construction and refurbishment of earthen irrigation channel banks

There are several ways a channel can fail to provide its required level of service or reachthe end of its useful life. These failure modes include:- lack of capacity- inefficiency- obsolescence- level of service (falls below acceptable levels)- structural integrity- redundancyEarly in the design phase, there needs to be agreement between the channel owner and thedesigner on the required life or durability of the channel. There is no simple or singleanswer to how long this period should be. It will depend on the particular circumstances,and life cycle cost analysis is a very useful decision-making tool to use. Refer to Section13.It should be recognised that the service requirements of a channel may change over time,and under some circumstances, the best solution may be one which permits someflexibility to address change and goes some way towards minimising life cycle costs. Forexample, a lower cost less durable channel where there is some uncertainty about futurerequirements.The designer needs to think broadly, from concept through detail, to the quality ofconstruction.12.4 Life-Cycle Cost ConsiderationsThe design of channels should include consideration of future operation, maintenance andrenewal costs, as well as initial construction costs so that an overall economy is achieved.Changes in design may be desirable in order to reduce future operation and maintenanceproblems, even though this may require increases in initial costs.The design service life of a channel bank should not exceed the projected economic lifeof the channel system or irrigation scheme. Otherwise the project will have overcapitalisedin an asset which will have a physical life beyond its required service life. Alower cost, shorter life bank design may be more appropriate to match the design andservice lives.Possible combinations of design variables should be considered and the volume ofearthworks, capital and maintenance costs etc worked out, with sectional dimensions andbed gradient chosen to achieve the lowest life cycle cost.A poor channel design can lead to excessive scour, weed growth, silting and seepagelosses, resulting in reduced channel life, progressive increase in maintenance costs orextensive remediation works.Refer to Section 13, Life-Cycle Cost Analysis.12.5 Objectives in DesignThe design of channels in erodible material can be a complex process involving a rangeof parameters based on functional requirements and the physical aspects of earthconstruction.Construction and Refurbishment of Earthen Channel Banks August 2002 - Edition 1.0 12-4