FIELD OBSERVATIONS OF HYDRODYNAMIC CONDITIONS AND SUSPENDED PARTICULATE MATTER IN THE SOUTHERN NORTH-SEA

As a component part of multi-disciplinary field experiments investigat ing the physical, biological and chemical processes involved in the re suspension and transport of suspended particulate matter (SPM), this p aper presents results from measurements of the local hydrodynamic cond itions 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 media n grain diameter, D-50, for sediments comprising the bed was 250 and 1 00 mu m, respectively. Observed SPM at both field sites comprised orga nic particles characterised by average settling velocity values, (w) o ver bar(s), in the range 10(-5) to 10(-1) mm s(-1). At Site A, SPM tim e-series exhibited a characteristic double-peaked form attributable to the local resuspension and the tidal advection of SPM. at quarter-diu rnal and semi-diurnal frequencies, respectively. For time-average comb ined 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 hydrodynam ic conditions indicating that SPM was limited by the local availabilit y of material for resuspension. In contrast, well-established faunal c ommunities at Site B indicated a stable environment where the mobilisa tion 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 Si te B. Estimates of (U) over bar(wc) at Site A exceeded the expected t hreshold for mobilisation of the bottom sediments for approximately 30 % of the total time. Measured bedload transport rates, Q(b), in the ra nge 0.05 x 10(-7) to 5.78 x 10(-7)m(2) s(-1) were obtained through ana lysis of sea bed photographs. (C) 1998 Elsevier Science Ltd. Al! right s reserved.