Abstract: Hydraulic calculations of the flow in channels and overbank areas of flood plains require an
evaluation of roughness characteristics. The Manning roughness coefficient (n), Chesy coefficient (C) and
Darsy-wesbach coefficient (f) are used to describe the flow resistance or flow retardation in canalized flow. In
catchment areas, surface flow is shallow and non-canalized, thus velocity and roughness coefficients can’t be
determined. Flow depth and bed slope are the most effectual parameters of flow retardation in gravel-bed lands.
In this study, a functional relationship between velocity of flow and bed slope is developed in order to covert
velocity of flow to bed slope for estimation of roughness for non-canalized flow in gravel beds. Four rounded
particles with different median sizes have been tested under different flow conditions in a 10 meter long flume.
Coarse particles were placed in the middle of the flume. Water surface profile and flow velocity were measured.
The results showed that the friction factor was considerably decreased, thus flow was less retarded with
increase of bed slope as a result of increase in flow velocity. Also, friction factor decreased with increase of flow
depth due to greater submergence of bed particles.