Field Study of Flow Resistance in Step-pool Streams (Case study of Dizin River)

Abstract

A series of steps and pools are ubiquitous bed forms in mountain stream channels, occurring where gradients exceed 2% and materials are in the gravel to boulder size range. Flow resistance, re ected by roughness elements, appears to be an important controlling factor in bed load transport rates and mean ow velocity. To estimate ow 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 a low-ow condition, while three-dimensional velocity measurements were made in low, medium, and high ow conditions. Gradient variations are in the range of 7% to 14%. As ow resistance is a function of geometry, bed material size, longitudinal slope, and hydraulic radius, dimensional analysis was conducted to develop a non-dimensional relationship for ow resistance in step-pool reaches. Thereafter, it was calibrated for the measured dataset of Dizin river and validated for Rio Cordon dataset. Comparable results of validation with a river located in a di erent environment suggest that ow resistance features in semi-arid and humid streams may have similar e ects on a non-dimensional resistance coe cient.