ORIGINAL PAPER Meander geometry, hydraulics and sedimentary structures—a case study of the Usri River section, Jharkhand, India Barendra Purkait 1 & Subhajit Sinha 1 Received: 2 January 2019 /Accepted: 1 April 2019 /Published online: 26 April 2019 # Saudi Society for Geosciences 2019 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 m 3 /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 Chézy 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. Keywords Usri River . Hydraulic properties . Point bars . Sedimentation Introduction The geometry of rivers like width, depth, slope, flow velocity, and planform is a manifestation of complex interaction of discharge sediment load sediment types and valley slope, with flow continuity, flow resistance, bed load, bank competence, and so on (c.f., Yalin 1972; Hey 1976 Kennedy et al. 1984; Doronzo and Dellino 2010; Cetin 2013). Their studies had generated empirical equations containing the abovementioned interacting variables in the laboratories and their simultaneous solutions are in good agreement with the flume experiments. The principle behind the equations may be well understood today, but those are far from applicability to the natural system as many of the variables have been either assumed or not taken into consideration. Much of the empirical relations are partial- ly supported by field observations. The planform of a river has a control from the motion of fluid and the sediment dispersal and storage points (Bridge 1977; Olesen 1984). The change in the fluid dynamics and sediment dispersal and storage pattern primarily generates the morphological change in alluvial riv- ers; of course other variables do play their role (e.g., Doronzo and Dellino 2010; Cetin 2013). The morphological change of alluvial rivers of fluctu- ating discharge is well known because much of the river during low discharge is exposed almost or completely giving a unique opportunity to study the sedimentology of the exposed channel bars vis-à-vis the meander geom- etry. A number of quantitative relations among river mor- phology, hydraulic parameters, and sedimentation have Editorial handling: Domenico M. Doronzo * Subhajit Sinha subho.ecstasy@gmail.com Barendra Purkait baren.purkait@gmail.com 1 Department of Geology, University of Calcutta, 35, Ballygunge Circular Road, Kolkata 700019, India Arabian Journal of Geosciences (2019) 12: 300 https://doi.org/10.1007/s12517-019-4456-3