construction and refurbishment of earthen irrigation channel banks

The product of the velocity and area is equal to the dischargeQ = 86.4 x V x AQ is the flow rate in ML/dayA is the cross-sectional area of waterway in m 2The Manning formula will give reasonably accurate results for open channels if theappropriate co-efficient of roughness is used and the channel slope is less than 0.10.Table 12-3 gives ranges of n for earthen channels in situations most frequentlyencountered with irrigation systems. The tabulated values have been compiled from anumber of technical references, based on experiments and observations of straightuniform earthen channels.WettedPerimeterDescriptionUnlined Channel – well-constructed straightuniform sectionCo-efficientof RoughnessnEarthFormationsLarge carrier channels (above 2500 ML/d) –with slick uniform section, free from aquaticgrowthMain supply channels (250 to 2500 ML/d)-newly constructed - bare soilSmall channels- less than 250 ML/d with someaquatic growth0.018 - 0.0200.020 - 0.02250.025 - 0.030Channels with considerable aquatic growth 0.030 - 0.035Channels with very heavy aquatic growth 0.040 – 0.050Table 12-3Values of Manning’s n for Open Channels12.11.1 Manning’s n – Co-efficient of RoughnessThe estimation of realistic values of the roughness coefficient n is an importantfactor in channel design.For a given channel the rate of flow is inversely proportional to the roughness of theperimeter surface. The co-efficient of roughness used in the design of channelsrepresents an evaluation of the degree of roughness of the wetted perimeter and itsnet retarding effect on the flow of water.The estimation of n warrants critical consideration and judgement in the evaluationof factors affecting its value.Channel roughness does not remain constant with time or even with depth of flow.An earthen channel may have one n value when first put into service and anotherConstruction and Refurbishment of Earthen Channel Banks August 2002 - Edition 1.0 12-13