FIELD-MEASURED HYDRAULIC RESISTANCE CHARACTERISTICS IN VEGETATION-INFESTED CANALS

RealiStiC estimates of hydraulic resistance are essential to the proper design and analysis of irrigation canal networks. Extensive field studies in Egypt provide data on the magnitude and variability of hydraulic resistance in earthen irrigation canals infested with aquatic weeds. Values of Manning resistance coefficient, n, were calculated from 280 measurements at selected cross sections in 23 stable canals with emergent ditch-bank vegetation. The temporal sample mean of monthly, values of n estimated within a year ranged from 0.017-0.062 for cross sections where five or more measurements were made. The temporal sample coefficient of variation ranged from 0.03-0.42. Dependence of n on flow regime (hydraulic depth and product of average velocity and hydraulic radius) was explored. Studies also were conducted on nine canals containing submerged vegetation. These studies included 312 sonar measurements of vegetation density as well as 156 measurements to estimate n at selected cross sections. Regression analysis revealed that variability in flow regime and in vegetation density contribute significantly to variability in n. Temporal mean of monthly values of vegetation density within a year ranged from 0.06-0.25, with temporal coefficient of variation between 0.17 and 0.92. Vegetation density at measured cross sections showed a clear seasonal pattern. The spatial mean and coefficient of variation in vegetation density along a canal were found to range monthly from 0.06-0.36 and from 0.18-0.76, respectively. Temporal mean values of n in canals with submerged vegetation ranged from 0.028-0.074, with temporal coefficient of variation between 0.13 and 0.57.