Preliminary findings of a study of the upper reaches of the Tamar Estuary, UK, throughout a complete tidal cycle: Part I: Linking hydrodynamic and sediment cycles

Abstract

Preliminary results are presented from an experiment in the turbidity maximum region of the Tamar Estuary, UK, to acquire detailed particle information of the suspension in relation to the main physical and biochemical driving processes throughout several complete tidal cycles. Results presented here focus on the relationship between the evolving suspension characteristics and physical mechanisms through one tidal cycle. The experiment took place over 5 days during April 2003. Insitu floc properties (including floc settling velocity, size, shape and effective density) were measured using the video-based INSSEV system. Profiles of full water column velocity, salinity, temperature and suspension concentrations were supplemented with detailed near bed measurements of turbulence and sediment concentrations. Measurements were made during several days approaching spring tides and the results exhibit a strong contrast in flood and ebb hydrodynamics which is reflected in the observed vertical distribution of sediment and particle characteristics. The flood tides were short and well-mixed with high values of turbulent shear stress causing rapid erosion and resuspension at the onset of the turbidity maximum. Suspension concentrations were relatively evenly distributed vertically reaching concentrations of 1.1 kg/m 3 at the INSSEV height of 0.5 m. In contrast ebb tides exhibited tidal straining and development of a concentrated benthic suspension layer with a lutocline in the region of the INSSEV height. Concentrations at this height reached 1.4 kg/m 3 while the INSSEV sampling height was just below the lutocline. Particles within the turbidity maximum on the ebb tide were typically much larger than the flood, with up to 30% greater than 400 μm just inside the lutocline. Turbulent shear stress measurements during the ebb exhibited a significant reduction in the region of the lutocline due to sediment stratification.