Notes on the estimation of resistance to flow during flood wave propagation

Abstract

Abstract. The paper discusses methods of expressing and evaluating resistance to flow in an unsteady flow. Following meaningful trends in hydrological sciences, the paper suggests abandoning, where possible, resistance coefficients in favour of physically based variables such as shear stress and friction velocity. Consequently, an acknowledged method of friction velocity evaluation based on the relations derived from flow equations is examined. The paper presents both a theoretical discussion of various aspects of friction velocity evaluation and the application of the method to field data originating from artificial dam-break flood waves in a small lowland river. As the method is prone to many errors due to the scarcity and the uncertainty of measurement data, the aim of the paper is to provide suggestions on how to apply the method to enhance the correctness of the results. The main steps in applying the method include consideration of the shape of the channel, the type of wave, the method of evaluating the gradient of the flow depth, and the assessment of the uncertainty of the result. Friction velocity and the Manning coefficient are compared in terms of resistance to flow variability during flood wave propagation. It is concluded that the Manning coefficient may be a misleading indicator of the magnitude of resistance in unsteady flow, and to be inferior to physically based variables in such cases.

Reconstructing extreme post‐wildfire floods: a comparison of convective and mesoscale events

“…grain roughness, form drag due to bedforms and channel geometry, vegetation resistance, as well as added fluid resistance due to sediment transport) into a single roughness parameter (Trieste and Jarrett, ; Lumbroso and Gaume, ). Field guides and empirical equations have been developed to facilitate more accurate estimates of Manning's n , but these often perform poorly in small mountain streams (Marcus et al , ; Smith et al , ; Ferguson, ; Mrokowska et al , ) as evidenced by the wide range of Manning's n values estimated by these different methods for our reach (0.03 to 0.13, excluding XS7).…”

Dissolved oxygen and water temperature dynamics in lowland rivers over various timescales

“…The other most classical example is the hysteretic relation between water level and discharge during backwater events (Herschy, 2009;Hidayat et al, 2011). As has been well documented, single-parameter rating curves developed for particular hydrometric stations fail to reflect the real discharge under unsteady flow occurring during backwaters or flood propagation waves (Hidayat et al, 2011;Mrokowska et al, 2014;Petersen-Overleir, 2006;Rowiński et al, 2000). To compare, the studies of Moog and Whiting (1998) have shown the hysteresis between river discharge and bed load transport, whereas Williams (1989) reported hysteresis between discharge and sediment concentration rates during flood events.…”