Vegetated flows in their environmental context: a review

This paper reviews current understanding of vegetated flows in fluvial channels. First, the physical aspects of this understanding are considered, starting with flow resistance estimation, where the methods used in practice are, to a certain extent, disconnected from theoretical mechanical understanding. Theoretical, semi-empirical and empirical approaches aiming to connect them are considered. Although progress has been made, these approaches have not yet been able to provide theoretically robust, logistically amenable methods that can be put into practice. This is because of the variety and complexity of fluvial channel vegetation, which make it difficult both to generalise theoretical approaches and to make empirical approaches logistically tractable. Mean flow and turbulence structure in fluvial vegetation, and their influence on particulate and solute transport are then reviewed, noting that progress in these areas has been less problematic. This is explained as an issue of scale. Studies of flow structures and transport are relevant to scales to which hydrodynamic instruments and concepts are best suited, whereas flow resistance studies are primarily relevant at reach scale, not the vegetation patch scale at which they are usually studied. Therefore, there is a need to find ways of parameterising vegetative flow resistance at reach scales. This requires understanding of how vegetation is spatially distributed at this scale. To obtain this the primary controls on vegetation distribution are then reviewed and its relationships to hydrodynamics, water quality, hydro-morphology and habitat structure considered. It is concluded that this scale-appropriate, inter-disciplinary approach is likely to be the best way of improving flow resistance measurement, whereas work at smaller scales on the hydrodynamic structure of vegetated flows can continue to provide new insights into hydrodynamic influences on ecology and solute and sediment budgets.