MPC Research Reports
Report Details

Title:	Response of Bed Shear Stress in Open-Channel Flow to a Sudden Change in Bed Roughness
Authors:	Muhammad Farrukh Jamil, Francis C. K. Ting, and Monika Kafle
University:	South Dakota State University
Publication Date:	Oct 2024
Report #:	MPC-24-567
Project #:	MPC-688
TRID #:	01946165
Keywords:	channel flow, channels (waterways), hydrodynamics, roughness, shear stress, velocimeters

Abstract

The determination of bed shear stress plays a pivotal role in understanding the fluid dynamics in both natural and engineered channels. The commonly employed logarithmic law provides a mathematical formula to compute the bed shear stress in open-channel flows. However, its applicability in flow through transition remains relatively unexplored. In this study, laboratory 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 along 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 various methods. The primary objective was to investigate the effects of bed roughness and water surface profile on the variation of bed shear stress in gradually varied flows through the transition. It was found that the evolution of bed shear stress was related to both 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 flows was 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 condition when the flow depth became uniform. It was also found that the choice of 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 STR transitions.

How to Cite

Jamil, Muhammad Farrukh, Francis C. K. Ting, and Monika Kafle. Response of Bed Shear Stress in Open-Channel Flow to a Sudden Change in Bed Roughness, MPC-24-567. North Dakota State University - Upper Great Plains Transportation Institute, Fargo: Mountain-Plains Consortium, 2024.