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Intertidal mudflats can experience rapid morphological changes, and are both sources and sinks of fine cohesive sediment within an estuary. Successful environmental management of these regions depends upon in-situ measurements, which help specify the interactions between the active processes and so allow the development of predictive models that the management practices require. The Profiles of Sediment Transport system (POST) has been developed in order to make high frequency measurements of velocity and suspended sediment concentration profiles in very shallow water (i.e. when depth, h < 1.0 m). Electromagnetic current meters and optical sensors were miniaturised to allow measurements within a few centimetres of the sea bed and provide fine scale resolution of vertical profiles. Two in-situ experiments, located in the Severn and Humber estuaries, examined the response of a mudflat to changing environmental factors, and in particular, studied the influence of local waves and tidal currents in very shallow water of depth (h)< 1.0 m. A value of 0.127 Nmˉ²was considered to be representative of the critical erosion shear stress (Tint) at Portishead, while at Skeffling τait was estimated to be about 0.31 Nmˉ². The effects of wave and current action were quantified and expressions were used to describe the relationships between velocity, bed shear stress and concentration. An expression relating near bed concentration to mean velocity (U ) at Skeffling was simply: Concentration (gLˉ¹) = 1.908 U + 0.193 when h< 1.0 m. R² = 0.730 The physical processes causing erosion and deposition across two mudflats have been identified, and the predictive expressions are considered to provide first order approximations for sediment concentrations and transport behaviour, for similar conditions at other North European sites. The results showed that the shallow water periods at the beginning and end of tidal coverage were extremely important in determining the surface character of the mudflat, and any erosion was most marked at these times. Small waves can be crucial to erosion because of their large contribution to the bed shear stresses in shallow water. Ignoring biological and chemical variables (both of which can control of erosion), it is proposed that for typical temperate environmental conditions, some form of mudflat erosion is likely when h< 1.0 m, and either the significant wave height (H2) is greater than 0.25 m, or the near bed velocities exceed 0.2 msˉ¹.

Laboratory studies show that turbulence controls the size of flocs by disrupting those flocs which exceed a critical diameter. Estuarine floc sizes have been shown to vary with the spring/neap cycle and turbulence has been suggested as the mechanism. A survey of the tidal variations of cohesive sediment floc size distributions and turbulence parameters has been undertaken in the Tamar estuary in south-west Britain. In-situ particle size distributions have been obtained using a 'marinised' version of the 'Malvern' laser diffraction sizing system. Turbulent current speeds were obtained using 10 cm diameter annular electromagnetic current meters. Velocity data is analysed using the inertial dissipation method to provide turbulent dissipation rates. Turbulence and size data, along with profiles of current, salinity, temperature and suspended solids concentration, record the passage of turbidity maximum and salt intrusion over four complete tidal cycles. Time series of observed particle size distributions vary smoothly over timescales of about one hour and these variations are linked to the flow conditions. Eight subsections of the tidal cycle were selected over which size distributions and flow conditions were slowly varying and the size distributions were time averaged over these subsections, and the resulting distributions compared. Size distributions in the turbidity maximum are strongly influenced by the mean current speed and this is found to be due to the different resuspension characteristics of newly formed aggregates and consolidated primary particles. Distributions are less dependent on tidal range at other stages during the tidal cycle. The size dependence of settling velocities strongly influences the size distribution of particles reaching the bed during the final stages of erosion of the salt intrusion, when the salt/fresh interface descends at a rate less than the settling velocity of large flocs but greater than that of small particles. This tends to sharpen the downstream edge of the turbidity maximum and preferentially retain floc aggregates in the upper reaches of the estuary.

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The development and field testing of a particle tracing technique for the prediction and monitoring of cohesive sediment transport is described. Natural, chemical and water soluble dye tracers have been used for many years to determine water circulation in order to predict sediment transport. Radioactive and fluorescent particles have been used widely to predict sediment particle transport, but have been restricted mainly to non-cohesive sand and gravel transport studies due to the difficulties of preparation, handling, disassociation of the label from the particle and labour-intensive analysis. The development of a fine cohesive tracing technique therefore offered a significant advancement for the understanding and prediction of fine cohesive sediment and pollutant dynamics in aquatic environments if a sediment analogue could be developed. The physical properties, including size, surface charge, fiuorescence and settling velocity of natural fine cohesive sediment were analysed in order to passively and actively adsorb organic fluorescent dyes onto the sediment surface; the tests were largely unsuccessful. The physical properties of artificial fluorescent particles as sediment analogues were examined and found to have a close correlation to natural sediment. Analysis of the fluorescent particles in mud suspensions on an Analytical Flow Cytometer offered an automated and accurate method of tracer concentration determination at low dilutions. A preliminary field study was carried out in a small pool with encouraging results. A study in a shallow freshwater lake was carried out to determine the sediment dynamics in the lake. A depth-averaged model of the wind-driven circulation within the lake was used to interpret the distribution of tracer. Secondary transport and deposition clearly led to an accumulation of sediment and internal loading in the lake driven by hydrodynamical forcing. A study of the particle residence time and deposition-resuspension processes in the turbidity maximum of a macro-tidal estuary. Fluorescent particles were released into the turbidity maximum and were advected down-estuary on the ebb tide and up-esiuary on the flood tide. The residual mass budgets indicated a significant deposition of the particles in the upper estuary at slack high water. The particles were detected in estuarine surface waters 1 week after release. The fluorescent particles behaved in a similar way to the suspended sediment in both the lacustrine and estuarine study and were considered as sutiable tracers for cohesive sediment.

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