Sediment deposition along the channel margins of a reach of the middle River Severn, UK

Abstract

This paper investigates vegetational, hydrological and geomorphological influences on sedimentation within the river margin of a 1.7 km, sinuous reach of the River Severn, UK. The empirical study was designed to investigate the impact of the following factors on riparian zone sediment deposition: (i) land use, (ii) channel planform location, (iii) the flood stage at which inundation occurs, (iv) submersion time, (v) flood event timing and ordering. Sedimentation was observed using sediment traps located on 48 sample plots within four sites (one on pasture, one within a poplar plantation and two within mixed riparian woodland) during four flood events. A total of 177 observations of sediment deposition were analysed for total dry weight of sediment, particle size and organic content. All sample plots were located within a 30 m wide ribbon of the riparian zone, immediately above the limit of perennial vegetation (i.e. the edge of the low flow channel). Patterns and rates of sedimentation differed between flood events. Whilst significant differences were found in sedimentation between the four sample sites, these were interpreted to be more a function of planform location than land use. The most marked variations were found in the quantity, particle size and organic content of deposited sediment according to riparian zone cross-sectional morphology and elevation. Highest sedimentation occurred within a 1 m elevation band that correlated with the channel-ward face of a morphological bench. Percentage sand content of deposited sediment exhibited a change from approximately 35% in a band extending from the low flow channel to a 1 m elevation above the morphological bench edge to approximately 10% above this level. Organic matter content was low (less than 5%) in sediment deposited up to a 1 m elevation above the edge of the morphological bench. Above this level, there was a gradual increase in organic matter content. These observations suggest a link between the form of the riparian zone and the sedimentation process. The relative influence of vegetational, hydrological and geomorphological influences on sedimentation characteristics are also discussed.