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, C̄, 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, D50, for sediments comprising the bed was 250 and 100 μm, respectively. Observed SPM at both field sites comprised organic particles characterised by average settling velocity values, w̄s, 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, Ū∗wc values >0.3 cm s-1, C̄ 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 C̄ and Ū∗wc was observed at Site B. Estimates of Ū∗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, Qb, in the range 0.05×10-7 to 5.78×10-7 m2 s-1 were obtained through analysis of sea bed photographs.