No abstract

The bed of the step-pool rivers is adapted to the flow conditions in terms of the topographic position and the steep slope of the path in such a way to dissipate the water energy, along the river. The beds of these rivers is sinusoidal like the meander rivers. However, as the river banks are mostly rocky in mountainous areas and the erosion in these areas is less, the bed fluctuation of these reaches is mainly vertical. When the water flow streams from the step crest into the pool, most of the water energy dissipates. In addition to the height difference, other factors contribute to the energy dissipation in the step pool reaches. The present study examines the energy dissipation by determining its effective factors, including step height (H), step length (L), hydraulic radius (R), sediment material diameter (d84), velocity in the step (V), etc. by field measurements in three reaches of mountain rivers. In the same vein, an equation is provided to estimate energy dissipation in the step pool reaches, by considering measurable effective parameters.

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.