Bulk properties of intertidal sediments in a muddy, macrotidal estuary

Measurements are presented of the bulk and mineralogical properties of intertidal sediments along the axis of the Tamar Estuary, southwest England. Bulk and dry density data indicate a significant increase in consolidation of the surface layer of the intertidal mudflats progressing down-estuary from the turbidity maximum region to the mouth. There are very significant trends of both density and estimated critical eroison shear stress with distance. Cross-sectionally averaged, bed shear stresses due to tidal currents are computed using a hydrodynamical model. These tidal stresses generally increase along the axis of the estuary from mouth to head and reach a maximum in the upper reaches. The increasing consolidation towards the mouth appears to result from the small bed shear stresses due to tidal currents in the lower reaches and the increasing proportion of coarse material there. Bed shear stresses are large in the upper reaches and regular intratidal resuspension and transport occur during spring tides, with little time available for consolidation following deposition during slack-water periods. The silt and clay fraction of the intertidal sediments increases from the mouth to the turbidity maximum region near the head (60 to > 99% dry weight). The particulate organic carbon (POC) content, assuming this to be approximated by loss on ignition, similarly increases from about 2% of dry weight near the mouth to about 8% in the turbidity maximum region. The POC content of a sample is largely dependent on the proportion of fine sediment within the sample, regardless of its position. The lower estuary is associated with intertidal sediments having relatively low silt and clay content and low associated organic material. It appears to be a fairly stable zone between the marine and estuarine environments. The upper estuary, in the turbidity maximum region, appears to be almost homogeneous in both POC (8.1 ±0.6%) and silt content (95±5%) during summer. The central region is an area of great variability in the size fraction, sediment type and POC content.