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C. Jago and C. Bull (2000)

Quantification of errors in transmissometer-derived concentration of suspended particulate matter in the coastal zone: implications for flux determinations

Marine Geology, 169(3):273-;286.

Optical beam transmissometers were tested at two shallow water sites off the English coast of the southern North Sea. Concentration and particle size of suspended particulate matter (SPM) or suspended sediment were measured over tidal cycles using transmissometers and laser sizer. SPM concentrations and horizontal fluxes, derived from calibrated transmissometer data, were compared with measured concentrations and fluxes, determined gravimetrically from filtered samples.SPM displayed a bimodal particle size spectrum with primary modes at 20 and 125–250μm. The first site, in the Wash embayment, was dominated by tidal resuspension with semi-diurnal addition of a coarse resuspension mode to a fine background mode. The second site, adjacent to the mouth of the Humber estuary, was dominated by tidal advection of the turbid Humber plume so that SPM concentration and quality varied diurnally as a strong lateral concentration gradient (from turbid, plume water to clear, plankton-rich shelf water) advected along the coast.Short-term variability of particle size and type diminished the correlation between optical beam attenuation and SPM concentration. Transmissometer calibration was improved by adopting a two-part correlation. At the resuspension site, the two parts were separated by onset and cessation of resuspension, one part corresponding to SPM in long-term suspension, the other to resuspended SPM. At the turbid plume site, the two parts were separated by advection of the salinity front so that one part characterised lower salinity, turbid plume water, the second part higher salinity, clear shelf water.Calculated flood and ebb SPM fluxes were inaccurate (up to 23\% error) using conventional transmissometer calibration, but error reduced to an average 3\% using two-part calibrations. There were large errors (up to 51\%) in net tidal SPM flux estimates, but these reduced to an average 16\% using two-part calibrations.

Suspended sediment, Transmissometer, Suspended particulate matter
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