Settling fluxes of cohesive sediments measured by sediment traps in a semi-enclosed embayment with strong tidal environments

Abstract

To understand the variations of vertical settling flux in a spring-neap tidal cycle in strong tidal environments, an anchored vessel with a Hydro-Bios Multi-Sediment Trap and a Nortek Acoustic Doppler Velocimeter (ADV) was deployed in the sub-tidal flat in Luoyuan Bay, a typical semi-enclosed embayment, from January 16 to 31, 2010 (from spring to neap to spring again). The observed results reveal that there is a strong relationship between the Signal-Noise Ratio (SNR) recorded by the ADV and the synchronous suspended sediment concentration (SSC) obtained by the filtrated method. The time-series of SSC and settling velocity were then estimated. The measured results indicated that the maximum tidal range during the observation was 7.03m, which occurred during the spring tide, and the minimum value during the neap tide was 3.58m. The near-bottom (1.1m above the bed) current speed, SSC, bed shear stress, and settling velocity decreased from spring to neap tide, and the maximum values were 0.556m/s, 53.40mg/L, 0.806N/m2 and 0.26mm/s, respectively. The horizontal flux varied from 0.04g/m2s to 21.77g/m2s; the horizontal fluxes over a tidal cycle decreased from spring to neap tide, and the net horizontal flux of suspended sediment was 880.66kg/m2 with a net transport direction of 292° during the entire observation time. The daily vertical settling fluxes, calculated based on mass balance, varied from 17.03g/m2d to 56.86g/m2d, which were lower than those obtained by the sediment trap (18.50–332.80g/m2d). According to the analyzed results of the hydrodynamic data, when the current speed exceeds 0.20m/s, the flow can penetrate the whole sediment trap and the bottom quiescent layer no longer exists. The horizontal flux plays an important role in the trap collection efficiency in strong tidal environments, which can lead to overestimation and a maximum value of 585% during the spring tide. A constant settling velocity (mean value during the observation) may underestimate the daily settling fluxes by approximately 22.28–51.80% during the neap tide and overestimate the fluxes by approximately 19.61–30.86% during the spring tide.