Meander geometry, hydraulics and sedimentary structures—a case study of the Usri River section, Jharkhand, India

Abstract

The Usri River is a low sinuous ephemeral waterway of 90-km length, characterized by fluctuating stream discharge. During summer months (April–May), the river gets dried up almost completely exposing the point bars and the stream bed. It maintains an almost uniform gradient of 1:664 except in the lower 8–9-km reach, where the gradient changes to 1:150. The maximum bankfull width varies from 75 to 604 m, bankfull depth 1.30 to 3.02 m, stream bed slope 0.00125 to 0.00275, sinuosity 1.34 to 1.48, and bankfull discharge 210.40 to 351.91 m3/s. This study uses an enlarged data set comprising previous works and present study. The enlarged data set is used (1) to measure the different meander parameters of the Usri River; (2) to interrelate the meander parameters of the Usri according to the rules which have been empirically established in other parts of the world; (3) to derive a new equation from Chezy and Manning equations (Rouse 1961) and used in estimating the relative maximum discharge for two successive years, where the discharge hydrographs are not available; and (4) to establish an empirical relation between relative discharge and relative volume of sedimentation in the point bars. In the present study, two new linear equations have been established considering the ripple heights and cross-bedding thickness with water depth, one for small and large ripples when combined and the other for small ripples. Subsurface sedimentary structures in the point bars were studied by making trenches. It is observed that the frequency percentages of tabular type of cross-beddings increase downstream than the trough, lenticular type, possibly indicating the aggrading nature of the river with reducing effect of scouring downstream. The aggradation of sediments downstream is also reflected in the sigmoidal curve of the river profile, where the stream bed slopes of the studied four sites are considered.