|A comparison of floc properties observed during neap and spring tidal conditions|
Manning, A.J.; Dyer, K.R. (2002). A comparison of floc properties observed during neap and spring tidal conditions, in: Winterwerp, J.C. et al. (Ed.) Fine sediment dynamics in the marine environment. Proceedings in Marine Science, 5: pp. 233-250
In: Winterwerp, J.C.; Kranenburg, C. (Ed.) (2002). Fine sediment dynamics in the marine environment. Proceedings in Marine Science, 5. Elsevier: Amsterdam. ISBN 0-444-51136-9. XV, 713 pp.
In: Proceedings in Marine Science. Elsevier: New York. ISSN 1568-2692; e-ISSN 2352-2860
cohesive sediment; flocculation; turbulent shear stress; floe size; settling velocity; effective density; suspended particulate matter; INSSEV instrument; Tamar estuary; turbidity maximum
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It is recognised that in order to properly understand how suspended particulate matter behaves during different tidal conditions within an estuary, high quality in-situ data is of a prime requirement. This paper initially presents floc data sets collected in the upper reaches of the Tamar estuary in south-western England. All floc samples were obtained using the in-situ sampling device INSSEV. The floc data was supplemented by simultaneous time series of near-bed profiles (using the high frequency POST system) of turbulent shear stress (TSS), suspended particulate matter (SPM) and current velocity. To enable a comparison of typical spring and neap tidal conditions, respective data sets were collected (on a sub-tidal duration) on 24(th) June 1998 and 5(th) August 1998. The spring tides experienced nearly twice the annual mean river flow (similar to40 m(3)s(-1)), and salinity did not exceed 0.5 at anytime during sampling. The afternoon flood saw surface currents approaching 1.1 ms(-1), and a maximum TSS of 0.7 Nm(-2) (at 25 cm). Throughout this period a concentrated benthic suspension layer developed, which displayed a peak particle concentration of 6 gl(-1) (50 cm above the bed) and a lutocline similar to40-60 cm above the bed. For the 5(th) August the annual mean river flow allowed the near-bed salinity at Station A to reach 8 during the afternoon flood. Surface currents did not exceed 0.55 ms(-1) and the SPM remained under 190 mgl(-1), with the exception of the turbidity maximum (TM) formation at sampling Station A 1.5 hours into the flood, where the near-bed SPM rose to 1.15 gl(-1). The maximum flood TSS 25 cm above the bed was 0.74 Nm(-2) and occurred just prior to the TM formation. An abundance of fast settling macroflocs (> 160 microns) from spring tides, accounted for a time series average of 89% of the mass settling flux (MSF). Whereas during neap tides, the macroflocs contributed 16% less to the MSF rate. This was partly due to a time series averaged macrofloc settling velocity of 4.6 mms(-1) from the spring tidal data; 2.8 mms(-1) higher than for neap tide conditions. During the TM passage at spring tides, macroflocs reached 1.5 mm in diameter; these floes had settling velocities of up to 16.6 mms(-1), but effective densities were less than 50 kgm(-3), which means they would be prone to break-up when settling to a region of high shear. At the opposite end of the scale, low SPM and quiescent conditions severely restricted floc production. A multiple parametric analysis identified both the TSS and SPM concentration as significant controllers of the settling velocity of the macroflocs, and these parameters must be included within any quantitative empirical algorithms.