UNCERTAINTIES IN SUSPENDED SEDIMENT CONCENTRATION ANDTRANSPORT DUE TO VARIATIONS IN SEDIMENT SIZE

摘要

The sensitivity of optical backscatter sensors (OBS) to fine particles in suspension oftenrncauses difficulties in measuring total sediment mass concentration and transport. Small quantitiesrn(by mass) of mud mask much larger suspensions of sand resulting in significant errors in thernestimation of transport. Sediment samples from a timed settling tube (TST) were used to assess therninfluence of the size distribution on the concentration and transport estimates from OBS sensors atrna coastal location where the bed sediments had a wide range of sizes. Very large differences inrnsuspended concentration were obtained (factors of more than 10) when the OBS was calibratedrnagainst samples from layers in the TST compared with calibration against a ‘bulk’ (deploymentaverage)rnsediment sample. The largest errors occurred at times when the currents were strongest,rnresulting in large transport errors. Better results can be obtained by using the OBS as an indicatorrnof fine sediment only. It is concluded that reasonable estimates (±10%) of the fine (as opposed tornthe total) sediment in suspension can be obtained when OBS sensors are calibrated against the finerncomponent (mud and silt only) of the bed. Transport estimates at the OBS height will be similarlyrnimproved: integration over the whole water column to obtain flux will have the further uncertaintyrnresulting from the form of concentration profile assumed.