Floodplains: the forgotten and abused component of the fluvial system

Abstract

Abstract. River restoration is strongly focussed on in-channel initiatives driven by fisheries interests and a continued desire for river stability. This contrasts greatly with the inherently mobile nature of watercourses. What is often overlooked is the fact that many rivers have developed floodplain units that would naturally operate as integrated functional systems, moderating the effects of extreme floods by distributing flow energy and sediment transport capacity through out of bank flooding. Floodplain utilisation for farming activities and landowner intransigence when it comes to acknowledging that the floodplain is part of the river system, has resulted in floodplains being the most degraded fluvial morphologic unit, both in terms of loss of form and function and sheer levels of spatial impact. The degradation has been facilitated by the failure of regulatory mechanisms to adequately acknowledge floodplain form and function. This is testament to the 'inward looking' thinking behind national assessment strategies. This paper reviews the state of floodplain systems drawing on quantitative data from England and Wales to argue for greater consideration of the floodplain in relation to river management. The database is poor and must be improved, however it does reveal significant loss of watercourse-floodplain connectivity linked to direct flood alleviation measures and also to altered flood frequency as a result of river downcutting following river engineering. These latter effects have persisted along many watercourses despite the historic nature of the engineering interventions and will continue to exacerbate the risk of flooding to downstream communities. We also present several examples of the local and wider values of reinstating floodplain form and function, demonstrating major ecological gains, improvement to downstream flood reduction, elevation of water quality status and reductions in overall fine sediment loss from farmland. A re-think is required regarding our approach to managing floodplains and funding floodplain restoration, arguing for greater recognition of the natural role of the floodplain as a resource for upstream flood management and as an agent for overall biotic improvement in line with restoration objectives.

Aerial Laser Scanning Data as a Source of Terrain Modeling in a Fluvial Environment: Biasing Factors of Terrain Height Accuracy

“…An accurate DTM can be exploited to delineate trajectories of moving water during high and retreating floods, to find appropriate places for ploughing or animal grazing, to identify environmental conditions which provide good habitats for valuable species, or even to help with the detection of heavy metal hotspots [2,4,[35][36][37][38]. However, the most important locations of floodplains are usually impervious places with dense vegetation combined with permanent or periodical water cover (swales, oxbow lakes, peats, or marshes) [39]. Accordingly, field surveys cannot be effective; furthermore, photogrammetry also has problems due to the vegetation cover and its ability to produce only a digital surface model (DSM) [40,41].…”