a 1 (vertical) : 3 (horizontal) batter slope.22.5.8 Operation and MaintenanceIf the operation of a channel system requires frequent filling and emptying or causesfrequent water level changes, a hard-surface lining will normally perform best. Withearth linings and earth-covered membrane linings, these conditions would speed upthe deterioration process considerably and would require increased maintenance.Most earth type linings do not prevent weed growth and do not withstand velocitieshigher than unlined channels. As well, de-silting operations must be carefullyconducted to avoid damaging the lining.The maintenance of earth linings and earth-covered membranes, such as weed controland silt removal, will therefore be very similar to unlined channels, and these highermaintenance costs should be assessed in a life-cycle cost analysis in comparison withthe higher installation cost of hard-surface linings.The adoption of exposed membrane linings or soil sealants may be limited by thehazard of livestock, de-silting operations or vandalism.When existing channels are lined, the time available to carry out the work mayinfluence the choice of lining. If the off-season shutdown is too short for the channelto dry out sufficiently for installation of a hard-surface lining, over-excavation andinstallation of a compacted earth lining or a covered membrane may be the bettersolution.22.5.9 Channel Size and ImportanceIn smaller channels the economy of compacted earth lining becomes less attractive.Generally, the larger the channel, the lower the unit cost because heavy earth movingequipment can be used effectively, while the size and manoeuvrability of equipmentsuitable for working in smaller channels is much more restricted. Another reason isthe unfavourable ratio of over-excavation to total excavation in smaller crosssections. Covered geomembrane linings are more suited to smaller channels.Construction and Refurbishment of Earthen Channel Banks August 2002 - Edition 1.0 22-16