Formation and Effects of Alternate Bars

Abstract

Major river regulation works have often resulted in alternate bar formation. Experimental work is described which provides more information on this phenomenon. Alternate bar formation is closely associated with meandering, inferring that alternate bar formation is caused by flow induced periodic disturbances of the horizontal velocity profile. Based on these considerations a criterion has been developed which defines the upper limit of alternate bar formation, and this has been satisfactorily compared to other formation criteria and experimental data. The lower limit is given by the initiation of bedload transport. A simple relation between slope, channel width and grain size is then deduced to indicate whether for a given channel, alternate bars can develop for any discharge. This relation takes into account armoring effects. Another equation is proposed to predict the scour depth resulting from alternate bar formation. Such scour often induces costly river maintainance works. Finally, some interesting results on flow resistance have been found, leading to a tentative equation for the intermediate roughness range.