Cohesive sediment erosion and deposition was investigated over a twenty-two month period (1996 to 1998) on an intertidal mudflat within the Conwy Estuary, U.K. Observations were made in situ on a new, purpose built survey station, and in a laboratory flume - which collectively measured: mudflat elevation via levelling; sedimentological properties via bio-geophysical tests; tidal elevation and current flow via a velocity gradient unit and an electromagnetic current meter; water column salinity and temperature using a conductivity, temperature and depth sensor; suspended particulate matter dynamics via optical backscatter sensors, a transmissometer, pumped sampling, a field flume, and settling
velocity tubes; and finally, deposition patterns via sediment traps and a novel 'wiping' nephelometer, first reported in Ridd and Larcombe (1994).
The planar, steeply sloping, Type 2a (Dyer, 1998), intertidal mudflat at Conwy, merges shorewards with a saltmarsh; the lower mudflat is bounded by sandflats. Sediments consist of clayey muddy sand with high organic contents (> 5 %). Porosity and wet density values are 59 % and 1729 kg m^-3, respectively. A key morphological result revealed 8 cm of net deposition over eighteen months. Hydrodynamic results show a semi-diurnal macrotidal asymmetry. Maximum flood and ebb velocities (and associated bed shear stresses, ^τ0) were: U_0.65 = 0.44 m s^-1 and U_0.95 = 0.36 m s^-1 (4.08 and 2.93 N m^-2), respectively.
There was little evidence of sediment stratification in the water column. Summer and winter concentrations near the bed were 5- 65 and 50- 75 mg 1^-1, respectively. Roughness lengths (Z₀) of 0.43 mm from laboratory flume tests equate with field values of 0.44 mm. This supports the definitions for Zo given by Soulsby (1997). Sporadic erosion occurred on the ebb, with no evidence for a seasonal variation in the critical shear stress for bed failure ( "Cc)- Incipient motion occurred at ^τ0 = 0.04 N m^-2, whilst laboratory flume tests
show values for ^τc which are consistent with in situ values: 0.57 and 0.54 N
m^-2, respectively. Erosion rates were of the order of 10^-5 kg m^-2 s^-1, and are characterized as Type I (Mehta and Partheniades, 1982).
Deposition was quantified, at high frequencies (minutes), on the flood of most tides. The average threshold shear stress for deposition was 0.10 N m^-2.  Most deposition (90 % ) occurred by the time of flow reversal; deposition surpassed erosion. It is proposed that the consequent mudflat accretion rate was mediated by tidal and seasonal processes. An association is shown between the magnitude of deposition and the phase of the spring-neap cycle. Peak deposition rates during spring and neap tides were 24 and 6 mg cm^-2 tide^-1, respectively. An anomalous seasonal peak in deposition occurred during the spring bloom;
levels doubled to 42 mg cm^-2 tide^-1. Particle aggregation shortly before high water, probably due to Phaeocystis binding, gave rise to increased settling velocities of I mm s^-1. A logarithmic relationship is presented between settling velocity and concentration. The overall pattern of sediment accretion on the mudflat is shown by both the concurrent: low resolution levelling data and the higher resolution deposition rate data.