construction and refurbishment of earthen irrigation channel banks

12.16 Channel Cross SectionThe cross section selected for a channel must be able to carry the maximum design flowand satisfy the appropriate relationships between bed width, water depth, batter slopes,freeboard, bank dimensions and operation and maintenance. The cross section may, atvarious points along the channel, be partially or entirely in cut or fill.Designing the cross section of a channel means determining the geometric shape for twomajor elements:- the water section- the above-water sectionThe components of a channel cross section are shown in Figure 12-9.fencecrestbankbermfreeboardbermbankcrestfencebedwaterwayFigure 12-9 Channel Cross-SectionMany channel design methodologies are primarily concerned with the hydraulic aspectsof the water section, and less attention is given to the above-water section. Theseoptimisation methods determine a channel cross-section which minimises hydraulicresistance or alternatively, determining the least cost channel dimensions. However, theyfocus primarily on the water section, and do not take into consideration the practicalconstruction, operation and maintenance requirements of the total channel cross-section.Many of the variables related to channel geometry used in these methods, such as flowdepth, channel width, channel bed slope and batter slopes, are in fact independentvariables for earthen channels and are determined by the combination of the physicalsituation of the channel; practical considerations, economic conditions, bank stability, andoperation and maintenance practices.The definition of the best cross-section for a channel is not the one which conveys a givendischarge with the minimum cross-sectional area or the least cost cross-section. Rather,the best cross-section is the one that has the lowest life cost, including construction,operation, maintenance and renewals of the below-water section as well as the abovewatersection of the channel.To reduce total life cycle costs, the design approach must incorporate the above-watersection as well as the water section and each channel needs to be evaluated individuallyfor the most economic combination. Refer to Section 13, Life Cycle Cost Analysis.Earthen irrigation channels in Australia are almost exclusively constructed with atrapezoidal cross-section.The proportions of a channel cross section below the water level will vary considerablydepending on the channel capacity, grade and the soils along the channel alignment.Construction and Refurbishment of Earthen Channel Banks August 2002 - Edition 1.0 12-34