Abstract
Step–pool reaches are common bed forms in mountainous streams exceeding 3% gradient and provide an important means of flow resistance in steep rivers. Step-pool channels are characterized by longitudinal steps formed by large boulders which are constituted into discrete channel-spanning accumulations that separate pools comprising finer grain. Flow resistance, reflected by roughness materials, appears to be an important control on bed load transport rates and mean flow velocity. To estimate flow resistance some morphological features and velocity were measured in the step-pool channel of Dizin River, located in Karaj River watershed in Iran. Topographic surveys and bed sediment sampling were made in low flow condition while three-dimensional velocity measurements were made in low, medium and high flow conditions. As flow resistance is a function of geometric, bed material size, longitudinal slope and hydraulic radius, dimensional analysis was conducted to derive a non-dimensional relationship for Chezy coefficient in step-pool reaches. Thereafter, it was calibrated for the measured data set of Dizin, Ammameh and Rio Cordon. Comparable results of calibration with a river located in a different environment may suggest that flow resistance features in semi arid and humid streams may have similar effects on non-dimensional Chezy coefficient.
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