This paper summarizes current practices for the estimation of flow resistance caused by floodplain vegetation in emergent flow conditions. The current state-of-the-art for the parameterization of vegetative form drag and associated flow resistance was explored with a view on practical applicability. Specifically, the dissimilar resistance behaviour of simply shaped rigid elements and foliated natural vegetation was emphasized by compiling and reanalysing data published by the authors’ research teams as well as others. It was shown that describing the key hydraulic properties of plants, geometry, and flexibility, with species-specific parameters is superior to the rigid cylinder analogy commonly used in hydraulic engineering practice. The discussion on the limitations of many existing approaches for the determination of vegetative flow resistance is intended to advance the use of modern practices such as the parameterization of vegetation density with the leaf area index, a parameter that can be derived using remote sensing techniques.

There is no standard flow resistance equation for the determination of mean flow velocity in mountain streams. The reason lies in the morphology of mountain streams, i.e. steep slopes, large roughness elements, bed forms and water depths of the same order of magnitude as the bed material size. Logarithmic, Froude and power-law approaches to determine flow resistance are discussed with respect to the roughness parameter which is usually a characteristic grain size. As a result of the irregular nature of gravel-bed profiles it is shown that the structure of these stream beds cannot be described sufficiently by a characteristic percentile of the grain size distribution. Statistical properties of a series of bed profiles are investigated in order to quantify the effect of roughness on flow resistance. The standard deviation of the bed elevations is introduced as characteristic roughness length and its applicability is verified by the analysis of experimental data. Based on this roughness parameter, an approach for the determination of flow resistance is derived which allows for spatial averaging of the flow field. Conclusions concerning the influence of bed forms on flow resistance are drawn with the use of the concept of the "at-a-site" hydraulic geometry.

The paper briefly outlines the double-averaging methodology for studying environmental rough-bed flows. It focuses on the applications of this methodology in environmental hydraulics by providing several examples illustrating advantages of this methodology over conventional approaches. Examples include: (1) identification of specific flow layers and flow types; (2) vertical distribution of the double-averaged velocity between the roughness tops and troughs; (3) vertical distribution of momentum fluxes and sinks for typical roughness types due to turbulence, mean flow heterogeneity, secondary currents, form drag, and viscous drag; (4) estimates of form-induced (dispersive) stresses and evaluation of their structure using quadrant analysis; and (5) closure development for mass-transfer-uptake processes for stream periphyton. These examples illustrate the advantages of the double-averaging methodology over conventional approaches as well as highlight its potential for studying flows over very rough beds, hig...