Document Type

Thesis - Open Access

Award Date


Degree Name

Master of Science (MS)

Department / School

Civil and Environmental Engineering

First Advisor

Francis Ting


The estimation of bed shear stress plays a pivotal role in understanding the complexities of flow dynamics in both natural and engineered channels. The commonly employed log law formula offers a mechanism to compute the bed shear stress in open channel flows. However, its applicability in flow transition remains relatively unexplored. In this thesis, an in-depth investigation into the behavior of the velocity profiles and bed shear stress during the transition from Rough to Smooth (RTS) and Smooth to Rough (STR) beds is undertaken. Experiments were conducted on smooth-to-rough (STR) and rough-to-smooth (RTS) transitions in an open channel flume under different composite water surface profiles. The velocity field was measured at the channel centerline using a Particle Image Velocimetry (PIV) system. The bed shear stress was determined from the measured velocity profile and water depth using several methods. The primary objective was to investigate the effects of bed roughness and water surface profile on bed shear stress in gradually varied flow. This study found that in gradually varied flow through a roughness transition the evolution of bed shear stress was related to both the water surface profile and bed roughness. In both RTS and STR transitions, the bed shear stress adjusted to the new bed condition almost immediately even though the velocity profile away from the bed was still evolving. Unlike external and close-conduit flows, however, the bed shear stress in free-surface flow is also affected by the local water depth, and thus the composite water surface profile created by the channel transition. The bed shear stress development closely followed the variation in local water depth, ultimately reaching equilibrium when the flow depth became uniform. It was also found that the choice of the displacement height of the mean velocity profile played an important role in determining the bed shear stress on a rough bed using the logarithmic law, and thus the development of bed shear stress in smooth-to-rough transition.

Library of Congress Subject Headings

Open-channel flow.
Shear flow.
Strains and stresses.


South Dakota State University



Rights Statement

In Copyright