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This thesis integrates existing work on surface accretion rates with more recent advances in the understanding of creek hydrodynamics. Concepts drawn from various disciplines are formulated into a revised framework within which marsh sedimentation may be better understood. Channel flux studies that treat the marsh as a 'black box' contribute little to our understanding of marsh functioning, whatever the accuracy of their execution. Marshes are better conceptualised as complex bio-sedimentary systems, characterised not only by an intimate relationship with adjacent tidal waters, but also by numerous internal pathways along which transport of water and materials may take place. Data relating to surface sediments, surface sedimentation, channel and over-marsh hydrodynamics, and suspended sediment composition and settling behaviour have been obtained within a relatively mature back-barrier marsh, 54 ha in area, at Scolt Head Island, on the north Norfolk coast. Tidal range averages 3.2m at neaps and 6.4m at springs. The annual sediment input to the marsh surface is estimated at 675 tonnes, equivalent to a mean accretion rate of approximately 0.26 cm/year. The broad pattern in sedimentation reflects surface topography via its control over inundation frequency. Locally, however, proximity to the creek system as an intermediate sediment source determines the rate and nature of sedimentation. The formation of composite particles via the action of flocculation and organic binding agents determines the depositional behaviour of fine cohesive sediment introduced to the marsh. Particle fall velocities are thereby enhanced, with settling from over-marsh tidal flows being a continuous process and not confined to slack water. Reworking of newly deposited material by deposit feeding gastropods appears to be intense over much of the marsh, and may be an explanation for the paucity of sedimentary structures. The marsh surface acts as a topographic threshold separating markedly different spring and neap creek flow regimes. Morphological development of the creeks is effectively confined to ebb-dominated spring tides, when export of sand occurs. Though the creeks act as effective conduits for material transport, a large proportion of the total spring tidal prism may be exchanged directly over the marsh edge. These results may be viewed in the context of present concern over rising sea levels linked to human-induced global warming. Scenarios for future sea-level change vary widely, though most estimates put the present rate of eustatic rise at approximately 1-2 mm/year. Subsidence along this coast is around 1mm/year. Thus many marshes exist in delicate equilibrium with present relative sea level. A numerical model incorporating quasi-continuous deposition, annual tide data, and eustatic and crustal movements successfully simulates historic marsh sedimentation along the north Norfolk coast and provides an insight into the possible effects of future eustatic changes.

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The objective of this research was to investigate the sediment behaviour in a compound channel and, in particular, the transfer of sediment between a main channel and its flood plain. A review of the literature on compound channels showed that, whilst research on fixed boundaries had been carried out or was in hand, there was no evidence that the loose boundary situation had previously been studied.Experiments were conducted in a loose boundary, outdoor sand channel of symmetrical compound section. The channel was of straight alignment, 50 m long, with an overall width of 3 m. Pumping equipment was available for recirculation of the water and the sediment.Samples of suspended sediment were collected from the shallow and deep sections and analysed by Coulter Counter to obtain the particle size distributions. The distribution curves were found to be very similar for the main channel and the flood plains.Sand from the channel was labelled by fluorescent dye and inserted in the bed of the main channel so as to simulate a point release. Bed samples were collected at sections 5 m and 10 m downstream of the injection point and examined under UV light for their tracer proportions. It was found that the cross-sectional distribution of tracer concentration was approximately Gaussian indicating that there had been some sediment transfer to the flood plains.A two-dimensional diffusion model, which accounts for the movement in the longitudinal and lateral directions, has been applied to describe the transport and dispersion of the tracer particles. A best-fit overlay with the experimental results enabled the longitudinal and lateral dispersion coefficients to be established. The model results, for the distribution of tracer, are depicted in 2 and 3-dimensional form at a sequence of time intervals for up to 2 hours after tracer release.It was concluded that under steady state conditions there would be a constant transfer of sediment from the main channel to the flood plains; in fact, analysis showed that approximately 40% of a continuous tracer injection in the main channel would be transferred to the flood plains.

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