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

Abstract

Thispaperassessestheimportanceofdepositionandstorageoffine-grained(c.< 150m)sedimentonthefloodplainsand bedsofthemain(non-tidal)channelsoftheRiverTweed(4390km 2 ),Scotland,andtwoofitstributaries(RiverTeviotand 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 016 to 218 kg m ˇ2 a ˇ1 (013to2 2m ma ˇ1 ).Themeanforthe10transectsinvestigatedwas129kgm ˇ2 a ˇ1 (1 3m ma ˇ1 ).Thetotalamountoffine sediment deposited was estimated to be about 44 000 t a ˇ1 . The fine-grained sediment stored in the channel bed was quantifiedusingresuspensiontechniques.Averagevaluesforindividualsitesrangedfrom012to096kgm ˇ2 .Themeanfor the10sitesinvestigatedwas056kgm ˇ2 .Thetotalamountofsedimentstoredonthechannelbedofthemainchannelsystem 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 loadfor the River Tweed at the downstream gauging site atNorham, indicates that floodplainsedimentationandchannelbedstoragerepresentabout40and4percent,respectively,oftheannualloadoffine sedimentdeliveredtothemainchannelsystem.Erosionofchannelbankswillreintroducetheequivalentofabout30percent 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 throughfluvialsystemsandtheinterpretationofdownstreamsedimentyields.Copyright#1999JohnWiley&Sons,Ltd.