中規模河床形態に関する研究

Abstract

The meso-scale river bed configuration, which includes alternating bars, double rowbars, braided streams and so on, affects not only large scale river bed variation but alsolateral bank erosion. It is, therefore, one of main subjects in river mechanics to clarifythe characteristics of its length scale and behaviour.In this paper, the meso-scale bed configuration and its geometrical characteristics arediscussed by the use of previous experiments and ours which was conducted in two pris-matic flumes and a gradually expanding flume. As a results of the investigation, it isclarified that both bar height-stream width ratio ZB/B and its length-width ratio lB/Bdepend mainly on Froude number Fr and depth-width ratio h/B and nearly independ onbed shear stress-critical shear stress ratio τ/τ. On the basis of the geometrical considerationof bars, meso-scale bed configuration is classified into four groups: double row bar (braidedstream), alternating bar, semi bar and short diagonal bar, and hydraulic condition oftheir formation is inspected by means of dimensional consideration. Interesting result isobtained from this inspection that channel slope has no primary effect on their formationunder the condition 1<τ/τc<12 and a new parameter, h/d/(B/d)2/3 or {h/d/(B/h)2}1/3, isintroduced for the criterion of bed configuration. Consequently, domain of formation for eachbed configuration is shown on h/d-B/h plane.