Deposition and storage of fine-grained sediment within the main channel system of the River Tweed, Scotland

Abstract

This paper assesses the importance of deposition and storage of fine-grained (c.`150 "m) sediment on the floodplains and beds of the main (non-tidal) channels of the River Tweed (4390 km 2 ), Scotland, and two of its tributaries (River Teviot and Ettrick Water). Caesium-137 analysis of floodplain sediment cores has been used to estimate average rates of overbank sedimentation during the last 30 to 40 years. Average values for individual transects ranged from 0Á16 to 2Á18 kg m À2 a À1(0Á13 to 2Á2 mm a À1 ). The mean for the 10 transects investigated was 1Á29 kg m À2 a À1 (1Á3 mm a À1 ). The total amount of fine sediment deposited was estimated to be about 44 000 t a À1. The fine-grained sediment stored in the channel bed was quantified using resuspension techniques. Average values for individual sites ranged from 0Á12 to 0Á96 kg m À2. The mean for the 10 sites investigated was 0Á56 kg m À2. The total amount of sediment stored on the channel bed of the main channel system at the time of sampling was estimated to be about 4300 t. Comparison of these estimates of floodplain and channel storage with the estimated suspended sediment load for the River Tweed at the downstream gauging site at Norham, indicates that floodplain sedimentation and channel bed storage represent about 40 and 4 per cent, respectively, of the annual load of fine sediment delivered to the main channel system. Erosion of channel banks will reintroduce the equivalent of about 30 per cent of the floodplain-deposited sediment back into the channel. The residence time of the fine-grained sediment stored on the channel bed is probably less than one year, but that of sediment deposited on the floodplain is likely to be considerably longer. Conveyance losses associated with overbank deposition have important implications for the routing of sediment through fluvial systems and the interpretation of downstream sediment yields.

Asynchronicity of fine sediment supply and its effects on transport and storage in a regulated river

“…Walling et al 1998Walling et al , 2006Owens et al 1999;Wilson et al 2004;Collins et al 2005;Collins and Walling 2007b). High values of storage tend to be found in small agricultural catchments (e.g.…”

Metal contamination of bed sediments in the Irwell and Upper Mersey catchments, northwest England: exploring the legacy of industry and urban growth

“…By isolating a known area of river bed, the use of this cylindrical sampling apparatus permits estimation of fine-grained sediment storage at each site using Eq. (1) (Owens et al 1999):…”

Control of sediment deposition rates in two mid-Atlantic Coast tidal freshwater wetlands

“…In contrast, the percentage of marsh and unvegetated mudflat increases nearly three-fold from about 3% above Walkerton to 8% between Walkerton and Gleason (Table 1). Marshes and mudflats are large reservoirs of sediment that can be resuspended, transported, and deposited further downstream (Ham & Church, 2000;Owens, Walling, & Leeks, 1999;Stanley & Hait, 2000). For example, about 40% of the river suspended sediment load of the River Tweed in Scotland is stored in the floodplain, compared to 4% stored in the river channel (Owens et al, 1999).…”