Roughness Manning Coefficient Variation in Irrigation Open Channels by Changing Width and Roughness Surface in the Armfield C4MKII Equipment

Abstract. Velocity in open channel depends inversely proportional to the roughness. This roughness is measured by "Manning friction coefficient n" and is selected arbitrarily for channels and hydraulics structures design, it is considered constant and depend on available information and designer experience. However, actually this coefficient depends on several factors. Nevertheless, constant n is used for both steady and unsteady flow. This work was done in the ARMFIELD C4MKII Channel device located in the Hydraulics Laboratory in the Irrigation Department at the Universidad Autónoma Chapingo having 5m length, 0.076m width, variable slope and acrylic walls with stainless steel bottom using rectangular cross section. Measurements were carried for flow rates from 1.35 to 0.44 l/s, slopes from 0.258-2.198%, bottom width from 0.076 to 0.0532m, measuring values of depth 0.01 m apart for all M2, S2 profiles in all conditions. Different channel flow surfaces were used from manufactured conditions to 1200 and 80 sandpaper and artificial grass. Computed data of n were done using discharge and depth values measured for different slopes, n value increases when the slope rises, n increases when width is reduced, but not clear for the smallest width bottom. The values of n were insensitive in smooth channels with some instabilities, however, strong roughness conditions (artificial grass) showed results with good agreement with those reported in the literature for pipes and channels. As a result, Designers computing hydraulic profiles M, S, C, H and A, must be cautious selecting n because they might be wrong finding erroneous profile lengths.