Welsh floodplain studies: The nature of floodplain geometry

Abstract

Field studies of floodplain geometry are necessary if the extent and patterns of inundation are to be related to discharge magnitudes. This is considered in relation to the floodplain geometry of parts of the three Welsh rivers — the Ystwyth, the Rheidol and the Tywi. A photogrammetric approach has been used and tested, together with the development of computer procedures for automated plotting of floodplain profiles on any required scale. The complex and locally variable relief (1.7 to 3.3 m) of the floodplains studied is shown to be related to former braided and meandering river activity, some of it within the past 130 years for which reasonable historical evidence is forthcoming. Relief results dominantly from the presence of abandoned channel loops, from point bars and from former channel bedforms now incorporated into the floodplain surface. Local aggradation and incision are also evident. Variation along the rivers is such that a single discharge-related flood-stage is not identifiable: instead there are sequences of flooding and emptying beginning at relatively low discharges and related to floodplain relief. Some consequences for flood studies and floodplain management are suggested.