GEOSTATISTICAL ANALYSIS OF SEDIMENT DEPOSITION IN 2 SMALL TIDAL WETLANDS, NORFOLK, UK

Attempts to assess the sedimentary status of coastal wetlands have typ ically utilized either residual sediment transport via major drainage channels ('creeks') or the rate of vertical substrate accretion. The f irst method neglects the interaction between channel and surface sedim entation and may be inappropriate where significant water movement occ urs across wetland margins. The second method requires areal interpola tion from a limited number of cores, sediment traps or marker horizons , yet the effects of different spatial and temporal sampling strategie s are rarely considered. This paper presents an analysis of spatial pa tterns of sediment deposition within two tidal wetlands on the eastern coast of England. Extensive deployments of surface-mounted sediment t raps over individual tidal cycles provide new insights into the spatia l scales over which particle settling varies. sedimentation is appropr iately considered as a regionalized variable and estimation of 2-dimen sional semivariograms allows the spatial scale of variability to be in corporated into interpolated 'sedimentation surfaces'. This enables mo re accurate estimation of mass fluxes than is possible from convention al arithmetic averaging or measurements of mass flux via drainage cree ks. Rapid particle settling during lateral and apical overtopping of t he creeks results in coherent patterns of sedimentation at spatial sca les of the order 20-200 m, depending on the developmental stage of the wetland surface-channel system. Comparison of these findings with ver tical accretion averaged over several years indicates an increasing de pendence of sedimentation rate upon inundation frequency (and therefor e elevation) as the time-averaging period is lengthened. Failure to co nsider these spatial and temporal sampling requirements may result in misleading assessments of wetland vulnerability to future accelerated sea-level rise or to changes in sediment supply.