Field observations of hydrodynamic conditions and suspended particulate matter in the southern North Sea

As a component part of multi-disciplinary field experiments investigating the physical, biological and chemical processes involved in the resuspension and transport of suspended particulate matter (SPM), this paper presents results from measurements of the local hydrodynamic conditions and the average concentration of SPM, (C) over bar, obtained at two field sites in the southern North Sea of Europe. At Sites A and B, average water-depth, d, was 29 and 47 m, respectively, and the median grain diameter, D(50), for sediments comprising the bed was 250 and 100 mu m, respectively. Observed SPM at both field sites comprised organic particles characterised by average settling velocity values, (w) over bar(s), in the range 10(-5) to 10(-1) mm s(-1). At Site A, SPM time-series exhibited a characteristic double-peaked form attributable to the local resuspension and the tidal advection of SPM. at quarter-diurnal and semi-diurnal frequencies, respectively. For time-average combined wave-current, w-c, bed shear velocity, (U) over bar(*wc) values >0.3 cm s(-1) (C) over bar was largely unrelated-to measured hydrodynamic conditions indicating that SPM was limited by the local availability of material for resuspension. In contrast, well-established faunal communities at Site B indicated a stable environment where the mobilisation of the bottom sediments was infrequent. With no wave stirring at the bed and weak tidal currents during the experiment, no correlation between observed (C) over bar and (U) over bar(*wc) was observed at Site B. Estimates of (U) over bar(*wc) at Site A exceeded the expected threshold for mobilisation of the bottom sediments for approximately 30% of the total time. Measured bedload transport rates, Q(b), in the range 0.05 x 10(-7) to 5.78 x 10(-7)m(2) s(-1) were obtained through analysis of sea bed photographs. (C) 1998 Elsevier Science Ltd. Al! rights reserved.