TECHNICAL NOTE Discharge Measurements and Roughness Coefficient Estimation in a River. The Case of Strymonas River in Northern Greece Evangelos Hatzigiannakis 1 & Dimitrios Pantelakis 1 & Ioannis Hatzispiroglou 1 & George Arampatzis 1 & Andreas Ilias 1 & Andreas Panagopoulos 1 Received: 24 March 2015 /Accepted: 24 December 2015 /Published online: 11 January 2016 # Springer International Publishing Switzerland 2016 Abstract In order to study river flow, the discharge and the channel bed roughness should be estimated. Discharge has been calculated by the continuity equation. The roughness coefficient of the Manning equation has been used with a view to estimate the bed roughness. In the literature, different values of the Manning roughness coefficient are determined for various flow conditions and geometric characteristics of river sections or different Manning roughness coefficient values are derived from calibration of various numerical models. Measurements of the flow velocity, the flow depth and the cross section area have been performed at three sections along the River Strymonas, which is located in the plain of Serres in Northern Greece. Measurements have been made over the three bridges once a month for a period of 16 months. A modern flow meter has been used in order to measure flow velocity. The monitoring results have shown that the variation of the roughness coefficient, considering the river bottom slope stable, with the hydro-geometric characteristics of the flow is noteworthy and the selection of a constant coefficient value for the simulation of the flow in Strymonas river would not be satisfactory. Keywords Manning equation . Bed roughness . River discharge . Strymonas . Flow meter 1 Introduction Flow measurements in open channels are essential in water resources management (Corato et al. 2014; Velísková et al. 2014). Discharge (Q), flow depth (y), velocity (U) and Manning Environ. Process. (2016) 3:263–275 DOI 10.1007/s40710-015-0120-4 * Dimitrios Pantelakis pantelakisdim@gmail.com 1 Hellenic Agricultural Organization FElgo-Demeter_, Institute of Soil and Water Resources, 57400 Sindos Thessaloniki, Greece