Rates of contemporary overbank sedimentation and sediment storage on the floodplains of the main channel systems of the Yorkshire Ouse and River Tweed, UK

Rates of contemporary overbank sedimentation and sediment storage on the fl oodplains of the main channel systems of the Yorkshire Ouse and River Tweed , UK, have been investigated, so that the estimates of sediment flux obtain ed for these rivers within the framework of the LOIS Community Research Pro gramme can be considered in relation to their overall suspended sediment bu dgets. Caesium-137 measurements have been used to establish the recent rate s of accumulation of fine-grained overbank sediment deposits associated wit h sediment cores collected from a series of transects across the floodplain s of the study rivers. Average annual sedimentation rates over the past 30- 40 years estimated for individual cores ranged from 0 to c. 1.60 g cm(-2) y r(-1) in the Ouse Basin and from 0 to c. 0.70 g cm(-2) yr(-1) in the Tweed Basin. In general, highest accumulation rates occur closest to the channel and values decrease with increasing distance from the channel, although for some transects very low accumulation rates also occur near the channel. Av erage overbank sedimentation rates for individual transects ranged from 0.0 10 to 0.554 g cm(-2) yr(-1) in the Ouse Basin and from 0.016 to 0.218 g cm( -2) yr(-1) in the Tweed Basin, with the mean for all transects being 0.206 and 0.129 g cm(-2) yr(-1), respectively. The total annual storage of fine-g rained sediment on the floodplains bordering the main channel systems of th e study rivers have been estimated by extrapolating the sedimentation rates derived for the individual transects to the adjacent floodplain reaches an d calculating the total mass of sediment deposited upstream of the tidal li mits. The resulting values are 70 970 t for the River Ouse (including the R iver Wharfe) system and 43 920 t for the River Tweed. These values of flood plain storage have been compared with the annual suspended sediment loads a t downstream gauging sites for 1995 and 1996 provided by the LOIS Core Moni toring Programme for the River Ouse and estimated from Harmonized Monitorin g Programme data for the River Tweed. This comparison indicates that 39 and 40% of the total suspended sediment delivered to the main channel systems of the rivers Ouse (to Skelton) and Tweed (to Norham), respectively, is dep osited and stored on their floodplains. For the River Wharfe, the conveyanc e loss is c. 50%, and this higher value is thought to reflect the low sedim ent loads recorded for 1995 and 1996, which are likely to underestimate the longer term average values with which the estimates of sediment storage sh ould be compared. The results obtained have important implications for the interpretation of downstream suspended sediment fluxes in the study rivers in terms of sediment mobilization and transfer within the upstream drainage basin. Copyright (C) 1999 John Wiley & Sons, Ltd.