PDF《alluvial channel》

University of Wollongong Research Online Faculty of Engineering - Papers (Archive) Faculty of Engineering and Information Sciences

2005 Flow resistance and bed form geometry in a wide alluvial channel Shu-Qing Yang University of Wollongong, shuqing@uow.

edu.au Soon-Keat Tan Nanyang Technological University, Singapore Siow-Yong Lim Nanyang Technological University, Singapore http://ro.uow.edu.au/engpapers/5352 Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library: [email protected] Publication Details Yang, S., Tan, S. &

Lim, S. (2005). Flow resistance and bed form geometry in a wide alluvial channel. Water Resources Research,

41 (9), 1-8. Flow resistance and bed form geometry in a wide alluvial channel Shu-Qing Yang1 Division of Civil and Environmental Engineering, College of Engineering, Korea Maritime University, Busan, Republic of Korea Soon-Keat Tan Maritime Research Center, School of Civil and Environmental Engineering, Nanyang Technological University, Singapore Siow-Yong Lim School of Civil and Environmental Engineering, Nanyang Technological University, Singapore Received

25 April 2005;

accepted

20 May 2005;

published

22 September 2005. [1] This paper explores the underlying mechanism of flow resistance in a wide alluvial channel with bed forms. On the basis of published data, it is shown that the grain roughness can be taken as equal to

2 times the median diameter of the bed sediment. An empirical equation for the bed form roughness has been proposed, and it depends on the bed form height and bed form steepness. The influence of the bed form length and height on the total bed shear stress and energy slope is deliberated, and empirical expressions for the length of the separation zone behind the bed forms are also proposed. The study proposed an equation to compute the total bed shear stress as a function of the grain and bed form roughness as well as the important role of the bed form geometry in the overall flow resistance in alluvial channels. The model is tested and verified against

670 flume measurements and

1540 field observations. The computed and measured energy slopes are in good agreement with close to 71% of all data sets within the ±20% error band. Citation: Yang, S.-Q., S.-K. Tan, and S.-Y. Lim (2005), Flow resistance and bed form geometry in a wide alluvial channel, Water Resour. Res., 41, W09419, doi:10.1029/2005WR004211. 1. Introduction and Background [2] In an alluvial channel, the various regimes of bed forms are the results of complicated interactions between the overlying flow and the mobile bed sediments. The physics of bed form is complicated because the flow boundary is not fixed but changes dynamically according to the sediment characteristics, channel shape and flow strength, among other factors. The variable bed forms modify the flow resistance and therefore the stage-discharge relationship of the channel conveyance. [3] The mobile bed resistance depends on many interre- lated factors including the skin or grain resistance and form drag or bed form resistance. The former is dependent on the depth of flow and grain size at the boundary surface while the latter is the resistance associated with the eddy forma- tions and secondary circulations set up by the flow over the bed form. Whereas the flow resistance for a given flow depth and velocity in a rigid boundary channel is approx- imately constant with time, it is not so for a mobile bed channel with bed forms. The flow resistance in the latter needs to consider the contribution of both the grain and bed form resistance. Generally, the equation for total shear stress acting on a sand bed is given by to ? rgRS ?1? where to is total bed shear stress, r is fluid density;